Crystallizing immunoglobulin presenting as polychromatic crystalline keratopathy: An unusual clinical presentation of monoclonal gammopathy of undetermined significance (MGUS)

Polychromatic crystalline keratopathy (PCK) is a rare complication of elevated serum immunoglobulin, sometimes associated with multiple myeloma and monoclonal gammopathy of undetermined significance (MGUS). A 49-year-old woman presented to her ophthalmologist with new visual changes. Slit lamp examination was diagnostic for PCK, and since crystals of PCK are sometimes associated with monoclonal gammopathy, the patient was referred to a hematologist for a diagnostic work-up after serum protein electrophoresis revealed a monoclonal band. A bone marrow biopsy revealed atypical plasma cells with eosinophilic crystalline inclusions, and complete work-up confirmed an underlying MGUS. This case is unusual because the patient had a low disease burden based on percentage of plasma cells and M-spike; however the M-protein was crystallizing within the plasma cell cytoplasm and in the patient's cornea. To our knowledge, PCK with such unusual marrow findings has not been previously reported in the pathology literature.