Massive plasmocytosis due to methimazole-induced bone marrow toxicity

Pancytopenia is a rare complication of the thionamide therapy reported seco ndary to aplastic anemia, the bone marrow being invariably hypocellular. We present a case of a 16-year-old female with Graves' disease who presented with massive bone marrow plasmocytosis mimicking multiple myeloma, The pati ent had already been on methimazole for a month when she was admitted to th e Pediatric Unit with the diagnosis of sepsis, CBC revealed pancytopenia. B one marrow aspirations showed hypocellular-normocellular bone marrow, 98% o f plasma cells. At that time, MMI was discontinued and the patient was star ted on broad-spectrum antibiotics, dexamethasone, and G-CSF, Bone marrow as piration day +4 stili showed hypo-normocellular marrow, with remaining 6% p lasma cells. Myeloma screen was negative; ANC >1,000 at day +7, platelets,> 50,000 at day +24, Twenty-four months after patient's discharge, her clini cal condition, CBC, and bone marrow remained normal. To our knowledge this is the first report of pancytopenia due to MMI, where the usual hypoplasia found is replaced by massive plasmocytosis, (C) 2001 Wiley-Liss, Inc.