Inclusion body myositis: peripheral nerve involvement. Combined morphological and electrophysiological studies on peripheral nerves.

The occurrence of neuropathy in 5 cases of inclusion body myositis (IBM) was studied. The intramuscular nerve branches showed variable ultrastructural changes in all cases. The observed changes were loss of axons, wallerian degeneration and axon terminal atrophy. EMG with concentric needles showed myopathic motor unit potentials in all cases. A reduced interference pattern due to loss of motor units was found in all cases. All but one patient showed fibrillation potentials in several muscle groups. Motor nerve conduction velocities and F-wave latencies were pathological in two of the cases, in which there were motor unit potentials with increased amplitude and duration as evidence of reinnervation. Sural nerve biopsy in one of these cases revealed slight neuropathy of the axonal type. These findings support the concept of peripheral nerve involvement in many cases of IBM.