Frequency of EMG abnormalities in idiopathic inflammatory myopathies under the EULAR/ACR classification criteria

The European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) classification criteria for idiopathic inflammatory myopathies (IIM) have been widely used in recent times. However, no studies have focused on electromyography (EMG) findings of IIM, considering the criteria. This study aimed to elucidate the frequency of EMG abnormalities, particularly fibrillation potentials and positive sharp waves (Fib/PSW), the most objective EMG findings of IIM. Clinical and EMG records of adult patients who were clinically diagnosed with polymyositis (PM), dermatomyositis (DM), amyopathic DM (ADM), or inclusion body myositis (IBM) were retrospectively reviewed and classified according to the EULAR/ACR classification criteria. The frequency of Fib/PSW in EMG was investigated in the recruited cases. Seventy-nine patients with clinically diagnosed IIM (44 with PM, 17 with DM, 7 with ADM, and 11 with IBM) were recruited. After classification using EULAR/ACR, 75 satisfied definite or probable IIM (61 and 14, respectively), and the frequency of Fib/PSW in this group was 95%. Furthermore, the remaining 4 patients with insufficient IIM probability also showed Fib/PSW. Fib/PSW may also be seen in cases with insufficient IIM probability not satisfying the criteria. EMG may help detect muscle involvement in these cases through Fib/PSW.