Hyponatraemia: an overview of frequency, clinical presentation and complications.

Hyponatraemia (defined as a serum sodium concentration <136 mmol/L) is the most frequently encountered electrolyte disturbance in clinical practice. It is classified according to volume status (hypovolaemia, hypervolaemia or euvolaemia), reflecting the relative proportions of water and sodium within the body. The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is the most common cause of euvolaemic hyponatraemia. Although hyponatraemia is associated with poor prognosis and increased length of hospital stay, it is often poorly managed and sometimes underdiagnosed and undertreated. This article provides an overview of the frequency, pathophysiology and complications associated with this common clinical condition.