Syndrome of inappropriate antidiuresis caused by ectopic vasopressin production in uterine sarcoma: a case report

Syndrome of inappropriate antidiuresis (SIAD) is characterized by impaired free-water excretion resulting in euvolemic hyponatremia and a common cause of hyponatremia. It can occur as a paraneoplastic syndrome associated with various malignancies. However, SIAD caused by uterine sarcoma is extremely rare. A 78-year-old woman was evaluated for abnormal uterine bleeding, and endometrial biopsy findings were suspicious for uterine sarcoma. During preoperative evaluation, severe hyponatremia (serum sodium level; 117 mEq/L) was detected. Clinical examination revealed no signs of dehydration or edema, and the laboratory findings were compatible with a diagnosis of SIAD. The patient underwent total abdominal hysterectomy and bilateral salpingo-oophorectomy. Histopathological examination revealed a mixed tumor composed of undifferentiated uterine sarcoma and endometrioid carcinoma. Immunohistochemical staining for arginine vasopressin (AVP) demonstrated positive staining in the sarcoma cells, suggesting ectopic AVP production by the tumor. Following the surgery, serum sodium levels improved and plasma AVP concentrations decreased. This case represents a rare instance of paraneoplastic SIAD caused by ectopic AVP production in uterine sarcoma and highlights the importance of considering malignancy in the evaluation of unexplained hyponatremia.