Hyponatremia refers to a lowering of the blood sodium content that can have severe health consequences. At its extremes, it can be life threatening. Hyponatremia that occurs during or after any exertional event is termed exercise-associated hyponatremia (EAH). The WMS EAH practice guideline was recently updated to provide the most current information for safe fluid hydration in effort to prevent, or when necessary, to medically manage individuals with mild to severe life-threatening symptoms due to hyponatremia. EAH can occur during either passive activity (e.g., rafting on the Colorado River) or endurance activities in the wilderness setting. Tragically, rare EAH fatalities can occur, which are preventable with effective public education that targets overconsumption of fluids during physical activity. This summary covers EAH risk factors, prevention, patient assessment and medical management in the outdoor setting and the acute care when arriving at a hospital.
The first reports of EAH occurred in the early 1980s in ultra distance endurance runners, triathletes, and hikers in activities lasting longer than 6 hours and typically in hot weather. These weather conditions resulted in many participants consuming large amounts of water or other sport hydration beverages to replace extensive loss of body fluids from sweating during the activity in an effort to prevent significant dehydration and heat-related illness. More recently, EAH has been reported to occur in both cool or hot weather conditions during a wide range of shorter duration sport and recreational activities, such as open water swimming, cycling, football and other team sports, yoga classes, canoeing, and mountain biking, to name a few.
The common factor to EAH development in all of these activities is an excessive amount of fluid consumed that exceeds the body’s ability to excrete it. The result is a low blood sodium concentration that can first be noticed up to 24 hours after stopping physical activity. In most of these cases, an excess in the hormone arginine vasopressin is also present that limits the ability of the body to excrete water. A much less common form of EAH is associated with excessive body water and blood sodium losses due to high sweat volume resulting in a depletion of blood sodium level as indicated by a body weight loss.
The major risk factors for developing EAH are listed in Table 1. The single most important risk factor is prolonged overconsumption of fluids (for example, water or sports drinks), an intake in fluid volumes greater than body water losses through sweat.
Table 1. Risk factors of the development of exercise-associated hyponatremia
Overdrinking water, sports drinks, and other hypotonic beverages
Weight gain during exercise
Exercise duration >4 hours
Event inexperience or inadequate training
Slow running or performance pace
High or low body mass index (indicator of obesity or underweight)
Readily available fluids during exercise events
Education of event participants, support crews, and medical personnel
There are many misconceptions regarding hydration needs during exercise that foster the belief that individuals should “drink as much fluid as possible.” This common, but dangerous recommendation for hydration during exercise has been made widely available to the public through television, radio, and Internet, or from product advertisements. Not surprisingly, the quality and details of information is very poor. Correct education on hydration guidelines and the prevention EAH is critical. Education programs should stress the following key points (Table 2) as an EAH prevention and management strategy.
Table 2. Prevention Strategies to Mitigate Exercise-Associated Hyponatremia
Participants should drink according to the dictates of thirst.
Endurance athletes should get an estimation of their individual fluid needs during prolonged exercise by assessing body weight losses per hour. These body weight losses generally represent fluid loss.
Reduce the availability of fluids among the event routes to prevent overhydration.
Sodium and/or salty snacks should be freely available for consumption along with the appropriate fluid intake, particularly in long hot events, where persons not used to heat and humidity (non-heat acclimatized) may be participating. NOTE: this strategy will not prevent EAH when combined with overdrinking
EAH education should be provided among event participants, event directors, support crews, park rangers, and first responders and Emergency Medical Service (EMS) transport personnel.
EMS providers should also have a basic understanding of EAH and how to recognize and manage mild to severe symptoms.
Conduct pre-site visits to the receiving medical facilities and emergency department triage staff to discuss EAH awareness, clinical management and collaboration with event medical staff.
Prehospital Assessment and Treatment
When mild to severe symptoms of EAH are a concern and when blood sodium concentration can be obtained and analyzed onsite, e.g., use of an i-STAT blood analyzer, the recognition (diagnosis) of EAH becomes straightforward (diagnosed when the sodium level is < 135 meq/L). However, rapid, on-site laboratory testing is rarely available, making the diagnosis of EAH challenging. Making a field diagnosis of EAH in such situations is dependent on obtaining an accurate history of total fluid intake. When a suspected EAH patient provides details about excessive fluid intake, EAH should strongly be considered as a possible diagnosis. Additionally, it is recommended to use all available clinical information during the assessment of the patient’s hydration status (history of fluid intake, food intake, presenting signs and symptoms, body weight, and urine output).
The medical treatment of a patient with EAH depends on correctly diagnosing the condition. EAH must be routinely considered as part of other possible conditions, most commonly moderate to severe heat illness. Moderate to severe heat illness has many overlapping signs and symptoms with EAH (see Table 3 and Table 4). As a result, EAH is all too frequently thought to be either heat exhaustion or exertional heat stroke in an individual presenting with collapse or exhaustion during or shortly after prolonged physical activity. Accurately determining EAH from heat exhaustion or exertional heat stroke is critical since incorrectly treating one as the other condition can make a patient worse. Beyond an accurate fluid intake history (and possibly urine output history), treatment options should be determined based on the presence of non-overlapping signs and symptoms. See Figure 1 for a formula (algorithm) management approach to make an accurate field diagnosis and treatment decision based on symptoms of either mild/severe EAH or moderate/severe heat illness.
Table 3. Signs and symptoms of mild exercise-associated hyponatremia
Weakness* malaise* and fatigue*
Bloated feeling and body weight gain
Nausea and vomiting*
Variable urine output *‡
*Overlapping signs and symptoms with heat exhaustion / dehydration
‡ Urine output is often unreliable for diagnostic purposes because of variable Arginine vasopressin effects. Thus, urine output in itself should not be relied upon to make a diagnosis of EAH v. Heat Exhaustion / Dehydration.
Table 4. Signs and symptoms of severe exercise-associated hyponatremia
Altered mental status (confusion / altered sensorium, ataxia, collapse)*
Dyspnea (non-cardiogenic pulmonary edema)
Frothy sputum (non-cardiogenic pulmonary edema)
*Overlapping signs and symptoms with heat stroke