CHRONIC HYPONATREMIA

: RESET OSMOSTAT, CHALLENGES IN DIAGNOSIS

Abstract

INTRODUCTION/BACKGROUND
Hyponatremia remains one of the most common electrolyte imbalances encountered. Hyponatremia has many causes, and it requires systematic investigation. SIADH is one of the most common causes. Reset osmostat is a rare and poorly recognised cause of mild to moderate hyponatraemia, with a presentation similar to SIADH.

CASE
We present the case of a 69-year-old male with a history of cerebrovascular disease who had been diagnosed with SIADH after extensive workup for chronic hyponatremia. He had been on fluid restriction and oral salt for years but with little effect on his serum sodium. Given his stable mild hyponatraemia, he was investigated for possible reset osmostat. Patient underwent an oral water loading test in daycare. He was given 1000 ml (15 ml/kg) of water to drink within 30 minutes and then monitored for 4 hours. Serum and urine osmolality, serum electrolytes and urine output were obtained at baseline, then hourly for 4 hours. Urine volume was also measured hourly. His baseline serum sodium was 125 mmol/L, which dropped to 122 mmol/L at 2 hours, then returned to baseline (125 mmol/L) at 4 hours. The serum osmolality was 266 mOsm/kg, which dropped to 259 mOsm/kg and returned to baseline (266 mOsm/kg) after 4 hours. The urine osmolality at baseline was 233 mOsm/kg and dropped to 123 mOsm/ kg midway through the test. Urine volume was greater than 300 ml/hour throughout the test, and the patient excreted more than 1000 ml of urine in total.

CONCLUSION
The results showed that the patient successfully excreted the water load, diluted his urine, and maintained serum sodium levels at the end of 4 hours. Although there is no consensus guideline, the findings in this case would be consistent with reset osmostat. Patients with reset osmostat usually do not require treatment. It is worthwhile to consider this diagnosis in a small subset of patients with a prior diagnosis of SIADH.