Exercise-Associated Hyponatremia: Serum Sodium, Symptomatology, Severity, and Sport Specificity

Exercise-associated hyponatremia (EAH) is an important cause of preventable morbidity and mortality. EAH refers to a low blood sodium concentration [Na⁺] of <135 mmol·L^-1, during or within 24 h of sustained endurance exercise. The current EAH literature contains ambiguities among field studies and unresolved clinical issues. Seeking clarity and resolution, we conducted manual searches of two large electronic databases using pre-defined inclusion criteria and discovered 1516 article titles and abstracts. Subsequent reviews of 345 full-length articles identified 56 eligible field research studies that reported 220 EAH cases during seven outdoor endurance activities (5-29.5 h duration). Our evaluations of these EAH cases generated the following seven findings. First, a greater percentage, not the absolute number, of women experienced EAH than men. Second, event specificity may account for much of the wide range of EAH symptoms and signs (SAS) reported among different outdoor activities. Third, out of 220 reported cases of EAH, none were asymptomatic when [Na⁺] was <130 mmol·L^-1. Fourth, the absolute value of [Na⁺] is not a reliable predictive index of EAH clinical severity or presentation. Fifth, running or hiking resulted in far more EAH cases and published epidemiological studies than cycling, swimming, and triathlon events. Sixth, the most common mild EAH complaints were nausea, weakness or lethargy, dizziness, headache, and extremity swelling. Seventh, the most common SAS of moderate-to-severe EAH (ie, suggesting hyponatremic encephalopathy) included altered mental status, vomiting, seizure, agitation/restlessness, collapse, and loss of consciousness. In conclusion, these findings should inform pre-event medical planning, on-site medical staff briefings, as well as the diagnosis of EAH severity in field settings. We also propose that our inventory of position statements and consensus documents will meet the needs of athletes and coaches who seek dependable information regarding risk factors and prevention.