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Exercise-Associated Hyponatremia in the Tropics
26 September 2011 (online)
Response to Professor Shepard’s Letter to the Editor:
We appreciate the interest shown by Professor Shepard in our study , and we thank the Editor for the opportunity to respond to the comments made.
With the limited number of hyponatremic runners admitted to the medical tent, we acknowledged the drawback of using incidence rate (X cases per 10 000 participants) to estimate the prevalence of exercise associated hyponatremia in our study. Nevertheless, reporting our findings using incidence rate allowed direct comparisons to be made with previous studies  .
While we agree with Professor Shepard that an individual with blood sodium concentration of 134 mmol/L is usually asymptomatic and this value would normally be considered biochemical rather than physiological, exceptions have been reported . More importantly, regardless of if this value was “true”, this runner was symptomatic upon presentation at the onsite medial tent. In the absence of other biochemical derangements (hypoglycemia, hyperkalaemia, hyperthermia etc.), our assessment fulfilled the diagnostic criteria for exercise associated hyponatremia (blood sodium under the normal range of the lab/device performing the test). Notwithstanding the possibility of imprecision of the iSTAT device that would have lead to a misdiagnosis, the management of the hyponatremia depends more on clinical symptoms rather than the actual blood sodium concentration.
Because the contemporary, widely promulgated, message warns runners to stay “well hydrated”, we agree with Professor Shepard that some of the runners might have over drank prior to the event and commenced the race hyponatremic (<135 mmol/L). This assumption is reinforced by common observations of long queues for toilets at race starts and visits to the toilets during the first few kilometers of the race. We unfortunately did not obtain pre-race samples (a limitation that was highlighted in our paper) but would like to emphasise that participants were advised to stay well hydrated before and during the event. In addition, the risks of exercise associated hyponatremia were made known to the runners prior to the race. On this note, race organisers may further increase the awareness of this illness by restricting opportunities for copious fluid ingestion.
We are pleased to acknowledge that Professor Shepard concurred with the key message of our study about the importance of fluid regulation during distance running, even when competing under tropical conditions. These observations further demonstrate that an overzealous approach to prevent dehydration may lead to a rare, but life threatening illness associated with the opposite fluid balance extreme – water intoxication.
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