Hiponatremia

Hyponatremia is a serum sodium concentration below 135 mmol/L. It is the most common electrolyte disorder encountered in clinical practice and can range from asymptomatic to life-threatening. Hyponatremia is usually defined in the setting of hypotonic hyponatremia, in which serum osmolality is reduced; other forms include hypertonic and isotonic hyponatremia.

Most hyponatremia is hypotonic and is classified by extracellular fluid volume status as hypovolemic, euvolemic, or hypervolemic. Common causes include hypovolemic: renal losses (diuretics) or extrarenal losses; euvolemic: SIADH, hypothyroidism, psychogenic polydipsia; hypervolemic: heart failure, cirrhosis, nephrotic syndrome. Other contributors include adrenal insufficiency, renal failure, low solute intake (beer potomania), excessive water intake, and certain medications. Pseudohyponatremia and hypertonic hyponatremia should be considered when lab results do not fit the clinical picture.

Diagnosis relies on serum osmolality, urine osmolality, and urine sodium, along with volume assessment and medication history. Pseudohyponatremia occurs with severe hyperlipidemia or hyperproteinemia; hypertonic hyponatremia results from hyperglycemia or mannitol. Correct classification informs treatment.

Management depends on acuity, symptoms, and cause. Severe symptoms (seizures, coma) or very low sodium require prompt hypertonic saline with neurologic monitoring. In many patients, fluid restriction and addressing the underlying cause are first steps. Hypovolemic hyponatremia is treated with isotonic saline; euvolemic/hypervolemic hyponatremia with fluid restriction and targeted therapy. Correct sodium gradually to avoid osmotic demyelination syndrome; typical targets limit correction to 6-8 mmol/L in 24 hours and 18 mmol/L in 48 hours. Vasopressin receptor antagonists are used selectively in some cases.

Prognosis depends on cause and duration; chronic hyponatremia can impair cognition and gait, while acute hyponatremia poses a higher risk of neurologic injury if not treated promptly.