Collapse at the finish line: exercise-associated hyponatraemia
During a 5-hour trail race at 28 °C, a 62 kg runner drinks 6.0 L of plain water and eats only two salty crackers. Her sweat rate is estimated at 0.8 L/h. She finishes with a headache, nausea and confusion. In the medical tent her body mass is 2.2 kg higher than at the start. Plasma sodium is 126 mmol/L (reference 135–145), plasma osmolality 262 mOsm/kg (reference 275–295), urine osmolality 480 mOsm/kg and urine sodium 38 mmol/L. Blood glucose and kidney function are normal; she takes no diuretics and has no history of kidney or adrenal disease. While waiting 40 minutes for the doctor she is given another 500 mL of water, and her headache worsens. The doctor considers exercise-associated hyponatraemia and asks whether this is simply sodium loss from sweating.
1. Explain why plasma sodium fell, using water balance and the urine data. Why is the urine osmolality of 480 mOsm/kg important?
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Water balance
Her mass rose by 2.2 kg, so she retained about 2.2 L of water. She drank 6.0 L and, at an estimated sweat rate of 0.8 L/h over 5 h, lost about 4.0 L, plus a small amount of sodium in sweat. The net effect is dilution of body sodium: sodium content changed relatively little, but the volume in which it is dissolved increased.
Why urine osmolality matters
Urine osmolality of 480 mOsm/kg is much higher than plasma osmolality of 262 mOsm/kg. That means ADH is acting on the collecting ducts and the kidneys are reabsorbing water rather than excreting free water. In dilutional hyponatraemia with normal kidneys, ADH should be suppressed and urine should be dilute, below about 100 mOsm/kg. The concentrated urine shows that water excretion is inappropriately impaired. Nausea, stress and exercise can stimulate ADH. Normal glucose and kidney function make other causes less likely.
2. Explain why giving more water worsened her symptoms and outline the immediate management priorities.
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Why more water worsened symptoms
Adding 500 mL of water further lowered plasma sodium and plasma osmolality. Water moves into brain cells by osmosis, causing cerebral oedema, which explains the worsening headache and confusion. Seizures or reduced consciousness would make this a medical emergency.
Management priorities
- Stop all oral fluid intake.
- Assess airway, breathing, circulation and neurological status.
- Measure plasma sodium urgently and repeat it.
- If there are severe symptoms such as confusion, seizures or coma, give hypertonic 3% saline to raise plasma sodium by a few mmol/L, with close monitoring.
- If symptoms are mild, restrict fluid and observe.
- Correct sodium slowly, generally by no more than 8–10 mmol/L in 24 h, to avoid osmotic demyelination.
More hypotonic fluid is contraindicated.
3. A teammate says she should take salt tablets and drink as much as possible. Evaluate this advice.
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The advice combines a weak measure with a dangerous one. Salt tablets may replace some sodium lost in sweat, but they do not prevent dilutional hyponatraemia if fluid intake is greater than sweat loss, and they may increase thirst and therefore water intake. The evidence that salt tablets prevent exercise-associated hyponatraemia is limited.
The main problem in this runner is overdrinking: 6.0 L in 5 hours is more than her sweat loss, as shown by the 2.2 kg weight gain. The safer advice is to drink to thirst, avoid drinking more than about 500–800 mL/h unless sweat rates are very high, and use body-mass change to check fluid balance. Salt tablets are not a substitute for reducing fluid intake, and in a symptomatic runner the priority is to stop fluids and seek medical treatment.