Sensory action potentials and biopsy of the sural nerve in the neuropathy of nonmalignant IgMk plasma cell dyscrasia.

Electrophysiological findings in 10 patients with polyneuropathy and nonmalignant IgMk plasma cell dyscrasia are reported. The amplitude of sensory action potentials and maximum conduction velocity along the sural and median nerves are related to the structural changes found at the biopsy and to the immunologic results. Quantitative analysis of the sural nerve showed a loss of large myelinated fibers in all 8 patients, in whom the nerve could be examined. The sural action potential as well as conduction velocity were undetectable in 4 cases, diminished in 5 and normal in 1. The primary mechanism underlying nerve damage in these neuropathies is discussed. It is suggested that the IgM paraprotein may act heterogeneously on the peripheral nerve's component.