It is unfortunate that hyponatremia recently made headlines for causing the death of a radio show contestant. In this case, a young woman forced herself to drink almost 2 gallons of water in just over 3 hours (2). As her body fluid was diluted, her plasma levels of sodium (Na+) became extremely low. A low level of plasma Na+ is termed “hyponatremia.” Although this woman died of water intoxication by fluid overload, the cause of hyponatremia can be multifaceted and circumstantial (12). It can be a medical emergency that leads to death.
In 2007, the American College of Sports Medicine (ACSM) released an updated position stand on “Exercise and Fluid Replacement” (18). This position stand, along with other studies, identifies what populations are at an increased risk for developing hyponatremia, the signs and symptoms of hyponatremia, and strategies for preventing the development of hyponatremia.
Hyponatremia can occur in any population and setting. It can occur in resting and exercising individuals. The most common cause of hyponatremia for individuals at rest is consuming a large amount of hypotonic fluid in a short amount of time. Circumstances leading to this situation are rare (e.g., participating in a contest or hazing) (2,15). Hyponatremia occurring in exercising individuals is called exertional hyponatremia (EH). EH occurs most often in settings where individuals drink ad libitum (i.e., at free will) more fluid than necessary. Although EH can occur in a variety of athletic settings, there seem to be common threads between reported cases. EH is more prevalent when exercise duration exceeds 4 hours (1,9,13) and when an individual is exercising at a low intensity and drinking frequently (3). EH occurs more often in women, perhaps because of their smaller body size, lower sweat rates, and more stringent adherence to drinking guidelines (1,3,5). Regardless, EH can occur in either gender. Reports of EH commonly come from those running in marathons (1,14), competing in Ironman distances (19), or hiking (6,7). Although reports of EH occurring in other settings is scarce or absent, it is important to remember that it can occur in any setting. This is especially pertinent for sports that practice or compete for several hours at a time or multiple times throughout the day.
Hyponatremia is medically defined as a plasma Na+ level <130 mmol·L−1, but symptoms may not be present until Na+ levels become <125 mmol·L−1 (3,12). The etiology underlying EH is still under much debate (17), but at least 2 situations contribute to the development of EH (Figure 1). When athletes drink fluid in excess (i.e., drink more fluid than lost in sweat), EH may occur when the body fluid becomes diluted (termed “water intoxication”). This is the most common cause of EH, but EH may also occur when an individual does not drink adequate amounts of fluid while exercising (i.e., becomes dehydrated) (11). Because Na+ is a component of sweat, prolonged exercise or concentrated sweat (especially Na+ concentrated sweat in non-acclimatized individuals) contributes to blood Na+ levels. Large losses of Na+ through sweat replaced with a hypotonic fluid (i.e., water) may increase the risk of EH (16).
Hyponatremia can only be diagnosed by measuring plasma Na+ levels, but this measurement may not be readily available. Likewise, athletic trainers or other medical personnel may not be present at every practice. Therefore, coaches and athletes should be aware of the signs and symptoms surrounding EH (i.e., know when to contact medical personnel). Signs and symptoms of EH include disorientation, altered mental status, coma, headache, vomiting, bloating, lethargy, and swelling of the extremities (4,8,13). Untreated EH may lead to cerebral edema, respiratory arrest and, eventually, death. These symptoms may develop during exercise or several hours after exercise is discontinued (13).
The following guidelines provide a plan to reduce the risk of serious EH. The volume of fluid ingestion should not exceed water loss through sweat (10,18). An athlete should not weigh more after practice than he/she did before practice. Replacing sweat losses during practice can be accomplished by knowing one's sweat rate (i.e., sweat loss per hour, Table 1). Sweat losses should be replaced but not exceeded. The rehydration fluid can be water or a sports drink; most sodium will be replaced during meals via solid foods. Individuals that are “salty sweaters” (i.e., not acclimatized to the heat), have a history of exertional heat cramps, and/or those that consume diets low in Na+ may want to consider the ingestion of extra salt. Research has shown that the ingestion of a electrolyte-containing sports drink during exercise attenuates the fall in Na+ during exercise (20), but others dispute this (21).
In conclusion, hyponatremia is a medical condition that occurs when Na+ levels decreases to <130 mmol·L−1. This can occur at rest or during exercise, the latter called EH. It usually occurs during long duration, low-intensity exercise but has the potential to occur in any setting. The cause of EH is usually the result of overdrinking but may occur in dehydrated individuals. The appearance of the signs and symptoms of EH (i.e., disorientation, altered mental status, coma, headache, vomiting, bloating, lethargy and swelling of the extremities) (4,8,13) should prompt immediate medical attention. Reducing the risk of EH includes rehydrating with Na+ containing fluids but, more importantly, the avoidance of overdrinking should be emphasized. Due to the life-threatening potential of EH, the low prevalence of EH should not undermine its seriousness.□
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