IgM myeloma, a distinct entity in the spectrum of B-cell neoplasia.

The distinction between multiple myeloma and Waldenström's macroglobulinemia can usually be made on the basis of clinical, histologic, and immunologic findings. However, some patients have features of both diseases. Two patients who had IgM monoclonal gammopathies and plasma cell neoplasia are presented. Both had bone lesions, monoclonal IgMk, and bone marrow infiltration with plasma cells. The presence of plasma cells was verified by electron microscopy. Immunoperoxidase studies in both cases showed positive staining with mu and kappa antisera only, suggesting that these plasma cells were the source of the IgMk protein. Using the criteria of monoclonal IgM, plasma cell neoplasia, and bone lesions, 28 similar cases were found. The analysis of clinical data revealed an increased incidence of lytic bone lesions, decreased IgG and IgA, renal failure, hypercalcemia, and Bence-Jones proteinuria, as are commonly seen in multiple myeloma. It also demonstrated an increased incidence of hyperviscosity symptoms, lymphadenopathy, hepatosplenomegaly, and mucous membrane bleeding, as are often seen in Waldenström's macroglobulinemia. Other common findings were anemia and plasma cell leukemia. These data suggest that, although rare, IgM myeloma should be considered a distinct clinical entity in the spectrum of B-cell malignancies with characteristics of both multiple myeloma and Waldenström's macroglobulinemia.