Demyelinating neuropathy due to primary IgM kappa B cell lymphoma of peripheral nerve.

A 53-year-old man presented with a painful, demyelinating sensorimotor peripheral neuropathy with lymphomatous infiltration on sural nerve biopsy, but no evidence of systemic lymphoma. The neuropathy responded to cytotoxic therapy. Seven years later he developed generalized lymphadenopathy due to B cell lymphoplasmacytoid lymphoma, with a subpopulation of cells expressing a monoclonal pattern of IgM kappa. The lymphomatous infiltrate in the original nerve biopsy showed similar monoclonal IgM kappa reactivity. The mechanism of demyelination of the peripheral nerves may be similar to that described in patients with IgM kappa monoclonal gammopathies.