The interview with the patient reveals that their total water intake since 6:00AM has been six liters. His plan was to start the day with “a full tank.” The patient ate the energy bars and the salt tablet and drank the water to keep their energy level high. He has no concept of the risk of over-hydration. He does have a history of “bonking” in previous multi-hour exercise sessions which he attributes to not eating and drinking properly.
After an hour of rest with only sips of water the patient urinates copious clear urine and states they can walk. The patient has not eaten due to nausea. They are A+0x4 and state that they no longer feel weird or shaky. They think they must have “hit the wall.” The patient reports a dull headache. The dizziness has resolved. You begin to walk to the trail head. The day remains hot and you both are now very stiff from the hour of inactivity.
The walk to the trailhead is uneventful. The patient drinks a half-liter of water enroute, urinates copiously once more, and eats another energy bar. He arrives at the trailhead feeling tired and sore--in his words, “wasted.”
We don’t know exactly what was going on here. The patient never saw a physician. He convinced himself he had a touch of the flu. Our suspicion is over-hydration (hyponatremia), the subject of this discussion.
As a reminder, the primary risk factor for exertional hyponatremia (a relative low blood sodium due to dilution) is excess fluid intake. A common scenario is intake greater than 1-1.5 liters an hour for four hours. There may be influence from inappropriate anti-diuretic hormone levels causing fluid retention. The concept of a “salty sweater” causing hyponatremia is probably a myth. NSAID (ibuprofen) use impacts the kidneys and is a concern, but this risk remains unclear.
We prevent exertional hyponatremia with judicious hydration. The mantras of “hydrate or die” or “drink to stay ahead of thirst” may get you into trouble. Thirst is a good indicator of fluid need so “drink according to thirst.” Sodium supplements during exercise have not been shown to prevent exertional hyponatremia in the face of over-hydration. Experience is critical. We must understand our individual fluid needs when we exercise in different environmental conditions.
The field treatment begins with an accurate fluid intake history. If over-hydration is suspected, slow or stop fluid intake until symptoms resolve. Salty snacks are appropriate, but by themselves will not correct exertional hyponatremia. Patients with neurological signs and symptoms, e.g. altered mental status, should be promptly evacuated.