Question

A marathon is a 26.2-mile race. People often train for months leading up to the race in order to successfully complete this type of long-distance running challenge. Sometimes, runners aren’t able to finish the race or runners' collapse. Your challenge is to use your findings from research articles and data sets in order to determine and explain what is causing some marathon runners to collapse.


These days, many amateur athletes participate in marathons. One such amateur athlete -- a 40-year-old woman running a marathon for the first time, runner #0358 -- got very sick and disoriented during the second half of a New York City marathon. She kept running the race, and she even took pictures with her family along the racecourse, but she later had no memory of running the second half of the race! Runner #0358 went home, then became very ill and disoriented, eventually ending up in a coma. While the runner did recover after a while, this event concerned the race organizers. The marathon was held on an unusually hot, humid, and sunny day. The race started at 10:00 AM, but some runners were on the course for 4 or 5 hours. During that time, the temperature got as high as 28◦ C (about 82◦ F), and the sun was out all day. Many runners ended up seeking medical care either at a stop along the racecourse, or at the end of the race.


What could have happened to her to make her so sick? Below is additional information about challenges that athletes may face with strenuous exercise in extreme conditions.


Athletes need more sodium because it is lost while sweating. The amount of sodium excreted in sweat is large and depends on many factors, such as body mass, training level, temperature, clothing, gender, and heat/humidity acclimatization. The level of sodium in your body can become too low or too high based on the amount of water in your body. If the amount of water you consume isn’t equal to the amount you lose, you can become dehydrated (too little water).


Hyponatremia occurs when sweating causes disproportionate sodium loss or an excessive intake of plain water, which dilutes the level of sodium in the blood. This is why it’s imperative to drink sports drinks containing electrolytes and not plain water for any physical activity over an hour.


If your blood sugar drops to a low enough level and you don't elevate it with some sort of fast-acting carbohydrate such as fruit juice or a sports drink, you may begin to experience symptoms of hypoglycemia. Hypoglycemia can cause symptoms such as headache, shakiness or trembling, excessive sweating, mental confusion and heart palpitations.


Exercise is one of the strong factors disturbing the maintenance of body temperature. Some conditions, such as heavy exercise in thermal extremes, could rapidly lead to dangerous internal temperatures. The thermal load posed by the environment or by heavy exercise may be too great for the capacity of the regulators. Or the regulator could be deranged due to extreme temperatures.

Based on this why might runner 0358 end up in a coma?

Answer 1

Short answer — the most likely cause is exercise‑associated hyponatremia (EAH) causing cerebral edema and acute encephalopathy (confusion → coma). Heat illness (exertional heat stroke), severe hypoglycemia, or dehydration/shock are other important possibilities and can overlap, but the facts given point most strongly to EAH.

Why EAH fits this case
- Risk factors present: first marathon, female, long time on course (4–5 hours), hot/humid day (increases sweat losses) — all are associated with EAH in marathon runners. Slower, inexperienced runners are a common group who develop EAH because they tend to consume large volumes of plain water over many hours.
- Typical clinical pattern: mental confusion, disorientation and amnesia during the race with later worsening after finishing is classic for hyponatremia. Symptoms can progress after the race as sodium becomes more diluted and cerebral edema worsens.
- Rapid recovery is also common when properly treated, which matches the eventual recovery described.

Physiology — how hyponatremia causes coma
- Sweat contains a lot of sodium. If an athlete replaces large sweat losses with mostly plain water (or very hypotonic fluids), the serum sodium concentration falls (dilutional hyponatremia).
- Low plasma sodium makes the extracellular fluid hypoosmolar relative to brain cells; water moves into brain cells, causing cerebral swelling (cerebral edema).
- Cerebral edema raises intracranial pressure and interferes with neuronal function → headache, nausea, vomiting, confusion, seizures and, in severe cases, coma and respiratory arrest.

How this differs from other conditions
- Exertional heat stroke causes core temperature typically >40 °C with CNS dysfunction. It often produces collapse on course, hot dry or very sweaty skin, and needs immediate rapid cooling. Heat stroke can coexist with or be confused with EAH.
- Hypoglycemia causes acute confusion and can lead to coma, but it is usually rapidly reversible with glucose and is more likely to occur in people who haven’t eaten or who are on glucose‑lowering drugs.
- Severe dehydration/shock will cause low blood pressure, dizziness and potential loss of consciousness but usually not the specific pattern of progressive confusion and delayed deterioration seen with EAH.

Key clinical implications (what to do)
- Suspect EAH in athletes (especially slower runners, women) who become confused/encephalopathic during or soon after prolonged exercise and who may have been drinking large volumes of water.
- On scene/hospital priorities differ:
- If core temperature is dangerously high → urgent rapid cooling (cold water immersion, ice packs) for exertional heat stroke.
- If severe neurologic symptoms suggest hyponatremia → check serum sodium immediately and treat severe symptomatic hyponatremia with controlled administration of hypertonic saline (3%) under medical supervision.
- If hypoglycemia → give fast-acting glucose (juice or IV dextrose).
- Never give large volumes of plain water to a confused, vomiting, or collapsed athlete; doing so can worsen hyponatremia.

Prevention
- Drink to thirst and use electrolyte-containing sports drinks during long events rather than large amounts of plain water.
- Acclimatize to heat, practice hydration plans during training, weigh before/after long runs to estimate fluid loss, and be aware that “more fluid” is not always safer.
- Race organizers should provide electrolyte options and medical screening/education, especially on hot days.

In summary: given her sex, long race time, hot/humid conditions, gradual confusion during the second half and delayed but severe deterioration, exercise-associated hyponatremia leading to cerebral edema is the most plausible explanation for runner #0358’s coma — though heat stroke or hypoglycemia should also be considered and treated appropriately.