Hyponatraemia is the most common electrolyte disorder encountered in clinical practice. Symptomatic hyponatraemia reflects brain damage because of cerebral swelling. Some coexisting factors such as extreme ages, hypoxia and female sex are associated with poor prognosis. In this report, we describe the case of a 75-year-old patient who suffered from hyponatraemic encephalopathy after elective vaginal hysterectomy under spinal anaesthesia. After being transferred to the ward, she developed nausea, vomiting, hypertensive crisis and intense anxiety. These symptoms were followed by grand mal seizure. Serum sodium level was 108 mmol/l. She also presented hypoxia, considered an aggravating factor, which was probably caused by the combination of benzodiazepine intake and cerebral oedema. However, fast raise of serum sodium level was achieved by immediate treatment with hypertonic saline, and she was discharged home without any sequelae.