[Hyponatremia in clinical practice]

Hyponatremia is defined as serum sodium level below 135 mEq/l; this electrolyte disorder can be associated with low, normal or high plasma tonicity. Hyponatremia with normal plasma osmolality, pseudohyponatremia, has little clinical value. Hyponatremia with increased plasma osmolality results from hyperglycemia or mannitol infusion. Patients with hyposmotic hyponatremia may be normovolemic, hypovolemic or hypervolemic; it is most important to know clinical history, physical examination that focuses on volume assessment and laboratory evaluation that includes urine osmolality and urine sodium concentration. Severe hyponatremia is associated with neurological complications and occasionally with mortality; for mild hyponatremia water restriction is usually sufficient, but in serious cases hypertonic saline infusion should be administered. Rapid correction of severe hyponatremia can cause brain demyelination; to prevent brain damage the rate of correction should be no more than 0.5 mEq/l/h (10-15 mEq/l/24 h).