The Incidence, Risk Factors, and Clinical Manifestations of Hyponatremia in Marathon Runners

OBJECTIVE: To report on the incidence, identify the risk factors, and clarify the clinical manifestations of acute hyponatremia in marathon runners. DESIGN: An observational and retrospective case-controlled series. SETTING: The medical care area of the 2000 Houston Marathon. PATIENTS: Marathon finishers treated in medical area receiving intravenous fluids (N=55), including a more detailed analysis of 39 runners completing a retrospective questionnaire. MAIN OUTCOME MEASURES: Vital signs, serum electrolytes, and finish time were analyzed via ANOVA studies between all non-hyponatremic (NH: N=34)) and hyponatremic (H: N=21)) runners. Fluid intake, training variables, NSAID use, and Symptomatology were further analyzed to delineate all significant differences between groups. RESULTS: There were no significant differences in vital signs, training variables, or NSAID use between H and NH groups, although there was a trend towards the less experienced runners presenting with lower post-race sodium levels. H runners had lower potassium K (p=.04), chloride Cl (p<.001), and blood urea nitrogen BUN (p=.004) levels than NH runners. There was a significant inverse linear relationship between both finish time versus Na (r2 =.51) and total amount of fluid ingested versus Na (r2 =.39). The total cups of water (p=.004), electrolyte/carbohydrate solution (p=.005) and total amount of fluid ingested (p<.001) were significantly higher in H compared to NH runners and the degree of hyponatremia was related in a dose dependent manner. Vomiting was observed more frequently in H than NH runners (p=.03). CONCLUSION: 21 runners presented to the medical area of the Houston Marathon with hyponatremia (.31% of entrants). Excessive fluid consumption and longer finishing times were the primary risk factors for developing this condition. Vomiting was the only clinical sign differentiating hyponatremia from other conditions that induce exercise-associated collapse.