Hyponatremia

Abstract

Hyponatremia is biochemically defined by a blood sodium concentration ([Na⁺]) below the normal reference range for the laboratory performing the test (typically <135 mmol/L). The clinical relevance of a below normal blood [Na⁺] is largely determined by the severity of the clinical signs and symptoms associated with cellular swelling. Severe hyponatremia may induce seizures, coma and cardiopulmonary arrest. However, mild to moderate hyponatremia may present with more non-descript symptoms such as lethargy, restlessness, disorientation, headache, nausea and vomiting, muscle cramps, and depressed neural reflexes. Low [Na⁺] can be caused by: 1) fluid overload (hypervolemia); 2) abnormal fluid retention (euvolemia); or 3) volume depletion (hypovolemia) or some likely combination. Morbidity and mortality from hyponatremia has been documented in infants fed dilute formula, children forced to drink excessive amounts of fluid as punishment, athletes who drink excessively during exercise, compulsive water drinkers (psychogenic polydipsia), and hospitalized patients receiving excessive amounts of intravenous fluids. Poor clinical outcomes and delayed recovery have been documented in patients with hyponatremia, compared to those who maintain normonatremia. Thus, the secret stories of hyponatremia often whisper tales of harm (overzealous fluid ingestion or administration) with the intention of good (prevent dehydration) except in the severely deranged (child abuse) or demented (schizophrenic psychogenic polydipsia).

ABBREVIATIONS: [Na⁺] – sodium concentration, ADH - anti-diuretic hormone, AVP – arginine vasopressin, EAF - exercise associated hyponatremia, ECF – extracellular fluid, RAAS - renin-angiotensin-aldosterone system, SIADH - syndrome of anti-diuretic hormone secretion