What is the primary treatment for severe hyponatremia in a marathon runner with a sodium level of 120 mmol/L and high urine sodium?

Exercise-Associated Hyponatremia in Marathon Runners

Primary Treatment Recommendation

For a collapsed marathon runner with severe hyponatremia (Na 120 mmol/L) and high urine sodium, immediately administer 3% hypertonic saline as bolus therapy—specifically 100 mL boluses given over 10 minutes, repeatable up to three times at 10-minute intervals until symptoms resolve. 1 This represents exercise-associated hyponatremia (EAH), a dilutional hyponatremia caused by excessive fluid consumption relative to sodium stores during prolonged exercise. 1

Understanding the Clinical Scenario

Pathophysiology

• EAH is a dilutional hyponatremia resulting from increased total body water relative to exchangeable sodium stores, not primarily from sodium loss. 1
• The high urine sodium (typically >20 mmol/L) reflects physiologic natriuresis in response to volume expansion from excessive fluid intake during the race. 1
• This is euvolemic to hypervolemic hyponatremia, not hypovolemic, despite the runner appearing dehydrated from exertion. 1

Risk Factors Present

• Marathon running time typically >4 hours 1
• Excessive fluid consumption during the race 1
• Altered renal function from prolonged exercise 1

Immediate Management Algorithm

Step 1: Assess Symptom Severity

• Severe symptoms (confusion, seizures, coma, altered mental status requiring collapse): Requires immediate hypertonic saline 1, 2
• Mild symptoms (nausea, bloating, headache) with normal mental status: Can use oral hypertonic solutions 1

Step 2: Hypertonic Saline Administration for Severe Cases

• Administer 100 mL boluses of 3% NaCl over 10 minutes 1
• Repeat up to three times at 10-minute intervals until severe symptoms resolve 1
• Target correction: 4-6 mEq/L increase within the first 6 hours 1, 2
• Maximum correction limit: 8 mEq/L in 24 hours to prevent osmotic demyelination syndrome 1, 2, 3

Step 3: Fluid Restriction

• Implement strict fluid restriction during transport or initial treatment 1
• Do NOT administer isotonic (0.9%) saline as it may worsen hyponatremia in EAH 4
• Avoid hypotonic fluids entirely 1

Step 4: Monitoring

• Check serum sodium every 2 hours during initial correction phase 2
• Monitor for symptom resolution 1
• Watch for signs of overcorrection 2, 3

Alternative Treatment for Mild Cases

For runners with mild symptoms and ability to drink: Oral hypertonic solutions can achieve recovery within 30 minutes. 1 In one study, 16 runners with EAH recovered within 30 minutes using concentrated oral hypertonic solutions. 1

Critical Pitfalls to Avoid

Do NOT Use Normal Saline

• Isotonic (0.9%) saline is contraindicated in EAH as it may fail to raise or even lower serum sodium. 4
• A case series demonstrated that despite administering 0.9% saline, some marathon runners showed falls in serum sodium concentrations. 4
• The high urine sodium indicates this is dilutional hyponatremia, not volume depletion requiring isotonic fluids. 1

Avoid Overcorrection

• Never exceed 8 mmol/L correction in 24 hours (or 12 mEq/L in susceptible patients). 2, 3
• Overcorrection can cause osmotic demyelination syndrome with dysarthria, dysphagia, quadriparesis, seizures, coma, or death. 2, 3
• If overcorrection occurs, consider desmopressin or free water to relower sodium. 2, 5

Recognize Delayed Presentation

• EAH presentation may be delayed—some runners finish with normal mental status then become confused later. 4
• Symptoms can develop up to 24 hours post-race. 1
• There is no correlation between running time and sodium level severity. 4

Why This Differs from Other Hyponatremia

Not SIADH

• SIADH requires euvolemia with no recent fluid overload 2
• EAH involves acute water intoxication from excessive drinking during exercise 1

Not Cerebral Salt Wasting

• CSW shows true volume depletion with hypotension and tachycardia 2
• EAH runners are volume expanded from excessive fluid intake 1

Not Hypovolemic Hyponatremia

• Despite appearing dehydrated from exertion, these patients have excess total body water 1
• High urine sodium confirms this is not volume depletion 1

Mortality and Morbidity Context

EAH has caused six deaths in the USA and UK, making rapid recognition and appropriate treatment critical. 1 The incidence among marathon runners ranges from 3-22%. 1 Severe hyponatremia with altered mental status can progress to cerebral edema, seizures, coma, and death if untreated. 1, 6

The key to preventing mortality is avoiding isotonic saline and using hypertonic saline promptly for severe symptoms while respecting correction rate limits. 1, 2