Activation of muscle
This is the voluntary contraction of a muscle or muscle group that results in the movement of a joint(s). Muscle activation in the context of the electrodiagnostic examination is similar to activation in performing manual muscle testing during the clinical examination. In the latter, maximum force is elicited to determine strength. In EMG however, the focus is on minimal recruitment via gradated, steady, voluntary muscle activation.
The observation of progressive activation in EMG is referred to as recruitment. Maximum activation is referred to as the interference pattern. In motor unit action potential (MUAP) analysis, it is the observation of the MUAP discharges at relatively low levels of activation that is critical in measuring their features (e.g. amplitude, area, duration, phases, etc.). Hence the emphasis in EMG is to facilitate the patient’s ability to achieve and maintain minimal to moderate controlled activation of specific muscles.
In this program, localization of muscles in this program is typically based on activation. Thus, it is important to understand their origins, insertions and actions. In the following text, the most important aspects of their origins and insertions are given along with recommended methods of activation. Many muscles have more than one action or direction of movement of a joint. The method of activation given in this text is usually for a specific muscle's principal action (or that action which can be most easily utilized with minimal limb movement).
Some patients may have difficulty with activating muscle(s). The examiner must then literally provide a "hand" to help them offer resistance. This can be avoided in most circumstances such that the examiner has one hand free at all times in order to manipulate the settings on the electromyograph. This is critical when performing special studies such as quantitative MUAP analysis and single fiber EMG where sustained, minimal activation of the muscle is necessary. Helpful "tricks" in activation (e.g., see adductor longus, deltoid, frontalis) are given where possible. Note that if the examiner does touch the patient with an ungloved hand, electrical interference may be encountered. Using gauze to avoid direct skin contact with the patient often eliminates this.
As in the clinical examination, the needle electrode examination must be modified if the patient has significant weakness. The examination shown in these videos is a basic one where the patient is assumed to have at least Grade 3 strength (Medical Research Council rating system; Guarantors of Brain, 2000). When greater weakness or complete paralysis is present, further modifications to the examination may be necessary. In such instances, the electrodiagnostic consultant may require even greater skill in navigating the electrode through the subcutaneous tissue and musculature.
It may be argued that localization of muscles may be made solely on surface landmarks and relative anthropometric measurements, making activation unnecessary in localization. No matter how carefully localization is made or by what strategy, muscle activation is still the sine qua non in producing the MUAP. Where atrophy or wasting of muscle may have occurred such as in subacute to chronic denervation, caution in interpretation must be made, especially with deeper muscles where landmarks may have changed due to reduction in muscle bulk.
On occasion, especially in larger limb muscles, the screen may be "silent" despite what clinically appears to be adequate activation by the patient. The trainee's usual response is to solicit even greater effort from the patient, all to no result except patient fatigue and discomfort. This phenomenon is usually due to misunderstanding the examiner’s instructions for activation. To the patient, activation of other muscles achieves the same desired result without the additional discomfort from the needle electrode (e.g., elbow flexion via the brachioradialis when the electrode is in the biceps brachii). If the needle electrode is placed correctly and the activation maneuver correct, MUAP discharges should readily appear at low levels of effort. Compensatory activation maneuvers by the patient, however, may be important clues to localization of the pathologic process (e.g., use of long extensor of the thumb to perform palmar abduction when the abductor pollicis brevis is weak)!
In this program, localization of muscles in this program is typically based on activation. Thus, it is important to understand their origins, insertions and actions. In the following text, the most important aspects of their origins and insertions are given along with recommended methods of activation. Many muscles have more than one action or direction of movement of a joint. The method of activation given in this text is usually for a specific muscle's principal action (or that action which can be most easily utilized with minimal limb movement).
Some patients may have difficulty with activating muscle(s). The examiner must then literally provide a "hand" to help them offer resistance. This can be avoided in most circumstances such that the examiner has one hand free at all times in order to manipulate the settings on the electromyograph. This is critical when performing special studies such as quantitative MUAP analysis and single fiber EMG where sustained, minimal activation of the muscle is necessary. Helpful "tricks" in activation (e.g., see adductor longus, deltoid, frontalis) are given where possible. Note that if the examiner does touch the patient with an ungloved hand, electrical interference may be encountered. Using gauze to avoid direct skin contact with the patient often eliminates this.
As in the clinical examination, the needle electrode examination must be modified if the patient has significant weakness. The examination shown in these videos is a basic one where the patient is assumed to have at least Grade 3 strength (Medical Research Council rating system; Guarantors of Brain, 2000). When greater weakness or complete paralysis is present, further modifications to the examination may be necessary. In such instances, the electrodiagnostic consultant may require even greater skill in navigating the electrode through the subcutaneous tissue and musculature.
It may be argued that localization of muscles may be made solely on surface landmarks and relative anthropometric measurements, making activation unnecessary in localization. No matter how carefully localization is made or by what strategy, muscle activation is still the sine qua non in producing the MUAP. Where atrophy or wasting of muscle may have occurred such as in subacute to chronic denervation, caution in interpretation must be made, especially with deeper muscles where landmarks may have changed due to reduction in muscle bulk.
On occasion, especially in larger limb muscles, the screen may be "silent" despite what clinically appears to be adequate activation by the patient. The trainee's usual response is to solicit even greater effort from the patient, all to no result except patient fatigue and discomfort. This phenomenon is usually due to misunderstanding the examiner’s instructions for activation. To the patient, activation of other muscles achieves the same desired result without the additional discomfort from the needle electrode (e.g., elbow flexion via the brachioradialis when the electrode is in the biceps brachii). If the needle electrode is placed correctly and the activation maneuver correct, MUAP discharges should readily appear at low levels of effort. Compensatory activation maneuvers by the patient, however, may be important clues to localization of the pathologic process (e.g., use of long extensor of the thumb to perform palmar abduction when the abductor pollicis brevis is weak)!
Anatomic Planes
In the anatomic position, a coronal plane vertically transects the body side to side resulting in anterior/ posterior segments. The horizontal or axial plane transects the body transversely resulting in rostral/ caudal, "cross-sectional" segments. In the axial or cross sections in this presentation, the convention is to look rostral (or toward the head from below). The sagittal plane extends anterior to posterior, vertically dividing the body into right/ left segments.Electrodes
This program emphasizes the placement of intramuscular needle electrode for direct recording of the myogenic signal. The two most commonly used are the concentric needle electrode and the monopolar needle electrode. Other less commonly used electrodes include the single fiber EMG electrode used for jitter and fiber density studies; the macro-EMG electrode used for macro-EMG and fiber density studies; and an intramuscular needle electrode that concomitantly records the myogenic signal and allows injections for therapeutic chemodenervation of muscle. The localizations given in this program are not synonymous with the placement of surface recording electrodes in performing motor conduction studies. A few of these are considered with a separate link in the program.
Isometric Contraction
This is the most common type of contraction or pattern of activation used in EMG. Tension is generated in a muscle via activation without a change in its length. Thus there is less chance of unintentional needle electrode movement in the muscle. Although standard isometric contraction in EMG does not appear to change muscle length by superficial observation, in clinical practice the needle electrode may "bow" or bend in some muscles (e.g., anterior tibial, flexor carpi radialis, etc.). In such muscles, it is preferable to activate the muscle first before inserting the needle electrode in an oblique direction (versus the preferred perpendicular approach). Resting activity can be examined later when the muscle is relaxed.Positioning of the Patient
Lateral Decubitus
The patient is positioned on their side. For purposes of this program, this position implies that the patient's trunk and legs are straight without any rotation of their shoulders or pelvis. Their head is supported by a pillow or similar device to keep the head and neck straight and in continuity with the long axis of their spine, without lateral deviation. In most instances, the limb or area to be examined in this position should be superior. Additional specific placement of the lower extremities is helpful in activating the patient (e.g., see multifidus, etc.).Prone
Supine
The patient is lying face upwards on the examination table.A variation of the supine position is the Lithotomy Position. This is covered separately in the Pelvic Floor and Related Muscles chapter.
Preparation of the patient
Patient tolerance is often proportional to their awareness of the procedure. The procedure is therefore explained to the patient on entry into the laboratory. Patient comfort and confidence in the examiner is a priority.
Appropriate gowning depends on which muscles are to be studied. It may be argued that if the study is directed to a cervical radiculopathy, only partial disrobing of outer garments such as a shirt or blouse with gowning is needed. The coincidental identification of a polyneuropathy, however, may necessitate study of a lower limb. In most circumstances, it is less disruptive if the patient is ready for this contingency rather than stopping while the patient disrobes further, especially if they require assistance. This also facilitates the preliminary clinical examination.
Subjective impressions of pain vary greatly between individuals. Given that the needle electrode examination will be considered at least minimally noxious by most patients, it is essential to ascertain that the procedure is explained fully prior to beginning the test. After completing the preliminary clinical examination and any nerve conductions or other studies, the patient should again be informed about the needle electrode examination. At this time it is helpful to repeat inquiry about any history of infection (e.g. HIV, hepatitis, etc.)/ risk factors for infection or anticoagulant use.
The electromyographer should review with the patient approximately 2-3 muscles to be examined at a time, including how to position and activate the muscle as well as demonstrating approximately where the needle will be placed. Although the patient may be anxious, it is much easier to "coach" the patient on activation while relatively relaxed, i.e., prior to needle insertion. Having patients participate with their muscle activation tends to distract them from the discomfort of the procedure.
Appropriate gowning depends on which muscles are to be studied. It may be argued that if the study is directed to a cervical radiculopathy, only partial disrobing of outer garments such as a shirt or blouse with gowning is needed. The coincidental identification of a polyneuropathy, however, may necessitate study of a lower limb. In most circumstances, it is less disruptive if the patient is ready for this contingency rather than stopping while the patient disrobes further, especially if they require assistance. This also facilitates the preliminary clinical examination.
Subjective impressions of pain vary greatly between individuals. Given that the needle electrode examination will be considered at least minimally noxious by most patients, it is essential to ascertain that the procedure is explained fully prior to beginning the test. After completing the preliminary clinical examination and any nerve conductions or other studies, the patient should again be informed about the needle electrode examination. At this time it is helpful to repeat inquiry about any history of infection (e.g. HIV, hepatitis, etc.)/ risk factors for infection or anticoagulant use.
The electromyographer should review with the patient approximately 2-3 muscles to be examined at a time, including how to position and activate the muscle as well as demonstrating approximately where the needle will be placed. Although the patient may be anxious, it is much easier to "coach" the patient on activation while relatively relaxed, i.e., prior to needle insertion. Having patients participate with their muscle activation tends to distract them from the discomfort of the procedure.
Patients often ask how many "sticks" or "pokes" they will receive with the "needle". The experienced examiner knows that it is impossible to give an exact number prior to the examination. A direct answer can be diplomatically avoided by a nonspecific response such as "several" or "not too many more". If asked again as the examination nears completion, the answer of "a few more" is at least encouraging. To offer numbers usually prompts the patient to start counting, only to feel mistrust if the quoted estimate is exceeded.
Preparation of the examiner
Preparation for the electrodiagnostic consultant starts with the development of a hypothesis or working diagnosis on the patient being examined. This is based on the referring physician's request, the electrodiagnostic consultant’s clinical assessment, and the nerve conductions or other testing completed to this point. The challenging and artful part of the needle electrode examination is to plan the core study of a few select muscles to address the problem with the least amount of patient position changes. Based on these results, the examiner proceeds to study additional muscles that will help prove or disprove the diagnostic impression.The efficiency and expedience of the needle electrode examination is proportionate to examiner experience. It is important that less experienced examiners refrain from turning insertion sites into what the author terms "inertion" sites. In this situation of the doldrums, the examiner becomes increasingly uncertain of the signal on the screen, their uncertainty accelerating in proportion to the length of time the electrode has been meandering in the muscle. Although most often encountered in trainees, most every electromyographer periodically experiences some degree of this phenomenon. Patients are sensitive to time. The examiner should always be mindful as to what point they have exceeded the usefulness in an insertion site. If examination of a specific muscle is a problem, it is best to move along to the next muscle. Such a problem can be resolved by substituting or studying other muscles, or returning to the same muscle at the end of the study when information from other muscles is available.
Needle electrodes
Technical issues concerning needle electrodes are covered elsewhere (Barkhaus, 1998; Barkhaus, 2005; Nandedkar, 2001; Nandedkar, 2002). The choice of concentric versus monopolar needle electrode is typically based on the examiner’s training and bias. Some historical reasons are no longer valid since almost all laboratories use disposable electrodes of either type. Both are available in variable lengths. In general, use of a 50 mm electrode of either type is often needed to examine the deeper, larger muscles in many adults, particularly the multifidus. Occasional use of a 75 mm is necessary, such as in obese patients. In the following discussion, the principles covered apply to both concentric and monopolar needle electrodes except where indicated (e.g., cannula potentials).
Some 25 mm concentric needle electrodes (sometimes referred to as "facial" concentric needle electrodes) are thinner (0.3 mm diameter) and have a smaller active recording surface (0.019 mm2) than other standard concentric electrodes (i.e., 0.46 mm diameter; 0.07 mm2 recording surface). Thus, their physical characteristics such as the higher resistance and reduced recording surface differ from the standard gauge concentric needle electrode. Actual differences measured in MUAP features such as the amplitude, duration, complexity, etc. between the standard and smaller concentric needle electrodes remain unresolved. As a general rule, however, MUAP amplitudes tend to be higher and may appear "neurogenic", particularly in distal muscle.
Some 25 mm concentric needle electrodes (sometimes referred to as "facial" concentric needle electrodes) are thinner (0.3 mm diameter) and have a smaller active recording surface (0.019 mm2) than other standard concentric electrodes (i.e., 0.46 mm diameter; 0.07 mm2 recording surface). Thus, their physical characteristics such as the higher resistance and reduced recording surface differ from the standard gauge concentric needle electrode. Actual differences measured in MUAP features such as the amplitude, duration, complexity, etc. between the standard and smaller concentric needle electrodes remain unresolved. As a general rule, however, MUAP amplitudes tend to be higher and may appear "neurogenic", particularly in distal muscle.
The advantages of the concentric needle electrode are that it requires only one placement for both the active and reference (i.e., cannula) recording electrode surfaces. There is also a large base of quantitative data on MUAP features for numerous muscles in subjects of various ages. Other electrodiagnostic consultants feel that the monopolar electrode is less noxious and better tolerated by patients. In our experience, patient tolerance is mainly contingent upon the experience and skill of the electrodiagnostic consultant.
Principles of insertion of needle electrodes
In this program's video demonstrations, the focus is on the insertion site. Also important is maintaining the continuity and ease of patient and physician movement during the examination. Keeping the electrode wires to the preamplifier over the examiner’s hand so that they, or the electrode, do not become dislodged, disconnected, or entangled facilitates a smooth examination.
At insertion, it is helpful for the examiner to slightly tense the skin at the insertion site using the thumb and index finger of the hand that is not holding the needle electrode. This allows quick penetration of the needle electrode through the skin and subcutaneous tissue. In some muscles, this maneuver also serves to identify and isolate the borders of the muscle under investigation. It has not always been possible to demonstrate the "skin tensing" maneuver while demonstrating insertion sites in the videos. While it is inserted, the author also tends to stabilize the needle electrode by positioning the gloved hand holding the electrode against the patient. This offers better control in maintaining needle electrode position.
When intrinsic muscles of the hand or foot are studied, try to insert the electrode in the less sensitive dorsal skin, rather than the glabrous skin of the palm or sole (the latter may also be quite callused in some individuals). In most instances the insertion of the needle electrode should be in a direction perpendicular to the orientation of the muscle fibers, i.e. skin. The needle electrode is inserted in a progressively deep track or corridor so as to sample both superficial and deep sites within the muscle.
At insertion, it is helpful for the examiner to slightly tense the skin at the insertion site using the thumb and index finger of the hand that is not holding the needle electrode. This allows quick penetration of the needle electrode through the skin and subcutaneous tissue. In some muscles, this maneuver also serves to identify and isolate the borders of the muscle under investigation. It has not always been possible to demonstrate the "skin tensing" maneuver while demonstrating insertion sites in the videos. While it is inserted, the author also tends to stabilize the needle electrode by positioning the gloved hand holding the electrode against the patient. This offers better control in maintaining needle electrode position.
When intrinsic muscles of the hand or foot are studied, try to insert the electrode in the less sensitive dorsal skin, rather than the glabrous skin of the palm or sole (the latter may also be quite callused in some individuals). In most instances the insertion of the needle electrode should be in a direction perpendicular to the orientation of the muscle fibers, i.e. skin. The needle electrode is inserted in a progressively deep track or corridor so as to sample both superficial and deep sites within the muscle.
In most muscles, the electrode placement should be midway between the midbelly or presumed endplate zone of the muscle and its origin or insertion. Although the electrode can be inserted more obliquely going in a direction more parallel to the muscle fibers, the risk is that in most muscles, particularly the larger ones, the same motor units will be sampled as the electrode crosses through fewer muscle fibers with the same length of penetration. Periodic activation before the needle is advanced through the corridor may reduce some patient discomfort. Exceptions occur, such as in the frontalis or orbicularis oculi where the muscle is a relatively thin sheet and insertion oblique to the skin surface are necessary.
In addition to a corridor directly perpendicular to the long axis of the muscle fibers, one can usually sample two additional corridors approximately 45 degrees to each side of the first corridor going away from the long axis of the muscle fibers. The corridor and angles are estimated from the surface of the muscle, not the skin. The electrode should not be re-directed in a corridor parallel to the long axis of the muscle fibers or placed in a new insertion site more proximal or distal to the original insertion site as this almost assures the likelihood of sampling the same motor units. Additional insertion sites should be made lateral or medial to the original site, far enough away so that the laterally angled corridors do not overlap. The exception to this is in the paraspinal musculature which is discussed in a separate section.
Even with perpendicular insertion, the appearance of the MUAP may change considerably as the needle electrode "views" it from different positions within the motor unit territory (Stalberg, 1991; Barkhaus, 2005). In figure 1, either electrode A or B may record the same MUAP(s) as the recording tip advances through the motor unit’s territory, yet these may look "different" at different positions within the motor unit territory, so long as the recording tip is within it. Assuming no change in the level of activation, the examiner may therefore have the false impression of sampling a greater number of motor units than are actually being seen, analogous to an artist painting the same subject but from different perspectives or other variables such as lighting. For example, the fin de siècle French impressionist Claude Monet painted a series of canvases depicting the same haystack at different times of the day, i.e., variation in lighting. While this same haystack in some paintings appears fairly similar, in other canvases it appears quite different. If in doubt as to whether a different motor unit is present, the needle electrode should be sufficiently advanced so as to pass through its territory.
This risk is further reduced by using additional corridors going medial or lateral as shown in figure 2, however, the only certain way is to exceed the boundaries of the motor unit territory. How large is a motor unit territory? This is variable depending on the muscle's size, i.e. smaller distal muscles versus larger proximal muscles. In the biceps brachii it is estimated to be 5-10 mm in diameter. If additional insertion sites are needed within the same muscle, it is recommended to make these far enough from the original site such that the corridors extending medial/lateral do not overlap.
In addition to a corridor directly perpendicular to the long axis of the muscle fibers, one can usually sample two additional corridors approximately 45 degrees to each side of the first corridor going away from the long axis of the muscle fibers. The corridor and angles are estimated from the surface of the muscle, not the skin. The electrode should not be re-directed in a corridor parallel to the long axis of the muscle fibers or placed in a new insertion site more proximal or distal to the original insertion site as this almost assures the likelihood of sampling the same motor units. Additional insertion sites should be made lateral or medial to the original site, far enough away so that the laterally angled corridors do not overlap. The exception to this is in the paraspinal musculature which is discussed in a separate section.
Even with perpendicular insertion, the appearance of the MUAP may change considerably as the needle electrode "views" it from different positions within the motor unit territory (Stalberg, 1991; Barkhaus, 2005). In figure 1, either electrode A or B may record the same MUAP(s) as the recording tip advances through the motor unit’s territory, yet these may look "different" at different positions within the motor unit territory, so long as the recording tip is within it. Assuming no change in the level of activation, the examiner may therefore have the false impression of sampling a greater number of motor units than are actually being seen, analogous to an artist painting the same subject but from different perspectives or other variables such as lighting. For example, the fin de siècle French impressionist Claude Monet painted a series of canvases depicting the same haystack at different times of the day, i.e., variation in lighting. While this same haystack in some paintings appears fairly similar, in other canvases it appears quite different. If in doubt as to whether a different motor unit is present, the needle electrode should be sufficiently advanced so as to pass through its territory.
This risk is further reduced by using additional corridors going medial or lateral as shown in figure 2, however, the only certain way is to exceed the boundaries of the motor unit territory. How large is a motor unit territory? This is variable depending on the muscle's size, i.e. smaller distal muscles versus larger proximal muscles. In the biceps brachii it is estimated to be 5-10 mm in diameter. If additional insertion sites are needed within the same muscle, it is recommended to make these far enough from the original site such that the corridors extending medial/lateral do not overlap.
Although usually unnecessary in the examination of some of the larger or superficial muscles, smaller or deeper muscles should be identified and isolated using activation to confirm localization prior to, and immediately after, needle electrode insertion. The insertion sites given for specific muscles in this program are recommended based on anatomy and experience. This assumes absence of other mitigating factors such as superficial veins, scars, vascular anastamoses for dialysis, superficial infections, etc.
If a muscle is palpated and activated, yet the needle electrode records no activity, re-positioning is necessary before checking insertion and spontaneous activity (see Activation above). In many muscles, initial confirmation of localization by way of brief activation is helpful. The muscle is then easily "deactivated" to permit assessment of insertion and spontaneous activity.
After examining insertion and spontaneous activity but before activating the muscle to assess the MUAPs, it is helpful to withdraw the needle electrode to a point just subcutaneous to the insertion point. The patient should then activate the muscle to a mild or moderate degree and the needle progressively re-inserted through the same corridor(s). Activation can be modulated so that the MUAPs can be optimally visualized on the display screen.
If a muscle is palpated and activated, yet the needle electrode records no activity, re-positioning is necessary before checking insertion and spontaneous activity (see Activation above). In many muscles, initial confirmation of localization by way of brief activation is helpful. The muscle is then easily "deactivated" to permit assessment of insertion and spontaneous activity.
After examining insertion and spontaneous activity but before activating the muscle to assess the MUAPs, it is helpful to withdraw the needle electrode to a point just subcutaneous to the insertion point. The patient should then activate the muscle to a mild or moderate degree and the needle progressively re-inserted through the same corridor(s). Activation can be modulated so that the MUAPs can be optimally visualized on the display screen.
If minor bleeding should occur after withdrawal of the needle electrode, compression with a gauze is quickly applied. Sometimes minor bleeding is delayed a few moments. It is an important point of examiner etiquette, skill, and regard for the patient that bleeding be minimized and promptly controlled. After each site is completed, the examiner may wish to routinely apply compression for a few moments. This need not slow the examination: while the examiner is applying gentle compression with one hand, the next site can be studied or prepared for study. Although needle electrodes are typically quite sharp, they quickly dull with repeated insertions, particularly when the tip is pushed too hard against bone (remember that the periosteum has pain afferents!).
Principles of intramuscular localization
Examiners may vary in where they position the needle electrode within the same muscle. This can have important effects on the recorded myogenic signal. The normal MUAP varies in shape depending on where along the longitudinal axis of the muscle fibers it is recorded, as well as where it might be within the motor unit territory itself (Stalberg et al, 1986; Nandedkar et al, 1988; Barkhaus and Nandedkar, 2005). By recognizing some of these variations of the muscle fiber’s long axis, it is possible to infer one's relative position within the muscle with respect to endplate region, tendon, etc. Such knowledge allows the electrodiagnostic consultant to optimally position the needle electrode and to be wary of potential misinterpretation of otherwise normal MUAPs.Endplate Region Motor Unit Action Potentials
This is a biphasic potential with an initial negative or upgoing phase. They may still be recorded when close to the endplate region but in the absence of endplate noise. Although acceptable for analysis, these MUAPs may be slightly shorter in duration than triphasic MUAPs recorded more distal to the endplate zone. Therefore these do not necessarily indicate a myopathic process. If the needle electrode is too close to, or within the endplate zone, endplate noise is usually seen and the patient typically indicates the site as discretely uncomfortable.Cannula Motor Unit Action Potentials
These are only recorded with concentric needle electrodes as monopolar electrodes have no cannula. These appear as large positive-going waves with occasional negative-going spikes within the main positive wave (figure 3). The negative spikes do not protrude above the baseline. These MUAP recordings derive from the cannula of the concentric electrode, hence their name. Cannula potentials are recorded when the recording tip of the electrode is outside of the motor unit territory or in smaller muscles, outside of the actual muscle. They may be large in amplitude and sound enlarged or "neurogenic" on the audio monitor. Ignore them. One might say that with cannula MUAPs, the examiner is "missing the point" (i.e., the sharp, negative upgoing component!). Reposition the electrode or activate the muscle so as to record muscle MUAPs generated from muscle fibers near the electrode's recording surface.Tendon Motor Unit Action Potentials
This waveform derives from the MUAP recorded near the tendon (figure 4). Occasionally a small "Gydikov potential" (Stalberg et al, 1986) may be seen, particularly with monopolar electrodes. This MUAP appears unremarkable in the initial positive-going phase and the negative-going main spike is of normal amplitude and duration. The positive-going after-wave is deep with a slow return to the baseline. This MUAP has been referred to as a "pelican potential" (Barkhaus and Nandedkar, 1997) because the profile of the positive after-wave resembles a pelican's beak in profile. The large "pouch" portion is close to the main spike of the MUAP, followed by the tapered point where it gradually returns to the baseline.These are more likely to be seen in smaller muscles and the multifidus where the electrode may more likely be in the tendinous region of relatively shorter muscle fibers. Duration measurement where the after-wave returns to baseline should be adjusted to where the sharp negative-going part of the slope tapers to a slower rate of return. These should not be construed as representing long duration "neurogenic" potentials. The tip-off is that the main spike is relatively narrow (or "normal") in duration relative to the longer duration positive after-wave. Genuinely enlarged MUAPs in reinnervation almost always have proportionate increases in the main spike (unless highly complex) and positive after-wave.
Variation in MUAPs between muscles
One of the major additions to this presentation is inclusion of sample MUAPs from almost every muscle discussed. We are unaware of any other similar resource. These samples are short segments at relatively low activation. Notice how different muscles have different "personae" (e.g., contrast the shape and size of a facial muscle or the external anal sphincter with that of the extensor indicis or biceps brachii).Distal limb muscles tend to have higher amplitudes and shorter durations compared to more proximal limb muscle. In the latter, MUAP amplitudes are typically lower and the durations longer. The differences in durations likely relate to the longer length of the muscle fibers in the proximal muscles. Note also in the various MUAP segments how complexity is more prominent in facial muscles and how they activate relatively rapidly for level of effort.
Activation is rarely perfect and needle electrode positioning and repositioning is often needed as shown in the segments. Repositioning of the needle electrode is easily noted by rapid change in the shape of the MUAPs. See if you can identify cannula MUAPs in the segments- they are there and we hope they become obvious to you with practice. Since we are not trigger-delaying the potentials, it is difficult to determine how many of the MUAPs may be superimpositions (Barkhaus & Nandedkar, 2005). We have tried to show these segments at as low activation as possible to avoid this effect.
Sampling size of MUAPs in muscle
The motor unit forms the basic functional unit of a muscle. In turn, a muscle may be considered a group of motor units. The estimated number of motor units in each muscle varies depending on its size and function. The motor unit generates the MUAPs that are recorded by the needle electrode. In clinical EMG, the examiner is typically recording from a relatively small portion of a muscle's total number of motor units. This depends on the amount of muscle actually explored by the recording electrode. The examiner must also appreciate that the MUAPs sampled represent the smaller, lower threshold motor units recruited at low levels of voluntary activation.
In quantitative analysis, a sample size of 20 MUAPs representing 20 different motor units is considered standard. Sample sizes of fewer MUAPs may distort the assessment of a muscle when mean values are used. The concept of "outliers" has been utilized in quantitative analysis (Stalberg, 1994). In this approach, identifying a minimum few abnormal MUAPs (> 20% abnormal, or three out of a sample of 20) may be sufficient to establish abnormality in lieu of the traditional mean value. A similar approach can be applied to routine subjective EMG. As a muscle is examined, the identification of 3-5 unequivocally abnormal MUAPs should be sufficient to determine the presence of pathology, allowing the examiner to move on to the next muscle. The examiner should ensure, however, that the abnormal MUAPs recorded represent different motor units. If the needle electrode has fully explored the corridors as described above, there should be minimal overlap between motor unit territories. If no abnormality is encountered after beginning study on a muscle using routine subjective EMG, then ongoing sampling and measuring about 20 different MUAPs should suffice to consider the muscle as being within normal limits. If abnormality is found, the MUAPs should be similar in their pattern of abnormality (e.g., durations of simple MUAPs should be similarly increased or decreased, not both).
Segmental innervation of muscle
The motor unit is ultimately represented in its respective spinal segment by a single anterior horn cell in the anterior gray matter of the spinal cord. The relative representation of a myotome or root in each muscle is critical to deducing segmental localization. Two to four nerve roots in various proportions may innervate a muscle’s total number of motor units. For example, most authors consider the biceps brachii to be predominantly supplied by the fifth and sixth cervical roots, with neither root being consistently predominant.
Schliak has advocated the concept of segmental pointer muscles of the limbs. Specific muscles are considered to precisely localize focal lesions of the spinal roots by virtue of their spinal segment or myotome. Unfortunately this concept has limited practical use as discussed above. Limb muscle is typically supplied by more than one myotome or spinal segment, one of which may be predominant. Use of comparative charts to determine segmental innervation (see Kendall et al, 2005) is helpful, but should be viewed with care as these innervations have been derived by different methods.
Principles and strategies in using the needle electrode to localize pathology
Pathologic processes should not be presumed to affect each motor unit in a homogeneous manner. The detection of pathology in a given muscle will depend on adequate sampling of MUAPs within the affected muscle, the severity of the pathologic process, and the time in the evolution of the pathologic process that the muscle is studied. Each of these factors may have a significant influence in how the assessment of a muscle may contribute to the localization of a pathologic process (see table).In radiculopathy, this has important implications. For example, assume a muscle is supplied 80% by L5 and only 20% by S1. In a S1 radiculopathy of mild to moderate severity, this muscle will likely show minimal abnormality on needle electrode examination. A common example is the frequent finding of mild reinnervation changes in the gluteus medius in a S1 radiculopathy.
This emphasizes the need for examining at least three muscles having common innervation by the same spinal segment under investigation. These muscles should have different peripheral nerve supply and be spread from a distal to proximal distribution. Hence a lumbosacral radiculopathy should not be diagnosed by examination of muscles distal to the knee which could also be explained on the basis of a polyneuropathy.
Multiple root lesions occur. There should be, however, caution in over-interpreting pathology in more roots or myotomes than may be actually involved in a single level radiculopathy. In the case of a L5 radiculopathy, the finding of mild reinnervation changes in the gluteus maximus as described in the above example does not necessarily mean that the S1 root is involved. It is explained more likely by the manifestation of the relatively minor contribution from L5 to the gluteus maximus, in addition to the severity of the lesion. This is an example of the limitations in the interpretation of the needle electrode examination. Further correlation and confirmation may be required from other investigations, such as imaging studies.
Multiple root lesions occur. There should be, however, caution in over-interpreting pathology in more roots or myotomes than may be actually involved in a single level radiculopathy. In the case of a L5 radiculopathy, the finding of mild reinnervation changes in the gluteus maximus as described in the above example does not necessarily mean that the S1 root is involved. It is explained more likely by the manifestation of the relatively minor contribution from L5 to the gluteus maximus, in addition to the severity of the lesion. This is an example of the limitations in the interpretation of the needle electrode examination. Further correlation and confirmation may be required from other investigations, such as imaging studies.
If an examination for radiculopathy is "positive" for pathological changes, additional ipsilateral limb muscles should be studied. This includes muscles having predominant innervation one segment caudal to, and one segment rostral to, the root level under investigation. Possible multiple root involvement should be excluded as opposed to manifestation of a severe single level radiculopathy, recalling that a single root may have variable amounts of innervation to multiple muscles (see above). If bilateral involvement must be excluded, it is reasonable to examine at least one contralateral muscle. Optimally this would be the counterpart of the most severely affected ipsilateral muscle. Other contralateral muscles should be examined as indicated.
Radiculopathy remains one of the most common indications for performing an EMG. In this presentation, the myotome or root segment supply of each muscle is given with the predominant segment in boldface (personal observations; see also Agur et al, Campbell, Dumitru et al, Kendall et al, Standring). The table lists frequently studied muscles in common radiculopathies. These are also shown in schematic presentation in a separate links throughout this presentation. Examination of the paraspinal muscle at the corresponding segment may also be used to validate a radiculopathy. There may be, however, a variable spatial divergence between the actual vertebral level and the segmental supply to the paraspinal musculature at a specific vertebral level. For example, the level of the multifidus supplied by the C7 root is often well caudal to the C7 vertebral body (personal observation, see also Standring). It is therefore easy to "miss" the affected paraspinal segment in a mild monoradiculopathy if insufficient sites are studied.
| Table. Limb muscles frequently studied in common radiculopathies Spinal Segment | |
| C5 | Deltoid, Rhomboid, Biceps Brachii, Supraspinatus, Infraspinatus |
| C6 | Biceps Brachii, Brachioradialis, Deltoid, Supinator, Infraspinatus Pectoralis (ClavicularPortion) |
| C7 | Triceps Brachii, Flexor Carpi Radialis, Extensor Digitorum Communis, Pectoralis (Sternocostal Portion) |
| C8 | First Dorsal Interosseous, Abductor Pollicis Brevis, Extensor Indicis, Flexor Pollicis Longus, Flexor Carpi Ulnaris |
| L2,3 | Iliacus, Adductor Longus |
| L4 | Vastus Lateralis or Medialis, Anterior Tibial |
| L5 | Gluteus Medius, Anterior Tibial, Peroneus Longus, Extensor Hallucis Longis, Extensor Digitorum Longus |
| S1 | Gluteus Maximus, Semimembranosus, Medial Gastrocnemius, Soleus |
| S2 | Abductor Hallucis, Abductor Digiti Quinti Minimi, First Dorsal Interosseous (Foot) |
| S3,S4 | Pelvic Floor and Related Muscles |
Pelvic Floor and Related Muscles
Introduction
Introduction: The clinical neurophysiologic study of the pelvic floor and related muscles (PFARM) of the body has been gaining popularity, commensurate with growing interest in the etiologies of various problems such as urinary and fecal incontinence, complications of vaginal delivery, etc. Clinical neurophysiologists working in this area often are working in collaboration with colleagues in gastrointestinal disorders, urology, gynecology, and urogynecology. Performance of these studies typically requires specialized training. Study of PFARM is essentially a separate procedure from what is thought of as routine EMG. In some institutions, the patient may be consented before undergoing this procedure. If the patient requires a more routine electrophysiologic study of the lower limb, etc., then this should be done prior to this study. It may also be helpful to have the patient advised to have some amount of bowel preparation when they are scheduled. This discussion does not include other procedures such as conductions or reflex studies. Detailed reviews of these procedures have provided by others (Roberts and Park; Fowler).
A few additional comments may be helpful. In routine EMG studies, the patient is vulnerable enough submitting themselves for needle electrode insertions in their trunk and extremities. In studying these muscles, the patient is submitting themselves to ultimate vulnerability- and exposure. They are also very likely quite anxious: needles and genitals! Thus PFARM examination should be treated with utmost consideration of the patient. In many ways, including optimal positioning, it might be thought of as analogous to the pelvic examination of the female patient. Before starting to prepare the patient, the patient should be completely informed of the procedure including purpose, risks, and benefits. As some needle insertions are deep, caution should be exercised if bleeding problems may be present or if the patient is on anticoagulants or antiplatelet agents.
During the procedure, there should be no reference to sexual function unless initiated by the patient. Such discussion should have been covered earlier when the history was obtained. Activation and/or relaxation should be requested in the context of bowel or bladder function (see muscle descriptions below). Throughout the description of the specific muscles, reference is made to having the patient activate or relax these muscles as in trying to hold back or have a bowel movement or urinate. Thus before starting the study, rehearse the patient such that they are selectively activating only these muscles, and not others such as the glutei. Activating the latter will result in unnecessary and excessive movement of the patient, thereby making it difficult to maintain position and make recordings. Use of audio feedback during these maneuvers may be useful in educating the patient.
As in routine EMG, the patient should be asked to make a final effort to void. The patient should change out of their clothing, including briefs or under shorts, and gown themselves, unless assistance is needed. For female patients, this should be provided by a female chaperone. Once the patient is gowned and positioned on the table, they should not be left unattended. The examination room should be appropriately heated and free of unnecessary sound and electrical noise.
As in routine EMG, the patient should be asked to make a final effort to void. The patient should change out of their clothing, including briefs or under shorts, and gown themselves, unless assistance is needed. For female patients, this should be provided by a female chaperone. Once the patient is gowned and positioned on the table, they should not be left unattended. The examination room should be appropriately heated and free of unnecessary sound and electrical noise.
As in routine EMG, the patient should be asked to make a final effort to void. The patient should change out of their clothing, including briefs or under shorts, and gown themselves, unless assistance is needed. For female patients, this should be provided by a female chaperone. Once the patient is gowned and positioned on the table, they should not be left unattended. The examination room should be appropriately heated and free of unnecessary sound and electrical noise.
Either the assistant or the clinical neurophysiologist may clean the area before starting, then cover the perineal area with an additional towel or sheet. This region should only be exposed when necessary, and only the portion under study should be exposed. Topical anesthetic cream (e.g., EMLA cream) may be used, but has been found to be primarily useful in studying the sphincter muscles. Injected anesthetic was not found to be of additional benefit in studying the urethral sphincter in the female
(Olsen et al, 1998). Before needle insertion, an alcohol wipe may be used on intact skin, but povidone-iodine is preferred on mucous membranes to avoid irritation and burning.
For female patients, a female chaperone/assistant is highly recommended, particularly if the examiner is a male. In this procedure, both the clinical neurophysiologist’s hands need to be gloved. Therefore an assistant is essential to help monitor the patient, as well as assist in manipulating the electromyograph during signal acquisition.
There should be an economy of insertions and electrodes. It is important to preplan the study as much as possible considering relative potential for contamination at insertion sites, as well as electrode type and length. From this perspective, we recommend going from female periurethral/vaginal insertions, to perineal insertions, to perianal insertions.
Choosing appropriate electrodes: while a shorter 25 mm electrode can be used to study the superficial muscles (female external urethral sphincter, external anal sphincter, and bulbocavernosus) a longer 50 or 75 mm electrode is necessary to study the deeper muscles (levator ani and male external urethral sphincter). Choice of electrodes can be either concentric or monopolar. Some prefer the concentric because it eliminates the need for a second reference electrode, particularly given the limited space in which to work. When a longer electrode is needed (particularly 75 mm), a monopolar electrode is thinner than its concentric counterpart and may be better tolerated. The examiner should be familiar with the differences between these electrodes as well as the 25 mm concentric "facial" electrode, which has a smaller recording surface than a standard concentric electrode. Computer assisted analysis has greatly expanded the ability to analyze the signals (Podnar et al, 1999; Podnar et al, 2000; Weidner et al, 2000).
The ground electrode may be placed anywhere in the region but should not interfere with the needle electrode. It may be useful to run a check of the recording quality after the initial insertion with the needle electrode just subcutaneous, and then again once muscle is entered. Once recording and activation maneuvers are started, the study should flow smoothly without interference.
Once the PFARM study is completed, the clinical neurophysiologist should ascertain the condition of the patient, thereafter allowing the chaperone/assistant to get the patient off the table, to the changing area. The clinical neurophysiologist should spend a few moments discussing at least preliminary findings as appropriate with the patient and responding to any questions or concerns. Patients may have some bleeding and require pressure after electrode withdrawal. Women should be cautioned that they may have "spotting" after the procedure but this is due to pooling of blood and is not vaginal or uterine in origin.
Despite busy schedules, the proper detailed documentation of the procedure and spending a little extra time with the patient may avoid intra- and post-procedure problems that may be more costly than a few minutes of the examiner’s time!
Myogenic Signal
The indications for PFARM studies (see below) essentially address disorders of motor overactivity or motor axonal loss resulting in denervation and reinnervation changes. The MUAPs in the sphincter muscles are typically smaller in amplitude, shorter in duration, and have more complexity than limb muscles. These features are probably due to the short lengths of the muscle fibers in these motor units as well as having a smaller innervation ratio than many of the limb muscles.
If one were to encounter such MUAPs in the biceps brachii, they might be considered suggestive of myopathic remodeling of the motor unit (see Barkhaus and Nandedkar, 2005). Although myopathic
disorders could affect PFARM musculature, it would be difficult to ascertain such changes in individual MUAPs because of the tendency for these to be tonically active. A fibrillation potential representing a single muscle fiber may be 3-5 msec in duration. Therefore making a case for myopathy based on short MUAP duration in these units would be difficult if not impossible. MUAP amplitude is highly dependent on needle electrode position (Nandedkar et al, 1988, Barkhaus & Nandedkar, 1996). The examiner must constantly make certain that their electrode position is optimally placed for recording maximal amplitude signal.
Recording MUAPs at low levels of activation will be the most helpful in looking for MUAP changes suggestive of pathology, particularly changes that imply reinnervation (increased amplitude/area and duration). Many newer electromyographs allow recording of the signal which can be saved and analyzed later offline. If the signal is stable and individual MUAPs easily identified, then one may also trigger and delay these MUAPs later offline to better quantitate them and assess stability. The latter may be helpful in assessing neurogenic conditions (Barkhaus and Nandedkar, 2005).
disorders could affect PFARM musculature, it would be difficult to ascertain such changes in individual MUAPs because of the tendency for these to be tonically active. A fibrillation potential representing a single muscle fiber may be 3-5 msec in duration. Therefore making a case for myopathy based on short MUAP duration in these units would be difficult if not impossible. MUAP amplitude is highly dependent on needle electrode position (Nandedkar et al, 1988, Barkhaus & Nandedkar, 1996). The examiner must constantly make certain that their electrode position is optimally placed for recording maximal amplitude signal.
Recording MUAPs at low levels of activation will be the most helpful in looking for MUAP changes suggestive of pathology, particularly changes that imply reinnervation (increased amplitude/area and duration). Many newer electromyographs allow recording of the signal which can be saved and analyzed later offline. If the signal is stable and individual MUAPs easily identified, then one may also trigger and delay these MUAPs later offline to better quantitate them and assess stability. The latter may be helpful in assessing neurogenic conditions (Barkhaus and Nandedkar, 2005).
Clinical Applications
PFARM studies should be thought of in the broader context of their use in the overall evaluation of a patient. Thus, the initial evaluation begins with a history and physical, which are the least invasive portions of the evaluation. As appropriate, the patient would then typically have other studies such as imaging, urodynamic testing, anal manometry, routine electrophysiologic testing, or autonomic testing performed. In performing initial routine electrophysiologic testing, (i.e. lower extremity), intrinsic foot muscles may be helpful in that they have significant S1-S3 innervation, particularly S1-S2 (Agur; Standring). For some clinical problems, assessment of these muscles may adequately complete the patient’s evaluation (see table).In evaluating for disorders of increased motor activity (e.g., vaginismus, constipation, pelvic pain, etc.) PFARM studies are helpful, not only for diagnosis, but also for localization in therapeutic chemodenervation.
| Table: Some common indications for PFARM study with commentary. Indication | Comment |
| Cauda Equina | PFARM study may be indicated as adjunct to routine study to ascertain lower sacral root involvement. |
| Coccydynia (tension myalgia form) | While generally diagnosed on history and physical examination, PFARM study may be helpful in identifying muscle overactivity. |
| Constipation | Overactivity of the puborectalis is a common cause of outlet obstruction and constipation. |
| Diabetic Neuropathy | Generally not indicated. |
| Fecal Incontinence | Superficial and deep external anal sphincter for incontinence of flatus and seepage. Include puborectalis for incontinence of solid stool. |
| Pelvic Pain | PFARM study may be helpful in identifying muscle overactivity as a component of a patient’s pain. Other indications evaluated as appropriate. |
| Proctalgia fugax | PFARM study may be helpful to exclude tension myalgia but should be normal in proctalgia fugax. |
| Pudendal Neuralgia | PFARM may be normal if no axonal injury. Include assessment of both motor branches of pudendal nerve. Consider sacral reflex testing. |
| Radiculopathy (S2,3,4) | Subcutaneous portion adequate for anal sphincter, include levator for direct sacral roots and intrinsic foot for non-pelvic muscle with S1-S2 innervation. |
| Sacral Plexopathy | Comprehensive PFARM study and also consider sacral reflex testing. |
| Urinary Retention | Consider evaluation of external urethral sphincter for dyssynergia during voiding (ideally during urodynamic testing which will also evaluate detrusor contractility). |
| Urinary Stress Incontinence (Female) | Generally not indicated for isolated urinary stress incontinence. |
| Urinary Stress Incontinence (Male) | Include external urethral sphincter. |
| Vaginismus | Pubovaginalis portion of pubococcygeus. |
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