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EAH usually presents in patients during or within hours of completing an endurance event (marathon) who have ingested excessive amounts of free water. Symptoms range from weakness and nausea to coma, seizures, and death.


  1. Defined as hyponatremia occurring within 24 hours of physical activity.

  2. Typically follows prolonged workouts of any kind, including half marathons, marathons, ultramarathons, sprint and full ironman events, football players, endurance cycling, and swimming events.

  3. The incidence varies widely. The overall rate of hyponatremia has been reported at 6% of participants, with symptomatic hyponatremia occurring in 1%.

  4. Secondary to a combination of both excessive fluid intake combined in some patients with inappropriate ADH release

    1. The leading risk factor is sustained excessive intake of hypotonic fluid in excess of fluid losses as manifested by weight gain during the event. Hyponatremia developed in 17% of runners who gained > 2 kg during the race, compared with < 2% of runners who gained < 2 kg.

      1. Other risk factors include long exercise duration and slow running pace.

      2. Ingestion of excessive water or carbohydrate sports drinks can both produce EAH. (Carbohydrate sports drinks are still markedly hypotonic compared with plasma.)

    2. Hyponatremia should suppress ADH. The finding that 44% of runners with EAH did not have maximally dilute urine suggests that SIADH contributes to hyponatremia in some patients.

  5. The key to understanding EAH is that it develops rapidly unlike most other causes of hyponatremia.

    1. The rapid development causes more severe symptoms at lesser degrees of hyponatremia. Falls in serum sodium of 7–10% can produce symptoms resulting in symptomatic hyponatremia even in patients with sodium levels of 125–130 mEq/L.

    2. The rapid onset of hyponatremia renders the plasma hypotonic relative to the brain, (which still has normal osmolality), leading to an osmotic influx of water into the brain and cerebral edema.

  6. Hyponatremia and cerebral edema cause neurologic symptoms, including confusion, headaches, vomiting, seizures, coma, herniation, and death. Symptoms may not occur immediately but develop over the first 24 hours.

  7. Noncardiogenic pulmonary edema can occur in patients with EAH.


  1. Prevention

    1. Athletes should be advised to weigh themselves before and after exercise and counseled to avoid excessive weight gain (> 2 kg).

    2. Thirst should be used as a guide to drinking during marathon events rather than fixed, regular, fluid intake.

    3. Sporadic weight checks during endurance events could also detect athletes with significant weight gain at risk for EAH.

  2. Treatment

    1. Individuals who collapse or have neurologic symptoms during or following endurance events should be immediately evaluated for EAH (as well as hypernatremia, hyperthermia, hypoglycemia, and myocardial infarction).

    2. It is critical to appreciate that the treatment of acute hyponatremia is different from that of chronic hyponatremia (Table 24-2). The hyponatremia in EAH develops rapidly and an aggressive approach to correction and treatment is safe and recommended. This contrasts with most hyponatremic patients who have chronic hyponatremia, in whom rapid ...

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