Description

Hyponatraemia is a common metabolic abnormality, particularly in hospitalised patients. Although identified as a low plasma sodium concentration, because of the interrelationship of sodium and water homeostasis, altered sodium levels are more often a reflection of change in water balance than sodium balance. Maintenance of normal balance requires a complex interplay of humoral factors, renal function and circulating blood volume. Age related changes in physiology, such as declining renal function and loss of tubular concentrating ability, together with the presence of other pathologies and multiple drug use, render the elderly particularly susceptible to disturbances in sodium and water homeostasis, in particular hyponatraemia. Correct management of hyponatraemia requires an understanding of the mechanisms involved in normal homeostasis, appropriate investigation to determine any underlying disturbance in such mechanisms and appropriate management. This review considers the mechanisms by which water and sodium homeostasis are normally maintained, how disorder of such mechanisms can result in hyponatraemia with particular reference to the elderly, and discusses an appropriate approach to clinical investigation and treatment.