Electrodiagnostic findings in myopathy may result from a number of factors, including abnormalities or changes in muscle fibers as well as the surrounding connective tissue. A wide range of findings is possible; therefore, the patient presenting with weakness and suspected myopathy can pose a diagnostic challenge to the electromyographer. If the abnormal pathology includes muscle degeneration and regeneration, one should anticipate abnormal membrane irritability on concentric or monopolar needle electromyography (EMG). Muscle fiber splitting, fiber atrophy with little or no degeneration, errors of muscle maturation syndromes, and impaired muscle membrane excitability may have needle EMG findings that one may not necessarily anticipate based on the underlying pathology. In this chapter a practical clinical approach to evaluating a person with weakness and question of myopathy, using standard electrodiagnostic techniques, is presented. A proper electrodiagnostic consultation includes a detailed history and physical examination with additional emphasis on the neuromuscular examination before beginning the electrodiagnostic study. After the electrodiagnostic study, the consultant should render an impression and make recommendation(s) for further workup or treatment options as appropriate.

The history establishes the basis for the entire evaluation, and thus it is the crucial first step in the electrodiagnostic study. Every disease has a cadence, and the history elicits the cadence that helps identify the specific disease. What is the presenting complaint? How long have symptoms been present? In the case of muscle weakness, is the onset acute, rapidly progressive, or slowly progressive? One should know if there are exacerbations and remissions or if the course is stable. It would be helpful to know if sensory symptoms accompany the weakness and whether or not the sensory symptoms predated or followed the weakness. A history of antecedent illness (bacterial or viral) might lead one toward evaluating for evidence of neurogenic etiology (e.g., Guillain-Barré syndrome) instead of a myopathy. What is the distribution of the weakness (1,2,3,4)? Is there a proximal or distal emphasis? Is the weakness segmental? Is there a time of day or a specific activity that aggravates the symptoms? Persons with myasthenia gravis tend to show increasing fatigue and weakness as the day progresses (5). Is there a family history of muscle weakness that might suggest a familial etiology for the weakness? Is fatigue an issue? If fatigue is present, the history must be expanded to evaluate for other causes of fatigue that are not necessarily direct muscle function problems (e.g., depressive illness). Does the person
complain of weight loss or weight gain? The loss of muscle fibers that might accompany a myopathy can result in weight loss, although extensive weight loss should lead one to consider the possibility of underlying neoplastic disease or neuropathy. In the presence of bulbar or extraocular weakness, mitochondrial myopathies should be considered (6).

One should inquire about activities of daily living. Can the person walk on level ground or on uneven ground, and can he or she climb stairs and arise to stand from a normal-height chair or from a couch? Can the person chew food without tiring, and is there difficulty swallowing? How about bathing, dressing, and toileting activities? The presence or absence of joint and/or muscle pain must be noted. There may be shortness of breath as a function of weakness of respiratory muscles or from lung disease (7) resulting from the underlying disease process (e.g., pulmonary fibrosis and pulmonary hypertension in systemic sclerosis [scleroderma]). Although systemic sclerosis is more likely to manifest as an acquired neuropathy (8), a myopathy can occur in scleroderma, from direct extension of fibrosis into muscle (9). Inflammatory muscle involvement in scleroderma may show clinical features similar to polymyositis, with significant proximal muscle weakness and high elevations of muscle enzymes (7).

The physical examination begins when one greets the patient or observes the gait as the person walks into the examining room. Some characteristic facies may be immediately recognizable because of obvious wasting of cranial and facial muscles. For example, in the person with myotonic dystrophy, the mouth tends to hang open, teeth often are in disrepair, bitemporal wasting is in evidence, and frontal balding is noted. A handshake might confirm myotonia of grip. In fascioscapulohumeral muscular dystrophy, facial weakness may show as the absence or relative absence of furrows, lines, or creases that would be expected for age. A rash could suggest inflammatory myopathy. The heliotrope rash of dermatomyositis and Groton’s sign come to mind (10).

The nerve conduction study in a patient with suspected myopathy should probably include two motor nerves in the upper limb and one in the lower. If one chooses to study only one motor nerve in the upper limb, it would be prudent to avoid an often-compromised nerve like the median. We recommend beginning the testing with one or more sensory nerves in the lower limb. Suspicion of a neuromuscular junction disorder must lead to repetitive stimulation testing at 2 Hz to 3 Hz (11). If Lambert-Eaton myasthenic syndrome (LEMS) is being considered in the differential diagnosis, one should deliver single supramaximal stimulation to the motor nerve being studied, with the muscle in the rested state (6). The patient then performs 10 seconds of exercise, followed immediately by a second supramaximal electrical stimulus to the motor nerve. Generally, motor nerve conduction studies are normal in myopathy. However, substantial muscle wasting underlying the recording electrode would lead to abnormally small amplitude of the recorded compound motor action potential (CMAP). As alluded to above, an abnormally small CMAP amplitude should lead one to consider repetitive nerve stimulation for possible myasthenia gravis or LEMS (see Chapter 15).

For needle EMG, we recommend studying two ipsilateral limbs, preferably on the dominant side, to allow for possible muscle biopsy evaluation on a limb that has not been explored with a needle electrode. The most characteristic electrodiagnostic findings in myopathy are best demonstrated during needle EMG. The act of inserting the EMG needle electrode into muscle disturbs a few muscle fibers, leading to a brief discharge of electrical activity. The duration of the discharge is to some extent related to the needle movement of the electromyographer. In inflammatory myopathy where there has been extensive muscle necrosis and regeneration, some muscle fibers might undergo functional denervation and reestablishment of motor endplates. The functional denervation manifests as abnormal membrane irritability with resulting
positive sharp waves and fibrillation potentials. The total number of muscle fibers is decreased in myopathy, with an increase of connective tissue and detritus, resulting in an increased distance between muscle fibers and the exploring needle electrode. The above combination contributes to the abnormally small amplitude of the motor unit potential (MUP) in inflammatory myopathy.

MUP duration is a function of the total number of muscle fibers that are available to contribute to the MUP. The slow, later phase of the MUP duration is derived from more distant fibers of the motor unit. In inflammatory and some other myopathies, there are smaller numbers of muscle fibers remaining in the motor units available to contribute, and the more distant units cannot contribute to the MUP, resulting in the short duration potential that is a hallmark of myopathy. Another hallmark of acute inflammatory myopathy is the presence of polyphasic MUPs. The normal motor unit comprises muscle fibers of different diameters. Larger muscle fibers tend to have faster propagation velocity along the muscle fiber itself than do smaller muscle fibers. In inflammatory myopathy there is a greater variation of fiber diameters than in the normal situation. Some muscle fibers are sick or damaged and are smaller in diameter than would be normally expected. This increased fiber diameter variation is thought to manifest as slight desynchronization of the MUP, resulting in polyphasic MUPs (12).

Another characteristic finding on needle EMG in myopathy is increased recruitment of MUPs. This is often manifested as the patient’s inability to voluntarily activate just one single MUP on command. Because the remaining motor units are smaller in size, sick, and weak, the patient’s effort to activate a single motor unit is unsuccessful. Smaller motor units generate less force than larger motor units (12). Instead of a single motor unit, many of the sick and damaged motor units must fire together to produce enough force to meet the demand for minimal activation. Therefore, at minimal contraction or turn-on frequency, many MUPs are immediately recruited, as recorded by needle EMG. At full effort of activation, the screen is filled with small-amplitude, short-duration, polyphasic MUPs that impart a high-frequency audio cue to the electromyogram.