Figure 1

Mechanisms underlying hypotonic hyponatremia. In most cases (middle panel) hyotonic hyponatemia results from a low effective arterial blood volume and is termed hypovolemic (or depletional) hyponatremia. The term syndrome of appropriate anti-diuresis has also been used to denote this condition. In childhood diarrhea, vomiting and febrile infections are the most common cause of hypovolemic hyponatremia. Persistently high levels of vasopressin or, exceptionally, an increased renal response to vasopressin cause the syndrome of inappropriate anti-diuresis (left panel), which is less frequent than the syndrome of appropriate anti-diuresis (hypovolemic or depletional hyponatremia). A peculiar form of depletional hyponatremia sometimes develops in patients with cerebral disease that mimics all of the findings in the syndrome of inappropriate anti-diuresis, except that renal salt-wasting is the primary defect with the ensuing volume depletion leading to a secondary rise in release of antidiuretic hormone (right panel). The ultimate causes of the three different conditions are "bordered".