Multiple myeloma presenting with retroperitoneal fibrosis and elevated IgG4.

The role of IgG4 in the pathogenesis of IgG4-related disease remains unclear, particularly regarding whether IgG4 actively induces histopathological changes or if its elevation is merely a byproduct of an immune response to specific antigens. We present a case of a 66-year-old man with asymptomatic myeloma and elevated IgG4 who subsequently developed retroperitoneal fibrosis as a manifestation of IgG4-related disease. This condition led to hydronephrosis secondary to ureteral obstruction caused by the retroperitoneal fibrosis. Remarkably, chemotherapy for multiple myeloma led to remission of both multiple myeloma and retroperitoneal fibrosis. While previous reports indicated that patients with IgG4 myeloma did not exhibit clinical evidence of IgG4-related disease, this case is distinctly different, as the patient showed characteristic symptoms of IgG4-related disease. This case provides a valuable new perspective on IgG4-related disease and its link to the development of multiple myeloma.