Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) Associated with Mediastinal Schwannoma

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the most common cause of euvolemic hypo-osmotic hyponatremia. There are several etiologies of SIADH including neuroendocrine tumor, pulmonary disease, infection, trauma, and medications. Here, we report a case of SIADH associated with a schwannoma involving the mediastinum in a 75-year-old woman who presented with nausea, vomiting, and general weakness. Laboratory testing showed hypo-osmolar hyponatremia, with a serum sodium level of 102mmol/L, serum osmolality of 221mOsm/kg, urine osmolality of 382mOsm/kg, urine sodium of 55 mmol/L, and plasma antidiuretic hormone (ADH) of 4.40 pg/mL. Chest computed tomography identified a 1.5-cm-sized solid enhancing nodule in the right lower paratracheal area. A biopsy specimen was obtained by video-assisted thoracoscopic surgery, which was diagnosed on pathology as a schwannoma. The hyponatremia was completely resolved after schwannoma resection and plasma ADH level decreased from 4.40 pg/mL to 0.86 pg/mL. This case highlights the importance of suspecting and identifying the underlying cause of SIADH when faced with refractory or recurrent hyponatremia, and that on possibility is mediastinal schwannoma

A Case of SIADH Diagnosed by Repeated Water Load Tests in a Hyponatremic Patient with Radiologically Suspicious Thymoma / 대한신장학회지

The causes of hypo-osmolar hyponatremic patients without edema are mainly related to either SIADH (syndrome of inappropriate antidiuretic hormone secretion) or hypovolemia and the evaluation of extracelluar volume status can be a clue to differentiate between the two. As a diagnostic andtherapeutic tool, positive response to isotonic saline load test is recognized in favor of hypovolemic hyponatremia but there is a pitfall due to similar response in a subset of euvolemic hyponatremia, SIADH in case that combined with hypovolemia or accompanied by low sodium intake. In such case, water load test may provide more help for exact evaluation of hypovolemia and SIADH. Hereby, we report a case of a 74 year old patient with doubtful thymoma radiologically presented to emergency room with symptomatic severe hyponatremia (108 mEq/L) consistent with clinical and biochemical features of hypovolemia and positive response to initial isotonic saline infusion. However, episodes of hyponatremia recurred despite euvolemic status, which was diagnosed as SIADH by water load test. We initially considered thymoma as a cause of SIADH but its pathologic finding was a thymic cyst. And hyponatremia with hypovolemic feature recurred but the final diagnosis came out as idiopathic SIADH by repeated water load test. His hyponatremia was completely corrected by strict water restriction (<500 cc/day).