This patient, who is experiencing nausea, headaches, and drowsiness following lithotripsy, has acute symptomatic hyponatremia, likely related to her recent surgery. Postoperative hyponatremia can develop within hours of surgery due to release of vasopressin related to surgical stress, the use of nonelectrolyte solutions (eg, glycine, sorbitol) for irrigation, or the infusion of hypotonic solutions during surgery.