Abstract
Normal plasma sodium (Na*) concentration ranges from 135 to 145 mEq/L. Exertional hyponatremia (EH) involves a decline of plasma Na* to less than or equal to 130 mEq/L during exercise that lasts longer than 4 hours (eg, hiking, marathon running, ultraendurance triathlons) [1,2]. In severe cases, the physiologic state of EH may progress to symptomatic hyponatremic illness (HI)'plasma Na* ≤ 120 mEq/L'that requires emergency medical treatment. The signs and symptoms of HI may include encephalopathy, nausea, vomiting, confusion, combative behavior, weakness, headache, peripheral edema (eg, watch, rings, or shoes may be tight), neck vein distention, puffy facial appearance, seizures, coma, and respiratory arrest. The incidence of HI in marathons and ultramarathons ranges from 0 to 2 per 1000 competitors [1]; HI has not been reported in shorter events (< 42.2 km footraces).
Two exercise-related forms of acute hypotonic hyponatremia have been observed among endurance athletes [1,3]. Hypervolemic hyponatremia involves fluid overload with a relatively small Na* loss; this expands the extracellular volume and results in edema. Hypovolemic hyponatremia involves a larger Na* loss with dehydration (no water excess and decreased body weight) and shrinkage of the extracellular volume, resulting from deficits of both total body Na* and water; edema is absent. Although both conditions involve reduced extracellular Na* concentration and expanded intracellular volume, all previously published cases of HI with encephalopathy (ie, brain dysfunction involving an altered mental state) have involved hypervolemic hyponatremia.