EMG Evaluation of the Motor Unit - Electrophysiologic Biopsy

5/24/13EMG Evaluation of the Motor Unit - Electrophysiologic Biopsy Medscape Reference Reference News Reference Education MEDLINE EMG Evaluation of the Motor Unit Electrophysiologic Biopsy Author: Paul E Barkhaus, MD; Chief Editor: Nicholas Lorenzo, MD more... Updated: Nov 30, 2011 Overview The motor unit (MU) is a part of the neuromuscular system that contains an anterior horn cell, its axon, and all of the muscle fibers (MFs) that it innervates, including the axon's specialized point of connection to the MFs, the neuromuscular junction. The neuromuscular system also includes upper motor neuron inputs to the lower motor neuron as well as afferent inputs from the periphery such as position receptors in joints, Golgi tendon organs, and muscle spindles. The result is the ability of the muscle to make various patterns of contraction under the control of the upper motor neurons in the central nervous system (CNS). In routine needle-electrode examination (ie, electromyography [EMG]) of voluntary muscle contraction, the electrodiagnostic consultant assesses the signature electrical signal generated by the MUs, termed the MU action potential (MUAP). In addition, signals generated by individual or non-MU groups of MFs, termed insertion activity (IA) and spontaneous activity (SA), are assessed. These are patterns of random or rogue sub-MUAP activity that may yield important clues in the interpretation of the EMG signal. In electrodiagnosis, consideration of all data is important. The focus of this article is on MUs and the MUAPs they generate, particularly in neuromuscular disorders in which remodeling of the MU may occur. To place these concepts in perspective, some review of basic clinical neurophysiologic concepts is necessary. This discussion also is predicated on the assumption that the reader has some basic understanding of EMG. Beginning with the pioneering work of Professor Fritz Buchthal and colleagues in the 1950s,[1] the technique of EMG in MUAP analysis has undergone much refinement. In the last 2 decades, the advent of computers in EMG and variations in needle electrodes and instrumentation have given greater capability to the electrodiagnostic consultant. This has created a renaissance in clinical studies of the MU, particularly with the work of Professor Erik Stalberg. The formal analysis or quantitation of the MUAP signal, termed quantitative analysis (QA), is currently not as cumbersome and time-consuming as in the past. It involves a discipline of quantitating the various features of the MUAPs. However, electrodiagnostic consultants have become accustomed to the expedient of subjective assessment of the EMG signal in routine analysis. A general trend also exists toward underutilization of new technology in instrumentation. emedicine.medscape.com/article/1846028-overview#showall 1/49 5/24/13 EMG Evaluation of the Motor Unit - Electrophysiologic Biopsy QA is not necessarily needed in every study, but careful analysis of the MUAP signal is essential. So why is an understanding of at least the principles of QA important? QA forms the foundation upon which routine subjective assessment of the MUAP waveforms is made. Over the decades, clinicians have become immured with a closed-mindedness that reflexively links the diagnosis to the waveforms (eg, myopathic MUAP, neurogenic MUAP, BSAPs [brief, small, abundant potentials]). Daube argued against this terminologic approach long ago.[2] The EMG signal should be assessed in the context of all electrodiagnostic data. A diagnostic impression should not be intended to bias the referring clinician against other reasonable possibilities. In every routine EMG study, therefore, a more formal or objective approach to MUAP analysis should be made. This is accomplished by the electrodiagnostician training his or her eye to truly measure the size, complexity, and stability of the waveforms. This also means not being a passive observer of the waveforms as they race across the screen, but reaching out and manipulating the EMG instrument's settings to extract more information from this same signal (ie, interacting with the instrument). The authors refer to this approach as objective-interactive EMG, a practical compromise to traditional QA.[3, 4] This article primarily addresses the changes in MUAPs that may be observed in neuromuscular disorders. The approach is to try and correlate the changes in the waveforms with what may be observed in the underlying pathologic changes in the MUs. To do this, the electrodiagnostic consultant must modify his or her assessment of the waveforms to one more objective than the simple subjective assessment usually performed on routine EMG. The electrodiagnostic consultant must become familiar with the limitations of the instrumentation and be able to interact with the recording equipment, from the level of entry at the recording electrode to its display on the EMG. Therefore, the goal of objective-interactive EMG is to extract reliable and reproducible information from the waveforms. In determining abnormality, consistency in abnormality should be established, as well as some impression of severity and timing. The latter refers most to neuropathic processes in which relative staging of chronicity in reinnervation is possible. All electrodiagnostic data, including other studies, such as nerve conductions that are not addressed in this article, must be used appropriately. Some may consider EMG to be a mature area of investigation and mundane in clinical application. The authors hope this article stimulates the electrodiagnostic consultants to make each study an exciting objective-interactive challenge and to interpret the findings in the context of an electrophysiologic biopsy. Table 1. Abbreviations Used in this Article (Open Table in a new window) ACh AP CMAP CN CFL EMG FF FR IA IP MF MN MON MU MUAP Acetylcholine Action potential Compound muscle action potential Concentric needle electrode Critical firing level Electromyography Fast twitch, fatigable (type II alpha MN) Fast twitch, fatigue-resistant (type II alpha MN) Insertion activity Interference pattern Muscle fiber Motoneuron Monopolar Motor unit Motor unit action potential 2/49 ATPase Adenosine triphosphatase emedicine.medscape.com/article/1846028-overview#showall 5/24/13 EMG Evaluation of the Motor Unit - Electrophysiologic Biopsy MUT NA QA S SA Motor unit territory Not applicable Quantitative analysis Slow twitch, slow fatigue (type I alpha MN) Spontaneous activity SFEMG Single-fiber EMG Anatomy and types of MUs The term MU, as defined by Sherrington,[5, 6] still is considered the anterior horn cell or alpha motoneuron (MN), its axon, and all the MFs it innervates, including the neuromuscular junctions. It remains the basic functional element of a skeletal muscle. Conversely, a muscle may be considered a grouping of MUs. The image below schematically illustrates 2 MUs in a muscle. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Schematic show s 2 MUs. A, Muscle fibers (MFs) are show n in side view . The endplate zone is central to the MFs (x). Distal region of the MFs on either side of their endplates is w here their action potentials are propagated (y), ie, aw ay from the endplates. B, Schematic representation of A on transverse view (ie, crosssection) demonstrates the overlap of MFs from different MUs and thus overlap in MU territories (of w hich only a portion is show n). C, MU action potential (MUAP) is recorded from the endplate region (see A, region x) and is characterized by its initial negative deflection. D, MUAP show s the typical triphasic appearance w ith an initial positive deflection. This implies that it is recorded along the MFs aw ay from the endplate zone (see A, region y) (sensitivity 100 mV per division vertical; sw eep 5 ms per division). With permission, copyright P.E. Barkhaus, MD, 2001. There is no known polyneuronal innervation of mature human muscle (ie, a MF is not innervated by more than one MN). Conversely, an MN may supply from 6-10 MFs in extraocular muscle to hundreds of MFs in a large proximal limb muscle such as the biceps brachii.[7, 8, 9] The result is a remarkable expansion from controlling unit (ie, MN) to the endpoint of its apparatus (ie, MFs), in both anatomic and physiologic terms. In addition to the function of the MU in motor control, the MN also has a trophic effect in maintaining the integrity of the MFs at its endplate. The endplate is that specialized region between the motor nerve terminal and the MF that mediates neuromuscular transmission (see image above, region x). The endplate region on the MFs contains the acetylcholine (ACh) receptors. When these receptors are activated successfully, the result is an endplate potential. Consequent to the endplate potential, an action potential (AP) is generated that spreads electrotonically down the length of the MF. As the AP travels down the MF membrane, the contractile apparatus is activated in turn. The endplate zone in a healthy muscle is fairly homogeneous in that the endplates are usually at the mid portion along the length of the MFs (see image above). This may vary depending on the shape of the muscle.[10] Such positioning of the endplates allows greater efficiency in the bidirectional spread of the AP along the length of the MF membrane. Relatively minimal data are available on MF in humans. Knowing how MF length was assayed experimentally and differentiating between anatomic length and functional length are also important.[11] MFs may vary considerably in length from approximately a few centimeters in the biceps brachii to less than 2 cm in smaller distal limb muscles. The configuration of a muscle is complicated further by the spatial arrangement of its MFs (eg, pinnate vs emedicine.medscape.com/article/1846028-overview#showall 3/49 5/24/13 EMG Evaluation of the Motor Unit - Electrophysiologic Biopsy staggered). MFs have AP propagation velocities of 2-6 m/s (mean value 3.7 m/s). The composition of a muscle by MF types (see Table 2 below) depends in part on the functional demands on that muscle. The gastrocnemius, which has more of a role in static postural maintenance, tends to be represented more heavily with type S (ie, slow twitch, fatigue-resistant) alpha MNs (see Table 2 below). In contrast, the first dorsal interosseous muscle of the hand participates in more rapid, phasic movements and has more type FF (ie, fast twitch, fast to fatigue) MNs. Different classification schemes have been offered for MNs and MFs, which also may have some interspecies differences (see Table 2 below).[7, 8, 12, 13, 14, 15] Table 2. Types of Alpha MNs and Their Corresponding MF Type* (Open Table in a new window) Alpha MN Types I II II Relative Characteristics (S = slow twitch, fatigue resistant) (FR = fast twitch, resistant to fatigue) (FF = fast twitch, fast to fatigue) Neuronal cell-body size Axonal diameter Axonal conduction velocity After-hyperpolarization time Firing rate Afferent input (ie, dendritic volume) Relative excitability threshold Twitch tension Contraction time Fatigability Force generated Histochemical Reactivity Smaller Smaller Slow Long Slow and regular on minimal effort Large Low Low, long >99 ms Slow Low Corresponding MF Types 1 NA NA Fast Intermediate Intermediate Intermediate Intermediate Intermediate, long Intermediate Relatively slow Moderately high 2A High Low Medium high High High High Larger Larger Faster Short Fast on strong effort Small High High, brief < 85 ms Fast High 2B High Moderate Low High High Low Alkaline ATPase (pH 9.4) Low Acidic ATPase (pH 4.65) High Oxidative enzymes Glycogen Phosphorylase Myoglobin High Low Low High ATPase = adenosine triphosphatase; NA = not applicable. emedicine.medscape.com/article/1846028-overview#showall 4/49 cross-section) based on scanning EMG studies. The recording area has a 150-mm radius.E.[11] SFEMG Fiber-density measurements in healthy muscle suggest that MFs (see image below. MD. D. w hich can extend through the MU territory.casaengineering.com/article/1846028-overview#showall 5/49 . Schematic representation of A on transverse view (ie. Newer techniques in quantitative histochemistry suggest a possible diversity in histochemical and biochemical properties even within the MFs of the same MU. copyright P. MU action potential (MUAP) is recorded from the endplate region (see A. The concentric needle electrode (B) records from the inner core at its tip. aw ay from the endplates. the MUT has an estimated diameter of 5-10 mm (ie. The muscle fibers (MFs) closest to its tip contribute to such features as the amplitude (small semicircle). This implies that it is recorded along the MFs aw ay from the endplate zone (see A. 19] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. the inference is that MFs from other MUs separate MFs within the same MU as in a mosaic pattern. region y) (sensitivity 100 mV per division vertical. MUTs vary in size in different muscles. B.Electrophysiologic Biopsy *Although related. electrode C) often are separated by 300 µm. 8. with approximately 1-5 MFs per MU (see image below. alpha MN subtypes are not equal to MF histochemical types.com. emedicine.[16] In humans. In a larger proximal limb muscle such as the biceps brachii. but it records from a moderate portion of the MU territory (circle). section B). approximately 20-25 MUs are represented. These MFs contribute to the duration feature of the motor unit action potential.[7. region x) and is characterized by its initial negative deflection. Schematic compares 3 recording needle electrodes relative to an MU territory in a muscle.[17] Data from animals suggest greater dispersion of the MUT in a muscle that also may change in size along the length of the muscle. on routine histologic cross-section of muscle those MFs belonging to individual MUs still cannot be discerned. crosssection) demonstrates the overlap of MFs from different MUs and thus overlap in MU territories (of w hich only a portion is show n). A. ie. The recording surface of the macro-EMG electrode (A) is its cannula. sw eep 5 ms per division). Schematic show s 2 MUs. With permission. The single-fiber EMG electrode is the most selective (C). C.medscape.[8] This reflects the extensive interspersion of MFs from different MUs. Image courtesy of w w w . The MU territory (MUT) is defined as "the area in a muscle over which the MFs belonging to an individual MU are distributed". Although an MU is homogeneous for the histochemical type of MF it contains. The endplate zone is central to the MFs (x).5/24/13 EMG Evaluation of the Motor Unit . 2001. Barkhaus. MUAP show s the typical triphasic appearance w ith an initial positive deflection. In a low-power histologic cross-section of muscle containing approximately 100 MFs. Distal region of the MFs on either side of their endplates is w here their action potentials are propagated (y). w ith a recording surface of 15-mm diameter on the side of the needle's shaft.[18] Since the mean diameter of a healthy MF is 50-60 µm. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Muscle fibers (MFs) are show n in side view . Muscle fibers (MFs) are show n in side view . fast MUs may change to slow MUs with increasing age and body size.[28] These specialized EMG techniques are discussed in the Equipment section below. these quantitative EMG and histologic findings suggest a drop out of MNs as aging proceeds beyond the fifth decade. C. it assumes the characteristics of the MN. aw ay from the endplates. polyneuronal innervation of MFs does not likely occur in the normal mature state. 25] Elderly individuals typically experience decrease in muscle bulk. or vice versa. Fast MUs also may be changed experimentally to slow MUs by constant electrical stimulation. By selective training in which force is generated. the MN exerts a trophic influence on the MFs and exhibits considerable plasticity. 7. particularly the type II MFs. 27] Second. B. 23. and castration. MFs may hypertrophy. ie.[22. Homogeneity is found in the MFs supplied by a given alpha MN. one neuron supplies a terminal axon to one MF. D.[18] This is the electrophysiologic analogue to MF type grouping but is based on the MU discharge rather than histochemical grouping.[26. S alpha MN) has only histochemical type I MFs in its MU (see image below). First. Distal region of the MFs on either side of their endplates is w here their action potentials are propagated (y).[25] Electrophysiologic evidence of MU remodeling in elderly individuals comes from 3 observations. Schematic show s 2 MUs.medscape.5/24/13 EMG Evaluation of the Motor Unit . During development. which under usual conditions is fixed. Hence. [21. 8] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.[20] Subsequent to early development in animals.Electrophysiologic Biopsy In addition. This is probably due to generalized atrophy of individual MFs rather than to a decrease in the total number of MFs. a histochemical type I MF may become a type II. Type II MF atrophy is a frequent change noted in aging. a type I (ie. denervated). thymectomy. the (muscle) fiber density as measured in SFEMG increases after the sixth decade. 24. Exercise training in rats did not change the proportion of MN subtypes (see Table 2 above). The exact mechanism of force generation in muscle and the functional connections of the MFs to the extracellular matrix are still incompletely understood. A similar phenomenon may occur transiently in reinnervation after denervation before full maturation of the reinnervation process occurs. 7. crosssection) demonstrates the overlap of MFs from different MUs and thus overlap in MU territories (of w hich only a portion is show n). Third. If an MF is orphaned (eg.[21. Muscle biopsy of healthy elderly individuals also exhibited evidence of MU remodeling. The endplate zone is central to the MFs (x). subsequently reinnervating it. This was based on the findings of targetoid MFs and mild degrees of MF type grouping. presumably in response to increased body weight and hence the need for more type S or tonic MUs. Polyneuronal innervation exists in early development but disappears in rats and kittens by the age of 6 weeks. Schematic representation of A on transverse view (ie. the concentric needle (CN) MUAP demonstrates an increase in duration with increased age. Data from investigators counting motor axons in anterior roots support the concept of MN loss. under such conditions.[22] though dying back of motor axons cannot be excluded. MU action potential (MUAP) is recorded from the endplate region (see A.com/article/1846028-overview#showall 6/49 . macro-EMG MUAP amplitude in the vastus lateralis and anterior tibial muscles becomes larger after the sixth decade. region x) and is characterized by its initial negative deflection. A. As stated above. running) generally does not result in MF hypertrophy. Therefore. emedicine. 8] Aging produces effects on the MU. They collectively demonstrate changes in the MU compatible with reinnervation as suggested by the MF type grouping on muscle biopsy. Cross-innervation experiments have demonstrated that MFs may change their histochemical type. though not to any significant degree until the individual is older than 60 years. Endurance training alone (eg. Hence. The former pattern is used in routine clinical EMG assessment of MUAPs (see image below).[7. 14.E. Physiology of the MU . and individual MUAPs cannot be identified w ithout reducing level of effort and use of trigger delay.E. 13. MD. smaller unit is recruited on sw eep 4. 32. they have a lower excitatory potential. a bias exists toward examination of the lower threshold. depending on the type of activation needed (eg. graded isometric contraction (ie. This implies that it is recorded along the MFs aw ay from the endplate zone (see A. in turn.[13. but only 4 phases are present. This is achieved by the size principle. 13. 15. Definite differences exist in other properties (see Table 2 above). 2001. With permission. This has a narrow range of approximately 1-3% as demonstrated by varying input stimulation. This pattern makes sense in that the type II MNs are activated later.Electrophysiologic Biopsy MUAP show s the typical triphasic appearance w ith an initial positive deflection. copyright P. but given the same number of excitatory inputs.[29] In humans. smaller type I MNs have relatively greater excitatory input given their smaller membrane size. the muscles that they supply. 14. Dendritic volume (ie. accounting for some of the differences between MU types I and II. discharging at 4 Hz. Therefore. a programmed pattern of activation of MUs is under central control. The size principle refers to alpha MN size relative to its order of recruitment in a population of MUs that comprises the fundamental organizational units of a muscle. slow isometric muscle contraction vs ballistic movement).medscape. sw eep 5 ms per division). copyright P. rate gradation refers to the discharge frequency of a specific MN.Activation and control of MU discharges Recruitment is defined as "the successive activation of the same and additional MUs with increasing strength of voluntary muscle contraction". type II alpha MNs have a larger membrane. This continues until the larger type II MNs reach their CFLs and are activated. With further effort. type S MUs. The alpha MNs are arranged somatotopically into groups in the ventral horn according to myotomes and. The baseline appears relatively full. It ceases to discharge when input drive to the MN pool drops below the CFL. Calibration settings are as indicated. ramp). 14] Although recruitment gradation refers to the number of alpha MNs being activated. as they are less resistant to fatigue. hence. number of excitatory synapses on the neuronal soma) is constant in the 2 types of alpha MNs. In healthy muscle.[30] See Muscular System Anatomy and Autonomic Nervous System Anatomy for more information. The percent proportion of supply of a given muscle by a specific root may vary among individuals as well as between sides in the same individual. 13. recruitment gradation) reaches its CFL and begins to discharge. slow postural movement vs rapid phasic movement). MU recruitment results in a strong efficient muscle contraction. In a smooth. Normal recruitment in deltoid muscle (A): First MU action potential (MUAP) is discharging at 13-15 Hz w hen a second. 15] emedicine. 15] Critical firing level (CFL) is a specific property of a particular MU that means a MN has a precise and reliable level or threshold of excitation at which it discharges. 31. an increased rate gradation occurs within that particular MN before central input to a second MN (ie. as studied by using macro EMG. Potential 1 appears polyphasic.5/24/13 EMG Evaluation of the Motor Unit . The soma of alpha MNs vary in size. region y) (sensitivity 100 mV per division vertical. 14.[7. in that the small type I MNs are activated initially. 2001. Patterns of recruitment may differ between various types of motor activation (eg. Barkhaus. allowing some protective effect in a single-level radiculopathy. MD. B show s 2 sw eeps recruitment in myopathy w ith mild voluntary effort. almost all muscles are supplied by more than 1 root. a muscle must be capable of prolonged activation that is relatively resistant to fatigue. A single MU is activated first.[7. Barkhaus.com/article/1846028-overview#showall 7/49 . Knight and Kamen reported that superficial MUs were larger relative to deeper MUs. 33] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. With permission.[16] It is a complex process that is under CNS control. This also may be termed functional threshold. A group of alpha MNs supplying a given muscle is organized in a vertical column within the ventral horn such that the rostral MNs supply the superficial and proximal MUs of the muscle and the more caudal MNs supply the deeper and more caudal MUs. In contrast. The authors do this to take their formalized method of analysis (ie. and 6 near the recording electrode. higher threshold MUs that are distant have smaller amplitude MUAPs (eg. this is based on the distance from the recording electrode. Healthy muscle in w hich muscle fibers (MFs) from 6 MUs are observed close to the recording electrode. A. again. Only 2 MUs have MFs close to the recording electrode. smaller unit is recruited on sw eep 4. To correlate with routine EMG needle examination. giving the erroneous impression of a reduced number of MUs. 32] In normal muscle. bias exists in assessing the waveforms of type I MNs.[38] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. The baseline appears relatively full. reflecting activation of the larger type II MUs. 2001. and individual MUAPs cannot be identified w ithout reducing level of effort and use of trigger delay. Their MUAPs are recognized easily by their relatively large size given their proximity to the needle electrode. some recruited MUs have MFs close to the needle electrode. how ever.5/24/13 EMG Evaluation of the Motor Unit . 31. Indications Electrophysiologic Biopsy The "electrophysiologic biopsy" is a concept introduced to emphasize that concordance should exist between the electrophysiologic findings and findings on traditional muscle biopsy in various disease processes (not that conventional biopsy actually would be performed in muscles affected by a focal structural process such as a radiculopathy!). discharging at 4 Hz. Models to demonstrate increased recruitment frequency. Barkhaus. MUAP amplitude reflects the size and distance of the closest MF to the recording surface of the needle electrode (see first image below. this does not necessarily reflect the actual MU size.[35] In practical application. Calibration settings are as indicated. MUAP amplitudes increase. At maximal levels of voluntary effort. section A is smaller than initially recruited MUAP). B show s 2 sw eeps recruitment in myopathy w ith mild voluntary effort. Potential 1 appears polyphasic.[3. Thus sharp MU action potentials (MUAPs) are recorded from only 2 MUs (MUs 1 and 5).medscape.Electrophysiologic Biopsy Recruitment frequency is a clinical electrodiagnosis method used to assess the firing rate of an MU. Courtesy of w w w . Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. as may be observed in a myopathy. MUAP amplitudes are relatively lower than at higher levels of activation. C show s loss of MFs from MUs 2. second recruited MUAP in second image below. objective-interactive EMG) to the next step of analysis and interpretation. copyright P. In myopathy. This serves as a reminder to the electrodiagnostic consultant of his or her responsibility to carefully analyze the data so as to accurately represent the underlying disease process. but only 4 phases are present. however. With permission. reflecting the loss of MUs. the recruitment frequency is reduced. B. the process is loss of MUs w ith subsequent reinnervation. 37] Therefore.com/article/1846028-overview#showall 8/49 . The firing rate of the first MU is measured immediately prior to the point at which the second MU is activated. an MU shows a rate gradation of 5-15 Hz before a second MU is recruited.casaengineering. the size principle is not demonstrated in CN EMG recordings. This technique can be performed only at a low level of motor unit activation when MUAPs can be clearly differentiated as they are recruited. MUs may discharge at rates approaching 50 Hz at maximal effort. Conversely. As force is increased. 34. This is done to comprehensively look at the electrophysiologic data to make inferences as to how the MU is remodeled in a particular disease process. 4.[36. This is because the recording is made at low levels of activation (ie. 3. emedicine.com. Thus. MD. Normal recruitment in deltoid muscle (A): First MU action potential (MUAP) is discharging at 13-15 Hz w hen a second. voluntary muscle effort). Special studies such as precision decomposition have demonstrated that in healthy human muscle. The reverse occurs in a neurogenic process: the recruitment frequency may be increased.E. section A). C. such abnormality is reflected in changes in the myogenic signal. This makes the needle electrode examination an excellent tool to sample sufficient sites when pathology may be minimal in severity or patchy in distribution. Recruitment of MUs is not covered in detail in this section. including alteration of their distribution within the MUT. of the electrode. This slight change in electrode position substantially affects the prominence of the turn on the negative slope.Electrophysiologic Biopsy EMG differs from other electrophysiologic studies. Myopathic Disorders The quintessential process in myopathies is the loss or dysfunction on MFs. show s the innervation of a regenerated MF from satellite cells via a collateral axonal branch from an adjacent MF. resulting in abnormal muscle function (see images below). Influence of needle electrode position in healthy (A) and myopathic (B) MU.medscape. The "electrophysiologic biopsy" approach samples not only multiple biopsy or sampling sites within a muscle. MF w ith segmental necrosis. since the normal "mosaic" distribution of the MFs within the MUT is altered in many neuromuscular disorders. patterns of MUAP activation (recruitment) should be evaluated. The recording electrode is also constantly being manipulated by the electrodiagnostic consultant so as to optimize the signal. D. as shown in image below. 39. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. this approach may be quite helpful in determining the actual biopsy site of choice for optimal diagnostic yield. such as EEG. Terminal axon branch from an adjacent nonaffected MF also could have provided the source for the reinnervating collateral branch. Note in normal MU (ie. Beyond the MUAP waveform. the AP propagating left emedicine. Note also in electrode position 2 in normal that 2 MUAPs are recorded w ith a slight change. In this instance the terminal axon branch (ie.com/article/1846028-overview#showall 9/49 . F. Innervation of a denervated MF (2) by means of a collateral al branch from an unaffected MF from another motor unit (1). The MUAP waveform is also unique in that it may offer clues as to how the MU is remodeled in a particular disease process. The latter is a physiologic procedure that passively records the signals from central neuronal generators using preset montages. In most situations. MF is split distal to the endplate. neuropathy. but permits "electrophysiologic biopsy" or sampling of other muscles as indicated (depending on the clinical problem). A.5/24/13 EMG Evaluation of the Motor Unit . SA. It requires special evaluation. disorders of neuromuscular transmission) are described in this section. and interpretation may be difficult. The reader is referred to other detailed discussions on MU recruitment. B. needle electrode position within the MUT influences the MUAP waveform. 2001. myopathy. Sampling is critical. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. myopathy). MD.E. EMG is quite different. With permission. Although a single arrow show s the AP propagating to the right. healthy MF.[3. Schematic of alterations in single muscle fibers (MFs) that contribute to changes in their action potentials (APs). Atrophy of the MF that results in a smaller AP. There is no standard "montage" to record myogenic activity both at rest and during volitional movement. A) how the MU action potential (MUAP) durations change little during movement of the electrode through the MU territory. innervated end to the right) has sent a collateral axon branch to the orphaned end (left) to reinnervate it. Barkhaus. and nerve conductions. as well as other data such as IA. copyright P. In cases where conventional muscle biopsy is needed (eg.[61] Also. E similar to D. almost a rotation. 44] The 3 major categories of basic neuromuscular disease processes (ie. an enlarged or reinnervated motor unit). With permission.Electrophysiologic Biopsy into the split goes dow n each branch. In the neurogenic pattern. Note in normal MU (ie. A.5/24/13 EMG Evaluation of the Motor Unit . angular MFs represent those that are still denervated and that have not yet undergone reinnervation. With permission. Normal pattern.E. This slight change in electrode position substantially affects the prominence of the turn on the negative slope. Schematic show ing typical alterations of muscle fibers (MFs) and their spatial arrangement in a myopathic process (bottom left) and neurogenic process (bottom right). Such splits may occur in hypertrophied MFs. These are based on alterations in muscle fiber size.medscape.casaengineering. MD. Barkhaus. Note also in electrode position 2 in normal that 2 MUAPs are recorded w ith a slight change. distribution in the MU territory. Conversely. An electrode recording from the split end w ould detect 2 APs. Dark. If muscle is affected significantly by the disease process.E. Courtesy of w w w . 2001. Electromyographic(EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. A) how the MU action potential (MUAP) durations change little during movement of the electrode through the MU territory. of the electrode. the schematics are not intended to show specific changes in disease processes that do not directly influence the motor unit action potential w aveform (eg. Specific disease processes w ithin each pattern may vary (eg.com. Enlarged MFs represent MF hypertrophy. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. copyright P. Influence of needle electrode position in healthy (A) and myopathic (B) MU. Barkhaus. MUAP amplitude may be normal if the recording electrode tip is near even 1 functioning MF and may emedicine. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. almost a rotation.E. not all myopathic processes exhibit MF hypertrophy). MD. amplitude calibrations as indicated). Various alterations in the MUAP w aveforms. copyright P. MUAP amplitude is reduced. particularly when MF atrophy is present (see section B of each image below). Also. and proximity to the recording electrode. 2001. shaded MFs represent those that w ould show fiber type grouping on histochemical staining. inflammatory cells). copyright P. MD. Barkhaus. B-E. Schematic of MU in a myopathic process w ith associated MU action potentials (MUAPs.com/article/1846028-overview#showall 10/49 . 2001. This technique does not reveal MFs innervated by a single MN (ie. With permission. w hich may be seen in either process and may be associated w ith MF splitting. duration also may be reduced. B-E. Schematic of MU in a myopathic process w ith associated MU action potentials (MUAPs. the myogenic signal may not be altered if the process involves primarily the subsarcolemmal structures (eg. and proximity to the recording electrode.casaengineering.Electrophysiologic Biopsy even be increased if the MF is hypertrophied (if normal membrane function is assumed). Influence of needle electrode position in healthy (A) and myopathic (B) MU. A. Courtesy of w w w . copyright P. Note also in electrode position 2 in normal that 2 MUAPs are recorded w ith a slight change. Various alterations in the MUAP w aveforms. in simple MUAPs. distribution in the MU territory. almost a rotation. With permission. Various alterations in the MUAP w aveforms. distribution in the MU territory.medscape.[62] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. MUAPs of increased complexity and duration are not correlated with chronicity of the process. and proximity to the recording electrode. 2001.com.casaengineering. contractile mechanism).com/article/1846028-overview#showall 11/49 . A) how the MU action potential (MUAP) durations change little during movement of the electrode through the MU territory. MD. Electromyographic(EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. amplitude calibrations as indicated). Loss of MFs alone does not result in polyphasic or serrated MUAPs.5/24/13 EMG Evaluation of the Motor Unit . Normal pattern.E. amplitude calibrations as indicated). Schematic of MU in a myopathic process w ith associated MU action potentials (MUAPs. These are based on alterations in muscle fiber size. Normal pattern.com. Note in normal MU (ie. In some myopathic processes. These are based on alterations in muscle fiber size. MF diameter variability produces increases in phases or turns by desynchronizing the main spike signal (see section B of the first image below and section E of the second image). of the electrode. MUAP area is reduced and. Barkhaus. Electromyographic(EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. This slight change in electrode position substantially affects the prominence of the turn on the negative slope. Courtesy of w w w . These in turn may increase the MUAP duration. as shown in section C of the image below. MF hypertrophy may result in MF splitting. A. emedicine. B-E. copyright P. In simulation studies. MUAP duration has the inherent lower limit of the duration of a single MF AP. shifting left. Small open arrow s on sw eeps 3-5 indicate low -amplitude potentials on the baseline that may represent a small MUAP w ith few remaining muscle fibers. the APs from more distant MFs that contribute to the initial and terminal components of the MUAP are lost. 5 ms horizontal per division): A show s 4 sw eeps on free run mode from a patient w ith myopathy. other possible causes for complexity in myopathic processes include innervation of satellite cells as they develop into MFs. Electrode is manipulated slightly.com. which has more than 200 MFs in its MUs. Hence. Courtesy of w w w .Electrophysiologic Biopsy Besides MF diameter variability.E. See section B of the first image below and section A of the second image. smaller potential is discharging slightly faster. Reduced recruitment.D show s trigger delay on MUAP 1. Finally. smaller muscles that have shorter durations under normal circumstances (eg. emedicine. Use of trigger delay is essential to ascertain each potential by visualizing it at least 3 times. The latter is observed in some healthy small muscles. to w here it superimposes on potential 1 on the fourth sw eep. ie. Tw o late components are appreciated readily (numbers 2 and 3 are separated by baseline). By slight manipulation of the recording electrode. firing MUs are recorded in a severely w eak muscle in a patient w ith myopathy (A) and a patient w ith neuropathy (B). superimposing on potential 1 on sw eeps 3 and 4. MUAP 1 is observed to the right on the top sw eep and partially on the bottom sw eep. If MF loss occurs. MFs may be innervated or reinnervated. Another sw eep show ing potential 1 is needed to verify it as a bona fide potential. Careful examination of the other complex w aveforms reveals subtle changes among them. Note the missing large-amplitude spike as indicated by the thin arrow on sw eep 4. In simulation studies. Trigger delay in instrumentation (calibration for A-E is 100 mV vertical. demonstrating recurrence of the w aveform 3 times is essential before ascertaining it as a MUAP. Second. The enlarged potential on the first sw eep (open arrow ) is the result of superimposition betw een the 2. MU action potential (MUAP) in A is complex and unstable. First potential occurs at almost the same position on each sw eep. laryngeal muscles) are particularly unsuitable in assessing myopathy. E. but this is not a prominent finding.casaengineering. B is a free run recording of rastered sw eeps from a patient w ith severe myopathy. In B the MUAP has high amplitude and is considered giant. Thus. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. and duration becomes shortened (see image below). Calibration is as indicated. thus confirming its complex w aveform. Single. the relationship between the number of MFs in a MU and MUAP duration is nonlinear. With permission. instability in MUAPs may occasionally occur in myopathic processes. Though uncommon. when loss of endplate areas occurs due to segmental necrosis (such as in inflammatory myopathy). indicating a steady firing rate of approximately 10 Hz. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Although this is actually observed in B sw eep 2. further demonstrating their instability. 2001. 100-200 MFs generated the normal MUAP duration. distal hand muscles. MD. a fibrillation potential. Barkhaus.5/24/13 EMG Evaluation of the Motor Unit . a temporal dispersion of 1-2 milliseconds can easily produce a MUAP with a duration of 4-5 milliseconds in a MU containing 5-15 MFs. trigger delay on potential 1 (C) show s a highly complex MUAP that is unstable even at standard settings (note change in pattern of peaks). At least 2 MU action potentials (MUAPs) are seen as denoted by the numbers.com/article/1846028-overview#showall 12/49 .medscape. and the trigger-delay now is set on potential 2. fast. These are easily overlooked unless the sensitivity is increased. A healthy larger muscle such as the biceps brachii. has a duration of 10-12 milliseconds. Complex MUAP (2) is observed shifting left on subsequent sw eeps. B. C show s loss of MFs from MUs 2.casaengineering. The result is the apparent recruitment of the first 2 MUs. amplitude calibrations as indicated). Single.com. These are based on alterations in muscle fiber size. Healthy muscle in w hich muscle fibers (MFs) from 6 MUs are observed close to the recording electrode. Thus sharp MU action potentials (MUAPs) are recorded from only 2 MUs (MUs 1 and 5). and 6 near the recording electrode. Schematic of MU in a myopathic process w ith associated MU action potentials (MUAPs. and the third image). as may be observed in a myopathy.casaengineering. Models to demonstrate increased recruitment frequency. A. the process is loss of MUs w ith subsequent reinnervation. Various alterations in the MUAP w aveforms. firing MUs are recorded in a severely w eak muscle in a patient w ith myopathy (A) and a patient w ith neuropathy (B). section A of the second image. Thus the second-order and third-order neurons controlling the higher threshold MUs appear to be earlier in onset of activation. MU action potential (MUAP) in A is complex and unstable. the first-order neurons controlling the early activated MUs are not observed because of severe MF loss (see section C of the first image below. Courtesy of w w w .5/24/13 EMG Evaluation of the Motor Unit . Recruitment in myopathic processes can be difficult to assess. showing a spurious pattern of reduced number of MUs firing rapidly for effort.com/article/1846028-overview#showall 13/49 . These are easily overlooked unless the sensitivity is increased.com. B-E. Courtesy of w w w . In B the MUAP has high amplitude and is considered giant. Reduced recruitment. The central drivers (ie. 3. A. and proximity to the recording electrode. further demonstrating their instability.com.[3] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Note the missing large-amplitude spike as indicated by the thin arrow on sw eep 4. emedicine.medscape. Small open arrow s on sw eeps 3-5 indicate low -amplitude potentials on the baseline that may represent a small MUAP w ith few remaining muscle fibers. distribution in the MU territory. Normal pattern. Calibration is as indicated.casaengineering. Only 2 MUs have MFs close to the recording electrode. This may be observed in moderately to severely affected muscles. In some myopathies.Electrophysiologic Biopsy Electromyographic(EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Careful examination of the other complex w aveforms reveals subtle changes among them. giving the erroneous impression of a reduced number of MUs. fast. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. lower MNs) are unaffected. The electrodiagnostic consultant should be cautious in describing neurogenic recruitment or IPs in the setting of MUAP waveforms that suggest amyopathic process. Courtesy of w w w . how ever. Note also that despite the large amplitude of this MUAP. The severity in dropout of MUs in a muscle may range from partial to complete loss.A large. whole MUs are lost by virtue of loss of MNs or their axons. the MFs of which are interspersed within their original MUT (see image below). 63] In a serial quantitative EMG study of sporadic inclusion body myositis. In processes such as axonal polyneuropathies. In normal muscle. This is accomplished by other surviving MUs. shifting left) at about 13 Hz in apparent isolation. In the bottom trace the sensitivity is increased to 100 microV/division vertical (no change in horizontal time base).[52. the axons generally degenerate or die back in segments. however. In such instances. making total loss of the MU more likely than partial loss of the MU. it may be normal or even increased.5/24/13 EMG Evaluation of the Motor Unit . Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.Electrophysiologic Biopsy Top . B. In summary. The latter observation makes sense in that the macro-EMG MUAP reflects the contribution of more MFs to its MUAP signal than that of the more selective (ie. other motor units typically w ould be recruited at this threshold (calibration 500 microV/division vertical. Portions of emedicine. 2000. Copyright. turns. A show s normal pattern w ith 2 MU action potentials (MUAPs. MU architecture in neuropathic process characterized by partial loss of MUs. show ing very small motor unit action potentials (MUAPs) in the baseline on either side of the large MUAP. the "thinning" of the main spikes of the MUAP). complex motor unit action potential (MUAP. increased phases.com/article/1846028-overview#showall 14/49 . 41. Abnormalities specific for myopathy include shortened MUAP duration or reduced area. 5 phases. Paul E Barkhaus. In general. The second is the reinnervation of orphaned MFs that have lost their controlling MNs. MD.[37] Although MUAP amplitude may be decreased in myopathy. approximately 2500 microV amplitude and 3 ms duration) firing at a progressively increasing rate (ie. arrow s denote duration) that might be recorded by a concentric electrode (blue bar). Surviving MU is reinnervating muscle fibers (MFs) from a denervated MU. partial losses of MUs may occur transiently as an axon degenerates. w ith permission. the spikes include essentially no area. smaller recording surface) concentric electrode MUAP. as these small potentials are overlooked easily or mistaken for baseline noise or fibrillation potentials. 10 ms/division horizontal). Neurogenic Disorders In neurogenic disorders. One way is MF hypertrophy. or linked components) may be observed as a sensitive but nonspecific finding of abnormality in early or mild myopathy. particularly area-to-amplitude ratio (ie.medscape. increased MUAP complexity (ie. the more selective concentric needle electrode was useful in detecting remodeling of the MUs but was not useful in correlating with clinical strength of the muscle studied (biceps brachii). the MU uses 2 basic compensatory mechanisms to regain function. This phenomenon may give rise to a mistaken "neurogenic" impression of the MUAP.[41] The macro-EMG MUAPs recorded by the less selective macro-EMG electrode correlated better with change in clinical strength between patients and over time. giving them a needle-like appearance. This is achieved by collateral sprouting from the terminal axons of the surviving MUs. Now assume a 50% loss of MUs. which is a transient lesion). surviving) MUs prior to the reinnervation phase.5 mm of the electrode tip. examination. C. More severe example of chronic ongoing MU loss w ith reinnervation by a surviving MU. diabetic polyneuropathy. the IP is reduced. yielding "distant. Increase in IA supervenes after several days.medscape. The other pattern is an ongoing chronic neurogenic process. more than half the baseline will still remain full. This is adequate to fill the display screen to show a full IP (ie. In such instances. Collateral reinnervation. and other electrophysiologic data such as nerve conductions to help decide which pattern is most likely. in such instances. Therefore. well-defined endplate zone of the surviving MU adopting these MFs now becomes more diffuse. if the activated MU is out of range of the recording electrode (though it may be activated at a distance remote from it. Show n are 2 MUAPs that might be recorded by a concentric electrode (blue bar). the MUAPs begin to exhibit changes in their waveforms that reflect the reinnervation process. motor neuron disease.Electrophysiologic Biopsy the territory of the surviving MU may have normal architecture if no reinnervation is occurring. >500 ms of a 1-s baseline epoch).5/24/13 EMG Evaluation of the Motor Unit . this results in 400 spikes per second (assuming a spike duration of 5 ms at a lower sensitivity setting on the amplifier). The area near the recording electrode tip is normally represented by MFs from approximately 20 MUs. Schematic representations of fractionation. Acute onset monophasic partial loss of MUs Acute onset.com. The once relatively narrow. degeneration of the distal axon occurs. roughly reflecting the severity of the lesion. emedicine. In the weeks after the acute insult. MUAP waveforms are normal during this acute period. These are enlarged in amplitude and duration (note difference in amplitude calibration compared w ith A: duration measurement at 100 mV w ould be much greater as show n in the second serrated MUAP below . monophasic partial loss of MUs may occur with partial trauma to a nerve trunk or root. Between onset and 7-10 days after onset. w here duration is denoted by arrow s). as show n by MF type grouping on biopsy. and SA such as fibrillation potentials occur at 21 days. MU structure initially is unchanged. Hence.casaengineering. This requires development of new collateral branches to these denervated MFs with the establishment of new synapses (see images below). nerve trauma[64] ). somewhat sooner in axons of shorter length (eg. the electrodiagnostic consultant may need to make great efforts to record MUAPs that are close to the recording electrode. Assuming actual acute axonal loss (vs temporary block from compression. For the electrodiagnostic consultant. For general categorization. Courtesy of w w w ." low amplitude activity on the signal baseline). in w hich previously reinnervated MFs (open circles) in an enlarged MU undergo denervation a second time. spondylotic radiculopathy).com/article/1846028-overview#showall 15/49 .[64] The motor axons involved fail to propagate their impulses from their central MNs past the site of the lesion. facial nerve). followed by reinnervation. it does not exist for recording purposes. the compound muscle action potential (CMAP) evoked distal to the lesion is normal despite clinical weakness. variable rates of progression may exhibit concurrent denervation and reinnervation (eg. In uncomplicated cases. Assuming a normal firing rate of 20 Hz at full effort (IP). is prominent.[65] The voluntary activity in the EMG in this initial period demonstrates a decrease in recruitment. By using the same values as given above. neurogenic processes can be divided into 2 basic types that share common characteristics but different patterns on EMG: acute-onset monophasic denervation that may range from partial to complete MU loss (eg. These terminal axons are responding to the trophic signals from the denervated or orphaned MFs from the lost MUs. as they represent the unaffected (ie. If the MU activated has no MF within this area. Before this time. The electrodiagnostic consultant must rely heavily on the history. MUAP becomes complex w ith increased duration (note also the late component). a reduced pattern of baseline may not be seen until more than half of the MUs are lost. orphaned MFs also begin to increase in diameter as reinnervation is established and they become functional. The MUAP amplitude is defined by MFs within 0. it does not contribute to the measurement of the IP. The once denervated. In turn. D and E. not all myopathic processes exhibit MF hypertrophy).Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. This is reflected in the variability of MF diameter as in section B of the image below (skinny orphaned MFs now joining a "family" of otherwise healthy MFs of normal diameter in a MU so that there is a spectrum in the conduction velocities along their MFs).5/24/13 EMG Evaluation of the Motor Unit . the schematics are not intended to show specific changes in disease processes that do not directly influence the motor unit action potential w aveform (eg. inflammatory cells). Schematic show ing typical alterations of muscle fibers (MFs) and their spatial arrangement in a myopathic process (bottom left) and neurogenic process (bottom right). Enlarged MFs represent MF hypertrophy.com/article/1846028-overview#showall 16/49 .E. arrow s denote duration) that might be emedicine. B. the AP propagating left into the split goes dow n each branch. In this instance the terminal axon branch (ie. A. innervated end to the right) has sent a collateral axon branch to the orphaned end (left) to reinnervate it. Innervation of a denervated MF (2) by means of a collateral al branch from an unaffected MF from another motor unit (1). E similar to D.medscape. healthy MF. F. w hich may be seen in either process and may be associated w ith MF splitting. Dark. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Also. In the neurogenic pattern. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Atrophy of the MF that results in a smaller AP. Specific disease processes w ithin each pattern may vary (eg. 2001. or linked components). MF w ith segmental necrosis. Barkhaus. Schematic of alterations in single muscle fibers (MFs) that contribute to changes in their action potentials (APs). show s the innervation of a regenerated MF from satellite cells via a collateral axonal branch from an adjacent MF. increased phases. an early but nonspecific finding. C. Viewed from the perspective of the MUAP. the waveform may begin to show increased complexity (ie. MF is split distal to the endplate. Such splits may occur in hypertrophied MFs. Barkhaus. turns. 2001. With permission.E. D. There is also dispersion of the endplate zone and the new terminal collateral axons to these reinnervated MFS have variable lengths and conduction times. shaded MFs represent those that w ould show fiber type grouping on histochemical staining. This technique does not reveal MFs innervated by a single MN (ie. Although a single arrow show s the AP propagating to the right. copyright P. angular MFs represent those that are still denervated and that have not yet undergone reinnervation. A show s normal pattern w ith 2 MU action potentials (MUAPs. an enlarged or reinnervated motor unit). An electrode recording from the split end w ould detect 2 APs. With permission. MD. copyright P. MD. Terminal axon branch from an adjacent nonaffected MF also could have provided the source for the reinnervating collateral branch. MU architecture in neuropathic process characterized by partial loss of MUs. com. Trigger (except in D) is alw ays at the third division. Collateral reinnervation. Barkhaus. Here. B and C show 4 and 10 superimposed sw eeps. 2001. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. how ever. D and E. B show s early change in architecture characterized by variation in muscle fiber (MF) size as reinnervation proceeds w ith grouping of the MFs. B. MUAP becomes complex w ith increased duration (note also the late component).com. reflecting the reinnervation process as the terminal sprouts from new synapses on the orphaned MFs (see sections B and C of the first image below and section C of the second image). A.casaengineering. MD. A show s healthy MU. Schematic representations of fractionation. MU action potential (MUAP) stability assess by using concentric needle (CN) electrode. These are enlarged in amplitude and duration (note difference in amplitude calibration compared w ith A: duration measurement at 100 mV w ould be much greater as show n in the second serrated MUAP below . E.5/24/13 EMG Evaluation of the Motor Unit . F show s increased instability w ith blocking. respectively. Instability persists until the synapses fully mature (typically many weeks to months). D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie. drop out or "block" of some of the component spikes).casaengineering. copyright P. as show n by MF type grouping on biopsy. is prominent.E.Electrophysiologic Biopsy recorded by a concentric electrode (blue bar). A. With permission. C. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. In contrast. As these MFs mature in the reinnervation process. C. Superimposed sw eeps from a healthy MUAP show ing stability. Show n are 2 MUAPs that might be recorded by a concentric electrode (blue bar). E show s increased instability or jitter. Blanket principle. This schematic show s MU remodeling after complete MU loss (eg. in w hich previously reinnervated MFs (open circles) in an enlarged MU undergo denervation a second time. Another early finding is instability of the MUAP. complete nerve transection). the successively discharging MUAPs change in shape even with standard filter settings). Courtesy of w w w . The nascent motor unit action potential show ing complexity and instability is show n in C. Portions of the territory of the surviving MU may have normal architecture if no reinnervation is occurring. B.medscape. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. They look superficially similar.E and F are 10 superimposed sw eeps from a patient w ith myasthenia. Surviving MU is reinnervating muscle fibers (MFs) from a denervated MU. calibration settings are as indicated. Four trigger-delayed emedicine. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. D is made w ith low -frequency filter at 2 Hz. Courtesy of w w w . w here duration is denoted by arrow s). particularly in the spike to the left. instability decreases such that it may require the use of high pass filtering to demonstrate this phenomenon (see image below). More severe example of chronic ongoing MU loss w ith reinnervation by a surviving MU. from a chronic neurogenic process show ing instability.com/article/1846028-overview#showall 17/49 . copyright P. This is reflected secondarily in the size of the surviving MUs that have provided reinnervation to the orphaned MFs (see section C of the image below. giving the erroneous impression of a reduced number of MUs. These are easily overlooked unless the sensitivity is increased. With permission.casaengineering. Several months to upward of a year after onset. Note that the amplitude is reduced and baseline "sharpened" w ith removal of the low frequency components. This is analogous to the MF type grouping seen on muscle biopsy. MUAP duration also increases.E. Calibrations are as indicated. Low -frequency filter is increased to 500 Hz in B.[67] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.com. This depends on a number of variables but predominantly on the degree of the original MU loss. the process is loss of MUs w ith subsequent reinnervation.medscape. Courtesy of w w w . This is particularly apparent in the main spike. sw eep is increased. note the calibration). fast. Successful reinnervation is reflected by enlarged but stable MUAPs with minimal to no fibrillation potentials. Reduced recruitment. MD. This recording is stable. Healthy muscle in w hich muscle fibers (MFs) from 6 MUs are observed close to the recording electrode. A. MUAP amplitude increases along with a concomitant increase in area. A recent study suggests that experimental partial peripheral motor nerve lesions may induce some changes in axon conduction properties of the surviving MNs. how ever. further demonstrating their instability.5 mm of the recording electrode tip.com. B. These changes in the MUAP waveform are specific to neurogenic processes. firing MUs are recorded in a severely w eak muscle in a patient w ith myopathy (A) and a patient w ith neuropathy (B). Note the missing large-amplitude spike as indicated by the thin arrow on sw eep 4.[22] emedicine. Courtesy of w w w . Careful examination of the other complex w aveforms reveals subtle changes among them. In B the MUAP has high amplitude and is considered giant. reinnervation becomes complete in that it reaches its maximum. Small open arrow s on sw eeps 3-5 indicate low -amplitude potentials on the baseline that may represent a small MUAP w ith few remaining muscle fibers. indicating increase in focal rearrangement of the MFs in the MUT within 0. 2001. MU action potential (MUAP) in A is complex and unstable. the remaining MUs are enlarged (see section B of each image below). increase in spike amplitude may occur in IP. C.casaengineering. Thus sharp MU action potentials (MUAPs) are recorded from only 2 MUs (MUs 1 and 5). Only 2 MUs have MFs close to the recording electrode. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. As the reinnervating MU acquires more MFs.Electrophysiologic Biopsy superimposed sw eeps of a motor unit action potential (A) recorded w ith a concentric needle electrode from a patient w ith myopathy. As reinnervation proceeds successfully.[66] The recruitment and IP may remain decreased because MUs are not replaced per se. rather. Calibration is as indicated. C show s loss of MFs from MUs 2. Hence. and 6 near the recording electrode. Models to demonstrate increased recruitment frequency.5/24/13 EMG Evaluation of the Motor Unit . Single. as may be observed in a myopathy. 3. shifting the smaller spike to the left. Barkhaus. IA and SA decrease.com/article/1846028-overview#showall 18/49 . Courtesy of w w w . Since all MUs are lost. is prominent. complete nerve transection). The reinnervating MUAP ultimately may become more enlarged than normal for that particular muscle. More severe example of chronic ongoing MU loss w ith reinnervation by a surviving MU. B show s early change in architecture characterized by variation in muscle fiber (MF) size as reinnervation proceeds w ith grouping of the MFs. as show n by MF type grouping on biopsy. In this situation. D and E. These MUs have abnormal architecture right from the beginning of their formation. The timing of increases in IA and SA is also the same except that it is more prominent.[68] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. the key to any recovery is successful regrowth of the axons from the proximal side of the lesion through the area of lesion. A show s normal pattern w ith 2 MU action potentials (MUAPs. As the number of successfully innervated MFs for such an MU increases. arrow s denote duration) that might be recorded by a concentric electrode (blue bar). no "normally" sized MUs are present at the beginning. MUAP becomes complex w ith increased duration (note also the late component). and only baseline activity is recorded on voluntary activation with the needle electrode. B. Surviving MU is reinnervating muscle fibers (MFs) from a denervated MU. In contrast.medscape. emedicine. initial complete transection of a nerve trunk or nerve root) demonstrates the same changes in the temporal sequence of loss of the distal portions of the axons as they degenerate (as discussed in the Acute onset monophasic partial loss of MUs section directly above). In this setting.casaengineering. Courtesy of w w w . w here duration is denoted by arrow s). complex.com. Acute complete loss of MUs Acute complete loss of MUs (eg. The difference lies in the complete loss of clinical function despite a normal evoked CMAP in the early post-onset period as already discussed.com/article/1846028-overview#showall 19/49 . the MUAP in turn becomes more enlarged. no voluntary activation occurs. These are enlarged in amplitude and duration (note difference in amplitude calibration compared w ith A: duration measurement at 100 mV w ould be much greater as show n in the second serrated MUAP below .casaengineering. the axon sends sprouts to innervate the MFs in a manner analogous to that already described. A show s healthy MU. The nascent motor unit action potential show ing complexity and instability is show n in C.[65] A major difference in the reinnervation process in this setting is the source of the reinnervating axons. MU architecture in neuropathic process characterized by partial loss of MUs. then distally along the path of the original axon.com. thus the reinnervation process forms MUs de novo (see image below). Schematic representations of fractionation. Collateral reinnervation. reflecting the complete loss of MUs. This schematic show s MU remodeling after complete MU loss (eg.5/24/13 EMG Evaluation of the Motor Unit . Assuming successful regrowth. Show n are 2 MUAPs that might be recorded by a concentric electrode (blue bar). short-duration MUAP waveforms. in w hich previously reinnervated MFs (open circles) in an enlarged MU undergo denervation a second time. Many factors affect this. These MUs initially are characterized as normal-to-reduced amplitude.Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. including the distance of the lesion to the muscle (regrowth of axons is approximately 1 mm/d) and how much the lesion has disrupted the anatomic path for the axons to regrow to the orphaned MFs in the muscle. C. Portions of the territory of the surviving MU may have normal architecture if no reinnervation is occurring. macro-EMG MUAPs may reach sizes approximately 20 times normal. the MUAPs become stable and SA becomes essentially normal. When reinnervation reaches its maximum (which is not necessarily equivalent to a complete functional recovery). as show n by MF type grouping on biopsy. While the latter may be observed in myopathy (eg.medscape. MUAP complexity). Show n are 2 MUAPs that might be recorded by a concentric electrode (blue bar). Collateral reinnervation. and 3) marked instability (ie. and duration increase to reflect the addition of their APs to the MUAP. These are enlarged in amplitude and duration (note difference in amplitude calibration compared w ith A: duration measurement at 100 mV w ould be much greater as show n in the second serrated MUAP below . MUAP amplitude. MUAP becomes complex w ith increased duration (note also the late component). [70] The expected chronic reinnervation changes were found on routine EMG. Therefore. patients do not appreciate any early change in strength. The changes in the MUAP waveform in the early period reflect 1) the variability in size and dispersion of the endplates (ie. This compensatory reinnervation process is so successful that as many as half of the MUs in a muscle may be lost before the reinnervation mechanism begins to fail and clinical weakness becomes manifest.[69] Meriggioli and Rowin reported a case of Kennedy disease that manifested as fatigue but normal clinical strength. In macro-EMG studies in such processes.com/article/1846028-overview#showall 20/49 . is prominent. implying an extraordinary ability to reinnervate and add MFs to a MU (see section C of the image below). Viewed in isolation when the details of the disease process are unknown. MUAP instability is not a prominent feature in myopathy. with the added presence of increased jitter on single fiber EMG. such as progressive radiculopathy secondary to spondylosis. Surviving MU is reinnervating muscle fibers (MFs) from a denervated MU. More severe example of chronic ongoing MU loss w ith reinnervation by a surviving MU. arrow s denote duration) that might be recorded by a concentric electrode (blue bar). The exception is that they are highly unstable within the setting of markedly increased IA and SA. as already described.Electrophysiologic Biopsy Because these MUAPs are formed de novo from a completely denervated muscle. 2) relatively fewer MFs in the initially formed MU (ie.5/24/13 EMG Evaluation of the Motor Unit . As additional MFs are reinnervated. The process is characterized by ongoing MU loss with compensatory reinnervation. they sometimes are termed nascent MUAPs (see section C of the image above). Note in section C of the image above how the configuration of the spikes varies on successive occurrences of the reinnervating MUAP. dating the onset is difficult because typically no identifiable precipitating event can be determined. The reinnervated MFs cluster (ie. Portions of the territory of the surviving MU may have normal architecture if no reinnervation is occurring. MF type grouping) typically by a single terminal axon to a greater degree than in lesions with partial axon loss (see section B of the image above). Sandberg and Stalberg reported a serial macro EMG study in old polio. C. In this pattern. the topography of the MFs in the newly formed MUT is quite different with respect to size of the original MUT. Ongoing processes of denervation Ongoing processes of denervation have ongoing concomitant reinnervation. showing a differential change in macro MUAP size between the biceps brachii and the anterior tibial muscles. the MUs enlarge to variable degrees compared to those observed in partial lesions. The authors believed that the latter finding of impaired neuromuscular transmission explained the patient's fatigue. MU architecture in neuropathic process characterized by partial loss of MUs. A show s normal pattern w ith 2 MU action potentials (MUAPs. Depending on the number of successful axons regrowing into the muscle. In this setting. denervation may be insidious such that compensatory reinnervation occurs in step with MU loss. immature terminal sprouts and synapses). w here duration is denoted by emedicine.[71] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. area. they may resemble superficially a MUAP from a myopathic process. B. reduced MUAP area and duration). inflammatory myopathy). These are typically diffuse processes such as polyneuropathies and motor neuron disorders but also may be focal. In such chronic processes. as may be observed in a myopathy. The presence of fibrillation potentials indicates duration of at least 21 days. depending on the relative activity of the disease process. and duration with variable complexity). This has been demonstrated on macro-EMG studies in which the recording is made from an area exceeding that of the MUT (ie. Fractionation therefore is not appreciated on CN or MON electrode studies. nonspecific changes in MUAP waveforms such as increased complexity and increased amplitude may be sensitive markers for abnormality but are nonspecific overall because these changes also may occur in myopathic processes.[52] Instability in such MUAPs indicates ongoing reinnervation. B. MUAP amplitude. The patient may assert that onset of weakness was recent. IA and SA may be increased.com/article/1846028-overview#showall 21/49 . Models to demonstrate increased recruitment frequency. One caveat may be made regarding the presence of fibrillation potentials. months). It makes sense that denervated muscle fibers (seen on muscle biopsy as small dark-staining angulated fibers) would generate relatively small APs that could be easily obscured at standard filter settings. that is.com. whereas enlarged MUAPs imply a chronicity of at least months. On initial study. more likely. Schematic representations of fractionation. emedicine. A.[42] Does this loss represent sporadic loss of MFs within the MUT or. Only 2 MUs have MFs close to the recording electrode.com. This compensatory mechanism eventually fails.[66] The concomitant presence of MUAP instability suggests a lesion is active.casaengineering. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. and 6 near the recording electrode. Therefore. Mean amplitude that is more than twice the upper limit also is considered a specific abnormality for neuropathy. Courtesy of w w w . This process is called fractionation of the MU (see sections D and E of the image above). the process is loss of MUs w ith subsequent reinnervation.casaengineering. Courtesy of w w w . Recruitment is said to be decreased. greater recording area than that using an intermediate selective CN or MON electrode). To summarize. as a result of reinnervation. This manipulation in electromyograph settings reduces baseline noise and "distant" myogenic activity. loss of limbs seems most likely. become relatively "less enlarged" due to loss of MFs within the reinnervated MU. how ever. selective loss of terminal branches (much like losing a limb on a tree to disease rather than a patchy loss of leaves)? Because the (muscle) fiberdensity measurements (by SFEMG study) remain unchanged (analogous to the highly localized measure of the leaves).Electrophysiologic Biopsy arrow s).[42] The electrophysiologic changes in the MUAP have similarities to the reinnervation process already described but with some important distinguishing characteristics. fibrillation potentials may appear absent. Such patterns may be observed in myopathy as described in the Myopathic disorders section above (see section C of the first image below and section A of the second image). Healthy muscle in w hich muscle fibers (MFs) from 6 MUs are observed close to the recording electrode. the MUAPs already are enlarged to varying degrees (ie. giving the erroneous impression of a reduced number of MUs. As already discussed. The specific changes in MUAP waveform that characterize a chronic neurogenic process are increases in duration of MUAPs with concurrent increases in amplitude and area. but clearly the changes recorded imply significant chronicity (ie. It is the process whereby enlarged MUAPs. electrophysiologic evidence exists for both denervation and reinnervation. area. 3. Thus sharp MU action potentials (MUAPs) are recorded from only 2 MUs (MUs 1 and 5). SA in the form of fibrillation potentials can occur in the setting of an acute lesion and take many months or longer to resolve. the number of high-amplitude spikes firing rapidly for effort is reduced. in w hich previously reinnervated MFs (open circles) in an enlarged MU undergo denervation a second time. To truly ascertain this. These alone do not indicate an active process. allowing a greater focus on the myogenic activity immediately near the recording electrode tip.medscape. the authors suggest increasing the high pass filter from the usual settings of 2-20 Hz to 500-1000 Hz.5/24/13 EMG Evaluation of the Motor Unit . D and E. In chronic processes. C show s loss of MFs from MUs 2. and the reinnervated enlarged MU also begins to fail. MU action potential (MUAP) in A is complex and unstable. Note the missing large-amplitude spike as indicated by the thin arrow on sw eep 4. if the MUAPs are enlarged. possibly. With permission. The image below summarizes how the increasing presence of fibrillation potentials relates to stability of the MUAPs. further demonstrating their instability. clinical history and examination. Barkhaus. Disorders of Neuromuscular Transmission Two important points must be considered in this category. Courtesy of w w w . such as MUAP size. Single. Small open arrow s on sw eeps 3-5 indicate low -amplitude potentials on the baseline that may represent a small MUAP w ith few remaining muscle fibers. Note that the degree of MUAP stability (number of unstable MUAPs) and density of fibrillation potentials imply different explanations w ith respect to progressive versus stable neurogenic processes.com/article/1846028-overview#showall 22/49 . As stated before.E.medscape. This further emphasizes the emedicine. If enlarged stable MUAPs are noted but without fibrillation potentials. focal. Special techniques often may be required to detect them. Calibration is as indicated. 2001. MD. The presence of fibrillation potentials without MUAP instability suggests recent onset of a neurogenic process or. such as in a radiculopathy. If the MUAPs are relatively normal in size. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Their pathology lies in the functional dynamics of the neuromuscular system (ie.Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. In the opposite situation (prominent fibrillation potentials and marked instability of typically mildly enlarged or enlarged MUAPs). The first is that these disorders generally are not picked up passively on basic electrodiagnostic testing. the spikes representing MUAP waveforms are of increased amplitude and area (see section B of each image above). To distinguish neurogenic processes. This emphasizes the importance of recording electrode position and the sampling of adequate sites in assessing MU loss. then a disorder of neuromuscular transmission should be suspected. Reduced recruitment. MUAPs are normal in size and no fibrillation potentials are seen. these relationships can be interpreted properly only after consideration of other variables. The presence of unstable enlarged MUAPs in the absence of fibrillation potentials suggests ongoing reinnervation without concomitant denervation. copyright P. of course. Careful examination of the other complex w aveforms reveals subtle changes among them. they must be suspected. fast.casaengineering. as already discussed. then this suggests either completed reinnervation or.com. MUAP stability) rather than the morphology (ie. other electrophysiologic data. MUAP waveform size or complexity). a very slowly progressive process. Ostensible dropout of MUs must be interpreted with caution in that the location of the recording electrode determines the number of MUs being recorded (see the first image above). firing MUs are recorded in a severely w eak muscle in a patient w ith myopathy (A) and a patient w ith neuropathy (B). active denervation with ongoing reinnervation is present in a relatively rapidly progressive process (eg.5/24/13 EMG Evaluation of the Motor Unit . The second point is that neuromuscular transmission disorders are not detected readily on routine needle electrode examination because they typically do not change the architecture of the MU. or diffuse. These are easily overlooked unless the sensitivity is increased. recent progression in a previously stable/reinnervated process. Under normal circumstances. In B the MUAP has high amplitude and is considered giant. Image show s the correlation of MU action potential (MUAP) stability and fibrillation potentials. and. such as in motor neuron disease). A. This change in electrode selectivity is analogous to performing a semi-quantitative SFEMG study. the electrodiagnostic consultant must interact with the instrumentation to convert the intermediate level selectivity of the recording electrode (CN. calibration settings are as indicated. MD. particularly when clinical weakness exists (see the first image below and sections E and F of the second image). Low -frequency filter is increased to 500 Hz in B. sw eep is increased. Trigger (except in D) is alw ays at the third division. the differentiating feature is that smaller complex MUAPs in myopathy are stable overall. E.E and F are 10 superimposed sw eeps from a patient w ith myasthenia. the variability is not observed easily. instability or intermittent blocking is observed readily in most instances.E. Hence. respectively. either of which may permit underestimation of jitter or instability. myasthenia gravis). recent literature has shown that CON or MON needle electrodes can be used in SFEMG to study jitter but not fiber density. Again. A. C. 2001.medscape. Note that the amplitude is reduced and baseline "sharpened" w ith removal of the low frequency components. In mild to moderately severe disorders (eg. resulting in similar electrophysiologic changes to the MUAP. from a chronic neurogenic process show ing instability. Calibrations are as indicated. drop out or "block" of some of the component spikes). A confirmatory electrophysiologic study for a disorder of neuromuscular transmission should rely on a more detailed quantitative study (eg. The complete details and differences between the two are beyond the scope of this discussion. D is made w ith low -frequency filter at 2 Hz. MON) to a highly selective one (ie. the variation in the MUAP waveform at standard settings is so florid that suspicion of a disorder of neuromuscular transmission can be made very quickly. SFEMG). F show s increased instability w ith blocking. increasing the low-frequency filter to 500-1000 Hz and increasing sweep speed to 0.5/24/13 EMG Evaluation of the Motor Unit . This recording is stable. Barkhaus. MD. MU action potential (MUAP) stability assess by using concentric needle (CN) electrode.E. Because of growing requirements for disposable needle electrodes. Occasionally. With permission. whereas those in disorders of neuromuscular transmission are characteristically unstable. Blocking is usually incomplete. Superimposed sw eeps from a healthy MUAP show ing stability. it cannot be appreciated since no APs are recorded. copyright P. how ever.Electrophysiologic Biopsy importance of the electrophysiologic biopsy as a comprehensive assessment of the MUAP signal. Blanket principle.com/article/1846028-overview#showall 23/49 . Four trigger-delayed superimposed sw eeps of a motor unit action potential (A) recorded w ith a concentric needle electrode from a patient w ith myopathy. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. E show s increased instability or jitter. 2001. They look superficially similar. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie.[72] emedicine. C. In such instances. the blocked MFs are analogous to the MF loss in a myopathy. B and C show 4 and 10 superimposed sw eeps. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. copyright P. B. An SFEMG electrode is more selective than a CN or MON needle electrode. The latter situation may occur in severe disorders of neuromuscular transmission.5-2 ms per division). With permission. If it is complete. shifting the smaller spike to the left. particularly in the spike to the left. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Barkhaus. By trigger delaying and superimposing successive sweeps. since repetitive motor nerve stimulation is a mainstay in the diagnosis of these disorders. In neuropathic processes. reduced in duration. significant change in amplitude or area of evoked CMAPs). Fluctuations in evoked CMAP amplitudes may occur in myopathic and neurogenic disorders. Barkhaus. and when present. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie. it may take effort to manipulate the needle electrode to find pairs of spike to assess stability (ie.E and F are 10 superimposed sw eeps from a patient w ith myasthenia. drop out or "block" of some of the component spikes).[57] Hence.E. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. the waveform is not as spiky (see the image below. chronic. This pattern of blocking is incomplete (or it cannot be appreciated) and is typically associated with large degrees of instability (ie. MU action potential (MUAP) stability assess by using concentric needle (CN) electrode. D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. Superimposed sw eeps from a healthy MUAP show ing stability. calibration settings are as indicated.medscape. A. What is the pathophysiology of the decremental CMAP response in a repetitive motor nerve stimulation study recorded with surface electrodes? This may be considered to be the large-scale manifestation of what is going on at the neuromuscular synapse: increased jitter. the surface electrode. Waveform changes in the CMAP in smaller. D is made w ith low -frequency filter at 2 Hz. calibration settings are as indicated. MU action potential (MUAP) stability assess by using concentric needle (CN) electrode. Superimposed sw eeps from a healthy MUAP show ing stability. jitter). In some cases of myasthenia (particularly those that are severe. MD. When the low-frequency filter is increased to assess stability. drop out or "block" of some of the component spikes). distal muscles usually are easier to detect than the broader. Trigger (except in D) is alw ays at the third division. E show s emedicine. D is made w ith low -frequency filter at 2 Hz. E. A. the MF loss is functional from blocking in transmission (ACh receptor loss outpaces ACh receptor regeneration). Some mention should be made of the CMAP. increased jitter may be a predominant finding (Meriggioli and Rowin. The CMAP is recorded with the least selective of all electrodes used. B. proximal muscles. significant pathology must be present to indicate a positive finding (ie.5/24/13 EMG Evaluation of the Motor Unit . E. copyright P. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. They look superficially similar. In the former. D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. jitter). Disorders of neuromuscular transmission usually exhibit normal MUAP configuration. This reflects marked instability and prolonged block in neuromuscular transmission. 2003). instability is common while reinnervation is incomplete (see sections B-D of the image below). particularly in the spike to the left. In some cases. Although the inference is that these MFs are denervated. they are usually minimal in density. respectively. from a chronic neurogenic process show ing instability. from a chronic neurogenic process show ing instability. With permission. B. C. Trigger (except in D) is alw ays at the third division. B and C show 4 and 10 superimposed sw eeps. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie. F show s increased instability w ith blocking.E and F are 10 superimposed sw eeps from a patient w ith myasthenia. but primarily blocking of neuromuscular transmission (see sections E and F of the image below). the MUAPs may appear small in amplitude. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. how ever. MUAP instability is associated with increases in IA and SA and enlarging MUAPs. or mildly complex. how ever. compare E and F with B-D). They look superficially similar. Therefore. A. longer duration CMAPs recorded from larger. respectively. In that setting. E show s increased instability or jitter. or untreated).[73. 2001. B and C show 4 and 10 superimposed sw eeps. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. fibrillation potentials are not reported commonly in myasthenia gravis.com/article/1846028-overview#showall 24/49 . but the trophic influence of the axon terminal on the MF is presumably preserved. A. instability is uncommon. C. 74] Therefore.Electrophysiologic Biopsy MUAP instability may be observed in both myopathic and neurogenic disorders. the MUAPs are normal in waveform. clinical weakness is apparent. but the blocking is less marked (eg. The MON needle electrode is fabricated by grinding a Teflon-coated metal wire to a sharp conical tip (see image below). 2001. somatoform disorders. particularly in the spike to the left. Some neuromuscular disorders that do not affect the synapse also may exhibit changes on repetitive motor nerve stimulation (eg. These represent instances in which the abnormality is outside of the MU's electrical function (ie.com/article/1846028-overview#showall 25/49 . The MU discharges are discharging relatively slowly for level of effort. Input to the lower motor neurons is compromised. reference. A common example is mild to moderately severe steroid myopathy in which the MFs are affected globally (eg. requiring careful assessment. Barkhaus. suggesting myotonia. copyright P. involvement of an upper motor neuron). This is the recording surface of the MON electrode. 39] emedicine. The muscle is contracted. then an approximately 2. socalled rippling muscle disease). Some are based in smaller notebook-type computers yet still have powerful analysis capability compared with that of older-generation analog electromyographs. case. steroid myopathy with particular involvement of type II MFs). If a neuromuscular disorder has its primary effect on the synapse. hypothetically and simplistically. but exhibit increased instability and blocking. but recruitment patterns may be decreased spuriously. resulting in reduced activation and therefore incomplete IP.[80] Changes in the EMG signal secondary to hysteria. MD.5/24/13 EMG Evaluation of the Motor Unit . This suggests a nonelectrical neuromuscular disorder. 79] In some myopathic processes. Brody syndrome[75.Electrophysiologic Biopsy increased instability or jitter. If CMAP amplitudes are already reduced. the MUAPs are typically normal in size and shape on routine analysis. they are blocking 50% of the time). If. Most of the new generation electromyographs are computer-based. F show s increased instability w ith blocking. and ground. Other disorders of neuromuscular transmission. Other Miscellaneous Disorders The clinical problem of delayed relaxation. motor neuron disease). the MUAPs actually may appear normal despite obvious clinical weakness. The EMG signal is concurrently fed into an audio amplifier. such as Lambert-Eaton myasthenic syndrome. periodic paralysis. With permission. The basic (or routine) technique of recording the EMG signal uses intramuscular needle electrodes. then a 20% decrement may be anticipated. 4. If. generation of its AP along the MF membrane). may occur in the setting of electrical silence. 40% of MFs are blocking (and of these. 20% of MFs exhibit blocking. yet no electrical activity appears on the screen.E. Severe blocking in neuromuscular transmission may not be appreciated easily from surface-recorded CMAPs other than for reduced amplitude of response. Instability alone is not the basis for the decrement of the CMAPs.[3. The opposite and very dramatic example is the patient with pronounced spontaneous muscle movements that are quite obvious clinically but are electrically silent (eg. In partial central disorders (ie.[3] Lukacs has reported changes in the EMG signal after ischemic stroke. incremental responses in presynaptic disorders). which is then displayed on a digital screen (usually a computer monitor). which by inference would occur at the subsarcolemmal membrane or contractile mechanism level (eg.[81] Equipment Recording of the EMG signal In most modern EMGs.0% decrement may be anticipated. A differential amplifier has 3 inputs: active.[77. less severe. 76] ). in another. then significant decrement may be difficult to appreciate. and malingering are not associated with changes in MUAP waveform. the myogenic or EMG signal is amplified by using a high-quality digital differential amplifier. 10% of the time). The 2 basic types are monopolar (MON) and CN needle electrodes. Two additional surface electrodes serve as a reference and ground. have special characteristics in the change of CMAP amplitude on repetitive nerve stimulation (ie. 78. and the recruitment and derecruitment pattern is erratic. In this setting.medscape. a useful technique in looking at changes in MU size and in monitoring certain neuromuscular disorders. Schematic compares 3 recording needle electrodes relative to an MU territory in a muscle. 39] The size of the recording surface affects the recording characteristics of the needle electrode. These MFs contribute to the duration feature of the motor unit action potential. The recording surface of the macro-EMG electrode (A) is its cannula. At the other end of the spectrum is the macro-EMG needle electrode. but it records from a moderate portion of the MU territory (circle). The tip of this electrode is ground to a 15° angle to expose the inner wire's surface (150 X 580 µm) as an ellipse. Courtesy of w w w . The concentric needle electrode (B) records from the inner core at its tip. The muscle fibers (MFs) closest to its tip contribute to such features as the amplitude (small semicircle). 4.) A third surface electrode is used for the ground input to the amplifier. The SFEMG electrode is the most selective. (In some images in this article.24 mm2 .Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. The exposed core at the tip of the CN is the recording surface.casaengineering. similar to the recording montage of the MON electrode.[18] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.com. emedicine. with the cannula serving as the reference. termed the cannula.casaengineering. Schematic show ing the recording areas of the concentric (A) and monopolar (B) needle electrodes. Photomicrograph show ing differences betw een the most commonly used needle electrodes: monopolar (MON) and concentric (CN).[41. w hich can extend through the MU territory. this is meant to represent the CN recording surface in side view. The recording area has a 150-mm radius. Although the recording surfaces of these 2 electrodes differ (MON is 0.[40] It is used to record the electrical signal generated by the entire MU.07 mm2 ).com.5/24/13 EMG Evaluation of the Motor Unit . Image courtesy of w w w . Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. they are similar compared to other needle electrodes used in QA (see first image below). which makes it the least selective recording needle electrode.casaengineering. The CN needle electrode is constructed from a fine metal wire inserted through a larger diameter metal cylinder. these techniques provide complementary information to supplement the routine needle EMG. The single-fiber EMG electrode is the most selective (C). Courtesy of w w w .com.medscape. with a recording surface of 0. with a recording surface of 27 mm2 . The inner core is insulated from the cannula.[3. a short solid line is shown as the recording surface.0003 mm2 . and it can differentiate reliably the APs generated by single MFs (see second image below).com/article/1846028-overview#showall 26/49 . w ith a recording surface of 15-mm diameter on the side of the needle's shaft. whereas CN is 0. 42] Collectively. [44] Examination of the EMG signal can initially be divided into 2 parts on routine assessment: involuntary and voluntary activity (see Examination of the EMG signal below). Therefore.5/24/13 EMG Evaluation of the Motor Unit . particularly "pogo-sticking" the electrode at various sites. Hence. 44] The 2 basic categories of IA are random (ie. MON-recorded MUAPs tend to have higher amplitude values and more phases). patterned signals may appear similar to those that may have been recorded on IA assessment. some differences might be expected in MUAP metrics. This electrode has a smaller recording surface than the standard CN (approximately 0.medscape. IA results from mechanical irritation of the muscle as the needle electrode passes through the muscle and is velocity-dependent with respect to electrode movement. making it appealing for those trying to compromise between use of a CN and an MON needle electrode. which may vary among manufacturers) and is shorter in length (25 mm). Use of the latter technique should be judicious from the standpoint of patient comfort and interpretation.[43] A further caveat involves use of the so-called facial (or pediatric) CN needle electrode. In pathological conditions.[44] Examples of SA may occur at the MUAP organizational level. fibrillation potentials).com/article/1846028-overview#showall 27/49 . Another example may be resting tremor. myotonic discharges. respectively. Voluntary Assessment of individually recruited MUAPs Size Shape Stability Assessment of activation pattern of MUAPs Recruitment Interference pattern (IP) SA is described while the needle electrode is held in a fixed position in rested muscle. Most of these signals are derived from single MFs or groups of MFs below the organizational level of the MU.[45. a slow-moving needle electrode generates little to no IA in healthy muscle. tend to provoke a greater (but still normal) amount of IA. including fasciculation potentials that are the spontaneous random discharge of single MUs.019 mm2 . the baseline of the signal should be smooth and uninterrupted. SA and IA are different in that they record APs generated spontaneously or by mechanical movement. Because of its smaller recording surface. [43] Its length limits its use in sampling MUs in larger muscle. trains of positive sharp waves. Identification and classification of IA and SA. Examination of the EMG signal Part I. a brief review of the EMG signal may be helpful because signals are occasionally difficult to identify or because they may overlap each other. are emedicine. they differ from each other in the waveforms they generate (eg. Technique EMG Signal Generation of the EMG signal Although this article emphasizes MUAPs. which represents periodic bursts of MUAP discharges as a result of abnormal central output from upper MNs (eg. which are based on the inherent physical properties of these needle electrodes. In healthy muscle. bursts of spikes of varying shape and duration) and patterned (eg. whereas fast movements. Involuntary Insertional activity (IA) Random Patterned Spontaneous activity (SA) Sub-MUAP MUAP Part II. The electrodiagnostic consultant must be aware of these differences when using these electrodes. Parkinson disease). complex repetitive discharges. particularly the rogue sub-MUAP and MUAP discharges. Two further categories are found under involuntary activity: insertional activity (IA) and spontaneous activity (SA).Electrophysiologic Biopsy Although the MON and CN needle electrodes have similar recording surfaces and both have intermediate selectivity. Calibration settings are as indicated. It is this second portion of the assessment (ie.Electrophysiologic Biopsy essential in making a comprehensive assessment of the EMG signal. Barkhaus. The second portion in the generation and assessment of the EMG signal includes 2 parts: the assessment of individual MUAPs and their patterns of activation. and the trigger-delay now is set on potential 2. SA. The patient gradually increases the force of contraction. emedicine. of the MUAPs) that is the focus of this article. smaller unit is recruited on sw eep 4. 2001. to w here it superimposes on potential 1 on the fourth sw eep. MUAP 1 is observed to the right on the top sw eep and partially on the bottom sw eep.E. Barkhaus. The EMG signal should contain only the discharges of a few MUAPs with adequate baseline between discharges to enable full identification of their size and shape. discharging at 4 Hz.[45. including how the MUAP reflects pathological changes in the MUs. B show s 2 sw eeps recruitment in myopathy w ith mild voluntary effort. copyright P. MD. copyright P. but only 4 phases are present. Use of trigger delay is essential to ascertain each potential by visualizing it at least 3 times. With permission. and individual MUAPs cannot be identified w ithout reducing level of effort and use of trigger delay. 5 ms horizontal per division): A show s 4 sw eeps on free run mode from a patient w ith myopathy. trigger delay on potential 1 (C) show s a highly complex MUAP that is unstable even at standard settings (note change in pattern of peaks). By slight manipulation of the recording electrode. Tw o late components are appreciated readily (numbers 2 and 3 are separated by baseline). Although this is actually observed in B sw eep 2. E. resulting in greater recruitment of MUs (see first image below) until full activation is achieved (ie. The bias in the routine assessment of MUAPs is that they represent the signals generated by low-threshold type I (S) MUs. B is a free run recording of rastered sw eeps from a patient w ith severe myopathy. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. and other electrophysiologic studies such as motor and sensory nerve conductions. Another sw eep show ing potential 1 is needed to verify it as a bona fide potential. see second image below). Trigger delay in instrumentation (calibration for A-E is 100 mV vertical. shifting left. The final portion of the assessment of the MUAPs lies in their activation pattern.com/article/1846028-overview#showall 28/49 . thus confirming its complex w aveform. Second. First potential occurs at almost the same position on each sw eep. Complex MUAP (2) is observed shifting left on subsequent sw eeps. Normal recruitment in deltoid muscle (A): First MU action potential (MUAP) is discharging at 13-15 Hz w hen a second. At least 2 MU action potentials (MUAPs) are seen as denoted by the numbers. At low levels of activation. 44] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. 2001. demonstrating recurrence of the w aveform 3 times is essential before ascertaining it as a MUAP. smaller potential is discharging slightly faster.D show s trigger delay on MUAP 1.5/24/13 EMG Evaluation of the Motor Unit . Electrode is manipulated slightly. indicating a steady firing rate of approximately 10 Hz. superimposing on potential 1 on sw eeps 3 and 4. The baseline appears relatively full. a presumed MUAP should be visualized 3 times before finally accepting it as a MUAP because of frequent superimposition between discharging MUAPs. The enlarged potential on the first sw eep (open arrow ) is the result of superimposition betw een the 2. With permission. Of course. Potential 1 appears polyphasic. These superimpositions may result in bizarre waveforms that may imply a pathological process in an otherwise normal study (see image below). IP.E. comprehensive assessment is not possible without considering other information such as IA. Individual MUAP measurement is made by having the patient slowly and minimally activate the muscle. MD.medscape. Influence of needle electrode position in healthy (A) and myopathic (B) MU. 1 s/division horizontal). this trace is from a patient w ith advanced s-IBM (calibration . 2001.500 microV/division vertical. Top trace . advancing the needle electrode the distance of the diameter of the MUT (eg. This slight change in electrode position substantially affects the prominence of the turn on the negative slope. complex motor unit action potentials (MUAPs). MD. To acquire MUAPs.com/article/1846028-overview#showall 29/49 . The "notch" just to the right of the second division mark show s a baseline shift from needle electrode movement. 45] Acquisition of the EMG signal with attention to sampling of MUAPs is based on knowledge of the MU and the MUT as already described. The needle electrode then is advanced in a linear track or "corridor" that is perpendicular to the long axis of the MFs. and the amplitude of the signal epoch is less than half of w hat might be expected in normal muscle.[30.[45] Scanning EMG studies demonstrate that the MUAP may vary considerably depending on where it is recorded in the MUT (see first image below). Sampling of MU action potentials within a muscle and signal acquisition Before starting a study. This ensures that the needle electrode passes through the maximum number of MFs proportionate to the length of the electrode that is in the muscle.5/24/13 EMG Evaluation of the Motor Unit . The middle trace is an interference pattern from a patient w ith severe s-IBM (calibration . 5-10 mm in the biceps brachii) may be necessary. segmental anatomy or myotomes and their potential variations must be considered. distal in most polyneuropathies vs proximal in most myopathies). from the early or far left portion of the middle sw eep (see "H" bar position betw een the middle and low er sw eeps). To ensure that a different MUAP is sampled at a given site.100 microV/division vertical. This epoch of signal actually show s the patient going from minimal activation at the left (beginning of the sw eep) to full effort on the far right. no amplitude change of "fullness" is seen going from minimal to full effort. of the electrode.Electrophysiologic Biopsy Interference pattern in biceps brachii. w ith permission. Note also in electrode position 2 in normal that 2 MUAPs are recorded w ith a slight change. the electrodiagnostic consultant must decide on a strategy of which muscles to sample. mononeuropathy). MD. A) how the MU action potential (MUAP) durations change little during movement of the electrode through the MU territory. With permission. 2000.E. emedicine. 1 s/division horizontal). This show s a relatively full baseline of small-amplitude.100 microV/division vertical. Note in normal MU (ie. Paul E Barkhaus. copyright P. radiculopathy. Barkhaus. The bottom trace is an expanded segment show ing interference pattern from biceps brachii. Copyright.Normal interference pattern at full effort (calibration . Sampling different MUAPs from different MUs may exhibit wide variation (see second image below). Overall. the pattern of muscles most likely affected is based on clinical examination (eg. In presumed diffuse processes.medscape. In focal processes (eg. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. 10 ms/division horizontal).[17] This variability is even more marked in neuromuscular disorders in which the normal mosaic distribution of the MFs in the MU is altered due to changes in the MFs themselves or in their distribution within the MUT. the patient minimally activates the muscle after the needle electrode is inserted through the skin to the surface of the muscle. almost a rotation. aw ay from the endplate zone).medscape. it is withdrawn to the surface of the muscle.com. This slight change in electrode position substantially affects the prominence of the turn on the negative slope. Motoneuron (MN) generates an action potential (AP) that in turn excites the muscle fibers (MFs) by means of neuromuscular transmission. C. With CN electrodes. Collectively.casaengineering. Cross-section schematic emphasizes dispersion of the MFs of that MU in the MU territory and their relative position to the recording needle electrode. B. Courtesy of w w w . Composite display of 25 MU action potentials (MUAPs) from a patient w ith myopathy. copyright P. 2001. of the electrode. Note also in electrode position 2 in normal that 2 MUAPs are recorded w ith a slight change. Note in normal MU (ie. almost a rotation. Three corridors are generally sufficient to sample a large limb muscle by using an electrode of suitable length (50 mm). Barkhaus. as demonstrated by "scanning" EMG studies (see second image below). Schematic show ing the generation of the MU action potential (MUAP).com/article/1846028-overview#showall 30/49 .5/24/13 EMG Evaluation of the Motor Unit . Influence of needle electrode position in healthy (A) and myopathic (B) MU. AP recorded from MF 3 is largest because of its proximity to the electrode. Propagation of the APs dow n the MF membranes is bidirectional aw ay from the endplates. With permission. A. Single MF APs recorded by the needle electrode (1-6 corresponding to the numbered MFs show n in A and B). emedicine. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.[45] Electrophysiologic Measurements of the MU When the MN discharges. The shape of the MUAP varies considerably depending on from where it is recorded within the MUT.[46] If more corridors are needed in the same muscle. The electrode then is angled approximately 30-45° medial or lateral to the axis of the first perpendicular corridor.E. Summation of the individual MF APs as they form the MUAP. To enter a different corridor once the needle electrode is at full depth of penetration. the electrode is moved to a new skin insertion site at a suitable distance medial or lateral to the previous site and the long axis of the MFs. D. MD. and the electrode again is advanced. Recording electrode is positioned betw een the endplate zone and the end of the MFs (ie. sampling MUAPs at each site. these single MF APs summate to form the MUAP (see first image below). the depth of sampling may affect amplitude because of the influence of the cannula on the signal. The AP from a single MF recorded extracellularly is triphasic in shape when recorded away from the endplate zone.Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Courtesy of w w w . Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. A) how the MU action potential (MUAP) durations change little during movement of the electrode through the MU territory.casaengineering. all of the MFs respond by generating an AP.com. crosssection) demonstrates the overlap of MFs from different MUs and thus overlap in MU territories (of w hich only a portion is show n). as when the single MF APs are recorded away from the endplate. C.Electrophysiologic Biopsy Although the typical MUAP is also thought to be triphasic. Schematic show ing the generation of the MU action potential (MUAP).casaengineering. The radial distance to the nearest MF determines the amplitude. which is high if the radial distance is less than 500 µm but lower when recorded from a distance of 2000-2500 µm (see the image above. aw ay from the endplates. The endplate zone is central to the MFs (x). MUAP show s the typical triphasic appearance w ith an initial positive deflection. Single MF APs recorded by the needle electrode (1-6 corresponding to the numbered MFs show n in A and B). Propagation of the APs dow n the MF membranes is bidirectional aw ay from the endplates. of the first image below). the electrodes are roughly equidistant from the endplates. A. Distal region of the MFs on either side of their endplates is w here their action potentials are propagated (y). B. 36. The MUAP recorded here is typically biphasic with an initial negative-going phase (see section C of the image below). Muscle fibers (MFs) are show n in side view . AP recorded from MF 3 is largest because of its proximity to the electrode. no initial wave of positivity or depolarization occurs. region y) (sensitivity 100 mV per division vertical.medscape. Because the recording is made at the endplate. emedicine. Recording electrode is positioned betw een the endplate zone and the end of the MFs (ie. Schematic representation of A on transverse view (ie. D. This implies that it is recorded along the MFs aw ay from the endplate zone (see A. copyright P. position x.E. 2001. Thus. Summation of the individual MF APs as they form the MUAP. Endplate noise on the baseline may be also noted. MD. Schematic show s 2 MUs.[3. This variation in time is termed temporal dispersion and is the result of several physiologic characteristics of the MU. it easily may be polyphasic or exhibit increased turns if phase interaction occurs between its component single MF APs (see sections C and D of the image below). An exception is a MUAP recorded from the endplate zone (see section A. B. compare the positions of MFs 3 versus 1 and 6 in Image 12B and their corresponding APs in section C of the image above). C.com/article/1846028-overview#showall 31/49 . Cross-section schematic emphasizes dispersion of the MFs of that MU in the MU territory and their relative position to the recording needle electrode.[49] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. the initial and terminal slow components are the same for all the MFs (see image above). The positive-to-negative-going spike of the AP is not synchronous. 49] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.5/24/13 EMG Evaluation of the Motor Unit . When the MUAP is recorded 2 cm or more from the endplate region.com. Motoneuron (MN) generates an action potential (AP) that in turn excites the muscle fibers (MFs) by means of neuromuscular transmission. A. MU action potential (MUAP) is recorded from the endplate region (see A. sw eep 5 ms per division). With permission. Courtesy of w w w . region x) and is characterized by its initial negative deflection. ie. signifying variable arrival times of the single MFs at the electrode (see image below). 39. aw ay from the endplate zone). Barkhaus. D. casaengineering. Recording electrode is positioned betw een the endplate zone and the end of the MFs (ie. from a chronic neurogenic process show ing instability. 2001. With permission. sw eep is increased. Single MF APs recorded by the needle electrode (1-6 corresponding to the numbered MFs show n in A and B). C. how ever. shifting the smaller spike to the left. copyright P. Note that the amplitude is reduced and baseline "sharpened" w ith removal of the low frequency components. E show s increased instability or jitter. MD. MU action potential (MUAP) stability assess by using concentric needle (CN) electrode. With permission. Why? As stated already. B. Calibrations are as indicated. The MUAP from an MU in a larger limb muscle is actually a theoretical ideal. drop out or "block" of some of the component spikes). Blanket principle. Low -frequency filter is increased to 500 Hz in B. Barkhaus. particularly in the spike to the left.Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. and the formal quantification of this is termed jitter in SFEMG. B and C show 4 and 10 superimposed sw eeps. Propagation of the APs dow n the MF membranes is bidirectional aw ay from the endplates. AP recorded from MF 3 is largest because of its proximity to the electrode.5/24/13 EMG Evaluation of the Motor Unit . MD. This is approximated in routine EMG by assessing the stability of the MUAPs (or the instability when it is increased.E and F are 10 superimposed sw eeps from a patient w ith myasthenia. D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. copyright P. Summation of the individual MF APs as they form the MUAP. 2001. A.medscape. the normal biologic system always has some variability. D is made w ith low -frequency filter at 2 Hz. D. A. Temporal dispersion is lower in muscles with shorter MF lengths. A. Trigger (except in D) is alw ays at the third division. Motoneuron (MN) generates an action potential (AP) that in turn excites the muscle fibers (MFs) by means of neuromuscular transmission. aw ay from the endplate zone). Cross-section schematic emphasizes dispersion of the MFs of that MU in the MU territory and their relative position to the recording needle electrode. C. Barkhaus. Courtesy of w w w . Schematic show ing the generation of the MU action potential (MUAP). (see section A of each image below).E. Some inherent variation also exists in neuromuscular transmission at the endplates. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. They look superficially similar. the needle emedicine. C. B. E. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. calibration settings are as indicated.com/article/1846028-overview#showall 32/49 . Superimposed sw eeps from a healthy MUAP show ing stability. Four trigger-delayed superimposed sw eeps of a motor unit action potential (A) recorded w ith a concentric needle electrode from a patient w ith myopathy.E. F show s increased instability w ith blocking. Hence.com. This recording is stable. This is best appreciated when the high pass filter is increased to 500 to 1000 Hz. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie. respectively. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. it does not record the contributions of all the MFs within the MUT (estimated to be 5. Although it has the largest recording area.[40.casaengineering. Rise time is defined as the time interval between the maximum initial positive peak to the maximum negative peak. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.com/article/1846028-overview#showall 33/49 . The single-fiber EMG electrode is the most selective (C). The muscle fibers (MFs) closest to its tip contribute to such features as the amplitude (small semicircle). But in generating the MUAP as recorded by the needle electrode. 2001. Schematic compares 3 recording needle electrodes relative to an MU territory in a muscle.Electrophysiologic Biopsy electrodes used to record the MUAPs have limitations and distort the "ideal" MUAP waveform signal. However.to 10-mm in diameter). MD. making the measurement points problematic. the traditional rise time value cannot be used since very few MUAPs affected by chronic reinnervation would be accepted. The measuring device (ie. The recording area has a 150-mm radius. Barkhaus.5/24/13 EMG Evaluation of the Motor Unit . the authors have proposed relaxing this criterion. but it records from a moderate portion of the MU territory (circle). by their method of questioning (ie. needle electrode) is of the user's construction. 28] This brings us back to the prefatory quote of this article. the CON or MON EMG signal). Therefore.5 milliseconds. the size of the individual MF AP it records is heavily dependent on the distance of the MF from the recording electrode. The concentric needle electrode (B) records from the inner core at its tip. copyright P. Rise time is not a feature of the MUAP but rather a criterion for MUAP quality when accepting MUAPs for measurement. Furthermore.com. 49] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. 39.medscape. Some automated algorithms do not select MUAPs on rise time but rather on slope. there is an appearance of homogeneity in the size and shape of the MFs.5 millisecond. Thus. Schematic show ing the MU action potential (MUAP).[59] emedicine. 36. The recording surface of the macro-EMG electrode (A) is its cannula. These MFs contribute to the duration feature of the motor unit action potential. Image courtesy of w w w .E.[58] The formidable-sounding macro-EMG needle electrode is the least selective needle electrode (see image below). 49] Most of this discussion covers the CN because less is known about the physical characteristics of the MON electrode. In anatomic cross-section of healthy muscle. With permission. The traditional rise time value should be less than 0. time interval between the first 2 arrows denoting turns). The CN electrode MUAP recorded in the biceps brachii has a recording area of approximately 2500 µm (this defines the MUAP duration feature as discussed below). it records waveforms that are actually much lower in amplitude than the routine CN MUAP.[3.[49] Even otherwise high-quality simple triphasic MUAPs may have rise times slightly greater than 0. when dealing with pathologic conditions. and to some degree distorts. w hich can extend through the MU territory. and the MUAP as recorded is not nature itself but a signal that the operator extracts. This is uncomplicated when one deals with simple triphasic waves (see image below. variation found in the APs generated by these single MFs based on their distance from the recording electrode. w ith a recording surface of 15-mm diameter on the side of the needle's shaft.[59. Highly complex waveforms (see section D of the image below) may have 2 or more turns or phases between the maximum initial positive peak and maximum negative peak. emedicine.E and F are 10 superimposed sw eeps from a patient w ith myasthenia. B and C show 4 and 10 superimposed sw eeps. respectively. drop out or "block" of some of the component spikes). It sometimes is referred to as a satellite or linked potential. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie.Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. MU action potential (MUAP) stability assess by using concentric needle (CN) electrode. [48] Phase Turn This is a deviation of the signal from. particularly in the spike to the left. Late This is a waveform that is time locked to the main MUAP but separated from it by an isoelectric component interval. calibration settings are as indicated. C. With permission. A. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. Trigger (except in D) is alw ays at the third division. In quantitative EMG. increases in jitter or instability of the CN MUAP may be best assessed by superimposing successive sweeps (see second and third image below this table). terming it jiggle. This measures the thickness of the main spike. Superimposed sw eeps from a healthy MUAP show ing stability. from a chronic neurogenic process show ing instability. MUAP Features [49.5/24/13 EMG Evaluation of the Motor Unit . more than 5 turns. Slight movement may easily change a turn onto a phase. Duration is defined as the time interval between the signal's deviation from the baseline and its return to the baseline. 50] (see also first image below Table 3) (Open Table in a new window) Feature Area Area-toamplitude ratio Duration Definition or Comment This is measured as the area under the rectified waveform within a defined duration.com/article/1846028-overview#showall 34/49 . E show s increased instability or jitter. When excess turns are present. until its return to. They look superficially similar.[37] Amplitude This is measured as the total peak-to-peak amplitude. the baseline. Stalberg et al recently have quantified the vertical instability of the MUAP. or a satellite potential. Variability This refers to changes in shape of the MUAP on consecutive discharges at standard filter settings. MD. the MUAP is termed serrated.medscape.) Complexity A MUAP is called complex if it has more than 4 phases. By changing the instrument settings to those used in SFEMG. D is made w ith low -frequency filter at 2 Hz. copyright P. A MUAP having more than 4 phases is considered polyphasic.E. F show s increased instability w ith blocking. usually a turn of 50 µV is considered clinically significant). A MUAP having more than 5 turns is abnormal. 2001. or vice versa. This is a change in polarity (must be defined in terms of µV. E. D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. B. Table 3. how ever. A. (The duration measure does not include such potentials. Barkhaus. polyphasic MUAPs are not included in duration measurements. Barkhaus. B. A. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.com. The main spike component of the MUAP depends on the temporal dispersion of the MF APs within 1 mm of the recording electrode tip.Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. shifting the smaller spike to the left. Courtesy of w w w .E. With permission. D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. how ever. Low -frequency filter is increased to 500 Hz in B. copyright P. MD. The number of phases and turns and main spike duration define and characterize the main spike. Trigger (except in D) is alw ays at the third division. Four trigger-delayed superimposed sw eeps of a motor unit action potential (A) recorded w ith a concentric needle electrode from a patient w ith myopathy.com/article/1846028-overview#showall 35/49 . Thus. With permission. They look superficially similar. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie. E. copyright P.casaengineering.[36.5 mm of the recording tip as shown in the image below (particularly on the distance to and diameter of the closest MF).E. B and C show 4 and 10 superimposed sw eeps.medscape. Data from simulation studies indicate that the CN MUAP peak-to-peak amplitude is determined by the MFs within 0. With permission.[36] emedicine. C. Blanket principle. 2001.5/24/13 EMG Evaluation of the Motor Unit . MU action potential (MUAP) stability assess by using concentric needle (CN) electrode. Schematic show ing the MU action potential (MUAP). This is estimated to comprise 5-10 MFs. C. Note that the amplitude is reduced and baseline "sharpened" w ith removal of the low frequency components. the most obvious feature of the MUAP is recorded from few MFs (typically 2-3) and from a disproportionately small area of the MUT. calibration settings are as indicated. Superimposed sw eeps from a healthy MUAP show ing stability. MD. sw eep is increased. A. MD. from a chronic neurogenic process show ing instability.E. This recording is stable. As temporal dispersion increases so also do the values of these features. F show s increased instability w ith blocking. particularly in the spike to the left. Schematic show ing the recording areas of the concentric (A) and monopolar (B) needle electrodes. Barkhaus. respectively. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. 2001. copyright P. minor movement of the electrode's recording tip may significantly alter MUAP amplitude. 37] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. drop out or "block" of some of the component spikes). 2001. D is made w ith low -frequency filter at 2 Hz. Not surprisingly. Calibrations are as indicated. E show s increased instability or jitter.E and F are 10 superimposed sw eeps from a patient w ith myasthenia. Barkhaus. In this instance the terminal axon branch (ie. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. D is made w ith low -frequency filter at 2 Hz. MD. section D). one might say. With permission. Schematic of alterations in single muscle fibers (MFs) that contribute to changes in their action potentials (APs). The second is the way in which they are distributed. By using the blanket principle[56] (ie. E. D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. B. within the MUT. or rather redistributed. A. MU action potential (MUAP) stability assess by using concentric needle (CN) electrode. 49. drop out or "block" of some of the component spikes).medscape. F show s increased instability w ith blocking. C. instability may be observed easily. E show s increased instability or jitter. sporadic delay or failure in AP generation occurs in 1 or more muscle MFs. from a chronic neurogenic process show ing instability. Remodeling of the MU in Neuromuscular Disorders The MU may remodel in neuromuscular disorders in 2 basic ways. MF w ith segmental necrosis. accumulation of storage products. "The MUAP is not the (MU) territory. on repeated discharges.5/24/13 EMG Evaluation of the Motor Unit . how ever. presence of inflammatory cells).Electrophysiologic Biopsy Area is determined by the MFs of an MU within 2. Other morphologic change may occur but does not have direct electrophysiologic implications on the MUAP per se (eg. innervated end to the right) has sent a collateral axon branch to emedicine.5 mm of the electrode tip (see image above). If. B and C show 4 and 10 superimposed sw eeps. only a portion of the total MFs of the MU are being reflected in what is termed the CN MUAP. increasing the low-frequency filter to 500 Hz and increasing the sweep speed to 0. as well as any increase in spikiness of the main spike component.[36] In healthy muscle.0 mm of the electrode tip and duration by those MFs within 2. This delay is analogous to the increased jitter and blocking quantified in SFEMG (see image below). This assumption also does not take into consideration that the recording surface of the needle electrode actually may be central or eccentric in its location within the MUT. copyright P.{Ref2}[39. Superimposed sw eeps from a healthy MUAP show ing stability. B. 2001. calibration settings are as indicated. particularly in the spike to the left. The Polish American philosopher and semanticist Alfred Korzybski once quipped. each MN discharge results in generation of an AP in all of the MFs of its MU.E. Trigger (except in D) is alw ays at the third division. A. the waveform varies in shape from 1 discharge to the next. Since the MUT may measure upward of 10 mm. They look superficially similar. Barkhaus. "The map is not the territory. healthy MF. respectively. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie." The MON electrode has a recording area similar to that of the CN (see image above).com/article/1846028-overview#showall 36/49 . C. The latter suggests subtle changes in the microarchitecture of the MU in the form of abnormal increase or dispersion of the MFs closest to the recording tip of the CN or MON electrode (see Remodeling of the MU in Neuromuscular Disorders below and the Indications section above). This may be subtle or unappreciated on standard filter and sweep speed settings (see image below. Alterations in MFs and MUs in neuromuscular disorders (see images below) Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. The first relates to intrinsic changes to the MFs. 50] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. A.E and F are 10 superimposed sw eeps from a patient w ith myasthenia." To make a bit of a pun for emphasis. MUAP duration values are similar for both electrodes.5-2 ms per division). Atrophy of the MF that results in a smaller AP. Barkhaus. Alterations in structure of MFs (ie. With permission. E similar to D. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. shaded MFs represent those that w ould show fiber type grouping on histochemical staining. the AP propagating left into the split goes dow n each branch. angular MFs represent those that are still denervated and that have not yet undergone reinnervation. w hich may be seen in either process and may be associated w ith MF splitting. Also. inflammatory cells). MD. segment isolated from endplate) Alterations in spatial arrangement of MFs within the MU Loss of MFs Collateral reinnervation of MFs (MF type grouping) Regeneration of MFs from satellite cells Table 4 below shows that similar electrophysiologic findings may be encountered in both myopathic and neurogenic disease processes.medscape. copyright P.casaengineering.com. changes that may alter MF membrane function) MF atrophy MF (longitudinal) splitting MF hypertrophy Segmental necrosis of MF (ie. This technique does not reveal MFs innervated by a single MN (ie. Courtesy of w w w . Although a single arrow show s the AP propagating to the right. Composite display of 25 MU action potentials (MUAPs) from a patient w ith myopathy. Schematic show ing typical alterations of muscle fibers (MFs) and their spatial arrangement in a myopathic process (bottom left) and neurogenic process (bottom right). D. not all myopathic processes exhibit MF hypertrophy). an enlarged or reinnervated motor unit). MF is split distal to the endplate. Specific disease processes w ithin each pattern may vary (eg. Although MUAP amplitude may be increased in myopathic processes. Barkhaus. In the neurogenic pattern.5/24/13 EMG Evaluation of the Motor Unit . Pathologic Correlates of MU Action Potential Features (Open Table in a new window) Pathologic Correlates emedicine. An electrode recording from the split end w ould detect 2 APs. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. With permission. 2001. show s the innervation of a regenerated MF from satellite cells via a collateral axonal branch from an adjacent MF. 2001. noncomplex) MUAPs (see image below). the schematics are not intended to show specific changes in disease processes that do not directly influence the motor unit action potential w aveform (eg. the key to proper interpretation is to look at all of the MUAP features collectively to make the correct assessment. copyright P. Enlarged MFs represent MF hypertrophy. F. which may be increased in both myopathic and neurogenic processes. it is rarely accompanied by any commensurate increase in area and certainly not in duration of simple (ie. An example is MUAP amplitude.E. MD.Electrophysiologic Biopsy the orphaned end (left) to reinnervate it.com/article/1846028-overview#showall 37/49 . Terminal axon branch from an adjacent nonaffected MF also could have provided the source for the reinnervating collateral branch. Table 4. However. Dark.E. Such splits may occur in hypertrophied MFs. Innervation of a denervated MF (2) by means of a collateral al branch from an unaffected MF from another motor unit (1). [60] Quantitative Analysis Reviewing the mathematical aspects of QA is beyond the scope of this article. the consideration of all MUAP features (in addition to other electrodiagnostic findings such as the IA. MF hypertrophy) and physiologic (eg. SA. nerve conductions. small AP) Reduced number of MFs Increased temporal dispersion of APs Abnormal volume conduction Atrophy of MFs (ie. offers optimal interpretation as to the actual pathologic process.medscape. a sampling of 20 MUAPs is considered adequate. Note that no single MUAP feature is pathognomonic for a particular disease process.Electrophysiologic Biopsy MU Action Potential Abnormality Increased Myopathic Processes Neuropathic Processes Amplitude MF hypertrophy (assuming normal membrane function) Synchronous firing of 2 or more MUs Regeneration of MFs MF hypertrophy (assuming normal membrane function) Synchronous firing of 2 or more MUs Reinnervation of more MFs Selective loss of small type I MUs Decreased Amplitude Atrophy of MFs (ie. small AP) Reduced number of innervated MFs Increased temporal dispersion of APs Abnormal volume conduction Increased Duration Increased variability of MF diameter MF hypertrophy Synchronous firing of 2 or more MUs Slow AP propagation in terminal axon branches Reinnervation of more MFs for MU Selective loss of small type I MUs Synchronous firing of 2 or more MUs Slow AP propagation in terminal axon branches Dispersion of endplate zone Decreased Duration Loss of MFs MF atrophy Fractionation of reinnervated MUs Polyphasia Increased variability of MF diameter Slow conduction in atrophic MFs Regeneration of MFs MF splitting Increased width of endplate zone Slow conduction in atrophic MFs MF splitting Slow conduction in newly formed terminal axon branches MU instability Innervation of regenerated MFs Reinnervation of orphaned MF segments Reinnervation Table 4 above gives the electrophysiologic correlates relating to these pathologic changes in morphologic restructuring. For instance. This implies overlap in both morphologic (eg. etc. Ultimately. MUAP duration typically is emedicine.). the standard protocol for that technique must be followed carefully to ensure accurate measurement. In general.[47] No matter which QA technique is used.com/article/1846028-overview#showall 38/49 . instability) features in both myopathic and neurogenic processes.5/24/13 EMG Evaluation of the Motor Unit . Copyright. Thus. often resulting in the unsuitable terms. the study becomes abnormal when the third outlier is encountered.5/24/13 EMG Evaluation of the Motor Unit .com. in neuromuscular disorders. Note the w ide range in size and complexity in the MUAPs. The electrodiagnostic consultant looks for MUAPs with features that would not be observed in normal muscle (eg. MD. Based on this. This occasionally happens. particularly amplitude. A further point is that the patterns of abnormality should be consistent. Methods in QA emedicine. In assuming a gaussian distribution of values. Paul E Barkhaus. Composite display of 25 MU action potentials (MUAPs) from a patient w ith myopathy. In other words. [3. Collective reference data involve the decision as to whether a subject should be included.[49.Electrophysiologic Biopsy measured at a sensitivity setting of 100-200 µV per division. The images below show composites of different MUAPs recorded from the biceps brachii in a myopathic process. finding "abnormality" that is not clinically significant is always possible. w ith permission. Stalberg et al have designated these MUAPs as "outliers". 50] In routine EMG or subjective analysis. increased number of phases). the testing of that particular muscle has reached diagnostic significance. along with percent polyphasic MUAPs. the range within ±2 standard deviations should include 95% of tested subjects. Careful scrutiny of all data generally reveals a predominant trend in abnormality in key MUAP features. mixed findings or neuromyopathy . This approach allows a greater economy of time. long duration. What is normal? In collecting reference data in asymptomatic subjects. and the study can be extended to another muscle or concluded.casaengineering. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. Composite of 20 motor unit action potentials (MUAPs) recorded w ith a concentric needle electrode from the biceps brachii of a patient w ith s-IBM.com/article/1846028-overview#showall 39/49 . yet still requires the examiner to quantify the MUAP signal. Courtesy of w w w . increased amplitude. A muscle may have no more than 10% outlier recordings. the chance always exists that a healthy subject may have an "abnormal" measurement. Mean values of MUAP features such as amplitude and duration traditionally have been used in QA. the electrodiagnostic consultant must confirm these by comparing his or her own reference data before using them.[51] They defined upper and lower limit values for MUAP features such as amplitude. 2000. Assuming a typical study sample of 20 MUAPs. look for a pattern or gestalt to help arrive at an interpretation or diagnostic impression. the strategy is different. both distal and proximal muscle). A special caveat extends to the pooling of EMG data in patients (ie.[48] MUAPs must be measured at the same instrumentation settings as the reference data. 39] If a laboratory uses published reference data. the duration values in a muscle should not be abnormal by being both below and above the reference ranges.medscape. Use of the term reference data is deliberate in lieu of normal. and polyphasic MUAPs are not included when mean duration is measured. At this point. Note the wide range in MUAP features (see Electrophysiologic Measurements of the Motor Unit in the Technique section above). Such a decision may be purely subjective. and usually only 1 MUAP can be collected per site Requires experience and practice in acquisition Analysis of MUAP features may be basic as above. the reader is referred to Nandedkar and Barkhaus.Electrophysiologic Biopsy To perform QA. 50] (Open Table in a new window) Method Manual Description or Comment Pioneered by Buchthal[1] Requires low activation of muscle Analysis essentially limited to MUAP amplitude. The main methodologies in QA are described in Table 5 below.com/article/1846028-overview#showall 40/49 . and phases Easily performed with most equipment but time-consuming Trigger delay Requires some modification to equipment: amplitude trigger. and averager[52] Measurements biased toward higher amplitude MUAPS. delay line. a standardized methodology or protocol is necessary. area) Time-consuming Multi-MU analysis [53] Acquires 1-6 MUAPs per site at minimal activation Therefore no bias toward larger amplitude MUAPs emedicine. In CN electrode analysis. duration. Table 5.medscape.[3. 39] If an electrodiagnostic consultant is using commercially available software for analysis. QA Methods in EMG[49. the authors' protocol excludes polyphasic MUAPs from duration measurement. For a more extensive review. or software may allow additional analysis of measurements (eg. he or she is obliged to understand the algorithm in how it makes measurements.5/24/13 EMG Evaluation of the Motor Unit . and they do not accept CN-recorded MUAPs less than 50 µV in amplitude for analysis. [3. By slight manipulation of the recording electrode. E. Although this is actually observed in B sw eep 2. MD. Use of trigger delay is essential to ascertain each potential by visualizing it at least 3 times. The blanket principle (or "poor-person's" SFEMG) is as follows: Payan[56] used the example of a blanket covering a statue to describe how an electrode of intermediate selectivity such as the CN or MON electrode could be changed to one of higher selectivity by altering the instrumentation. Tw o late components are appreciated readily (numbers 2 and 3 are separated by baseline). Second. Trigger delay also may be used to examine other waveforms such as iterative discharges (eg. indicating a steady firing rate of approximately 10 Hz.E. superimposing on potential 1 on sw eeps 3 and 4. 2001. this instrumentation adjustment is necessary to study MUAP stability. copyright P. myotonic discharges. interacting with instrumentation) to scrutinize waveforms may be essential during a study (see image below). 5 ms horizontal per division): A show s 4 sw eeps on free run mode from a patient w ith myopathy.com/article/1846028-overview#showall 41/49 . shifting left.5/24/13 EMG Evaluation of the Motor Unit . To use a different example. smaller potential is discharging slightly faster. Moreover. 39. First potential occurs at almost the same position on each sw eep. B is a free run recording of rastered sw eeps from a patient w ith severe myopathy. thus confirming its complex w aveform. Complex MUAP (2) is observed shifting left on subsequent sw eeps. complex repetitive discharges) in performing a study. its intermittent use in routine EMG (ie. trigger delay on potential 1 (C) show s a highly complex MUAP that is unstable even at standard settings (note change in pattern of peaks). Barkhaus. The enlarged potential on the first sw eep (open arrow ) is the result of superimposition betw een the 2. demonstrating recurrence of the w aveform 3 times is essential before ascertaining it as a MUAP.Electrophysiologic Biopsy Uses decomposition and averaging technique Acquisition and analysis very rapid Requires review and editing of data to ensure quality Good reproducibility Decomposition Allows acquisition at higher levels of activation with subsequent breakdown of signals into their component MUAPs Requires editing and validation of MUAPs by operator[54] Although trigger delay is a QA technique. MUAP 1 is observed to the right on the top sw eep and partially on the bottom sw eep. 55] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. With permission.D show s trigger delay on MUAP 1.medscape. The photographer emedicine. Acquisition is made with a single lens reflex camera that has a 55 mm (intermediate size) lens. Another sw eep show ing potential 1 is needed to verify it as a bona fide potential. Electrode is manipulated slightly. to w here it superimposes on potential 1 on the fourth sw eep. Trigger delay in instrumentation (calibration for A-E is 100 mV vertical. imagine standing in a precisely fixed position within a forest and wishing to take a picture of a tree that is directly before you. At least 2 MU action potentials (MUAPs) are seen as denoted by the numbers. and the trigger-delay now is set on potential 2. Barkhaus. Low -frequency filter is increased to 500 Hz in B. emedicine. stability of the MU may be appreciated readily in an objective manner by superimposing successive sweeps of the signal.Electrophysiologic Biopsy would like to refocus to appreciate closer details on the tree but cannot change position. a CN or MON electrode cannot substitute completely for the highly selective SFEMG electrode. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. With permission. Increasing the low-frequency filter reduces the electrode's recording territory by removing generally the contribution of the MFs between those contributing to turns and phases and those contributing to the duration measure at standard filter settings (see section A of the first image below). Note that the amplitude is reduced and baseline "sharpened" w ith removal of the low frequency components. Schematic show ing the recording areas of the concentric (A) and monopolar (B) needle electrodes. leaving only those APs of MFs very close to the recording surface of the needle electrode. This removes surrounding low-frequency components of the MUAP signal. This recording is stable.casaengineering.5/24/13 EMG Evaluation of the Motor Unit . 2001. In other words. By increasing the sweep speed (0. Calibrations are as indicated. In an analogous manner. close-up lens is not possible. modern EMGs have also provided better instrumentation to "zoom in" without breaking needle electrode position (ie. 39] Despite change in filter settings and other changes in instrumentation. see section A of the second image below). the "blanket" (ie.com. In the case of the MUAP. Four trigger-delayed superimposed sw eeps of a motor unit action potential (A) recorded w ith a concentric needle electrode from a patient w ith myopathy. Modern photography has provided the zoom lens. shifting the smaller spike to the left. stopping and breaking the fixed focus with the camera to change to a separate. Therefore. This is achieved by increasing the low-frequency (or high pass) filter from the 2-20Hz standard setting for routine MUAP acquisition to 500-1000 Hz. low-frequency components of the signal) is removed.51. The spikes or APs observed may represent single MFs or more than 1 MF.medscape. Blanket principle. losing the waveform of interest).[3. copyright P. Therefore. increasing the low-frequency filter reduces the electrode's recording territory by attenuating the contribution of the MFs beyond those contributing to turns and phases at filter standard settings. sw eep is increased. most of the microarchitecture of the MU can be observed immediately near the recording surface of the electrode to appreciate its "spikiness" (not the same as fiber-density measure in SFEMG)[57] and stability (analogous to jitter on SFEMG). C. Therefore. MD. Courtesy of w w w .[56] Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. and the inherent detail can be appreciated.E.5 ms per division). in addition to making quantitative assessments of the MUAP's basic metrics at standard settings. the electrodiagnostic consultant is interacting actively with the EMG to extract even more information (see images below).com/article/1846028-overview#showall 42/49 . C.E. The authors feel that this yields a more thoughtful interpretation of the EMG signal. A. Changes in the MUAP waveforms allow inferences to be made with respect to the integrity of the MFs and their MNs. In summary. Barkhaus. American Association of Neuromuscular and Electrodiagnostic Medicine. D show s 2 sample sw eeps of the same MUAP in early reinnervation w ithout the trigger. QA is the foundation upon which current understanding of EMG is based. D is made w ith low -frequency filter at 2 Hz. disease processes are better understood. E show s increased instability or jitter. Milwaukee Veterans Affairs Medical Center Paul E Barkhaus. it may be considered superfluous by many electrodiagnostic consultants.5/24/13 EMG Evaluation of the Motor Unit . This recording is stable. Four trigger-delayed superimposed sw eeps of a motor unit action potential (A) recorded w ith a concentric needle electrode from a patient w ith myopathy. and American Neurological Association emedicine. drop out or "block" of some of the component spikes). Therefore.com/article/1846028-overview#showall 43/49 . MD is a member of the following medical societies: American Academy of Neurology. Department of Neurology. Superimposed sw eeps from a healthy MUAP show ing stability. B and C show 4 and 10 superimposed sw eeps.medscape. from a chronic neurogenic process show ing instability. However. Because it is not needed in the majority of studies. how ever.E and F are 10 superimposed sw eeps from a patient w ith myasthenia. This is termed objective EMG. shifting the smaller spike to the left. 2001. 2001. C. copyright P. Director of Neuromuscular Diseases. Low -frequency filter is increased to 500 Hz in B. more data are extracted in a time-efficient manner. particularly in the spike to the left. Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy.E. MD. MD Professor. With practice. They look superficially similar. Therefore. B. understanding at least the basics is essential so that. this approach is termed objective-interactive EMG. With permission. Medical College of Wisconsin. visual impressions. The authors consider this to be an "electrophysiologic biopsy" of the MUs within a muscle (see Indications above). With permission. MU action potential (MUAP) stability assess by using concentric needle (CN) electrode. Note that the amplitude is reduced and baseline "sharpened" w ith removal of the low frequency components. Blanket principle. respectively. calibration settings are as indicated. >2 mV in amplitude).Electrophysiologic Biopsy Electromyographic (EMG) evaluation of the motor unit (MU): electrophysiologic biopsy. QA requires a discipline in acquisition and recording that may be time-consuming. training one's eyes to use the graticule markings on the monitor to make objective measures of the signals is a significant step up from making subjective. E. somewhat analogous to the traditional morphologic muscle biopsy. By interacting with the instrument. Barkhaus. copyright P. MD. Contributor Information and Disclosures Author Paul E Barkhaus. in turn. Taken together. F show s increased instability w ith blocking. Calibrations are as indicated. A. a qualitative description such as "large amplitude" becomes quantitative (eg. and F recordings are made w ith low -frequency filter increased from standard 2-500 Hz. the potential on the second sw eep is obviously shorter in duration as a result of instability (ie. Trigger (except in D) is alw ays at the third division. sw eep is increased. Departments of Neurology and Neurosurgery. The views expressed in this article do not necessarily reflect those of the Department of Veterans Affairs. University of Pittsburgh Medical Center-Shadyside Neil A Busis. PharmD. and American Medical Association Disclosure: UCB Pharma Honoraria Speaking. MD is a member of the following medical societies: American Academy of Neurology. Clinical Neurophysiology Laboratory. Glaxo Smith Kline Honoraria Speaking. Sleepmed/DigiTrace Honoraria Speaking. Paul E Barkhaus. American Academy of Sleep Medicine.medscape. University of South Florida College of Medicine Selim R Benbadis. University of Nebraska Medical Center College of Pharmacy. MD is a member of the following medical societies: American Academy of Neurology. MD Associate Professor of Neurology. American Clinical Neurophysiology Society. PhD CareFusion Disclosure: Nothing to disclose. and American College of Physician Executives Disclosure: Nothing to disclose.5/24/13 EMG Evaluation of the Motor Unit . MD Professor. Neurology Specialists and Consultants Nicholas Lorenzo. Director of Comprehensive Epilepsy Program.Electrophysiologic Biopsy Disclosure: Nothing to disclose. Minn: emedicine. consulting. PhD Adjunct Assistant Professor. MD Consulting Staff. MD is a member of the following medical societies: American Academy of Neurology and American Association of Neuromuscular and Electrodiagnostic Medicine Disclosure: Nothing to disclose. American Academy of Neurology. References 1. Coauthor(s) Sanjeev D Nandedkar. American Epilepsy Society. Editor-in-Chief. Additional Contributors The author. consulting. Lundbeck Honoraria Speaking. An introduction to electromyography. Selim R Benbadis.com/article/1846028-overview#showall 44/49 . MD Chief. consulting. Electromyography Laboratory. Disclaimer: This article does not necessarily reflect the views of the Department of Veterans Affairs or the United States Government. Buchthal F. American Association of Neuromuscular and Electrodiagnostic Medicine. consulting. Tampa General Hospital. Specialty Editor Board Dianna Quan. Division of Neurology. Department of Medicine. and Phi Beta Kappa Disclosure: e-medicine Honoraria Other Francisco Talavera. Rochester. consulting Chief Editor Nicholas Lorenzo. Director. Cyberonics Honoraria Speaking. Medscape Drug Reference Disclosure: Medscape Salary Employment Neil A Busis. consulting. University of Colorado School of Medicine Dianna Quan. MD is a member of the following medical societies: Alpha Omega Alpha. MD. Reprinted as Minimonograph #15. 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