A rare case of paraneoplastic dermatomyositis secondary to smouldering multiple myeloma referred for proteinuria.

Dermatomyositis (DM) is often associated with malignancies but less commonly with plasma cell dyscrasias. Myoglobin nephropathy is an uncommon complication of inflammatory myopathies. This case describes a patient who presented with proteinuria because of myoglobin nephropathy caused by dermatomyositis. The dermatomyositis was paraneoplastic with an underlying smouldering multiple myeloma (SMM) diagnosed on investigation. Case report. A 58-year-old mixed-race gentleman was referred for sub-nephrotic proteinuria. Further history taking revealed a 1-month history of proximal muscle weakness and a 5-month history of skin changes suggestive of dermatomyositis. Additional testing showed no associated albuminuria. There was haematuria on the urine microscopy which, with the accompanying raised creatine kinase levels, suggested myoglobinuria. Kidney biopsy confirmed myoglobin nephropathy. Further testing was done to exclude paraneoplastic DM. Investigations for solid organ malignancies were negative. Plasma cell dyscrasia screening revealed an IgA kappa monoclonal band, which was confirmed with serum free light chain testing. A bone marrow biopsy confirmed the diagnosis of SMM based on the revised working group diagnostic criteria for multiple myeloma. This case serves as a reminder of the importance of clinical history and examination. Investigation in this case revealed an extrarenal cause of the proteinuria, and recognition of the presentation of DM led to further investigation for secondary causes. Routine screening for plasma cell dyscrasias is recommended as part of DM work-up to allow the diagnosis to be made early, prior to organ damage.