Exercise-Associated Hyponatraemia

Exercise-associated hyponatraemia (EAH) is a blood sodium below 135 mmol/L developing during or up to 24 hours after prolonged physical activity, usually from drinking more fluid than the body can excrete.

It is a preventable cause of serious illness and death in marathons, ultramarathons, long-distance triathlons and military training, and it is often mistaken for dehydration. Because it is usually dilutional, giving more dilute fluid can be fatal. The task is to suspect it in the unwell or collapsed endurance athlete, measure sodium, treat severe cases urgently with hypertonic saline, and prevent it through sensible fluid advice.