IgM demyelinative neuropathy with amyloidosis and biclonal gammopathy.

A 59-year-old man developed a sensorimotor neuropathy of the upper and lower limbs, associated with a biclonal gammopathy, within the space of a few months. Each of two paraproteins was coupled with a distinct IgM kappa IgG lambda light chain. Examination of a nerve biopsy specimen by electron microscopy revealed a demyelinative process with a widening of the interlaminar space in the myelin sheath, as well as deposits of amyloid substance between nerve fibers. Direct immunofluorescence revealed the presence of IgM and of the kappa light chain in certain Schwann cells, while the lambda IgG was fixed to the amyloid deposits. Immunoperoxidase histochemistry showed a positive reaction in normal human nerve tissue to the immune serum IgM and kappa light chain. The findings suggest that the widening of the interlaminar space of the diseased myelin corresponds to an active fixation of immunoglobulin on the sheath of the Schwann cell. The presence of two light chains in this patient's gammopathy caused a dual pathology: the kappa chain, a demyelinative neuropathy, and the lambda chain, a primary amyloidosis, with deposits in the peripheral nerve and in the kidney.