Hyponatraemia in acute brain disease.

Hyponatraemia (HN) can result from a wide range of mechanisms, and therapy must be individualized. Two theories of the origin of HN in acute brain disease have prevailed. The first is the cerebral salt wasting syndrome (CSWS), where excessive natriuresis caused by some unknown cerebral natriuretic factor lowers the total sodium pool of the body and hence the plasma concentration. The second theory is the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), where an increase in total body water is caused by unphysiological secretion of ADH, lowering the concentration of sodium in the plasma. A third possibility is 'sodium shift', i.e. a displacement of sodium from the extracellular to the intracellular space with a simultaneous movement of potassium in the opposite direction. The morbidity and mortality associated with HN only arise in cases where the rate of development of HN was 0.5 mmol h-1 or more. Symptoms respond promptly when the HN is quickly corrected with furosemide and 3% sodium chloride.