Exercise-Associated Hyponatremia in the Tropics

Response to Professor Shepard’s Letter to the Editor:

We appreciate the interest shown by Professor Shepard in our study [1], 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 [2] [3].

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 [4]. 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.

• References

• 1 Lee JKW, Nio AQX, Ang WH, Johnson C, Aziz AR, Lim CL, Hew-Butler T. First reported cases of exercise-associated hyponatremia in Asia. Int J Sports Med 2011; 32: 297-302
• 2 Frizzell RT, Lang GH, Lowance DC, Lathan SR. Hyponatremia and ultramarathon running. JAMA 1986; 255: 772-774
• 3 Hew TD, Chorley JN, Cianca JC. Divine JG. The incidence, risk factors, and clinical manifestations of hyponatremia in marathon runners. Clin J Sport Med 2003; 13: 41-47
• 4 Hew-Butler T, Anley C, Schwartz P, Noakes T. The treatment of symptomatic hyponatremia with hypertonic saline in an Ironman triathlete. Clin J Sport Med 2007; 17: 68-69