Elevated brain temperature under severe heat exposure impairs cortical motor activity and executive function.

<p dir="ltr">Abstract</p><p dir="ltr"><b>Background</b></p><p dir="ltr">Excessive heat exposure can lead to hyperthermia in humans, which impairs physical performance and disrupts cognitive function. While heat is a known physiological stressor, it is unclear how severe heat stress affects brain physiology and function.</p><p dir="ltr"><b>Methods</b></p><p dir="ltr">Eleven healthy participants were subjected to heat stress from prolonged exercise or warm water immersion until their rectal temperatures (Tre) attained 39.5°C, inducing exertional or passive hyperthermia, respectively. In a separate trial, blended ice was ingested before and during exercise as a cooling strategy. Data were compared to a control condition with seated rest (normothermic). Brain temperature (Tbr), cerebral perfusion, and task-based brain activity were assessed using magnetic resonance imaging techniques.</p><p dir="ltr"><b>Results</b></p><p dir="ltr">Tbr in motor cortex was found to be tightly regulated at rest (37.3°C ± 0.4°C) despite fluctuations in Tre. With the development of hyperthermia, Tbr increases and dovetails with the rising Tre. Bilateral motor cortical activity was suppressed during high-intensity plantarflexion tasks, implying a reduced central motor drive in hyperthermic participants (mean Tre: 38.5°C ± 0.1°C). Global gray matter perfusion and regional perfusion in sensorimotor cortex were reduced with passive hyperthermia. Executive function was poorer under a passive hyperthermic state, and this could relate to compromised visual processing as indicated by the reduced activation of left lateral-occipital cortex. Conversely, ingestion of blended ice before and during exercise alleviated the rise in both Tre and Tbr and mitigated heat-related neural perturbations.</p><p dir="ltr"><b>Conclusion</b></p><p dir="ltr">Severe heat exposure elevates Tbr, disrupts motor cortical activity and executive function, and this can lead to impairment of physical and cognitive performance.</p>