Syndrome of Hypernatremia, Hypodipsia and Partial Diabetes Insipidus: A New Interpretation

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Abstract
A 17-yr-old girl is described with a space-occupying lesion in the region of the hypothalamus and evidence of secondary trophic hormone lack. Despite clinical evidence of slight dehydration and a serum sodium of 170–190 mEq/1 there was complete absence of thirst and the average fluid intake was 1200 ml per day. This abnormality persisted following replacement therapy with cortisol and thyroxine. Although plasma volume and total body water measurements were within the normal range there was evidence to suggest a deficit in both these body fluid compartments. Comparison of serum sodium and urine U/S Osm ratios indicated an impaired ADH release at all levels of serum osmolality. The maximum urine osmolality achieved following exogenous vasopressin (1200 mOsm/kg) was greater than that with fluid deprivation (720 mOsm/kg) confirming partial diabetes insipidus. Despite this, the osmotic threshold for ADH release measured during hypertonic saline infusion appeared to be normal (less than 295 mOsm/kg). Both this and lack of patient cooperation prevented a reduction in serum sodium when water loading was attempted. The disorder was effectively treated with chlorpropamide which allowed rehydration by reducing free water clearance. The term “essential hypernatremia” has previously been used to describe such hypodipsic patients and the abnormality ascribed to a resetting of the osmotic threshold for ADH release (“syndrome of hypodipsia and ADH reset”). It is apparent, however, that there is insufficient published evidence to support the concept of normal ADH release above an elevated osmotic threshold. It seems more likely that ADH secretion is impaired at all levels of serum osmolality and that urine concentration and effective water conservation is possible only when the serum osmolality is greatly increased. It is suggested that a more suitable term for this disorder is “syndrome of hypernatremia, hypodipsia and partial diabetes insipidus.”