IgD lambda multiple myeloma causing hyperammonaemia with possibly reduced ammonium excretion.

A man in his 70s presented with impaired consciousness due to hyperammonaemia while investigating multiple tumours in his brain, chest, spine and right adrenal gland. He did not have any disorders causing hyperammonaemia, such as liver dysfunction, urea-producing bacterial infection or any medications interfering with ammonium metabolism. Blood and urine tests in addition to tumour biopsy specimens confirmed immunoglobulin D lambda multiple myeloma. His general status responded to chemotherapy using bortezomib, dexamethasone and daratumumab, and he subsequently regained full consciousness and a normalised serum ammonia level. He, unfortunately, died of refractory multiple myeloma with hyperammonaemia. The autopsy specimen revealed lambda light-chain deposits in the distal tubule epithelium with cast precipitation and intact liver cells. Urine osmolality gap analysis suggested possibly reduced urinary ammonium excretion, but further investigation is necessary to elucidate the significance of pathological renal characteristics in multiple myeloma with hyperammonaemia.