Such rapid onset can occur with Acute psychogenic polydipsia Use of the recreational drug ecstasy (MDMA) Postoperative patients who received hypotonic fluid during surgery Marathon runners who replace sweat loss with hypotonic fluids Rapid-onset hyponatremia is problematic because the cells of the central nervous system have not had time to remove some of the intracellular osmolar compounds used to balance intracellular and extracellular osmolality.