Exercise-Associated Hyponatremia in a Marathon Runner
During a summer marathon with a wet-bulb temperature of 28°C, a 31-year-old runner drinks about 6 L of plain water over 4 hours and finishes in 4:10. She becomes confused and vomits. In the medical tent: BP 108/66, HR 112, temp 37.8°C, weight 58 kg (pre-race 57 kg). Serum Na 122 mmol/L, serum osmolality 258 mOsm/kg, urine osmolality 470 mOsm/kg, urine Na 48 mmol/L, glucose 92 mg/dL, BUN 8 mg/dL. She is not on diuretics, has no edema, and takes no NSAIDs. The clinician considers 3% saline, isotonic saline, or fluid restriction. A student argues that because she is volume-depleted from vomiting, isotonic saline is safest. The lab confirms the sodium value on a second sample. Her urine output is low.
1. Classify the hyponatremia from the serum and urine data and explain whether ADH action is appropriate. Why is the urine sodium not low despite vomiting?
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Serum osmolality 258 mOsm/kg is hypotonic (<275), so this is hypotonic hyponatremia. Urine osmolality 470 mOsm/kg is inappropriately high (>100), and urine Na 48 mmol/L is >30; together these indicate ADH-mediated water retention, not simply salt depletion. In true volume depletion, urine Na is usually <20 mmol/L unless vomiting, diuretics, or ADH are present. Here nausea, vomiting, pain, and exercise are nonosmotic ADH stimuli; excessive water intake plus impaired free-water excretion dilutes serum Na. ADH inserts aquaporin-2 in the collecting duct, increasing water reabsorption and concentrating urine. The high urine Na also reflects volume expansion and natriuresis. Thus ADH action is not appropriate for the low osmolality, but it is explainable by exercise-associated hyponatremia. She is not edematous, so heart failure, cirrhosis, and nephrotic syndrome are less likely.
2. Why can isotonic saline fail to correct and even worsen her sodium? Estimate her sodium deficit to a target of 130 mmol/L using TBW = 0.5 × weight (58 kg).
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TBW = 0.5 × 58 = 29 L. Deficit = (130 − 122) × 29 = 232 mmol Na. But the deficit formula assumes a stable distribution and is not a guide for acute symptomatic correction. Isotonic saline has osmolality about 308 mOsm/L. If ADH is high and urine osmolality is 470 mOsm/kg, the kidney can excrete the administered salt in a more concentrated urine while retaining free water; the net effect may be a fall in serum Na. This is why isotonic saline is unreliable in SIADH-like physiology. With confusion and vomiting, she has severe symptoms; guidelines favor a 3% saline bolus (usually 100–150 mL) to raise Na by about 2–3 mmol/L, then reassess, rather than fluid restriction alone. Correction should be limited to about 4–6 mmol/L in 24 hours.
3. If her Na rises from 122 to 132 mmol/L over 6 hours, what neurologic complication is she at risk for and why? What correction rate is generally safer?
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A 10 mmol/L rise in 6 hours is too fast. The brain adapts to chronic hyponatremia by extruding intracellular osmolytes; if extracellular Na is raised rapidly, water leaves brain cells and causes osmotic shrinkage. This can produce osmotic demyelination syndrome, with confusion, dysarthria, quadriparesis, or locked-in syndrome days later. General limits are ≤8 mmol/L in 24 hours, with 4–6 mmol/L in 24 hours often used for high-risk patients; acute symptomatic hyponatremia may be corrected faster only until symptoms improve, then slowed. The 3% saline bolus should be stopped once severe symptoms resolve or the Na has risen about 5 mmol/L, and the rate should be reassessed frequently.