Hyponatraemia is the most common electrolyte abnormality in hospital inpatients, and is associated with complications such as seizures, increased mortality and prolonged hospitalisation. The risk of symptoms, complications and death increases with severity of hyponatraemia. A wide variety of illnesses and therapies can cause hyponatraemia, and the treatment of the underlying condition is always important. There are three main pathophysiological causes of hyponatraemia. Hypovolaemic hyponatraemia is characterised by clinical and biochemical evidence of dehydration and is best treated by intravenous sodium chloride solution. Hypervolaemic hyponatraemia presents with fluid overload, which usually requires diuretic therapy. Euvolaemic hyponatraemia, of which the syndrome of inappropriate antidiuretic hormone secretion is an example, is traditionally treated with fluid restriction, although the new vasopressin antagonists, the vaptans, show great potential for future therapy. An accurate diagnosis of the cause of hyponatraemia is necessary for appropriate treatment. Severe hyponatraemia is potentially life-threatening and, if cerebral irritation is present, intensive therapy with saline infusion may be necessary; because of the risk of central pontine myelinolysis, this should always be given under specialised, highly experienced direction. Plasma sodium should be checked every two hours, and the rate of elevation of plasma sodium concentration should not exceed 0.5 mmol/l/h.
