of Exertional Heat Illness

ICD-10-CM code

ICD-10 category

Etiology

Signs and symptoms

T67.0

Heat stroke and sunstroke

Failure of body to dissipate heat

CNS dysfunction (e.g., irritability, confusion, coma, aggressive behavior)

Extreme hyperthermia (>40.0 °C)

T67.1

Heat syncope

Circulatory instability

Fainting episode due to a reduction in central venous return

T67.2

Heat cramp (exercise-associated muscle cramp)

Water and electrolyte imbalance

Painful, involuntary muscle contractions

T67.3

Anhydrotic heat exhaustion (includes prostration due to water depletion)

Water and electrolyte imbalance

Nausea, anorexia, fatigue/weakness, tachycardia, loss of concentration, prominent sensation of thirst, decreased urine output

Body temperature typically <40.0 °C

T67.4

Salt depletion heat exhaustion

Water and electrolyte imbalance

Nausea, anorexia, fatigue/weakness, tachycardia, loss of concentration, hemoconcentration, muscle cramps, vomiting

Body temperature typically <40.0 °C

T67.5

Unspecified heat exhaustion

Unspecified causes: physical exertion (diagnosis of exclusion)

Nausea, anorexia, fatigue/weakness, tachycardia, loss of concentration, dizziness, confusion, pallor, profuse sweating, minimal CNS disturbances

Body temperature typically <40.0 °C

T67.6

Transient heat fatigue

Psychological causes

Extreme tiredness, reluctance to perform physical work, deterioration of skilled performance in the absence of water and/or salt depletion

T67.7

Heat edema

Water and electrolyte imbalance

Pooling of fluid in interstitial spaces causes swelling in the extremities

T67.8

Other heat and light effects

Various causes

Signs and symptoms NOS

T67.9

Unspecified heat and light effects

Various causes

Signs and symptoms NOS

ICD-10-CM 10th revision of the international classification of diseases, CNS central nervous system, NOS not otherwise specified

CNS central nervous system, NOS not otherwise specified

While ICD-10-CM serves as the universal means of classifying medical conditions, some entities further classify heat-related illnesses for purposes of reporting and calculating incidence rates. For example, within the US Military, only hospitalizations, outpatient medical encounters, or records of reportable medical events with a primary or secondary diagnosis of the medical conditions that are considered heat illnesses (heat stroke [T67.0] and heat exhaustion [T67.3–5]) are classified as such [46]. The other taxonomies within code T67 do not qualify in the case definition of heat illnesses in this particular setting. Furthermore, issues related to the interpretability of the relevant ICD codes and/or correctly classifying heat-related illnesses to the appropriate ICD code exist, particularly when attempting to discern between the various etiologies of heat exhaustion (T67.3–T67.5) or between heat stroke and heat exhaustion in a field-based setting [47, 48].

Specific to heat stroke, it is classified by ICD-10-CM code T67.0. Heat stroke can be classified as either classic heat stroke (CHS) , which typically occurs in the elderly or infants during times of excessive heat (e.g., heat waves), or exertional heat stroke (EHS) which is caused by the inability to dissipate metabolically produced body heat during physical activity. Although CHS and EHS demonstrate differing etiologies, they are classified under the same universal coding, which makes it difficult to distinguish in large-scale epidemiological data [49].

Epidemiology

To further understand the impact of EHI in a given population, acknowledging overall and condition-specific epidemiological data is necessary. Examining emergency department (ED) data for treatment of heat illness in the United States shows that the annual incidence rate for heat illness is between 21.5 and 31.19 per 100,000 person-years and incidence rate for heat stroke is 1.34 per 100,000 person-years [21, 50–52]. These data also show that the incidence of heat illness and heat stroke is more likely to occur in men, during the summer months or during times of anomalously high temperatures for a given time of the year, and as age increases [21, 50–54].

Given that the aforementioned data reports cases across the lifespan, one must take into consideration the data points involving young children or elderly individuals. While these populations are at risk for heat illness, the injury insult is typically due to physiologic (e.g., incomplete development of thermoregulatory body systems in young children, decline in thermoregulatory capacity and/or presence of contributing comorbidities in the elderly) or external causes (e.g., no access to air conditioning or domiciles providing relief to extreme heat) that are exacerbated to exposure to extreme heat typically seen during heat waves [55–58] rather than physical exertion. When re-examining ED data on incidence of heat illness in age groups 15–34 and 35–64, ages that are most likely associated with recreational physical activity and/or occupational work, incidence rates increase to 41.8 and 32.8 per 100,000 person-years, respectively, with the former age group exhibiting a 39% higher incidence than the 35–64 age group [51]. This trend holds true when examining heat stroke incidence, in that of the 8,251 heat strokes treated in EDs in the United States from 2000 to 2010, 54% occurred in ages 20–59, which is typically more associated with recreational and occupational activities [21].

Heat Illness in Military Settings

EHI risk has been extensively studied within military operations due to the extent in which these injuries can affect the outcomes related to the preparation/training of warfighters and mission success during times of war. Research dating back to the late nineteenth century and early twentieth century began to characterize the factors responsible for the onset of EHI [4]. During World War I, while fighting in Mesopotamia, Macedonia, and South West Africa, British soldiers experienced heat-related hospital admissions rates of 0.1–77.39 per 1000 personnel, with the warmer summer months experiencing a larger rise of heat-related incidents [4]. Similar trends were also seen during World War II; higher incidence of heat illness occurring during hotter times of the calendar year and/or in geographical locations exposing the warfighters to thermal stress [4]. Following World War II, the US Armed Forces still exhibited increased rates of EHI; this was most notably due to the expanded reach of the US Armed Forces around the world and the increased numbers of individuals going through basic training [5]. While the incidence rates for heat exhaustion and heat stroke varied between 16.0–56.8 and 1.5–9.1 per 100,000 per year, the advancements in the development of environmental-based activity modifications greatly reduced the incidence of training-related EHI [5].

As research into the epidemiology of EHI in military settings has expanded [2, 3, 8, 27, 48, 59–67], we have a better understanding of where, and to what extent these EHI events are occurring. Similar to what was seen during both World Wars, incidence rates for EHI are higher during hotter periods of the year and/or at geographical locations exhibiting more extreme environmental conditions [2, 3, 8, 27, 59, 61–67]. These data also suggest variations in total EHI cases and incidence based on branch of service (Fig. 1.1), enlistment status (Fig. 1.2), and gender (Fig. 1.3). While the total number of EHI events is lower in recruits (i.e., military personnel pursuing basic training and initial stages of training upon enlistment), the overall incidence is much greater when compared to the other enlistment groups. While this may be due to a number of factors, the geographical locations of basic training in the United States and physical demands of training may be greater contributors of EHI events in these specific populations. Other considerations on risk of EHI in military populations pertain to the overall fitness of military personnel, particularly in new recruits, where data shows an increased risk of EHI in individuals that are overweight and less cardiovascular fit than their more fit, less overweight counterparts [10, 68].

../images/471692_1_En_1_Chapter/471692_1_En_1_Fig1_HTML.png — Fig. 1.1

Total cases of (a) heat stroke and (b) heat exhaustion within the separate branches of the US Armed Forces [27, 61–67]

../images/471692_1_En_1_Chapter/471692_1_En_1_Fig2_HTML.png — Fig. 1.2

Total cases and incidence rates of (a) heat stroke and (b) heat exhaustion based on enlistment status in the US Armed Forces. Recruit, which was not tracked until 2012, is defined as enlistee during the initial stages of training/preparation [E1–E4) [27, 61–67]

../images/471692_1_En_1_Chapter/471692_1_En_1_Fig3_HTML.png — Fig. 1.3

Total cases and incidence rates of (a) heat stroke and (b) heat exhaustion between male and female enlisted members of the US Armed Forces [27, 61–67]

Heat Illness in Athletic Settings

Within the athletics setting, the risk of EHI is typically present in situations where athletes are performing intense exercise in hot environmental conditions or required to wear protective equipment as part of their sport’s uniform. Sports such as American football, field hockey (particularly the goalie), and running events are sports where there is greatest risk. Within the United States, of the 54,983 EHIs treated in EDs from 1997 to 2006, 41,538 (75.5%) were sport related, with American football and recreational exercise (e.g., running) being the most common types of activity involved with EHI in men and women within the ≤19 and 20–39 age groups [52].

In-depth analyses of the incidence of EHI within secondary school and collegiate athletics have yielded a greater insight into where risk is greatest. Work by Kerr et al. [22] and Yard et al. [43] have shown the variability in EHI risk among secondary school athletics, with American football and girls’ field hockey having the highest incidence rate (Fig. 1.4) [69]. This can be largely attributed to the time of year in which these sports begin (i.e., the later summer to early fall within the Northern Hemisphere) and the protective equipment worn by those participating; in girls’ field hockey, only the goalie position wears protective equipment.

../images/471692_1_En_1_Chapter/471692_1_En_1_Fig4_HTML.png — Fig. 1.4

EHS fatality incidence rates in secondary school sports. (Adapted from Kerr et al. [22] and Yard et al. [43]). (From Adams [69], with permission)

Recent work by Kerr et al. [23] expands on this previous work showing that American football and girls’ field hockey remain the sports with highest risk of EHI among secondary school athletics. In addition, girls’ cross-country also exhibited EHI incidence rates that are more than two times greater than all other secondary school sports aside from American football and field hockey [23]. Moreover, girls’ (incidence rate [IR], 1.18 per 10,000 athlete exposures [AEs]) and boys’ (0.52 per 10,000 AEs) cross-country and American football (0.61 per 10,000 AEs) EHI incidence rates are largest in competition than all other secondary school sports [23].

Risk of EHI within collegiate athletics exhibits similar trends of that observed within secondary school athletics. American football and men and women’s cross-country have higher overall EHI incidence rates (1.55, 0.48, 0.35 per 10,000 AEs, respectively) with men and women’s cross-country having the largest EHI incidence rates during competition (4.01 and 3.69 per 10,000 AEs, respectively) [70] (Fig. 1.5). Most EHIs occurred during practices (72.8%) and during the preseason (64.7%) portion of a sport’s competitive seasons, with preseason practices having an EHI rate of 1.16 per 10,000 AEs compared with an EHI rate of 0.23 per 10,000 AEs during all other times of the season [70]. Furthermore, EHI rates were 2–4 times greater in warmer climates (1.05 per 10,000 AEs) than those of temperate (0.43 per 10,000 AEs) or cooler (0.26 per 10,000 AEs) climates [70].

../images/471692_1_En_1_Chapter/471692_1_En_1_Fig5_HTML.png — Fig. 1.5

Exertional heat illness incidence rates among National Collegiate Athletic Association sports from 2009 to 2010 through 2014 to 2015. ^bOnly sex-comparable sports (baseball/softball, basketball, cross-country, ice hockey, indoor track, lacrosse, outdoor track, soccer, swimming and diving, tennis). ^cExertional heat illnesses were not reported in women’s gymnastics and men’s indoor track, lacrosse, outdoor track, swimming and diving, and tennis [70]

American football is of particular concern for EHI given the construct of the sport; the sport requires individuals to don protective equipment covering roughly 75% of their body surface area, which is known to reduce the ability to dissipate body heat [71]. In addition, training for the competitive season begins in the late summer and early fall within the Northern Hemisphere and includes individuals of varying body size (i.e., amount of lean mass and fat mass), which may predispose the larger athletes to increased risk [25]. Over a 30-year period (1980–2009), there were 58 documented EHS-related deaths among American football athletes in the United States with a majority of these deaths involving athletes participating in this sport in the southeastern portion of the United States (Fig. 1.6) where they were exposed to more extreme environmental conditions [25]. Data [72] support that there is an average of roughly three American football players that die each year due to EHS. Examining EHS-related American football deaths in 5-year blocks dating back to 1975, a greater number of annual deaths are occurring as time progresses (Fig. 1.7), which is particularly alarming given the advancements made in science and medicine surrounding the prevention, recognition, and treatment of EHS to optimize survivability.