[Syndrome of inappropriate secretion of antidiuretic hormone associated with resection of pulmonary squamous cell carcinoma].

Syndrome of inappropriate secretion of antidiuretic hormone (SIADH) shows various symptoms based on hyponatremia and brings catastrophic outcome occasionally. A 75-years-old man was found to have an abnormal shadow of the right lung on chest X-ray. Bronchoscopic examination showed that the abnormal shadow was aterectasis caused by the squamous cell carcinoma of middle lobe bronchus, and middle and lower lobectomy of the right lung was performed. On the postoperative day 2, suddenly the serum sodium concentration decreased to 116 mEq/l and serum osmolarity also decreased to 246 mOsm/l. Inspite of hyponatremia, the urine sodium level elevated to 73 mEq/l. So the urine osmolarity elevated to 752 mOsm/l, too. In addition to these data, edema and dehydration were absent, and both renal and adrenal function were normal. So we diagnosed that this abnormal conditions-hyponatremia with corresponding serum hypoosmolality and an inappropriately high urinary osmolality due to continued sodium excretion was induced by SIADH. Fluid restriction and antibiotics therapy for pneumonia resulted in an appropriate rise in the serum sodium level to 138 mEq/l on the postoperative day 21. To avoid catastrophic outcome in the cases of hyponatremia after surgery, it is important to remember that hyponatremia may be induced by SIADH.