Hyponatremia: manifestations and treatment

All hyponatremic states have in common elevation of vasopressin. Without this the loss of salt would be followed by appropriate diuresis and normonatremia. If hyponatremia is triggered by a volume change as in heart failure or portal cirrhosis not only is ADH released but the mechanisms that control salt retention create an essentially sodium free urine, always less than 20 mEq/L. If the initial event is inappropriate ADH secretion whether it be cerebral disease, neoplasm, a pulmonary lesion or a growing list of drugs; there is no related signal for salt retention and urine sodium and tonicity are high, the latter usually higher than that of plasma. If salt loss is due to intrinsic renal disease, diuretics, osmotic or otherwise, or adrenal failure urinary sodium is variable depending upon the magnitude of the response to volume of salt retaining factors. Because hyponatremia is often present with major illness and because more than one factor may be involved in its genesis, the establishment of its origin and appropriate treatment remain a diagnostic and therapeutic challenge.