Abstract
The osmolality of body fluids is normally maintained within a narrow range. This constancy is achieved largely via hypothalamic osmo-receptors that regulate thirst and arginine vasopressin, the antidiuretic hormone (ADH). Anything that interferes with the full expression of either osmoregulatory function exposes the patient to the hazards of abnormal increases or decreases in plasma osmolality. Hyposmolality is almost always due to a defect in water excretion. Increased intake may contribute to the problem but is rarely, if ever, a sufficient cause. Impaired water excretion can be due to a primary defect in the osmoregulation of ADH (inappropriate antidiuresis) or secondary to nonosmotic stimuli like hypovolemia or nausea. The two types differ in clinical presentation and treatment. Resetting of the ADH osmostat is commonly associated with resetting of the thirst osmostat. Hyperosmolarity is almost always due to deficient water intake. Excessive excretion may contribute to the problem but is never a sufficient cause. Impaired water intake can result from a defect in either the osmoregulation of thirst of the necessary motor responses. Thirst may be deficient because of primary osmoreceptor damage as in the syndrome of adipsic hypernatremia or secondary to nonosmotic influences on the set of the system. They are distinguishable by the clinical presentation as well as the type of ADH defects with which they are associated. So-called essential hypernatremia due to primary resetting of the osmostat has been postulated, but unambiguous evidence for such an entity has not yet been reported.
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