Inclusion body myositis: no evidence for a neurogenic component

Because electrophysiologic, clinical, and histopathologic observations have suggested that inclusion body myositis (IBM) may have a coexistent neurogenic component, we used macro-electromyography (macro-EMG) to search for changes in the motor unit territory and signs of reinnervation. We studied 11 patients, aged 53 to 77 years (mean, 65.2 years), with typical, nonfamilial IBM lasting a mean of 8.5 years, and eight healthy volunteers aged 54 to 70 years (mean, 64.6 years), as control subjects. Nerve conduction studies showed focal abnormalities in 5 of the patients, but no evidence of a polyneuropathy. Concentric needle EMG in various proximal and distal muscles of the upper and lower limbs revealed short- or long-duration complex motor unit potentials (MUPs) or a mixture of both types of MUPs. Macro-EMG studies in the tibialis anterior muscle showed smaller macro-MUP amplitudes and areas in patients than in normal subjects. Four patients had abnormal macro-EMG studies with an increased number of small macro-MUPs, 1 patient had an equivocal study with large-amplitude but normal-area macro-MUPs, and the remaining 6 patients had normal studies. These findings are consistent with a primary muscle disorder similar to those seen in other myopathies. We conclude that macro-EMG does not support a coexistent neurogenic component in patients with IBM compared with normal subjects of similar age.