Electromyographs of the flexor digitorum profundus muscle are useful for the diagnosis of inclusion body myositis.

INTRODUCTION: The frequent observation of high-amplitude and long-duration motor unit potentials (MUPs) in inclusion body myositis (IBM) is problematic, because it may lead to a misdiagnosis of amyotrophic lateral sclerosis (ALS).

OBJECTIVE: To document the diagnostic utility of EMG from the flexor digitorum profundus (FDP) muscle for IBM.

METHODS: Quantitative analyses of MUP parameters were performed in the FDP and biceps brachii (BB) muscles from 7 biopsy-confirmed IBM patients.

RESULTS: In the FDP muscle, all MUP parameters were significantly decreased in IBM patients, which indicated the predominance of low-amplitude and short-duration MUPs in this muscle. In the BB muscle, most parameters were increased, suggesting the frequent contamination of high-amplitude and long-duration MUPs.

CONCLUSIONS: Low-amplitude MUPs in the FDP muscle indicate the presence of an advanced myopathy in this muscle that was extremely weak for all subjects. Examining the FDP muscle would reduce the chance of misdiagnosing IBM as ALS.