LAUSR

Hematologists often encounter electrolyte disturbances such as hyponatremia during chemotherapy. Hyponatremia can be elicited by several mechanisms: renal or extra‐renal sodium loss, fluid hyper‐ infusion or excessive water intake, and endocrine disorders such as hypothyroidism, adrenal insufficiency, and syndrome of inappropriate antidiuretic hormone (SIADH), which may be associated with pulmonary infection or tumor, disorders in the central nervous system, and concomitant medication. Severe hyponatremia can lead to life‐threatening events. Here, we report marked hyponatremia in a patient being treated with bortezomib. A 64‐year‐old Japanese man was admitted to our clinic with recurrent costal bone fractures. One month before admission, he developed appetite loss and lost 3 kg of weight. His body mass index on admission was 14.9. He had a history of retinitis pigmentosa with blindness. On admission, laboratory findings showed hemoglobin 8.2 g/dL, albumin 4.3 g/dL, creatinine 1.77 mg/dL, sodium 130 mEq/L (normal range: 135‐148 mEq/L), calcium 9.5 mg/dL, IgG 5,568 mg/dL (normal range: 870‐1700mg/dL). Protein electrophoresis showed amonoclonal peak, and β2‐microglobulin was 18.4 mg/L (normal range: 0.9‐1.9). A bone marrow examination revealed that 90% of nucleated cells were myeloma cells with unbalanced κ‐ and λ ‐type immunoglobulin. We diagnosed the IgG‐κ type of multiple myeloma stage III disease (International Staging System) with symptoms. He was treated with bortezomib (1.3 mg/m on days 1, 4, 8 and 11) and oral dexamethasone (20 mg on days 1, 4, 8 and 11). Because of the patient's thrombocytopenia, the administration of bortezomib and dexamethasone scheduled for day 11 was canceled. During the 1st course of treatment, the serum sodium level decreased to 125 mEq/L, and the patient had appetite loss. He was treated with fluid intake restriction (800 mL/day) and sodium chloride supplementary intake (12 g/day). The 2nd course of bortezomib was started on day 35 after the patient had recovered from the bone marrow suppression. On the second day after the patient was discharged, he was hospitalized again due to fever. The laboratory testing showed white blood cells 5.9 × 10/L, creatinine 0.57 mg/dL, sodium 110 mEq/L, potassium 3.2 mEq/L, and C‐reactive protein 0.5 mg/L. Liver function was normal. Other specific test results were as follows: blood osmolality 248 mOsm/kgH2O, thyroid function normal, non‐suppressed ADH levels (2.4 pg/mL) and normal brain natriuretic peptide (15.2 pg/mL). A urine examination revealed elevated osmolality (525 mOsm/kgH2O)