Electrophysiological and clinical examination of polymyositis: a retrospective analysis.

Background: We retrospectively analyzed electromyogram (EMG), laboratory and clinical data of 93 patients with polymyositis (PM) to help improve early diagnostic accuracy. Methods: Data were collected from hospitalized patients who were diagnosed with PM between January 1, 2006 and April 1, 2010 at Chinese People's Liberation Army General Hospital. Results: Eighty-six (92.47%) patients exhibited an isolated myopathic EMG pattern, 3 (3.23%) showed myopathy combined with neuropathy and 1 (1.08%) had diabetes mellitus. Among the 4 without myopathic EMG, 1 had inflammatory myopathy, and 3 had prior corticosteroid treatment with clinical improvement before EMG examination. Muscle biopsy, performed in 40 cases, inclusive of the 7 cases that did not show an isolated myopathic EMG patterns, revealed that 80% exhibited typical pathological features of PM, 17.5% showed nonspecific muscle fiber necrosis and 2.5% were normal. EMG showed that tibialis anterior and biceps brachii exhibited more abnormal EMG results than abductor pollicis brevis. Patients with disease duration >3 months had significantly higher alanine aminotransferase and reduced motor unit potential (MUP) amplitudes compared to patients with shorter disease duration. Statistical analysis revealed a significant association between disease duration and MUP amplitude reduction. Patients without interstitial lung disease showed significantly longer disease duration and higher creatine kinase and lactate dehydrogenase levels than patients with interstitial lung disease. Conclusions: Our results show that MUP is a sensitive yardstick for diagnosis of PM and is inversely related to disease duration. Our study also highlights that selecting specific muscles for EMG may improve diagnostic accuracy in PM.