Pathogenesis and diagnosis of hyponatremia

This discussion emphasizes two aspects of hyponatremia: classification according to effective osmolality of the body fluid, and distinction between appropriate and inappropriate ADH secretion. Assessment of the effective osmolality is important because the main deleterious effect of hyponatremia is cell overhydration, which occurs only when the effective osmolality is reduced. Since most cases of hyponatremia are associated with low effective osmolality, cell overhydration is a hallmark of acute hyponatremia. On the other hand, one must be aware of other types of hyponatremia in which effective osmolality is either normal or even increased. Inappropriateness of ADH secretion is defined as ADH secretion that occurs despite low effective osmolality and normal or expanded effective vascular volume. ADH secretion that occurs in hyponatremia is deemed appropriate if the effective vascular volume is low. The use of laboratory parameters is much more reliable in determining effective vascular volume than is careful physical examination.