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Z39.18 SUPPLEMENT — SODIUM BALANCE AND EXERCISE Exertional Heat Illness and Hyponatremia: An Epidemiological Prospective Robert Carter III Department of Human Factors, Research Center of the Army Health Service, La Tronche, France; Thermal and Mountain Medicine Division, United States Army Research Institute of Environmental Medicine, Natick, MA CARTER, R. Exertional heat illness and hyponatremia: an epidemiological prospective. Curr. Sports Med. Rep., Vol. 7, No. 4, pp. S20YS27, 2008. In active populations, heat illness remains a cause of exercise-related injury and death. There is evidence that hyponatremia also occurs, but less often than heat illness. Incidence rates of these conditions are determined by the population at risk and individual susceptibilities. Improved strategies are needed to identify high-risk individuals who are likely to develop either hyponatremia or heat illness. INTRODUCTION (1,2,5,7Y26) and the U.S. Army Research Institute of Environmental Medicine (USARIEM) Total Army Injury While performing demanding physical activity for long and Health Outcomes Database (TAIHOD). While it is durations, fluid and electrolyte imbalance is common in acknowledged that the populations at risk for HYPO and athletes, military personnel, and recreational hikers. The EHI may differ, reasonable comparisons are made by militaryand civiliancommunities have introducedextensive examining incidence rates to better understand relative heatmitigationmeasurestomanageheatstrainandreducethe magnitudeofeachcondition.Ithasbeenreportedthatthese risk of serious exertional heat illnesses (EHI). These heat two conditions have several overlapping clinical features, mitigationmeasuresincludefluidandelectrolytereplacement whichhasledtomisdiagnosisinsomerarecases.Thisarticle guidelines, vigilance, and identifying high-risk individuals. is not intended to persuade the reader of the relative Despite these measures, exercise in hot weather continues to importance of either condition. result in preventable injuries and deaths in young, healthy individuals. With existing emphasis on appropriate fluid intake during OBSERVATIONAL STUDIES OF WATER INTAKE exercise for the avoidance of dehydration, heat illness, and DURING EXERCISE associated performance decrements, there has been a subse- quent increase in reported exertional hyponatremia (HYPO) Many of the recent studies examining water intake during casesrelatedtoexcessivewaterintake,elevatedsweatingrates, exercise have been observational in nature, with much excessive sodium losses in sweat, and inadequate sodium attentionfocuseduponHYPO(2,3,6,27,28).Giventheacute intakeinsoldiers(1),athletes(2,3,4),andrecreationalhikers development of EHI and HYPO during exercise, observa- (5,6). The primary purpose of this article is to systemically tional field studies provide some unique aspects, since examine the epidemiological literature of fluid and electro- inducing these conditions in a laboratory setting is unethical lyte imbalances that occur during physical activity. The (29). One important advantage of observational studies to secondary purpose of this article is to examine signs and address questions related to water intake during exercise is symptoms of HYPO and EHI cases from the literature that they allow for the calculation of incidence and prevalenceratesaswellasinformationtogenerateadditional hypotheses regarding medically related issues, risk factors, Addressforcorrespondence:RobertCarterIII,Ph.D.,M.P.H.,FACSM,Department and behaviors of the individuals performing these activities. ofHumanFactors,CentredeRecherchesduServicedeSante´desArme´es,LaTronche, However, very few studies in the literature provide epide- France(E-mail:[email protected]). miological data on both HYPO and EHI incidence in the same setting. 1537-890X/0704/S20YS27 One disadvantage of observational studies is that they CurrentSportsMedicineReports Copyright*2008bytheAmericanCollegeofSportsMedicine are more susceptible to confounding and sampling bias. S20 Copyright @ 2008 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. Furthermore, it is difficult to establish causal links between system dysfunction, with high core temperatures usually but variables with the use of observation studies. The lack of not always greater than 40-C (104-F). Conversely, individ- controlinthistypeofstudydesignsuggeststhereisalwaysa uals with a rectal temperature greater than 40-C universally possibility that some unknown factor is exerting influence do not have a heat injury or heat stroke (29); the entire upon the outcome (29). clinicalpictureincludingmentalstatusandlaboratoryresults Case reports are not very helpful in estimating the extent must be considered together. Individuals with heat stroke of EHI and HYPO within a population. In fact, published have profound neuro-cognitive impairments that present case reports or media reporting of high-profile athletes or early and universally. In addition, heat stroke can be individuals who develop an illness (i.e., heat stroke) can complicated by liver damage, rhabdomyolysis, disseminated have a profound impact upon perceptions, medical advice, intravascular coagulation (DIC), water and electrolyte and future behaviors of individuals performing similar Bimbalances,^ and renal failure. athletic events or recreational activities. However, case reports are very reliable when characterizing clinical symp- Exercise-Induced Hyponatremia tomsandsignsandoftenprovidedetailedinformationthatis HYPO is defined as decreased blood sodium levels, either notavailableinlargeobservationalorcross-sectionalstudies. caused by overhydration, inadequate sodium intake, or Throughout this article, incidence rates are calculated or excessive losses in sweat (31); medical problems can result reported as time or person-time, whenever possible. Inci- in edema (cerebral or pulmonary) and death in rare cases. dence is the rate at which new events (i.e., EHI, HYPO, or Clinically significant HYPO is defined as blood sodium less exercise-induced dehydration) occur within a population at than 130 mmolILj1 (3). risk. The numerator is the number of new events (e.g., hyponatremia) arising in a specified time period. The denominator is the population at risk (e.g., marathon EPIDEMIOLOGY OF EXERTIONAL HEAT ILLNESSES participants) of an event during this time period. Incidence rates are sometimes expressed as person-time, but often MostoftheepidemiologicalstudiesofEHIintheliterature expressed as X cases per a given population base (e.g., per are from military populations (i.e., Army, Air Force, and 1000 or 100,000 participants). The use of person-time as Marines) and have focused on specific bases for relatively opposed to only ‘‘time’’ enables the investigator to handle brief periods and with relatively small populations (10,32). situations where there are multiple drop-outs in a study or Recently, Carter and colleagues documented 5246 EHI where researchers have not been able to follow an entire hospitalizations and 37 heat stroke deaths in the U.S. Army population at risk. Using person-time calculations, the from1980through2002(33).Theyshowedthatheatstroke follow-upperioddoesnothavetobethesameforallpersons hospitalization rates increased five-fold (1.8 per 100,000 studied. Person-time for a population is the sum of the times persons in 1980 to 14.5 per 100,000 persons in 2001). The of follow-up for each individual within a given population. reasons for this dramatic increase in heat stroke hospital- izations are unclear. Since 2003, recent deployments to hot regions of the world have resulted in significant increases in DEFINTIONS OF EHI AND HYPO both hospitalizations and outpatient care for EHI, and at leastsixheat-relateddeathshavebeenreported(unpublished Minor Heat Illnesses data from the Defense Medical Surveillance System). Minor heat-related illnesses include heat cramps and heat Previously, Gardner and colleagues reported that in the syncope. Heat cramps are associated with intense muscle U.S. military, 12% of exercise-related deaths are attributed spasms, typically of the leg, arm, and abdominal areas. Heat to EHI (32). Smalley and colleagues report that 51 cases cramps are believed to result from fluid and sodium deficits (1.3per1000persons)ofEHI occurred among basic trainees and occur mostly in persons with lack of heat acclimatiza- at Lackland Air Force Base (LAFB) in 1999. In addition, tion. Heat syncope results from excessive pooling of blood to they document seven heat stroke deaths among LAFB theskinandextremitiesandoccursmostlyindehydratedand trainees from 1956 to 1999 (34). Of the 217,000 Marine re- inactive persons with lack of heat acclimatization. cruits that trained from 1982 to 1991, 1454 individuals suffered EHI, which is believed to be one of the highest Serious Heat Illnesses incidence rates (67 per 1000 persons) among the military Serious heat illnesses represent a continuum on the services (35). severity scale and include heat exhaustion, heat injury, and heat stroke (30). These serious heat illnesses have many Exertional Heat Illness in Athletes overlapping diagnostic features. Heat exhaustion is a mild to Numerous studies have demonstrated that EHI is a major moderate illness characterized by an inability to sustain risk for competitive athletes exercising in hot weather cardiac output with moderate (>38.5-C, 101-F) to high conditions(36,37).However,moststudiesofEHIinathletes (G40-C, 104-F) rectal temperatures. It is frequently accom- have focused only on mortality and do not have sufficient paniedbyhotskinanddehydration.Heatinjuryisamoderate information to calculate incidence rates. Previously, Cooper tosevereillnesscharacterizedbyorgan(e.g.,liver,renal)and and colleagues evaluated EHI among American collegiate tissue (e.g., gut, muscle) injury, with high core temperatures footballathletesduringa3-monthperiod(AugustYOctober). usuallybutnotalwaysgreaterthan40-C(104-F).Heatstroke They reported a total of 139 EHI and an incidence rate of is a severe illness characterized by severe central nervous 4.19 per 1000 athlete exposures (AEs) with no cases of heat Volume7● Number4● Supplement EpidemiologyofHeatIllness S21 Copyright @ 2008 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. stroke or hyponatremia (38). The National Collegiate to three heat stroke cases occur each summer, and heat Athletic Association reported on the 2003Y2004 football stroke is the second leading cause of death (5). season with incidence rates of 0.18 per 1000 AEs and 0.01 per 1000 AEs for heat exhaustion and heat stroke, Risk Factors for Exertional Heat Illnesses respectively. However, only cases severe enough to miss a It is accepted widely that EHI does not have a causal practice session were reported. Recently, a 3-yr longitudinal relationship with any one factor and that a variety of study of 60 collegiate football programs throughout the individual factors, health conditions, medications, and United States reported 542 cases of EHI with an all-cause environmental factors can modify an individual’s risk for EHI rate of1.50per1000AEs. Specifically, theEHI ratefor serious EHI (Table 1). Furthermore, it should be noted that heat cramps was 1.15 per 1000 AEs, heat syncope was 0.10 notallriskfactorscanbeappliedacrosspopulations.Insome per 1000 AE, and heat exhaustion was 0.25 per 1000 AEs cases, risk factors may be unique to a given population (e.g., (39). During the 1996 U.S. Olympic Track and Field Trials membersofthemilitary,hikers,orathletes)andmaychange (Atlanta, GA) and the Atlanta Olympic Games, EHI with time. Serious EHI occurs not only in high-risk incidence rates were 2.84 per 1000 AEs and 2.93 per 1000 populationsbutinlow-riskpopulationswhotakeappropriate AEs, respectively (40). precautions and are exposed to conditions they have been EHI can often occur among athletes competing in long- exposed to many times before. This suggests that some distance running events and race-walking events. These victims are inherently more vulnerable on a specific day, or athletescollectivelyaccountedfor30.6%(19/62)attheU.S. some unique event triggered EHI or abnormal rectal temper- OlympicTrials and52.9%(62/117)ofthetotalEHIcasesat ature responses (8). the Atlanta Olympic Games (40). Many studies have Historically,suchunexpectedEHIcaseswereattributed to reported the occurrence of serious EHI during long distance dehydration (which impairs thermoregulation and increases running events, but are not sufficient to calculate incidence cardiovascularstrain),butitisnowsuspectedthataprevious rates. event (e.g., sickness or injury) can make an individual more Several studies have shown that EHI and heat stroke susceptible to serious heat illness (8). One theory is that deaths also can occur among youth athletes (41,42). The previous heat injury or illness might prime the acute phase NationalCenterforCatastrophicSportInjuryResearchWeb response and augment the hyperthermia of exercise, thus site (www.unc.edu/depts/nccsi/) documented that more than inducing unexpected serious heat illness (30). Figure 1 90 high school football players died of EHI between 1955 illustrates an example in which thermoregulatory responses and 2005, and three heat stroke deaths occurred in 2006. were altered by an acutely occurring medical condition During the USA Cup Soccer Tournament, it was reported thattheincidencerateofEHIwas2.8per1000player-hours (41). Another observational study examined 3028 athletes TABLE1 who participated for 7 d in 13 different sports at 8 separate Riskfactorsforexertionalheatillnesses. sites during the 1985 Junior Olympic Games in Iowa City. During this event, EHI accounted for 17% of the 121 TransientConditions medical events serious enough to remove an athlete from Situational competition (42). Taken together, these studiesdemonstrate Lackofheatacclimation that young, healthy athletes in many high-intensity sports are at significant risk for EHI. Lowphysicalfitness Excessivebodyweight Dehydration Exertional Heat Illnesses during Recreational Activities >1-haerobicintenseexercise Studies of EHI occurrence during recreational activities Alcohol and noncompetitive athletic events are limited; however, Peerpressure/motivation available evidence suggests that in particular, inexperienced Medical participants can be at significant risk for EHI. Townes and colleagues examined injuries and medical treatment during a Febrileillness multi-day recreational bike tour and reported that 85 of the Gastroenteritis(diarrhea,emesis) 2100 individuals (41 per 1000 persons) who participated Self-medication(prescription,recreationaldrugs,ergogenicstimulants) were treated for dehydration and EHI (43). Another study reported that 117 of the 2650 participants (44 per 1000 Sicklecelltrait persons) in the California AIDS ride were treated for EHI Inflammation (44). Backer and colleagues conducted a retrospective Malignanthyperthermia analysis of desert hikers (~250,000 persons per year) that needed medical treatment in Grand Canyon National Park. Historyofheat-relatedinjury They showed that 109 of the 116 individuals who required Environmental medical attention were treated for EHI (6). In the Grand Prolongedheatwaves Canyon National Park, the incidence rate of EHI in desert Highdailytemperature+humidity hikers range from 3 to 4 per 1000 persons. Furthermore, one S22 CurrentSportsMedicineReports www.acsm-csmr.org Copyright @ 2008 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. physical condition, dehydrated, and exposed to environ- mental stress (14). Erythrocyte sickling can reduce blood flowandoxygen-carryingcapacityofredbloodcellsandlead to endothelial damage, intravascular coagulation, and local tissue damage due to hypoperfusion (46). Persons with susceptibility for malignant hyperthermia (MH) may be at greater risk for EHI (45). EPIDEMIOLOGY OF EXERTIONAL HYPONATREMIA Recent studies have documented that HYPO is a potentially life-threatening medical situation occurring dur- ing exercise in otherwise healthy individuals (2Y4,6,29). Populations with a low to moderate risk for HYPO include Figure 1. Idiosyncratic hyperthermia observed in association with the military communities and recreational hikers (6), infection. A U.S. Army soldier with no history of heat illnesses was whereas individuals engaged in marathon running and involved in a study requiring5 d of heat acclimation/familiarization (6). The environmental chamber temperature was 40-C, relative humidity ultra-endurance events are at far greater risk. The first was20%,andthewindvelocitywas1mIsecj1.Thevolunteerexhibited published report of HYPO appeared in the literature over normalrectaltemperaturetothe100-minexerciseboutintheheatond 20 yr ago (2). Since that time, several case reports, case 1and2.Ond3,thevolunteerreportedfortestingwithaslightelevation series, and cross-sectional (observational) studies (3,4) have in resting rectal temperature of 37.1-C compared with 36.6-C on the been published, with the majority of them from endurance previous2d,allofwhicharewithinnormalrange.Later,adiagnosisof cellulitiswasgivenforablister.Thevolunteerwasprescribed14doforal running events and military populations (16). Recently, one antibiotic therapy. Nineteen days later, the volunteer continued partic- study reported the occurrence of HYPO in desert hikers ipation in the study and walked in the heat for the fourth time with (Grand Canyon) (6). The reported incidence rate of HYPO physiologicalresponsessimilartod1and2,weeksearlier(8). was low (0.02 to 0.4 per 1000 persons), and no deaths were (i.e., local infection). A U.S. Army soldier’s rectal temper- reported (5). atures in response to walking in the heat on the day after diagnosis of an infected blister (cellulitis) were abnormally Hyponatremia during Athletic Events elevated.Anothertheoryisthatpriorinfectioncanproduce Several studies suggested that endurance athletes com- pro-inflammatorycytokinesthatdeactivatethecells’ability monly develop asymptomatic HYPO at the end of the event to protect against extremely high temperatures. (3,4,29).However,giventhelargenumberofendurancerace Most cases of EHI occur during summer months across participants each year, the incidence of clinically significant populations (military, athletes) (33,36). EHI incidence rates HYPO is likely in the range from 1 to 3 per 1000 persons. can vary from year to year, but more often occur when the TheactualmortalityrateofHYPOisnotknownbutislikely weather is the hottest. In general, not being heat acclimat- to be very low. ized has been identified as an important factor for heat Much attention related to HYPO has been directed to intolerance and heat illness across populations. drinkingguidelinesandadviceforathletesbythemedicaland Heat stroke fatalities in U.S. football and military have sciencecommunitiesasthecasualfactorforincreasedHYPO been speculated to be caused, in part, by increased use of incidence (31). Nevertheless, recently marathons and ultra- nutritional supplements. However, delayed or improper endurance competitions have seen a huge influx of inex- treatment is the likely major cause of many fatalities (36). perienced participants, which has been shown to be the The use of stimulants (e.g., ephedra, cocaine, heroin, and population more likely to experience HYPO (27). In the methamphetamine) has been associated as a risk factor in United States alone, there were more than 20 inaugural EHI. Stimulants increase heat production and may impair marathons in 2003. Furthermore, with the growth of heat dissipation, thus elevating core temperature (17). participants in these competitions has been a parallel Bodymassindex(BMI)andpoorfitnessarealsoimportant increase in the number of water stops (28). All of these risk factors for occurrence of EHI in military personnel, factors must be considered when examining the casual recreational hikers (5,6), andathletes.MarineCorps recruits pathway of HYPO. increasetheirriskforEHIbythree-fold witheither aBMIof Hyponatremia (water intoxication) during endurance greater than or equal to 22 kgImj2 or a 1.5-mile run-time of exercise was first described by Noakes and colleagues (2) more than 12 min (32). Furthermore, in recruits with both and has been further investigated recently (3,4). A cross- high body mass and slow running times, risk increased more sectional analysis involving a subset of runners (N = 488) than eight-fold. from the 2002 Boston Marathon found the incidence of Several genetic disorders may modify the risk of EHI HYPO at 13% (defined as serum sodium = 135 mEqILj1). (14,45,46). Case reports suggest that sickle cell trait (SCT) The authors also reported that 0.6% of runners developed may increase the risk of serious EHI (46). SCT is more severe HYPO (defined as serum sodium = 120 mEqILj1); prevalentinAfricanandcertainAsianpopulationsandthese however, the majority of them were mildly symptomatic or populations are at increased risk if they also are of poor asymptomatic, and several runners failed to complete the Volume7● Number4● Supplement EpidemiologyofHeatIllness S23 Copyright @ 2008 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. event (27). A prospective study from the New Zealand can delay significantly or prevent HYPO from occurring Ironman triathlon found the incidence of HYPO (serum (31). sodium G 135 mEqILj1) to be 18% (40). In that study, 12% Italsohasbeensuggestedthatnonsteroidalanti-inflammatory of the athletes with HYPO were reported to be symptomatic drugs (NSAID) are related to HYPO (28); however, no for HYPO (4). An observational, retrospective study involv- evidence yet has found a biological plausibility for this ing 5082 participants from the Houston Marathon reported relationship. Furthermore, a recent large observational study an incidence rates of HYPO in general (defined as serum shows that the prevalence of NSAID use was similar among sodium = 135 mEqILj1) of 4.1 per 1000 persons (N = 22) HYPO and non-HYPO participants (27). and0.4per1000persons(N=2)forsevereHYPO(defined Extreme environmental conditions (hot, humid, and as serum sodium = 120 mEqILj1) (47). abnormally cold) also may be contributing factors to HYPO A retrospective analysis of 2135 participants from eight (4,6,27). These conditions can influence physiological endurance events found an incidence rate of clinically responses (i.e., sweating) to exercise (31) and drinking significant HYPO (serum sodium = 130 mEqILj1) at 14 per behavior of individuals. Recently, increased availability of 1000 persons (3). Studies from other endurance events have drinking stations at athletic competitions and preexercise reported incidence rates of HYPO as high as 29%; however, overhydration also have been suggested to contribute to thesestudieslikelysufferfromsamplingbiasandoverestimate HYPO incidence (28). the Btrue^ nature of the problem (28). Furthermore, most of the HYPO studies are based upon participants seeking medicaltreatmentortakenfromparticipantswithsymptoms, EXERCISE EPIDEMIOLOGY: ROLE OF DEHYDRATION which likely do not represent the entire competing popula- tion.Takentogether,thesestudiessuggestthatasymptomatic Exercise-induced dehydration has been shown to be or mild HYPO can be a common occurrence in endurance associated with an increase in thermoregulatory strain sportsandmarathons,butsevere,clinicallysignificantHYPO leading to reduced exercise performance, one of several likely occurs less often, and deaths are rare (nine cases factors involved in EHI (33,36), and associated with HYPO reported worldwide since 1985). in some rare cases (31). Although there are well-controlled laboratory data demonstrating that dehydration can have Hyponatremia in Military Populations adverse physiological affects during exercise, relatively few Several investigations of HYPO in the military were studies report incidence rates of dehydration during athletic conducted, mainly in response to a cluster of serious HYPO competitions (48) and military deployments or training- cases and deaths that were reported in 1989 and 1996 (32). related activities (33). In 1997, five cases of serious hyponatremia and one death In general, data sufficient to calculate incidence rates are occurred during U.S. Army training in the southern United lacking; however, some data are available, which allow for States. However, death related to HYPO is extremely rare, investigation of this important question. Cheuvront and and the incidence rate of HYPO across the U.S. Army Haymes reviewed the extent of dehydration measured in population has been less than 1 per 100,000 persons for the numerousstudiesofmarathonrunners(N>400).Theyshow past10yr(33). Furthermore,data fromtheDefense Medical thatdehydrationintheserunnersrangessignificantly(j1.2% Surveillance System, which report hospitalizations and to j6.4% decrease in body weight) (48). Although this ambulatoryclinicalvisits,providevidencethattheincidence review of the literature provides important information rateofHYPOis0.01to0.03per1000person-yearsacrossall regarding observed fluid losses in many marathon runners, U.S. military populations (1997Y2005). Taken together, most of the studies reviewed by Cheuvront and Haymes these data suggest that clinically significant HYPO is not a provide only group means, and denominators were not major problem in the U.S. military. Thus the cluster of available to calculate incidence rates (48). severe HYPO cases and deaths that were reported previously Recently,Noakesandcolleaguesexaminedpooleddatafrom are likely not caused by widespread problems within the eight endurance competitions (distances ranging from 42 to military community. Risk Factors for Exertional Hyponatremia TABLE2 Riskfactorsforexertionalhyponatremia. The three risk factors for HYPO consistently reported in observational studies, case reports, and case series are IndividualFactors significant weight gain (fluid retention) during the event, Overdrinking longerfinishingtimes,andBMIoflessthan20(Table2)(27). Although the incidence rate at which women develop Exerciseduration(>4hcontinuousphysicalactivity) HYPO is higher than men, several studies have shown that Lowbodyweight(G20BMI) this sex difference is a function of smaller body size and Women(smallstature) longer racing time. Thus, women have increased time to Preexerciseoverhydration drink with lower fluid requirements (3,4,27). The most important factor in HYPO development is excessive fluid ExcessiveNa+losses(sweat) consumptionfor an individual’s fluid needs (2,3,16),not the InsufficientNa+infoodconsumption composition of beverage consumed. However, for long Fluidretention endurance events like Ironman competitions, a sports drink S24 CurrentSportsMedicineReports www.acsm-csmr.org Copyright @ 2008 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. 228km)andreportedbodyweightandbloodsodiumchanges. Significant dehydration during exercise also has been They showed that 1077 of 2135 (504 per 1000 persons) reported in non-athletic populations. Carter and colleagues athletes were more than 2% dehydrated (3). Speedy and report that the prevalence of dehydration in military EHI colleaguespreviouslyreportedplasmasodiumandbodyweight cases ranges from 25% to 30% (33). Heat exhaustion with changesin330ultra-enduranceathletesanddemonstratedthat dehydration is the most common form of heat illness seen themajorityofthoseathleteshadsignificantbodyweightlosses among hikers in the Grand Canyon (6). Taken together, and elevated plasma sodium levels (4). However, given the these data support that the occurrence of dehydration in nature of the ultra-endurance and multi-day competitions, endurance competitions, desert hikers, and in military body weight measurements may not reflect only fluid balance populationsisverycommon.Itislikelythatthedehydration alterations, but also food intake habits and nonspecific that occurs during exercise, especially in hot environments, exercise-related gastrointestinal complications (i.e., vomiting in part contributes to reduced exercise performance, early and diarrhea) (24,49). fatigue, and increased EHI risk. Figure2. FrequencyofsignsandsymptomsinEHIandHYPOcasesreportedintheopenliterature.Thepercentageofcaseswithmostcommonly reportedsignsandsymptomsofEHI(N=74)andHYPO(N=34)casesfromtheopenliterature(1,2,5,7Y13,15Y23,25,26)andfromtheTAIHOD.Other signsandsymptomsreportedforEHIincludedilatedpupils,abnormaleyemovement(oculardeviation),spontaneoushandmovements,aggressiveor irrational behaviors, spontaneous nose bleeding, postexercise muscle cramping, and hyperventilation. The symptoms and signs defined as CNS dysfunctionincludeallreportsofalteredconsciousness,coma,convulsions,disorientation,decreasedmentalstatus,andapathy.Hyperthermiaisdefined asrectaltemperaturehigherthan40-C. Volume7● Number4● Supplement EpidemiologyofHeatIllness S25 Copyright @ 2008 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. THE BURDEN OF DIAGNOSIS: SIGNS AND during a variety of recreational activities (i.e., biking, desert SYMPTOMS hiking), and deaths are not so uncommon. The reported incidenceofEHIisdeterminedbyahostoffactors,including It has been reported that HYPO and EHI have many the definition of EHI, the target population, and individual common signs and symptoms, which can lead to delayed susceptibility. There is evidence that HYPO occurs in diagnoses or misdiagnoses in some cases (6,16,28,36). Many endurance athletes, military personnel, and recreational studies report signs and symptoms of HYPO and EHI; activities with a small percentage of severe cases, and death however, how often these signs and symptoms are consis- is very rare. Most cases of HYPO are likely due to tently observed in presenting cases has not been well overdrinking, which is influenced by individual variability described. Figure 2 illustrates the percentage of cases with for required fluid intake. In addition, a host of other most commonly reported signs and symptoms of serious EHI individual and environmental factors are in the casual (N = 74) and HYPO (N = 34) cases from the literature pathway to include inexperience with selected athletic (1,2,5,7Y13,15Y21,23,25,26) and from the U.S. Army competitions and recreational activities (e.g., desert hiking). Research Institute of Environmental Medicine (USARIEM) The signs and symptoms have considerable overlap, but Total Army Injury and Health Outcomes Database (TAI- with vigilance by well trained medical personnel, a correct HOD). Other signs and symptoms reported for EHI and diagnosis is highly achievable. Despite substantial interests in HYPO but not included in the analysis include dilated EHI and HYPO, more work is required to determine the true pupils, abnormal eye movement, spontaneous hand move- occurrenceofthese medical conditionsin populations atrisk. ments, aggressive or irrational behaviors, spontaneous nose Moreover, better measures and prevention strategies are bleeding, and postexercise muscle cramping. The symptoms neededtoreducetheincidenceratesofbothHYPOandEHI. andsignscollectivelydefinedasCNSdysfunctionincludeall Additional studies are needed to examine incidence rates reports of altered consciousness, coma, convulsions, disori- and risk factors of both HYPO and EHI across diverse entation,decreased mentalstatus,and apathy. Hyperthermia populations. Furthermore, longitudinal studies are needed to was defined as a rectal temperature higher than 40-C. examine trends in incidence and prevalence of EHI, Unfortunately, due to multiple overlapping features, dehydration, and HYPO, in particular across athletic and differential diagnosis based upon only signs and symptoms recreational settings. may be challenging. The early signs of clinical EHI and HYPO may be nonspecific. Additional information such as rectal temperature, laboratory values (i.e., blood sodium, Acknowledgments urine, or blood osmolality), and knowledge of fluid intake patternsmaybeneededtoconfirmdiagnosis.Althoughmost Disclaimer:Theviewsexpressedinthisarticlearethoseoftheauthorsand casesofEHIreportedrectaltemperaturesover40-C,insome donotreflecttheofficialpolicyoftheDepartmentoftheArmy,Department cases lower values were reported. ofDefense,ortheU.S.Government. It should be noted that self-reported fluid intake alone should not be used for diagnosis of HYPO, since individuals are commonly inaccurate at estimating previous fluid intake. References For example, one patient self-reported drinking 3 L of water over a 7- to 8-h period when exposed to ambient temper- 1. Holman, N.D., and A.J. Schneider. Multi-organ damage in exertional heatstroke.Neth.J.Med.35:38Y43,1989. atures as high as 112-F, which alone may propose dehy- 2. Noakes, T.D., N. Goodwin, B.L. Rayner, et al. Water intoxication: a dration or EHI. However, further medical evaluation found possiblecomplicationduringenduranceexercise.Med.Sci.SportsExerc. thatthepatient’sbloodsodiumvaluewas126mEqILj1with 17:370Y375,1985. a measured rectal temperature of 96.5-F (5). 3. Noakes, T.D., K. Sharwood, D.B. Speedy, et al. 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