Inflammatory demyelinating lesions in two patients with IgM monoclonal gammopathy and polyneuropathy.

We report two patients with polyneuropathy and IgM monoclonal gammopathy in whom peripheral nerve biopsy showed the widening of myelin lamellae which is characteristic of IgM paraproteinaemic neuropathy. Moreover, certain myelinated fibres were invaded by histiocytes overloaded with myelin debris, and in some instances elongated macrophage processes could be seen peeling away the myelin lamellae. The latter ultrastructural features are characteristic of inflammatory demyelinating polyneuropathies in both human and experimental pathology. Such an association has not been reported to date in human pathology, but could explain the prevalence of inflammatory demyelinating lesions in experimental models of IgM paraproteinaemic neuropathy. These two cases seem to bridge the gap between inflammatory demyelinating polyneuropathies and polyneuropathies associated with IgM monoclonal gammopathy.