Electromyography

The term electromyography is a general term that refers to methods of studying electrical activity of muscle. Various EMG techniques exist, each with its own indications, advantages, and limitations. Of the many EMG techniques, only a few have use for the study of pelvic floor disorders. EMG techniques study neuromuscular activity of striated muscles, using a recording electrode that is inserted into or placed on the surface of a muscle. Bioelectric potentials generated by the depolarization of the skeletal striated muscle are picked up by this electrode and then filtered and amplified. They are displayed on an oscilloscope for visual analysis and fed through a speaker system so they can be monitored acoustically. Modern computer-based equipment allows for conversion of the signal into digital data that can then be easily stored, processed, and analyzed.

In order to fully understand the use of EMG, it is important to understand the structure and function of the so-called “motor unit.” Lower motor neuron cell bodies reside in the anterior horn of the spinal cord. The axon from this neuron will branch and innervate several muscle fibers scattered in a mosaic pattern throughout the muscle. All of the muscle fibers from this neuron will be activated together and are considered the motor unit.

Electromyographic studies used to evaluate pelvic floor disorders can be separated broadly into two categories: kinesiologic EMG (kEMG) and motor-unit EMG . Kinesiologic EMG is used to simply assess the presence and timing of activity or inactivity of a muscle, usually the urethral or anal sphincter. It is used in conjunction with physiologic tests, such as urodynamics and anal manometry, to assess sphincter relaxation during voiding or defecation. Kinesiologic EMG is also used for biofeedback during pelvic muscle rehabilitation for treatment of urinary or fecal incontinence. In contrast, motor-unit EMG is a diagnostic test used to assess the neuromuscular function of a muscle. It can differentiate normal muscle from denervated/reinnervated or myopathic muscle. Common techniques used for motor-unit EMG are concentric needle EMG (CnEMG) and single-fiber EMG (SfEMG). Motor-unit EMG that uses computer-assisted digital analysis to obtain a faster, more standardized, and detailed assessment of neuromuscular function than traditional needle EMG is known as quantitative EMG (qEMG). The general principles, techniques, interpretation, and limitations of each EMG technique as it applies to the investigation of pelvic floor disorders and sacral neurologic disease are outlined in the next section.

Kinesiologic EMG

Kinesiologic EMG is used to assess the activity or inactivity of a muscle during a defined activity. The technique provides information on the timing of a muscle contraction by recording the increase in electrical activity that occurs during a contraction through surface or wire/needle electrodes placed on or within the specific muscle(s) being investigated. This increase in electrical activity correlates moderately with muscle strength and has been used by some as a surrogate for measuring this parameter. However, kEMG cannot be used to detect neuropathic or myopathic changes within a muscle.

In the investigation of pelvic floor disorders, kEMG is used most frequently during urodynamic evaluations. Specifically, it is used to assess urethral sphincter and/or pelvic floor muscle activity during voiding. By simultaneously recording pelvic floor muscle activity using kEMG, and detrusor pressure using the urodynamic pressure catheters, detrusor/sphincter coordination can be assessed. The primary role of kEMG in this context is to detect the condition of detrusor sphincter dyssynergia in which a detrusor contraction is coupled with a simultaneous urethral sphincter contraction, resulting in abnormal voiding and urinary retention. Similarly, kEMG can also be used to investigate coordinated relaxation of the levator ani muscles during defecation. This EMG technique has been used by some to diagnose anismus, a condition characterized by an inappropriate contraction of the levator ani complex, particularly the puborectalis muscle, during defecation resulting in obstructed evacuation. Another common use for kEMG is to provide visual and/or audio biofeedback of pelvic floor muscle activity during a pelvic muscle exercise program for treatment of urinary or fecal incontinence. Pelvic floor rehabilitation using biofeedback has long been advocated as more effective than teaching pelvic muscle exercises with verbal instructions alone, although other studies have called this into question.

Kinesiologic EMG can be performed with various types of surface or intramuscular (needle or wire) electrodes. Surface electrodes measure the net electrical activity within a muscle. They have the advantage of being simple and noninvasive; however, they are prone to artifact and contamination from signals from other muscles. Because they measure net electrical activity of a muscle and are not capable of measuring individual motor unit potentials, surface electrodes cannot be used to diagnostically evaluate for denervation/reinnervation injury or myopathic diseases. Surface electrodes are used commonly for biofeedback for pelvic floor strengthening in the treatment of urinary or fecal incontinence. For measuring levator ani activity, two small circular skin electrodes are commonly applied on each side of the perineum at 1 cm anterior to the anus. Anal plugs with concentric ring electrodes are commonly used to assess external anal sphincter activity for biofeedback-assisted sphincter strengthening for the treatment of fecal incontinence. Similarly, ring electrodes placed on a Foley catheter at 1 cm distal from the balloon have been used to measure urethral sphincter activity. Electrical activity measured from surface or ring electrodes are amplified and visually displayed on a computer screen and/or audibly projected through a speaker system to provide the patient with immediate feedback during a contraction. Often surface electrodes are also placed on the gluteus or abdominal muscles so that patients can learn to appropriately contract their pelvic floor/sphincter muscles while avoiding simultaneous contraction from other surrounding muscle groups.

To evaluate detrusor-sphincter coordination in a patient with voiding dysfunction, kEMG is performed in conjunction with a urodynamic evaluation. This test is poorly standardized, and either surface electrodes are applied to the perineum as described earlier or a wire electrode is placed periurethrally into the urethral sphincter. The electrical activity of the pelvic floor/urethral sphincter is then recorded during the voiding phase of the study.

In a neurologically intact subject, voiding is characterized by cessation of all sphincter EMG activity just before contraction of the detrusor muscle. In patients who have spinal cord lesions between the lower sacral segments and the upper pons, coordination between the detrusor muscle and the urethral sphincter can be lost. This detrusor sphincter dyssynergia is characterized by an increase in sphincter EMG activity during a detrusor contraction rather than a decrease, and these patients typically have severe voiding dysfunction and urinary retention.

As a diagnostic test, except for the evaluation of detrusor-sphincter coordination, the relevance of kEMG is unclear, and no standardized method for performing or interpreting kEMG has been accepted. In the artificial setting of the urodynamic laboratory, many neurologically intact women will contract their urethral sphincter during voiding because of embarrassment and/or the discomfort of the urethral catheters and electrodes. Therefore, it is important to ask the patient whether she is trying to stop voiding voluntarily so that detrusor sphincter dyssynergia is not diagnosed in error. In a study of 550 consecutive patients, found that dyssynergia was found only in patients with well-defined lesions of the suprasacral spinal cord. Therefore, the diagnosis of detrusor sphincter dyssynergia made in the absence of other neurologic deficits consistent with this level of injury should be suspect. Inappropriate urethral contraction during voiding can also be a learned behavior resulting in dysfunctional voiding.

As a method of biofeedback, kEMG is useful, but the limitations associated with surface electrodes still apply.

Concentric Needle EMG

CnEMG is a widely used diagnostic test for evaluating the neuromuscular integrity of skeletal muscles. It can be used to differentiate normal muscle from muscle that is neuropathic or myopathic. A concentric needle electrode consists of a stainless steel outer cannula within which runs a fine silver, steel, or platinum wire that is insulated except at its tip. The concentric needle is inserted into a muscle where the inner wire serves as a recording electrode, whereas the outer cannula serves as a reference electrode. Bioelectrical potentials are measured as voltage differences between the two electrodes that are then recorded, displayed, and analyzed. A CnEMG examination provides information on the insertional activity, spontaneous activity, motor-unit action potentials (MUAPs), and recruitment pattern (also known as interference pattern) of a muscle. Interpretation of each component provides the examiner with information about the presence and duration of neuropathic and/or myopathic injury to the examined muscle (see later).

CnEMG can be performed on an individual muscle, as on the anal sphincter when evaluating isolated fecal incontinence, or systematically performed on several muscles to determine the level of a neurologic injury. All muscles that are innervated by one spinal segment (level) are referred to as a myotome. Using CnEMG to systematically evaluate several muscles in different myotomes often permits localization of a lesion to the spinal root, nerve plexus, or an individual peripheral nerve. In a patient with bowel or bladder dysfunction in which a neurologic cause is suspected, a thorough evaluation often requires EMG of the lower extremity as well as pelvic floor musculature.

Technique

The clinical history and sacral neurologic examination should be reviewed carefully before beginning the EMG evaluation, because this will help determine which muscles should be examined. Ideally, the evaluation should be performed in a shielded room to prevent interference from other electronic devices or AC power cords. The patient is placed in a comfortable position that allows access to the pelvic floor musculature, usually the dorsal lithotomy or lateral decubitus position. A grounding surface electrode is applied. Most laboratories do not use local anesthetics in any form before insertion of the needle electrodes, although some authors advocate using a topical anesthetic, particularly for examination of the urethral or anal sphincter. After wiping the skin overlying the appropriate muscle with alcohol, the concentric needle electrode is inserted until the insertional activity of the muscle is noted, confirming that the electrode is within the muscle. The EMG investigation consists of analysis of electrical activity at rest (assessment of spontaneous activity), at slight voluntary contraction (MUAP analysis), and at strong contraction (interference pattern analysis). Generally, the most painful portion of the examination is the initial insertion through the skin, so several sites within the muscle should be sampled by moving the tip of the needle without removing it from the skin. Depending on the muscle, more than one skin penetration may be necessary. Both the visual output on the screen of the EMG machine and the audio output from the speaker are used to assess the quality of the recording and to identify electrophysiologic phenomena. Commonly used settings for CnEMG evaluation of the pelvic floor muscles are filter settings 5 Hz to 10 kHz; horizontal sweep speed 10 ms/div; and gain setting 50 to 500 mV/div.

Anal Sphincter

Both the subcutaneous and deep portions of the external anal sphincter (EAS) are accessible for CnEMG evaluation. To study the subcutaneous portion of the EAS, the needle electrode is inserted 1 cm outside the mucocutaneous junction of the anal orifice, to a depth of 3 to 6 mm beneath the skin. The deep portion of the EAS is assessed by inserting the needle at the mucocutaneous junction of the anus, at an angle of about 30 degrees to the anal canal axis, for a depth of 1 to 3 cm. The subcutaneous and/or deep portion of the EAS is sampled on both sides, although there is no discrete neuromuscular junction, so motor units from the left side may be sampled on the right side, and vice versa ( ). Ideally, 20 or more MUAPs should be sampled during the evaluation ( Fig. 14.1 ).

Concentric needle electromyography of the anal sphincter. The subcutaneous portion of the external anal sphincter (EAS) is examined by inserting the needle electrode (A) 1 cm outside the mucocutaneous junction of the anal orifice, to a depth of 3 to 6 mm beneath the skin. The deep portion of the EAS is assessed by inserting the needle (B) at the mucocutaneous junction of the anus, at an angle of about 30 degrees to the anal canal axis, for a depth of 1 to 3 cm. The subcutaneous and/or deep portion of the EAS are sampled in four quadrants, divided into the upper and lower, left and right portions of the sphincter (inset) .

(Reprinted with the permission of the Cleveland Clinic Foundation.)

“Anal mapping” has been used in the past to identify the location of a sphincter defect before an anal sphincteroplasty in the treatment of fecal incontinence. To identify the precise location of a sphincter defect, more needle insertions are typically required, usually at 9 o’clock, 10 o’clock, 12 o’clock, 2 o’clock, 3 o’clock, and 6 o’clock positions. Endoanal ultrasound has largely replaced anal mapping with EMG for the identification and localization of sphincter defects.

Urethral Sphincter

The external urethral sphincter can be studied by inserting the electrode into the sphincter muscle periurethrally or transvaginally. For the periurethral approach, the needle is inserted approximately 5 mm anterior to the external urethral meatus (12 o’clock) to a depth of about 1 to 2 cm. For the transvaginal approach, the posterior vaginal wall is retracted with a speculum, and the needle is inserted approximately 2 cm proximal to the external urethral meatus, off the midline, and directed laterally into the urethral sphincter. Although slightly more painful, the periurethral approach provides superior sampling of the urethral sphincter, obtaining as many as twice the number of MUAPs as the transvaginal approach. At least 10 MUAPs should be recorded to adequately evaluate the urethra.

Bulbocavernosus Muscle

The bulbocavernosus muscle is a readily accessible perineal muscle that is innervated by the perineal branch of the pudendal nerve. The bulbocavernosus muscle can be reached either transmucosally, with a needle insertion medial to the labia minora, or through the skin lateral to the labia majora.

Levator Ani

The iliococcygeus portion of the levator ani muscle complex is investigated using a transvaginal approach. The muscles are localized by first inserting two fingers into the vagina and asking the patient to contract. One side of the levator complex is isolated, and the electrode is inserted using the opposite hand. The ischial spine can be used as a fixed reference point and sites relative to this easily identifiable landmark can be chosen ( ). This is then repeated on the opposite side. The puborectalis muscle can be accessed from either a transvaginal or perineal approach. For the transvaginal approach, an index finger is placed just inside the vaginal introitus. The distal pubovisceralis muscles are palpated. Approximately 1.5 cm caudal to the insertion into the pubic bone, the needle electrode is inserted. It is important to discern which portion of the levator ani is being analyzed, because there are differences in the behavior and quantitative parameters at different sites ( ). For a perineal approach, a larger needle can be used and inserted approximately 1 cm posterior to the anus in the midline for a depth of approximately 3 cm.

Lower Extremity

Bladder and bowel dysfunction are not uncommon when there are lesions involving the lumbosacral nerve roots and/or lumbosacral plexus. These lesions typically also affect innervation of the lower extremity. Therefore, whenever a patient’s history and examination suggest lower extremity involvement as well as bowel and/or bladder dysfunction, the electrophysiologic evaluation should include the muscles of the lower extremity as well as the pelvic floor. A detailed description of the CnEMG evaluation of the lower extremity musculature is beyond the scope of this text. However, muscles that are often useful in such an evaluation include quadriceps (L3, L4; femoral nerve); adductor longus (L4; obturator nerve); tibialis anterior (L4, L5; deep peroneal nerve); gastrocnemius (S1, S2; tibial nerve); gluteus maximus (S1; superior gluteal nerve); gluteus medius (L5; inferior gluteal nerve); adductor hallucis brevis (S1, S2; tibial nerve); and the paraspinal muscles from L3 to S1.

Interpretation

CnEMG is a valuable tool for evaluating lower motor neuron disease and muscle disease. Lesions involving the upper motor neurons in isolation have a normal EMG evaluation. The initial assessment in any CnEMG examination is an evaluation for insertional activity , which is the electrical activity that occurs when the needle electrode is first introduced into a muscle or is moved, is the result of mechanical stimulation or injury of the muscle fibers, and usually stops within about 2 s of movement. The presence of insertional activity confirms that the electrode has been placed within the muscle. Absence of insertional activity with an appropriately placed needle electrode usually means a complete atrophy of the examined muscle. Once the needle has been properly placed, the patient is asked to completely relax the muscle being examined, and the presence or absence of spontaneous activity is assessed. Denervated muscle fibers may produce rhythmic spontaneous electric potentials, such as fibrillation waves or positive sharp waves. The presence of this spontaneous activity in a resting muscle is a sign of denervation. In the urethral sphincter, the anal sphincter, and the levator ani muscles, which all tonically contract even in the resting state, the only normal spontaneous activity is normal MUAPs. In limb muscles and the bulbocavernosus muscle, which do not tonically contract, there should be a complete absence of spontaneous activity at rest. The pelvic floor muscles achieve complete electrical silence during voiding and defecation. Spontaneous activity that is found in pathologic conditions includes fibrillation potentials, positive sharp waves, complex repetitive discharges, fasciculations, myotonia, myokymia, and neuromyotonia ( Fig. 14.2 ) . Fibrillation potentials are biphasic muscle fiber potentials that occur spontaneously and typically have an amplitude of between 20 mV and 300 mV and a duration of less than 5 ms. They usually fire rhythmically at a rate of 2 to 20 Hz. Fibrillation potentials develop 2 to 30 days after an injury and can occur with a motor nerve lesion as well as with primary muscle diseases, such as acute muscle injury, muscular dystrophies, and inflammatory myopathies. Positive sharp waves are biphasic waves with an amplitude of 20 to 500 mV and a 1- to 5-ms duration. They are more common than fibrillations and have a similar clinical significance. Complex repetitive discharges (CRDs) are spontaneous high-frequency discharges with a regular firing pattern beginning and ending abruptly that can be bizarre in appearance. They can be found after chronic partial denervation, muscular dystrophy, inflammatory myopathies, and in some metabolic disorders. The striated urethral sphincter appears to be particularly likely to develop CRDs, which have been found in association with urinary retention in young women (Fowler’s syndrome) and even in some neurologically normal women.

Examples of abnormal spontaneous activity seen during concentric needle electromyography.

As described previously, the motor unit is the smallest functional unit of the motor system and consists of a motor neuron, its axon, and all of the muscle fibers innervated by the axon. The motor unit in a normal human limb muscle consists of several dozen muscle fibers lying within an area of 5 to 10 mm diameter. The MUAP is a compound potential representing the sum of the individual action potentials generated in the few muscle fibers of the motor unit that are within the pickup range of the recording electrode. After a determination about spontaneous activity is made, the patient is asked to slightly contract the muscle being examined (if needed), and attention is turned to MUAP analysis . The shape, amplitude, duration, number of phases, and stability of each MUAP should be assessed. Most EMG units have a trigger-and-delay mechanism that allows MUAPs to be “frozen” for more careful analysis. Ideally, 20 or more MUAPs should be sampled for each muscle. This is often difficult for striated urethral sphincter, where analysis of 10 MUAPs is often all that can be achieved due to the small size of the muscle. Muscles that have been denervated and reinnervated, and muscles that are myopathic, have distinct MUAP characteristics from those of normal muscle ( Fig. 14.3 ). The morphology of a MUAP reflects, among other things, the number and local concentration of muscle fibers comprising a motor unit. After a neuronal injury, the muscle fibers innervated by that neuron begin to atrophy. Adjacent nerve fibers attempt to reinnervate these denervated muscle fibers, resulting in a neuron that now supplies a greater number of muscle fibers. This creates a MUAP with larger amplitude, longer duration, and a greater number of phases and turns. Because these changes depend upon reinnervation by an adjacent motor neuron, large complex polyphasic MUAPs are not typically seen until 3 to 6 months after a nerve injury. In the setting of an acute nerve injury, reinnervation has not had time to occur and MUAP morphology is normal. Myopathic injury results in a loss of muscle fibers and therefore a motor unit with fewer muscle fibers. This results in a MUAP with shorter duration and smaller amplitude than normal.

A normal motor unit (top left) consists of a motor neuron, its axon, and all of the muscle fibers that it innervates. A normal motor unit action potential (MUAP) (top) is characterized by its amplitude, duration, and number of phases (usually three to four). Denervation of a motor unit (middle) results in death of some motor fibers and reinnervation of others via collateral sprouting from an adjacent motor unit axon. This results in a more complex MUAP with greater amplitude, duration, and number of phases. Myopathy (bottom) results in death of muscle fibers without subsequent reinnervation, resulting in MUAPs with decreased amplitude.

After performing MUAP analysis, the patient is asked to increase her contraction effort, and motor unit recruitment is assessed. In normal muscle, as the force of voluntary contraction increases, motor units are recruited in a specific order that is determined by the thresholds inherent to the individual unit. Therefore, a sequential increase occurs in the firing rate of MUAPs as well as a sequential acquisition of new higher threshold MUAPs as the force of voluntary contraction increases. Analyzing this pattern of recruitment, often called an interference pattern , provides additional information about the health of a muscle and its innervation. An interference pattern is called “full” or “complete” when the number and firing rate of the individual MUAPs becomes so great that they can no longer be distinguished and the baseline is obscured ( Fig. 14.4 ). Recruitment is normal when a complete interference pattern occurs at maximal effort. In neuropathic disorders, a reduced number of motor units is present so a decreased recruitment and an incomplete interference pattern are present, although the remaining units fire rapidly. In myopathic disorders, muscle fibers are reduced and the number of motor units is normal. A complete interference pattern is seen, but it occurs at much less than maximal effort, and the amplitude to the MUAPs is reduced.

Normal interference pattern (top); neuropathic interference pattern characterized by increased amplitude, reduced number of motor unit action potentials (MUAPs), and a rapid rate (middle); myopathic interference pattern characterized by decreased amplitude and a normal number of MUAPs (bottom); myopathic interference patterns become “full” at lesser degrees of muscle contraction.

The chronicity of a nerve lesion can be determined by the CnEMG features present. In an acute nerve lesion, reinnervation has not occurred so the MUAPs have normal morphology. The number of motor units is reduced, however, so recruitment is decreased. Between 2 and 30 days after an injury, spontaneous activity, such as fibrillation potentials and positive sharp waves, become apparent and insertional activity becomes longer. Between 3 and 6 months after an injury, reinnervation has occurred so the MUAPs become larger and more complex. The location of a nerve lesion is determined by the pattern of muscles demonstrating denervation. For instance, neuropathic CnEMG findings in the external anal sphincter, but normal findings in the levator ani muscle and bulbocavernosus muscle, suggest an isolated lesion involving the inferior rectal branch of the pudendal nerve. In contrast, if neuropathic findings were found in the external anal sphincter, the right levator ani muscle, the right gluteus maximus and gluteus medius muscles—but not on the left side or in the L5, S1 paraspinal muscles—this would suggest a right-sided lumbosacral plexus lesion.

Traditionally, CnEMG has depended largely on an examiner’s auditory and visual impression for analysis. In an attempt to improve the accuracy, reliability, and speed with which CnEMG can be performed, quantitative techniques have been developed. Although the first form of qEMG was introduced in the 1950s, it did not become widespread until recently, when newer EMG machines became computerized. The technical aspects of performing a qEMG evaluation are identical to conventional CnEMG described earlier. The difference is in the analysis, where computerized digital analysis is performed on the collected EMG data, allowing a much more detailed evaluation of neuromuscular function in a shorter period. Two complementary forms of qEMG are (1) analysis of multiple motor unit action potentials (multi-MUAP analysis) and (2) interference pattern analysis. In several recent studies, qEMG has been applied to the investigation of the pelvic floor muscles.

In multi-MUAP analysis , the examiner identifies a period of crisp EMG activity during a slight-to-moderate muscle contraction. The computer uses a system of template matching to automatically extract, quantify, and sort any well-defined MUAPs according to their shape. This allows a rapid acquisition of data, such that the 20 to 30 MUAPs needed to adequately describe a muscle can be obtained in only a few minutes. Several MUAP parameters are then quantified, including amplitude, duration, area, number of phases, number of turns, rise time, and spike duration. Thickness (area/amplitude) and size index (2 × log [amplitude + area/amplitude]) are automatically calculated by the computer and assist in differentiating between normal and myopathic or neuropathic muscles. Normative data for the external anal sphincter, urethral sphincter, and levator ani muscles have been published and are available for comparison.

Interference pattern analysis is a broad term used to describe one of several automatic quantitative techniques for evaluating muscle recruitment. Computerized analysis of interference patterns is particularly useful because the EMG signal recorded at even a moderate contraction is too dense to be accurately assessed by visual inspection. The most popular technique for interference pattern analysis is turns/amplitudes cloud analysis ( Fig. 14.5 ). This method of analysis is even faster than multi-MUAP analysis (2–3 min) and does not require a standardized force of contraction, eliminating an important source of variability.

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Urodynamics: Indications, Techniques, Interpretation, and Clinical Utility
2. Urethral Injection of Bulking Agents for Intrinsic Sphincter Deficiency
3. The Effects of Gynecologic Cancer on Lower Urinary Tract Function
4. Lower Urinary Tract Fistulas

Stay updated, free articles. Join our Telegram channel

Join