Physical exertion and athletic competition performed in environmental extremes are challenged by the risk of premature fatigue and heat illnesses. Heat cramps, heat exhaustion, and heat stroke are recognized by the medical professions as the traditional infirmities associated with exercise in hot environments (American College of Sports Medicine, Casa et al). In recent years, exercise-associated hyponatremia has emerged as an additional concern when inappropriate fluid intake, physiological dysfunction, and/or excessive sweat loss combine during exercise to disrupt sodium balance and induce this state. Estimating the occurrence of hyponatremia in endurance events is challenging. For those who develop symptomatic hyponatremia, which will be explained in this review, occurrence is very low at between 0.1 and 4% (Montain et al 2001). However, nearly one-third of those who report to the medical tent for assistance after a race may have hyponatremia (Montain et al 2001).
This brief review will define hyponatremia, describe the physiological ramifications and clinical outcomes, identify and discuss potential causes of the problem, identify treatment and perhaps most importantly provide recommendations for preventing the development of the condition.
Definition and Physiology
Hyponatremia is defined as a blood sodium concentration that drops below 135 mmol/L (Montain et al). The normal range in blood is 135 to 142 mmol/L. It is possible for individuals at an endurance event to report to the medical tent with blood concentrations below 135 mmol/L and be asymptomatic. Symptomatic hyponatremia occurs when blood concentrations drop below normal, and the signs and effects develop, including swelling of fingers and limbs and edema, headaches, confusion, imbalance, convulsions or seizures, going into a coma, and even dying of respiratory distress.
This pathology, while represented by measurement in the blood, is a condition affecting all of the body fluid found outside of our cells (extracellular fluid space, or ECF). Sodium and chloride are the main electrolytes in the ECF and normally serve to hold water in this space and help maintain body water balance. In the case of hyponatremia, water is no longer restricted to ECF in the appropriate amount, and excessive water shifts into cells, including nerves in the brain. Swelling and nervous-system dysfunction follow and lead to the symptoms previously identified. The edema elsewhere in the body is also explained by the inability to maintain water volume in the ECF space as water shifts into cells in the limbs and hands.
Potential Causes and Contributing Factors
In theory, hyponatremia could develop three ways: by a significant loss of sodium, an excessive expansion of body water volume, or a combination of both events. Overconsumption of water that produces the simple dilution of sodium covers the water expansion possibility, but it is not complete. Normally the expanded blood volume and decrease in blood osmolality should stimulate the kidneys to excrete excess water as urine. It has been proposed that an inappropriate secretion of arginine vasopression (AVP), also known as antidiuretic hormone, explains the lack of urine production to eliminate the water. Elevated AVP prevents water excretion thereby promoting water retention and diluting blood sodium. Some speculate that this may be a consequence of the trauma and inflammation that may occur to the body during events such as the marathon that block the suppression of AVP production. However, cases studies and experimental studies show an inconsistent relationship between fluid overload and AVP levels. Others suggest this might actually be an appropriate AVP response – to begin conserving fluids during exercise that could lead to a fluid deficit – just not in the correct situation.
Risk factors that have been identified in association with hyponatremia include being female, having a small body mass, being over exuberant with drinking and consequently gaining weight during the race, being a slow runner and on the race course for over four hours, possibly losing excess sodium in the sweat and not replacing it during the race (Almond et al). The occurrence of hyponatremia is more common in females than in males regardless of its occurrence during exercise or non-exercise occasions. One could speculate that the reproductive hormones estrogen and progesterone are the culprits by promoting water retention. However, research fails to implicate either of these hormones as causative (Stachenfeld and Taylor). Possibly because females have a smaller mass and body water volume compared to men, it may be easier to dilute the extracellular space with water consumption, particularly when sweat rates are low. Slower runners may generate less heat, and sweat less. If unaware of drinking to match sweat loss (and no more), excessive drinking behavior in hopes of randomly maintaining hydration status puts them at risk.
Runners who appear to develop hyponatremia need first to be diagnosed with the measurement of blood sodium. Medical tents at race courses typically use the handheld i-Stat® (Abbott Diagnostics), which allows a finger stick for sampling and will take blood for a direct assessment in 1-to-2 minutes. Blood-gas analyzers commonly found in clinical chemistry units can also be used. Flame photometers are typically used in research but do not provide the real-time evaluation needed in critical-care situations.
Upon making a positive diagnosis, physicians may administer diuretics to reduce excess water and give hypersaline intravenously to elevate extracellular sodium. The patient needs to be kept under medical observation until blood sodium has returned to normal and symptoms subside.
Recommendations for Prevention
With many unknowns regarding the cause of hyponatremia and the outcome of death being devastating, it behooves participants in long duration events to establish strategies to eliminate the risk if and when fluid ingestion is expected if not required during the race. Determining one’s sweat rate during warm and cool conditions will help establish the amount of fluid needing to be replaced rather than leaving it to chance. Weighing one’s body mass before a training run and after having emptied the bladder and reweighing after the run before emptying the bladder will provide a gross estimate of sweat lost. Accounting for fluid intake will give the precise and accurate estimate of sweat loss. The majority of the mass lost during of the training run will be due to sweat loss and provide an estimate of the sweat rate when divided by the hours or decimal of hours for the training session. Knowing the sweat rate and fluid loss for several environmental conditions that a runner may face on the day of competition allows the runner to apply the fluid replacement plan least likely to result in water accumulation.
Some race directors have reduced the number of aid stations that provide fluid and the chance to fluid overload. This can be a challenge if race day weather conditions are extremely hot, and the volume of fluid available for all participants is inadequate. Other race directors have salt packets or tablets placed on the aid station tables for participants to replace sodium during the race. At this point, it is not clear whether either action has been effective in reducing the incidence of hyponatremia during races. The most important action is for each participant to estimate his or her individual fluid needs and not exceed them.
As participation continues to grow in endurance events, the probability is there will be an increase in the numbers of participants experiencing hyponatremia. Of particular concern are the charity runners who are less savvy about sweat rate, fluid requirements, and environmental effects on their biology. It is important for race directors, coaches, and participants to be aware of this risk, know the signs and symptoms, and most importantly, take precautions to reduce the risk of hyponatremia occurring.
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