Background: Hyponatremia is a common fluid-electrolyte disturbance, particularly in patients with neurologic disorders, in part because of the major role the central nervous system (CNS) plays in the regulation of sodium and water homeostasis.
Review summary: The classification of hyponatremia is based on an assessment of serum sodium concentration ([Na+]), serum and urine osmolality, and body volume status. In most cases, hyponatremia is associated with hypotonicity, which causes water to move into the brain. Adaptive responses limit the impact of cerebral edema in chronic hyponatremia, but CNS symptoms and death may occur in response to rapid or large decreases in serum [Na+]. The prompt correction of serum [Na+] is mandatory in symptomatic patients, but overly rapid correction must be avoided to limit the risk of myelinolysis. In neurologic disorders, euvolemic hyponatremia (usually caused by the syndrome of inappropriate secretion of antidiuretic hormone) must be distinguished from hypovolemic states such as cerebral salt wasting because the treatment of the 2 conditions differs. Vasopressin antagonists represent a new approach to the treatment of euvolemic and hypervolemic hyponatremia secondary to arginine vasopressin dysregulation.
Conclusion: The optimal treatment of hyponatremia is controversial, but appropriate treatment must be determined according to the osmolality and volume status of the patient. If left untreated, serious CNS complications and adverse outcomes, including an increased risk of death, can occur.