Exaggerated Vasopressin Secretion and Attenuated Osmoregulated Thirst in Human Survivors of Hyperosmolar Coma

Aims/hypothesis: To test the hypothesis that subnormal thirst sensation could contribute to the development of the hypernatraemia characteristic of hyperosmolar coma, we studied osmoregulation in survivors of hyperosmolar coma.

Methods: Eight survivors of hyperosmolar coma, eight control subjects with Type II (non-insulin-dependent) diabetes mellitus and eight healthy control subjects underwent water deprivation during which measurements of thirst, plasma osmolality and vasopressin were taken.

Results: Water deprivation caused greater peak plasma osmolality in the hyperosmolar coma group (301.7 +/- 2.7 mmol/kg) than in Type II diabetic (294.3 +/- 3.2 mmol/kg, p < 0.01) or control group (296.9 +/- 3.0 mmol/kg, p < 0.01) and a greater increase in plasma vasopressin concentration (hyperosmolar coma, 5.8 +/- 1.3 pmol/l, Type II diabetes, 1.8 +/- 1.3 pmol/l, p < 0.001, control subjects, 2.2 +/- 1.8 pmol/l, p < 0.001). Thirst ratings were lower following water deprivation in the hyperosmolar coma group (3.5 +/- 0.8 cm) than in Type II diabetes (7.7 +/- 1.6 cm, p < 0.001) or control subjects (7.4 +/- 1.3 cm, p <0.001), and the hyperosmolar group patients drank less in 30 min following water deprivation (401 +/- 105 ml) than Type II diabetic (856 +/- 218 ml, p < 0.001) or control subjects (789 +/- 213 ml, p < 0.001).

Conclusion/interpretation: Survivors of hyperosmolar coma have subnormal osmoregulated thirst and fluid intake, which might contribute to the hypernatraemic dehydration typical of the condition.