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

Background: 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.

Methods: Eleven healthy participants were subjected to heat stress from prolonged exercise or warm water immersion until their rectal temperatures (T_re) 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 (T_br), cerebral perfusion, and task-based brain activity were assessed using magnetic resonance imaging techniques.

Results: T_br in motor cortex was found to be tightly regulated at rest (37.3°C ± 0.4°C (mean ± SD)) despite fluctuations in T_re. With the development of hyperthermia, T_br increases and dovetails with the rising T_re. Bilateral motor cortical activity was suppressed during high-intensity plantarflexion tasks, implying a reduced central motor drive in hyperthermic participants (T_re = 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 T_re and T_br and mitigated heat-related neural perturbations.

Conclusion: Severe heat exposure elevates T_br, disrupts motor cortical activity and executive function, and this can lead to impairment of physical and cognitive performance.