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Multiple Sclerosis Alters Dynamic Cerebral Autoregulation during Heat Stress

Multiple sclerosis (MS) is a neurological disease marked by demyelination within the central nervous system that results in individuals with MS experiencing a myriad of symptoms, including cognitive dysfunction and orthostatic dizziness. Adding complexity, many individuals with MS (60–80%) experience temporary worsening of their symptoms upon increases in core temperature (Tcore), an occurrence known as Uhthoff's phenomenon. This study tested the hypotheses that individuals with MS: 1) have impaired cerebral autoregulation under normothermic baseline conditions, and 2) have exacerbated impairments in cerebral autoregulation as a result of a passive heat stress compared to healthy controls (CON). Seven subjects with relapsing‐remitting MS (age: 44±7 yrs; height: 174±9 cm; weight: 74±17 kg) and 7 matched healthy controls (CON; age: 44±6 yrs; height: 176±11 cm; weight: 81±14 kg) were outfitted in a tube‐lined water perfusion suit in which they were exposed to a normothermic baseline (NT: 34 °C circulating water) condition for 10 minutes followed by a ~60‐minute whole‐body heat stress (WBH: 48 °C circulating water) until a 0.8 °C increase in Tcore was achieved. Continuous measurements of mean middle cerebral artery blood velocity (MCAv; transcranial Doppler ultrasonography) and mean arterial pressure (MAP; finger photopleythsmography) were taken throughout each thermal condition. Transfer function analysis of beat‐to‐beat fluctuations in MAP and MCAv was used to quantify dynamic cerebral autoregulation (dCA) during NT and WBH. All data from transfer function analyses were examined at very low frequency (VLF: 0.02–0.07 Hz), low frequency (LF: 0.07–0.20 Hz), and high frequency (HF: 0.20–0.30 Hz) ranges. Estimates of coherence exceeded 0.5 at LF and HF during NT, and all frequencies during WBH. Contrary to our hypothesis, LF phase (MS: 0.37±0.20 vs CON: 0.55±0.37 radians, p=0.29) and LF gain (MS: 0.83±0.21 vs CON: 0.89±0.12 cm/s/mmHg, p=0.52) were similar during NT. No differences in LF phase during WBH (MS: 0.73±0.30 vs CON: 0.66±0.30 radians, p=0.66). However, differences in LF gain were observed between groups following WBH (MS: 1.26±0.47 vs CON: 0.79±0.21 cm/s/mmHg, p=0.03). These preliminary data demonstrate a negative impact on estimates of transfer function gain that manifest with increases in Tcore, likely indicating MS impairments in dynamic cerebral autoregulation during heat stress.