Needle electromyography (EMG) findings in patients with myopathy can be classified as "irritable" or "non-irritable," depending on the presence or absence of "abnormal spontaneous activity" in the form of fibrillation potentials, positive sharp waves, or complex, repetitive discharges. An irritable myopathy suggests either an active necrotizing myopathic process and/or unstable muscle membrane potential. Although inflammatory myopathies are traditionally considered to show irritable myopathic changes on EMG, the precise incidence of these changes is not known. We retrospectively reviewed the EMG findings in 178 consecutive patients with biopsy-confirmed myopathy. Inflammatory myopathy was defined on the basis of biopsy findings combined with clinical features. Needle EMG showed "irritable" myopathic features in 66% of the patients in the inflammatory histology myopathic group and in 30% patients with non-inflammatory histology (p <0.001). Overall, irritable EMG findings predicted an inflammatory histology in only 44% of patients. The degree and distribution of irritability on EMG did not improve the probability of finding inflammatory myopathy. By the same token, non-irritable EMG findings also did not exclude inflammatory histology. We conclude that irritable myopathy on EMG as defined above is not very sensitive and not very specific for predicting an inflammatory histology.
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