MYOPATHIES

Polymyositis/Dermatomyositis

Polymyositis/dermatomyositis has been described in children ranging in age from infancy to adulthood. Children may result with progressive proximal muscle weakness, dysphagia due to involvement of pharyngeal musculature, dyspnea, and muscle tenderness. A classic

skin rash may or may not be present. Creatinine kinase values are often markedly elevated. Classic EMG findings include: (a) increased insertional activity with complex repetitive discharges; (b) fibrillations and positive sharp waves; and (c) low-amplitude, polyphasic, short-duration MUAPs recruited rapidly in relation to the strength of contraction.

Congenital Myopathies

Congenital myopathies are a heterogeneous group of disorders usually presenting with infantile hypotonia, normal cognitive status, and primary structural abnormalities of the muscle fibers, which are elucidated on histologic and electron microscopic evaluations of muscle biopsy specimens. Patients usually develop proximal rather than distal muscle weakness, which is nonprogressive and static. These myopathies are described in Chapter 18. NCSs are generally normal; however, there may be mild reductions in CMAP amplitudes. On needle EMG, findings are either normal or there may be mild, nonspecific changes, usually of a myopathic character (small-amplitude, short-duration polyphasic MUAPs). The only congenital myopathy consistently associated with abnormal spontaneous rest activity is myotubular (centronuclear) myopathy. In this disorder, the EMG reveals myopathic MUAPs with frequent complex repetitive discharges and diffuse fibrillation potentials.

Dystrophic Myopathies

The dystrophic myopathies are extensively described in Chapter 18. EMG is rarely used in present times for the diagnostic evaluation of a suspected dystrophic myopathy due to molecular genetic testing and the importance of muscle biopsy in differentiating among Duchenne muscular dystrophy, Becker muscular dystrophy, and limb girdle muscular dystrophies. EMG in dystrophic myopathies is characterized by low-amplitude, short-duration polyphasic MUAPs (see Figure 6.14). Recruitment is myopathic in nature with increased recruitment or "early" recruitment demonstrated with slight effort. The interference pattern is usually full. Complex repetitive discharges (see Figure 6.15) and abnormal spontaneous rest activity may be present, reflecting membrane instability.

Metabolic Myopathies

Nonspecific myopathic EMG findings may be demonstrated in metabolic myopathies. For example, absent maltase deficiency shows increased insertional activity, complex repetitive discharges, low-amplitude short-duration MUAPs, profuse fibrillations, and positive sharp waves. Carnitine deficiency, a disorder of lipid metabolism,

FIGURE 6.14 Low-amplitude short-duration polyphasic motor unit action potential in a 14-year-old girl with limb-girdle muscular dystrophy.

demonstrates increased recruitment for effort, decreased amplitudes of MUAPs and occasional fibrillations. EMG may be normal in many metabolic myopathies such as carnitine palmitoyltransferase I (CPT I) deficiency.

Myotonic Disorders

Myotonic disorders such as myotonic muscular dystrophy and Schwartz-Jampel syndrome may show myotonic discharges with either a positive sharp wave or fibrillation configuration, and a waxing and waning firing frequency. The myotonic discharges are often described as exhibiting the sound of a "dive bomber." There may be profuse fibrillations and positive sharp waves. MUAPs are often low amplitude and short duration. There may be more involvement of distal musculature than proximal musculature in myotonic muscular dystrophy. Again, with a known family history of myotonic muscular dystrophy, confirmation of the diagnosis in an individual with classic clinical features can be expeditiously and cost-effectively confirmed in the EMG laboratory. However, clinical trials frequently require molecular genetic confirmation of myotonic muscular dystrophy (DM1 versus DM2 and other myotonic disorders), so EMG is becoming less utilized diagnostically.

A recent study assessed the spectrum of disorders associated with electrophysiologic myotonia in a pediatric electromyography laboratory (137). Records of 2,234 patients observed in the Electromyography Laboratory at Boston Children's Hospital from 2000 to 2011 were screened retrospectively for electrophysiologic diagnoses of myotonia and myopathy. Based on electromyography, 11 patients manifested myotonic discharges alone,

FIGURE 6.15 Complex repetitive discharges in a dystrophic myopathy.

8 exhibited both myotonic discharges and myopathic MUPs, and 54 demonstrated myopathic MUPs alone. The final diagnoses of patients with myotonic discharges alone included myotonia congenita, paramyotonia congenita, congenital myopathy, and Pompe disease (acid maltase deficiency). The diagnoses of patients with both myotonic discharges and myopathic MUPs included congenital myopathy and non-Pompe glycogen storage diseases. Myotonic discharges are rarely observed in a pediatric electromyography laboratory, but constitute useful findings when present. The presence or absence of concurrent myopathic MUPs may help narrow the differential diagnosis further.

Linear Scleroderma

Muscle atrophy and asymmetric extremity growth are common features of linear scleroderma (LS). Extracutaneous features are also common and primary neurologic involvement, with sympathetic dysfunction, may have a pathogenic role in subcutaneous and muscle atrophy. One recent study (138) investigated nerve conduction and muscle involvement by electromyography in pediatric patients with LS. A retrospective review of LS pediatric patients who had regular follow-up at a single pediatric center was conducted from 1997 to 2013. Electromyograms (EMG) were performed with bilateral symmetric technique, using surface and needle electrodes, comparing the affected side with the contralateral side. Abnormal muscle activity was categorized as a myopathic or neurogenic pattern. Nine LS subjects were selected for EMG, two with Parry-Romberg/hemifacial atrophy syndrome, seven with LS of an extremity, and two with mixed forms (linear and morphea). Electromyogram analysis indicated that all but one had an asymmetric myopathic pattern in muscles underlying the linear streaks. Motor and sensory nerve conduction was also evaluated in upper and lower limbs and one presented a neurogenic pattern. Masticatory muscle testing showed a myopathic pattern in the atrophic face of two cases with head and face involvement. Thus, in this small series of LS patients, the investigators found a surprising amount of muscle dysfunction by EMG. The muscle involvement may be possibly related to a secondary peripheral nerve involvement due to LS inflammation and fibrosis. Further collaborative studies to confirm these findings are needed.