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 positions. 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 msec/div; and gain setting 50–500 μV/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. Typically, the subcutaneous and/or deep portion of the EAS are sampled in at least four quadrants, roughly divided into the upper and lower, left and right portions of the sphincter. Ideally, 20 or more MUAPs should be sampled during the evaluation (Fig. 11-1).

Figure 11-1 Concentric needle EMG 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. Endo-anal 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 and pubococcygeus portions of the levator ani muscle complex are most easily 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 in at least two sites on the muscle. This is then repeated on the opposite side. Although there is no standardized location for needle insertion into the levator ani muscles, one technique that has been proposed uses the ischial spine as a fixed reference point and samples two sites relative to this easily identifiable landmark (Weidner et al., 2000). The first site is located 2 cm caudal and medial from the ischial spine. The second site is 2 cm farther medial. The puborectalis muscle can be accessed from a perineal approach. A 75-mm electrode is required 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. Insertional activity 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 seconds 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. 11-2). Fibrillation potentials are biphasic muscle fiber potentials that occur spontaneously and typically have an amplitude of between 20 to 300 μV and a duration of less than 5 msec. 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 μV and a 1- to 5-msec 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.

Figure 11-2 Examples of abnormal spontaneous activity seen during concentric needle EMG.

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, 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. 11-3). 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.

Figure 11-3 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.

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