A Boy with Reset Osmostat Who Developed Chronic Hyponatremia due to Hypothalamic Injury Caused by a Giant Arachnoid Cyst

What is already known on this topic?Reset osmostat (RO), a subtype of syndrome of inappropriate antidiuretic hormone secretion, is a rare cause of hyponatremia characterized by a decrease in the plasma osmolality threshold for antidiuretic hormone excretion. What this study adds?For the first time, we report an 11-year-old boy diagnosed with RO and a giant arachnoid cyst in the prepontine cistern.Sodium correction was considered unnecessary for RO, but chronic hyponatremia must be treated for the risk of decreased bone density and growth obstacles.