Are All Exertional Heat Strokes Equal?

Exertional heat stroke (EHS) is commonly studied as a single clinical entity. While most EHS cases are associated with situational factors, few (complex) cases recur and/or lack clear contextual explanations. This study aimed to investigate thermoeffector responses by comparing individuals with complex EHS to those with simpler, context-related cases.

56 participants were divided into three groups: 18 controls with no history of heat illness (CONT), 38 EHS (ALL-EHS), subdivided into simple EHS (SIMP-EHS; n=21) and complex EHS (COMP-EHS; n=17). Following anthropometric measurements, participants completed a heat tolerance test (HTT), with rectal temperature (Trec), whole-body sweat loss and oxygen consumption recorded.

Heat intolerance was identified in 39%, 50%, 33%, and 65% of participants in the CONT, ALL-EHS, SIMP-EHS, and COMP-EHS groups, respectively. Final Trec was higher in COMP-EHS (38.8±0.63°C) than in CONT (38.3±0.42°C; p=0.021) and SIMP-EHS (38.3±0.52°C; p=0.009), but did not differ between ALL-EHS (38.5±0.62°C) and CONT. Compared with heat-tolerant individuals, heat-intolerant individuals had higher body mass index and body fat and lower surface area-to-mass ratio and sweat loss (p<0.001).

When considered as a homogeneous group, EHS did not differ from controls in Trec or heat-intolerance rate. However, when subgroups were considered, complex EHS cases showed a distinct pattern with greater Trec. Classical predictors of heat intolerance were observed in heat-intolerant participants but did not differ among EHS groups, suggesting other factors are involved. Overall, these findings suggest that individuals with complex EHS may have greater vulnerability to heat stress. Therefore, when systematic HTT screening is not possible, the HTT may be particularly relevant for complex EHS and should be considered as one tool among others to inform return-to-duty decisions.