The Marathon Runner's Sodium Balance
A 42-year-old woman runs a 4-hour marathon on a cool day, drinking 4 L of plain water at aid stations. At the finish she is confused and nauseated. Her plasma sodium is 126 mmol/L (normal 135-145), plasma osmolality is 268 mOsm/kg (normal 275-295), and urine osmolality is 480 mOsm/kg with urine sodium of 35 mmol/L. She has gained 1.5 kg since the start. She is not thirsty. A teammate who finished earlier with similar weight gain but plasma sodium of 139 mmol/L is alert and oriented. Both drank the same volume.
1. Explain why this runner developed hyponatremia while her teammate with similar fluid intake did not. Which measurements support your explanation?
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The key is dilution exceeding excretion, not fluid volume alone.
- She gained 1.5 kg ≈ 1.5 L of retained fluid, so intake exceeded losses.
- Plasma osmolality (268) is below normal, confirming hypo-osmolar hyponatremia - a true dilutional state.
- Urine osmolality of 480 mOsm/kg is higher than plasma (268). If she were appropriately suppressing ADH, urine would be maximally dilute (≈50-100 mOsm/kg). A concentrated urine during hypo-osmolality means ADH is inappropriately high, so free water is reabsorbed rather than excreted.
- Urine sodium of 35 mmol/L shows she is still excreting sodium, which further lowers plasma sodium.
The teammate's plasma sodium of 139 with the same intake indicates that person excreted the excess water (suppressed ADH, dilute urine). The difference is renal free-water clearance, not the drinking volume. Possible drivers of the high ADH: nausea, pain, stress, or exercise-associated non-osmotic ADH release.
2. Her serum sodium is 126 mmol/L and she is symptomatic. Should she be given hypertonic saline rapidly to correct to 140 mmol/L within 2 hours? Justify your answer with a target rate and a risk.
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No - rapid full correction is dangerous.
- Symptomatic hyponatremia may warrant hypertonic (3%) saline, but the goal is to relieve symptoms, not normalize the number.
- Standard safe limit: raise plasma sodium by no more than ~8-10 mmol/L in 24 hours (often ~4-6 mmol/L in the first hours if severely symptomatic).
- Correcting from 126 to 140 (14 mmol/L) in 2 hours vastly exceeds this and risks osmotic demyelination syndrome (central pontine myelinolysis), which can cause irreversible neurological damage.
- A reasonable plan: small bolus of 3% saline to stop symptoms, then reassess; aim for a modest rise, then allow free-water diuresis once ADH falls.
So the judgement is: treat the symptoms with hypertonic saline if needed, but cap the rate. The number 140 is not the target; safety of the rate is.