Collapsed Ultramarathon Runner with Hyponatremia
A 42-year-old male is brought to the medical tent at mile 80 of a 100-mile ultramarathon on a hot, humid day (WBGT 28°C). He is confused, agitated, and has vomited twice. His crew reports he has been drinking large volumes of water at every aid station and has not urinated in 6 hours. Vital signs: HR 118, BP 102/68, RR 22, T 38.1°C, SpO2 98%. He is diaphoretic and his skin is cool and clammy. Point-of-care glucose is 110 mg/dL. He is able to answer simple questions but is oriented only to person. There is no history of head trauma. The race medical director must decide on immediate management and disposition.
1. What is the most likely diagnosis, and what is the immediate management priority?
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The most likely diagnosis is exercise-associated hyponatremia (EAH), likely with encephalopathy. The clues: excessive water intake, no urination for 6 hours (suggests ADH-mediated water retention), confusion, vomiting, and a core temperature of 38.1°C (not hyperthermic enough to explain encephalopathy). EAH is a dilutional hyponatremia caused by overdrinking and impaired free water excretion. Immediate management: assess for severe symptoms (confusion, vomiting, seizures, coma) which mandate hypertonic saline. Do not give hypotonic fluids. If point-of-care sodium is available and <135 mmol/L with severe symptoms, give 100 mL of 3% NaCl IV over 10 minutes; may repeat up to 3 times if no improvement. If sodium measurement is unavailable, treat empirically based on clinical picture. Also check for hypoglycemia (already normal) and consider other causes of altered mental status. Do not use diuretics. Avoid rapid correction beyond 10-12 mmol/L per 24 hours once symptoms improve. Transport to hospital for monitoring.
2. What are the key risk factors and prevention strategies for this condition?
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Risk factors: excessive fluid intake (drinking beyond thirst), prolonged exercise (>4 hours), low body weight, female sex, slow running pace, use of NSAIDs, and hot weather. Prevention: drink to thirst, avoid overhydration, use weight-based fluid replacement (e.g., 400-800 mL/hr depending on sweat rate), include sodium in fluids (though evidence for sodium supplementation is weak), and educate athletes on the dangers of overdrinking. Race organizers should provide educational materials and have sodium measurement available.