Electrophysiologic findings in amyloid myopathy.

Myopathy is an uncommon manifestation of systemic amyloidosis. A retrospective chart review of 17 patients seen between 1975 and 1997 with biopsy-proven amyloid myopathy was performed to characterize the electrophysiologic features. Nerve conduction study abnormalities occurred in 14 of 17 patients (82%). The most common abnormality was a low peroneal and/or tibial compound muscle action potential amplitude (n = 9). Mild abnormalities also occurred in seven other nerves. Repetitive stimulation at 2 Hz in 10 nerves was normal. Electromyography demonstrated fibrillation potentials in 69% of muscles, most frequently in the gluteus medius (90%) and paraspinals (87%). The fibrillation potentials were sparse in 60%. Short-duration, low-amplitude motor unit potentials (MUPs) were found in 72% of the muscles examined, were mild in 81%, and were more common in proximal muscles. Long-duration MUPs were found in 19% of muscles and a mixed population of MUPs in 4%. These findings are similar to those of common chronic, inflammatory myopathies.