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To report on the incidence, identify the risk factors, and clarify the clinical manifestations of acute hyponatremia in marathon runners.An observational and retrospective case-controlled series.The medical care area of the 2000 Houston Marathon.Marathon finishers treated in medical area receiving intravenous fluids (N=55), including a more detailed analysis of 39 runners completing a retrospective questionnaire.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.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 dependant manner. Vomiting was observed more frequently in H than NH runners (p=.03).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.