Hyponatremia: a real threat or the worst-case scenario?

Hyponatremia has been recognized as a common and potentially serious complication of ultra-endurance exercise. The incidence of exercise-associated hyponatremia has been reported to be as high as 29% and can be either symptomatic or asymptomatic.The term hyponatremia is defined as a serum sodium level below 135 mEq/L. However, it is also a term used clinically to refer to the syndrome that can occur at serum sodium levels less than 130 mEq/L. The subsequent disruption of osmotic balance between extracellular and intracellular space can promote movement of fluid into cells with subsequent cell swelling and signs from the central nervous system, the lungs and the muscles. Symptoms include confusion and disorientation, nausea and vomiting, pulmonary edema and cardiorespiratory arrest and even comma and death. There is no consensus in the literature regarding the case of exercise induced hyponatremia. Proposed mechanisms include excessive sodium loss from sweating, ingestion of low sodium fluids and solid foods during exercise, overload of the extracellular fluids by excessive water intake, impairment of renal function, loss of salt from the intravascular water by movement into a third space or the use of medications such as non-steroidal anti-inflammatory drugs. While most of these theories remain controversial or poorly supported by experimental data, the most compelling explanation for exercise induced hyponatremia is the fluid overload hypothesis. In a recent study of our lab in runners of The Spartathlon, a 246 km continuous running race, severely hyponatremic runners (plasma sodium levels below 130 mEq/L, 23% of the study participants who finished the race) showed a dramatic expansion of plasma volume, accompanied by a decrease in plasma osmolality for the hyponatremic runners only. Additionally, most surveys on fluid consumption in such events have revealed larger fluid intakes during the race for the hyponatremic athletes, compared to normonatremic athletes. Despite the fact that many of the reported individual cases of hyponatremia have been associated with severe and life threatening symptoms, a relationship between these symptoms and the presence of hyponatremia during ultra-endurance exercise has not been firmly established. It has been estimated that among athletes who manage to finish ultra-endurance events with serum sodium levels below 130 mEq/L, only about 60% seek medical attention. On the other hand, the incidence of hyponatremia among marathon runners requiring medical treatment at the end of the race have been reported to be as little as 5.6%. In our study population of the Spartahlon runners, 5 out of 27 runners examined at 100 km of the race where found to be severely hyponatremic. Of these runners, three managed to complete the 246 km race. These observations may argue against a causal relationship between hyponatremia and life threatening complications observed immediately after the completion of an ultra-endurance race. It is also possible that ultra-endurance athletes may have developed a kind of resistance to hyponatremia, as a result of training or selection procedures. Nevertheless, avoiding the development of hyponatremia during an ultra-endurance event should be of high priority. Strategies should include optimum drinking schedules according to the individual sweating rates and the environmental conditions, as well as the consumption of adequate amounts of sodium before and during the race. Such a program will decrease the possibility of hyponatremia and will simultaneously eliminate the risk of dehydration and heat stroke.

Suggested Reading plan

[1]. Kavouras S.A., Anastasiou C.A., Skenderi K.P., Tsekouras Y., Echegaray M., and Matalas A.L. (2004). Fluid and Electrolyte Balance in Ultra-Endurance Running. Med Sci Sports Exerc, 36(5) Suppl.
[2]. Montain S.J., Sawka M.N., and Wenger Bruce. (2001). Hyponatremia Associated With Exercise: Risk Factors and Pathogenesis. Exerc Sports Sci Rev, 29(3), 113-117.
[3]. Speedy, D.B., Noakes T.D., and Schneider C. (2001). Exercise-associated hyponatremia: A review. Emerg Med, 13, 17-27.