Hyperthermia impairs short‐term memory and peripheral motor drive transmission

Abstract

The aims of this study were to determine (i) the effect of passive hyperthermia on motor drive and cognitive function, and (ii) whether head cooling can limit the hyperthermia-induced alterations. Sixteen subjects were randomly exposed for 2 h to three different conditions: control (Con, 20• C), hot (Hot, 50• C) and hot head cool (HHC -where cold packs were applied to the head under Hot conditions). Three cognitive tests measuring attention and two measuring memory were performed. Neuromuscular testing included electrically evoked muscle action potentials (M-waves) and reflex waves (H-reflex) at rest and during brief (4-5 s) and sustained (120 s) maximal voluntary contractions (MVC) of the plantar flexors. All the tests were performed in the environmental room. During brief MVC, torque was significantly lower in both Hot and HHC as compared to Con (P < 0.05). The decrease in muscle activation was significant in Hot (P < 0.05) but not in HBC (P = 0.07). This was accompanied by peripheral failures in the transmission of the neural drive at both spinal (significant decrements in H-reflexes and V-waves, P < 0.05) and neuromuscular junction (significant decrements in M-waves, P < 0.05) levels. During sustained MVC, muscle activation was further depressed (P < 0.05) without any concomitant failures in M-waves, suggesting neural activation adjustments occurring probably at the supraspinal level. Cerebral perturbations were confirmed by significant decrements in both memory tests in Hot as compared with Con (P < 0.05) but not in simple tests (attention tests) that were not affected by hyperthermia. The decrement in memory capacity suggested the existence of frontal lobe activity impairments. Thus, HHC preserved memory capacity but not the visual memory.

Cognitive and perceptual responses during passive heat stress in younger and older adults

“…30 -50% RH) for 20 -120 min, in which internal temperature either did not change (16,17) or increased to a similar extent as that in the current study (ϳ1.5°C) (16,19,54). As a result, a rationale for our divergent findings is likely the waterperfused suit method with which we induced moderate heat stress.…”

“…2 and 3, Table 3). This finding is even more surprising given that performance on these exact tests was reduced in heat-stressed younger adults utilizing a similar sample size (range: 8 -18 subjects), e.g., OTS (16,17), RVP (19), PRM (19,54), and SSP (19,54). Thus, we are confident that our findings are not due to a lack of sensitivity, as has been speculated as a confounding factor in other similar studies (e.g., 3,61).…”

“…Various aspects of attention (19,22,65), memory (6,19,28,41,54), and executive function (16,17,67) are reduced in heat-stressed younger adults. Therefore, our findings that passive heat stress did not affect the measured indices of cognitive function are unexpected (Figs.…”

“…The chosen CANTAB tests assess aspects of attention, memory, executive function, and subjective perceptions and required ϳ30 -40 min to complete. Importantly, these tests have been used repeatedly in studies investigating cognitive changes in heatstressed younger adults (16,17,19,54). Subjects completed the battery of tests on five occasions: one time during visit 1 and two times during each of the two experimental sessions.…”

“…This chronological cognitive decline may contribute to the risk of deleterious outcomes during heat waves in older adults by, for instance, leading to poor decision-making. Interestingly, perhaps because of the deleterious impact of heat stress on cerebral blood flow (46,61) and/or disruptions in cerebral functional connectivity (66), many cognitive processes are impaired in heat-stressed younger adults [e.g., aspects of attention (19,22,65), memory (6,19,28,41,54), and executive function (16,17,67)], although this is not always observed (3,50,61). If heat stress-induced impairments in cognitive function are amplified with age, this might suggest that the contribution of cognitive factors to the risk of morbidity and mortality during heat waves would be exacerbated in older adults.…”