Exercise-Associated Hyponatremia (EAH)
Exercise-associated hyponatremia (EAH) is a potentially life-threatening condition defined as a serum or plasma sodium concentration below 135 mmol/L that occurs during or up to 24 hours after prolonged physical activity, primarily caused by excessive fluid intake relative to sodium losses. 1
Pathophysiology
EAH develops through a dilutional mechanism rather than excessive sodium loss:
- The fundamental mechanism is an increase in total body water disproportionate to the amount of exchangeable sodium stores 1
- This occurs when fluid intake exceeds the body's capacity for fluid excretion during prolonged exercise 1
- Non-osmotic arginine vasopressin (AVP) secretion is the unifying pathogenic stimulus for abnormal renal water retention in acute symptomatic EAH 2
Risk Factors
Several factors increase the risk of developing EAH:
- Excessive fluid consumption beyond total body fluid losses 1
- Exercise duration >4 hours (longer race times) 1, 3
- Female sex (more common in women) 3, 1, 4
- Low body mass index 3, 1
- Inexperience and inadequate training 3
- Slower pace during endurance events 3
- High or low BMI 3
- Readily available fluids during events 3
- Altered renal function during prolonged exercise 1
- Weight gain during exercise 3, 5
- Non-steroidal anti-inflammatory drug use 5
Clinical Presentation
EAH presents with a progressive spectrum of symptoms:
- Early/mild symptoms: bloating, nausea, vomiting, headache 1
- Moderate symptoms: altered mental status 1
- Severe symptoms: hyponatremic encephalopathy, seizures, coma 1
- Fatal outcome: death due to cerebral edema if left untreated 1
The symptoms of EAH often overlap with heat exhaustion and exertional heat stroke, making diagnosis challenging without blood testing 6.
Incidence
- Varies widely from 3-22% in marathon runners in Europe and USA 1
- Higher prevalence in ultra-marathon running compared to marathon running 4
- Very frequent in swimming events, more frequent in running and triathlon, and rather rare in cycling 4
- Geographically variable: very common in the USA, less common in Europe, and rarely reported in Africa, Asia, and Oceania 4
Prevention
The most effective prevention strategies include:
- Drink according to thirst - this is the safest and most individualized hydration strategy 3, 2
- Avoid excessive fluid intake beyond thirst sensation 1
- Monitor body weight to avoid weight gain during events 1, 5
- For events lasting several hours, consider specific hydration plans developed with a sports dietitian 3
- When consuming fluids during exercise, aim for 20-30 mmol/L sodium and 2-5 mmol/L potassium content 3
- Consume fluids at a modest rate (0.4-0.8 L/h for most athletes) 3
- Heat acclimatization may help attenuate declines in serum sodium 5
Management
Treatment depends on symptom severity:
- Mild-to-moderate EAH (without encephalopathy): fluid restriction or oral hypertonic saline solution 2
- Severe EAH (with encephalopathy): urgent treatment with intravenous 100 mL boluses of 3% saline until resolution of encephalopathy symptoms 2
Special Considerations
- EAH is more common in women, possibly related to the Varon-Ayus syndrome (severe hyponatremia with lung and cerebral edema) 4
- Environmental conditions significantly impact risk - races in hot temperatures show higher prevalence 4
- A definitive diagnosis requires blood sodium measurement, but treatment decisions may need to be made based on clinical presentation when testing is unavailable 6