EMG in Myositis
Source: Lyu RK, et al. Incidence of irritable electromyography in inflammatory myopathy. J Clin Neuromusc Dis 1999; 1:64-67.
Needle electromyographic (emg) examination of myositis nearly always reveals a triad of findings, comprising positive waves, complex repetitive discharges, and myopathic motor unit potentials.1 However, these abnormalities may not be as sensitive as previously thought. Among 178 consecutive, biopsy-proven cases of myopathy comprising 80 men and 98 women, 47 (26%) and 131 (74%) had inflammatory and noninflammatory histology, respectively, based on the presence or absence of inflammatory cells in the region of necrotic muscle cells, in the presence of type 2 fiber atrophy. Only 31 (66%) of inflammatory, and 39 (30%) of noninflammatory cases demonstrated spontaneous activity (positive waves, fibrillation potentials, or complex repetitive discharges). Interestingly, among the 10 patients with complex repetitive discharges alone, without positive waves or fibrillation potentials, none had inflammatory histology, indicating its lack of predictability in isolation. Overall, spontaneous activity predicted inflammation in only 44%, and was thus neither sensitive nor specific for myositis. —mr
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