Myopathy

Spontaneous Activity in Myopathies

Fibrillation potentials and positive sharp waves usually are associated with neuropathic disorders (i.e., neuropathy, radiculopathy, motor neuron disease). However, denervating potentials occur frequently in many myopathic disorders . They are thought to most likely occur as a consequence of segmental inflammation or necrosis of muscle fibers, separating a distal, healthy portion of the muscle fiber from the part attached to the endplate ( Fig. 38.2 ). Infarction of small intramuscular nerve twigs by surrounding interstitial inflammation also is speculated to be a possible cause of denervation in inflammatory myopathies. Although the presence of denervating potentials in a patient with myopathy often suggests the diagnosis of an inflammatory myopathy, denervating potentials can occur in a variety of myopathies ( Box 38.4 ), including some toxic myopathies, several muscular dystrophies, and other inherited myopathies. In chronic myopathies, complex repetitive discharges may also be seen.

The presence of myotonic discharges yields additional information. A myotonic discharge is the spontaneous firing of a muscle fiber that waxes and wanes in both amplitude and frequency. The morphology of a myotonic discharge is either a positive wave or a brief spike potential. This morphology is the same as that of acute denervating potentials (i.e., fibrillation potentials and positive sharp waves). This should not be surprising because myotonic discharges are generated by muscle fibers as well. Myotonic discharges can be differentiated from fibrillation potentials and positive sharp waves by the waxing and waning of both firing frequency and amplitude. Remember that fibrillation potentials and positive sharp waves, in contrast, fire at a very regular rate. Myotonic discharges may be seen in myotonic dystrophy (types 1 and 2), myotonia congenita, paramyotonia congenita, and hyperkalemic periodic paralysis. They can also be seen in other myopathies, including acid maltase deficiency (especially in the paraspinal muscles), myotubular (centronuclear) myopathy, some drug-induced myopathies (e.g., chloroquine, colchicine, cholesterol-lowering agents), and, occasionally, in PM.

The last type of “spontaneous activity” to recognize is a contracture , which is the complete absence of any EMG activity in a muscle while it is in the contracted state. Superficially, a muscle cramp and a contracture may appear similar clinically—the painful involuntary contraction of a muscle. However, during muscle cramps, which are neuropathic in origin, the EMG shows involuntary firing of MUAPs at a high frequency, whereas during a contracture, there is electrical silence. Contractures are seen only in rare metabolic myopathies (e.g., McArdle’s disease, CPT deficiency) and occur as a result of insufficient energy available to break the actin-myosin bonds and return the muscle to a relaxed state. The “cramps” experienced by patients with metabolic myopathies such as McArdle’s disease or CPT deficiency are, in fact, contractures.

Motor Unit Action Potential Analysis in Myopathies

Differentiating between myopathic and neuropathic disorders usually is primarily based on analysis of MUAP parameters ( Fig. 38.3 ). In most myopathies, there is dropout or dysfunction of individual muscle fibers that effectively decreases the size of the motor unit ( Fig. 38.4 ). In this situation, the actual number of motor units (i.e., anterior horn cells and axons) does not change. Only in the rare case of a very severe myopathy where every muscle fiber in a motor unit drops out does the effective number of motor units decrease.

Remember that there is a large normal variation in MUAP parameters. In borderline cases, it is advisable to measure at least 20 MUAPs and compare them with age-matched controls for the muscle sampled. Analysis of MUAPs, either subjectively or, more ideally, quantitatively, commonly allows a diagnosis of myopathy by noting specific changes in MUAP duration, amplitude, phases, and recruitment pattern.

MUAP duration is the most important parameter to measure in myopathy. Duration most closely reflects the total number of muscle fibers in a motor unit, including those muscle fibers at a distance from the recording electrode. The measurement of duration usually does not include linked potentials. In myopathy, duration characteristically decreases. The reduction in duration is best explained by the random dropout of muscle fibers ( Fig. 38.5 ). Of course, the finding of one brief MUAP does not make the electrodiagnosis of myopathy. Because there is a normal range of MUAP duration that varies depending on age and the muscle studied, one must sample several MUAPs to determine the mean duration. In myopathy, although the mean duration decreases, some of the MUAPs may be normal or possibly of long duration ( Fig. 38.6 ). In mild or equivocal cases, quantitative EMG of 10–20 MUAPs should be performed. In addition, it is important to remember that brief-duration MUAPs may be seen in conditions other than myopathy. Any disorder that effectively causes loss or dysfunction of individual muscle fibers (e.g., myopathy, NMJ disorders with block, disorders of the terminal axon) without affecting the motor neuron and its main axon can result in short-duration MUAPs ( Box 38.5 ). A similar situation occurs in early reinnervation after severe denervation, when only a few fibers have successfully reinnervated, resulting in nascent (early reinnervated) motor unit potentials, which are also short and small. This point again emphasizes that the entire EMG examination must be taken as a whole and interpreted in light of the nerve conduction studies, as well as the history and examination, before a diagnosis is reached.

Somewhat surprisingly, chronic myopathies may have very long MUAP durations or frequently have linked or satellite potentials. These findings likely are secondary to fiber splitting or collateral sprouting from reinnervation in those myopathies associated with necrosis and subsequent denervation. In the chronic or late stage of a myopathy, it may be very difficult to distinguish a myopathic from a neuropathic disorder based on the duration of the MUAPs alone.

MUAP amplitude depends on just the few muscle fibers that are very close to the needle electrode. In myopathy, the amplitude commonly is decreased, but it can also be normal or increased if the needle electrode is placed near split or reinnervated fibers.

MUAP phases often are increased (more than four phases) in myopathy, but this is a nonspecific finding. The number of phases is primarily a measure of synchrony, and polyphasia may be seen in both myopathic and neuropathic disorders. Presumably, many of the remaining muscle fibers are dysfunctional and do not fire as synchronously as normal.

One of the most important findings in a myopathy is the presence of an early recruitment pattern. In myopathies in which there is dropout of individual muscle fibers from a motor unit, the motor unit becomes smaller and subsequently can generate less force. Early recruitment refers to the inappropriate firing of many MUAPs to generate a small amount of force. In general, only the electromyographer performing the study can assess early recruitment. Assessing early recruitment requires knowledge of how much force is being generated for the number of MUAPs that are firing. In myopathy, the number of MUAPs firing (recruitment) is appropriate for the firing frequency (activation); what is inappropriate is the number of MUAPs firing for the degree of force generated.

Only very rarely is the recruitment of MUAPs actually reduced in myopathy. This occurs only in the setting of end-stage muscle disease if all the muscle fibers of a single motor unit are lost, thereby causing an actual reduction in the number of motor units. This results in a reduced recruitment pattern on EMG. This situation is extremely uncommon but may arise in some very chronic myopathies that involve certain muscles severely, such as the quadriceps in IBM.

Single-Fiber Electromyography in Myopathy

Single-fiber EMG (see Chapter 37 ) in patients with myopathy is commonly associated with increased jitter and blocking, especially in those myopathies associated with abnormal spontaneous activity. This finding emphasizes that increased jitter and blocking, although very sensitive to disorders of the NMJ, are not specific to these disorders. Any neuropathic or myopathic condition that involves any degree of denervation and reinnervation results in newly formed or dying NMJs, which in turn lead to abnormalities on single-fiber EMG. In differentiating a myopathy from a disorder of NMJ transmission, single-fiber EMG is most helpful in those cases where both nerve conduction and routine needle EMG findings are normal. In this setting, if the single-fiber EMG is abnormal, a disorder of NMJ transmission is more likely than a primary muscle disorder.