Needle electromyography, muscle MRI, and muscle pathology: Correlations in idiopathic inflammatory myopathies

Background: On needle electromyography (EMG), abnormal spontaneous activity is considered a feature of active myositis. However, the correlation between needle EMG and muscle pathology is not always clear. Moreover, the changes in EMG findings after corticosteroid therapy are not fully understood. Aim: To investigate the correlations among muscle pathology, needle EMG and muscle MRI findings in patients with idiopathic inflammatory myopathies (IIMs). Methods: The clinical features, laboratory results, needle EMG, muscle MRI findings and pathological features of 50 consecutive patients with IIMs who underwent muscle biopsies were reviewed. We also describe the changes in these findings between before and after corticosteroid treatments. Results: The most common IIM was dermatomyositis (40%). On needle EMG, fibrillation/positive sharp waves (Fib/PSWs) were observed in 72% of the patients. MRI abnormalities were observed in 86% of the patients. In the pathology study, 88% of patients had inflammatory markers. The incidence of Fib/PSWs did not differ between pretreatment patients (79%) and steroid-treated patients (62%). In the pathology study, perimysium/perivascular cell infiltration also did not differ between before and after treatment. However, endomysium cell infiltration was significantly less frequent in steroid-treated patients (0%) than in pretreatment patients (29%) (P < .05). Conclusions: Our study confirmed that Fib/PSWs on needle EMG and muscle MRI were sensitive guides for diagnosing IIMs, and Fib/PSWs might be correlated with muscle fiber injuries or segmental necrosis rather than inflammatory cell infiltration in the affected muscles. Before treatment and within at least 1 month after starting corticosteroid treatment, Fib/PSWs could be detected by needle EMG.