A Case of ‘Essential’ Hypernatraemia Due to Resetting of the Osmostat

Abstract

We describe a 24-year-old short, obese girl who has bizarre episodic neurological abnormalities related to hyperosmolality due to hypernatraemia. Investigation of osmoregulation by water loading and infusion of hypertonic saline revealed (i) hypodipsia with thirst onset raised to plasma osmolality of 332 mmol/kg and (ii) elevation of the threshold for vasopressin release (plasma osmolality 320 mmol/kg) but normal slope of the plasma vasopressin-plasma osmolality curve. Baroregulated vasopressin release was also grossly subnormal. Other hypothalamo-pituitary investigations showed deficiencies of releasing hormones for gonadotrophins and growth hormone, but prolactin response to combined hypoglycaemia and TRH was blunted She demonstrated other anomalies including hyperlipoproteinaemia and defective lymph drainage of the legs. Skull X-rays, together with computerized tomography and nuclear magnetic resonance scans of the hypothalamo-pituitary region and the rest of the brain were normal. We believe that this is the first case of essential hypernatraemia documented with direct evidence of resetting of the osmostat.

Chronic hypernatraemia and hypothermia following subarachnoid haemorrhage

“…Regression analysis of the hypertonic saline test results suggests destruction of the osmoreceptor (Robertson, 1983). While it may be argued that the osmotic stimulus was not sufficient to exclude resetting of the osmostat (Gill et al, 1985) it may be noted that the patient demonstrated adipsia despite a plasma sodium of 187 mmol/I and plasma osmolality of 397 mOsm/kg in circumstances producing the syndrome of muscle weakness (Maddy & Winternitz, 1971) and rhabdomyolysis (Opas et al, 1977). Nausea is a potent non-osmotic stimulus to AVP release (Rowe et al, 1979) and a rapid and marked rise in plasma AVP was provoked by the apomorphine test.…”

Osmoregulation in clinical disorders of thirst appreciation

“…The combination of hyperosmolality and dilute urine can lead to the misdiagnosis of cranial diabetes insipidus, particularly if thirst is not formally assessed. This rare condition is sometimes referred to as essential hypernatraemia (Gill et al ., 1985; Thompson et al ., 1987b). Computerised tomography and nuclear magnetic resonance scans of the hypothalamo‐pituitary region are normal and the aetiology is uncertain.…”

Severe Hypernatremia with Impaired Thirst

“…Unfortunately, in only two cases has the 'shift to the right' of the AVP/plasma osmolality curve been documented [87,102]. One patient was a 24-year-old obese young lady with bizarre episodic neurological abnormalities, hypodipsia and elevation of the threshold for vasopressin release (plasma osmolality 320 mosm/kg) [87], Baroregulated AVP release was also abnormal in this patient. The other patient was a 65-year-old woman with hyperna tremia who also exhibited nephrogenic diabetes insipi dus presumably because of lithium toxicity [102].…”