Exercise-associated hyponatremia is defined as a blood sodium concentration less than 135 mmol/liter (135-145 is considered normal). The sodium in a sports drink may delay the onset of hyponatremia, but drinking too many sports drinks can still result in hyponatremia.  This condition most commonly develops from drinking too much water or sports drink before, during, or after the marathon race. Drinking too much fluid dilutes the blood sodium concentration if the kidneys do not properly clear the extra body water and becomes dangerous if it causes the brain to swell. Significant brain swelling can lead to death.

Risk factors include

• Finish time > 4 hours
• High fluid intake defined as 2 full glasses at each water stop
• Poorly acclimatized to current temperature and/or humidity
• NSAID usage on race day (due to Increased affect of AVP on kidneys)
• Neutral or weight gain during an endurance event
• Female sex
• Short stature

Symptoms begin to appear when serum sodium levels fall below 135 mmol/L and water begins to flow into the body cells. Early symptoms of hyponatremia include weight gain, puffiness (e.g., swollen fingers, tight-fitting watch or rings), nausea, vomiting, progressively worsening headache, and a sense of “just not feeling right.” More serious symptoms include confusion, irritability, agitation, and seizures. Left untreated, hyponatremia can progress to serious brain and lung swelling, coma, and death.

The patient’s vitals signs are usually normal but may include a slightly elevated systolic blood pressure.  Treatment in the medical tent requires a blood draw to determine the serum sodium level.  This is measured by the I-Stat machine operated by the respiratory technicians (RT’s).

Sodium level: 130-134

• Elevate patient’s legs 12 inches above their core
• Give oral fluids with sodium or a salty snack
• Consider infusion of one liter of 0.9% normal saline if unable to tolerate oral fluids
• Getting the runner to urinate often corrects the sodium level
• Repeat sodium level after IVF and successful urination
• The runner must urinate prior to discharge from the medical tent

Sodium level: 125-130

• Elevated patient’s legs 12 inches above their core
• Infuse one liter of 0.9% normal saline
• Give oral fluids with sodium or a salty snack
• Recheck serum sodium level after IVF and successful urination
• Consider use of 3% hypertonic saline if altered mental status or respiratory status
• Consider transport to hospital for further management

Sodium Level: < 125

• Elevate patient’s legs 12 inches above their core
• Infuse 3% hypertonic saline 100 ml over 10 mins (Must be approved by the medical director and assistant medical director)
• Give supplemental oxygen via nasal canula
• Prepare for transport