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Exercise-Associated Hyponatremia in the Tropics
26 September 2011 (online)
I was interested to read the recent article of Lee and associates  suggesting that exercise-associated hyponatremia (EAH) could cause problems for the long-distance athlete, even if races were conducted in an environment as warm as Singapore. Their findings related to 42 and 84 km events, both of which were run under relatively unfavorable conditions (an average dry-bulb temperature of 29 ℃, with a relative humidity of 89% and a wind-speed of only 0.3 m/s). One of 4 659 completing marathoners and 7 of the 407 completing ultramarathoners reported to the medical tent with symptoms; the marathoner and 2 of the ultramarathoners had blood sodium levels <135 mE/L, and were thus diagnosed as cases of EAH. On the basis of this information, the authors reported (p. 299) “The incidence of EAH in the 84-km run (33.2 per 10 000) is>18-fold higher than the incidence of EAH in the 42-km runners (1.8 per 10 000).”.
No one would argue that 2 cases of EAH among 407 runners is statistically larger than 1 case among 4 659, but nevertheless I would question the wisdom of calculating incidences based on such limited numbers. Indeed, if one examines the data in more detail, a blood sodium of 134 mM/L was reported for the marathoner with presumed EAH. This is a very modest deviation from the normally anticipated range of 135–143 mM/L, and it would normally be classed as a biochemical rather than a clinical hyponatremia, without expectation of any related symptoms  . Moreover, we have no information as to whether the runner’s pre-race blood sodium levels were within the anticipated normal range . There is a statistical expectation that 1 in 40 individuals will fall outside of this range, so that given a field of 4 659 runners, as many as 116 competitors might begin their race with blood sodium readings <135 mM/L. To this uncertainty must be added the inevitable imprecision of blood sodium determinations   ; the standard deviation of duplicate measurements when using the I-STAT device is at least 1.5 mM/L  .
Recognizing these various issues, it is by no means certain that any of the 42 km runners developed EAH. In contrast, one of the two 84 km runners, who reported to the medical tent with symptoms of confusion and a blood sodium of 117 mM/L seems a clear-cut case of EAH; the other, with a final blood sodium of 131 mM/L and an 0.9 kg increase in body mass, but with no striking symptoms, is a less clearly proven diagnosis.
We are not told what advice the runners were given prior to their participation in this particular competition. Because of the warm conditions, they may perhaps have received little guidance. From the average changes in body mass observed over the event, we may infer that the majority of runners drank only a reasonable amount of fluid. Nevertheless a substantial minority (7–8%) gained some weight during both events (as judged from Fig. 3, a typical increase of 0.4–0.8 kg). Thus, there seems some merit to the authors’ main recommendation that the fluid intake of distance athletes should be regulated, even when they are competing under tropical conditions.
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