Hypoosmolar Hyponatremia as Presenting Symptom of a Rare Case of Olfactory Neuroblastoma.

Hyponatremia is a common electrolyte disturbance. The syndrome of inappropriate antidiuresis (SIAD), characterized by nonosmotic release of arginine vasopressin, is a typical cause of hyponatremia. Ectopic secretion of arginine vasopressin by a tumor is often associated with small cell lung cancer but can also be seen in other neoplasms. We present a rare case of a previously healthy 34-year-old woman, evaluated for refractory hypoosmolar hyponatremia consistent with SIAD. After excluding other causes of SIAD, imaging revealed contrast-enhancing mucosal thickening in the left maxillary sinus with expansion into the sphenoid and frontal sinuses, leading to the diagnosis of olfactory neuroblastoma. After surgical removal, hyponatremia resolved. Although treatment of olfactory neuroblastoma typically involves surgery and radiotherapy, there are no standardized therapeutic guidelines, due to the low prevalence of the disease. In summary, olfactory neuroblastoma is an extremely rare paraneoplastic cause of SIAD that should not be overlooked after excluding other possible etiologies.