Electromyographic findings in sporadic inclusion body myositis

Clinically oriented diagnostic criteria can be as specific for diagnosis of sporadic inclusion body myositis (sIBM) as pathological criteria, especially at the time of presentation. EMG may provide an convincing proof that a muscle biopsy should be performed. To compare the EMG results in patients with sIBM divided into subgroups based on the newest ENMC criteria for sIBM and to obtain the utility of EMG in the diagnostic process at the time of presentation. We retrospectively analysed 16 patients with sIBM for motor unit action potential (MUAP) morphology as well as occurrence and distribution of abnormal spontaneous activity (SA) in muscles. Abnormal SA was recorded in 62.5% of sIBM patients. We found statistically significant differences between subgroups in the incidence of polyphasic MUAPs and high amplitude outliers which were more commonly seen in the “clinico-pathologically defined sIBM”. Duration of MUAP in the tibialis anterior was significantly shorter in “probable sIBM”. “Pseudo-neurogenic” MUAPs, mainly in lower limb muscles, are more commonly seen in “clinico-pathologically defined sIBM” while myopathic MUAPs with prominent abnormal SA are recorded in patients diagnosed with “probable sIBM”. Both EMG patterns may be suggestive of sIBM and be an indication for further diagnosis.