Hyponatremia

Hyponatremia is a condition defined by a serum sodium concentration below 135 mmol/L. It reflects an excess of total body water relative to sodium and can arise from impaired water excretion, excessive water intake, or a combination of both. Clinical presentation ranges from mild symptoms such as nausea and malaise to severe neurological dysfunction including confusion, seizures, and coma. Acute hyponatremia develops within 48 hours and tends to be more dangerous than chronic hyponatremia, which evolves over days to weeks.

Classification commonly relies on volume status: hypovolemic hyponatremia results from simultaneous loss of sodium and water (e.g., diuretics, adrenal insufficiency); euvolemic hyponatremia features normal fluid status and is often due to SIADH, hypothyroidism, glucocorticoid deficiency, or primary polydipsia; hypervolemic hyponatremia occurs with fluid overload in conditions such as heart failure, cirrhosis, or nephrotic syndrome. Other related concepts include hypertonic hyponatremia, caused by high osmolality from hyperglycemia, and isotonic hyponatremia (pseudohyponatremia from abnormal lipids or proteins).

Diagnosis involves measuring serum osmolality, assessing volume status, and testing urine osmolality and urine sodium to distinguish causes. Treatment depends on severity, chronicity, and underlying etiology. Severe or life-threatening hyponatremia with neurological symptoms is treated with hypertonic saline to carefully raise serum sodium. For hypovolemic hyponatremia, isotonic saline can restore volume and sodium. In euvolemic hyponatremia (e.g., SIADH), fluid restriction is first-line, with additional therapies such as vasopressin antagonists in selected cases. Hypervolemic hyponatremia focuses on addressing the underlying condition and judicious fluid management.

Throughout, avoid rapid correction to minimize the risk of osmotic demyelination syndrome. Prognosis varies with cause, severity, and the rapidity of onset and treatment.