Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) and Precocious Puberty With a Third Ventricle Arachnoid Cyst

Arachnoid cyst (AC) is a rare defect of the central nervous system that accounts for 1% of all intracranial lesions, of which only 1% of reported cases are located in the third ventricle. Endocrine manifestations associated with AC include precocious puberty, growth hormone deficiency, and hypothalamic dysfunction. We report a child who presented with a visual field defect, hyponatremia, and precocious puberty related to a third ventricle AC. Hyponatremia as a complication of AC is rare. A literature review revealed two case reports of Syndrome of inappropriate antidiuretic hormone secretion (SIADH) associated with suprasellar AC. The pathophysiology of SIADH in AC is not well understood. Hyponatremia may worsen following endoscopic fenestration of the AC secondary to changes in intracranial pressure. In conclusion, hyponatremia with AC should be recognized during the preoperative and postoperative periods and may require treatment with hypertonic saline in addition to fluid restriction.