Hyponatremia in an Ultramarathon Runner
A 42-year-old woman collapses after a 100-km ultramarathon in 14 h. Ambient temperature is 28°C with 70% humidity. She weighs 1.5 kg more than at the start and reports drinking 6 L of plain water during the race. She is confused, nauseated, and has a seizure at the finish line. Serum sodium is 124 mmol/L; plasma osmolality 258 mOsm/kg; urine osmolality 480 mOsm/kg; urine sodium 40 mmol/L. Blood glucose and renal function are normal; she has no edema. She took no NSAIDs and has no history of SIADH. The medical team debates fluid restriction, isotonic saline, or 3% hypertonic saline. Her estimated sweat sodium loss was not replaced. Her serum creatinine is 0.8 mg/dL and blood urea nitrogen is 12 mg/dL.
1. Explain the physiological mechanism that produced hyponatremia in this runner. Include water intake, ADH, and free water clearance.
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She drank 6 L of plain water, a large hypotonic load. Normally a fall in plasma osmolality suppresses ADH, allowing dilute urine and free water excretion. Here, exercise stress, nausea, and pain cause non-osmotic ADH secretion. ADH retains water in the collecting duct, so free water clearance falls. The retained water dilutes extracellular sodium; her 1.5 kg weight gain is consistent with water retention. Sweat sodium loss also contributes, but the main driver is excessive water intake plus impaired water excretion.
2. Why is urine osmolality 480 mOsm/kg when plasma osmolality is only 258 mOsm/kg? What does this indicate about ADH action?
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Plasma osmolality of 258 should suppress ADH and produce dilute urine well below 100 mOsm/kg. A urine osmolality of 480 means ADH is acting despite low plasma osmolality. This is inappropriate ADH secretion (non-osmotic), often seen in exercise-associated hyponatremia. The kidney is reabsorbing water, which worsens the dilutional hyponatremia and explains why urine sodium is 40 mmol/L (volume expansion promotes natriuresis).
3. Compare fluid restriction, isotonic saline, and 3% hypertonic saline for this patient. Which is most appropriate, and what correction limit must be respected?
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Fluid restriction is appropriate for mild asymptomatic hyponatremia but not for a seizure. Isotonic saline is relatively hypotonic compared with the concentrated urine and, with high ADH, may not correct sodium and can worsen it. 3% hypertonic saline is indicated for severe symptomatic hyponatremia with cerebral edema; it raises serum sodium by 4–6 mmol/L rapidly to stop seizures. Correction should then be limited to ≤8–10 mmol/L in 24 h to avoid osmotic demyelination.