Blunted sweating does not alter the rise in core temperature in people with multiple sclerosis exercising in the heat

The purpose of this study is to determine whether thermoregulatory capacity is altered by multiple sclerosis (MS) during exercise in the heat. Sixteen MS participants (EDSS: 2.9 ± 0.9; 47 ± 8 yr; 77.6 ± 14.0 kg) and 14 healthy control (CON) participants (43 ± 11 yr; 78.6 ± 17.0 kg) cycled at a heat production of 4 W·kg^-1 for 60 min at 30°C, 30% relative humidity (RH) (Warm). A subset of eight MS (EDSS: 2.6 ± 0.5; 44 ± 8 yr; 82.3 ± 18.2 kg) and 8 CON (44 ± 12 yr; 81.2 ± 21.1 kg) also exercised at 35°C, 30% RH (Hot). Rectal temperature (T_re), mean skin (T_sk) temperature, and local sweat rate (LSR) on the upper back (LSR_back) and forearm (LSR_arm) were measured. All CON, and only 9 of 16 and 7 of 8 MS participants completed 60 min of exercise in Warm and Hot trials, respectively. All MS participants who were unable to complete exercise stopped with a ΔT_re between 0.2 and 0.5°C. The time to reach a ΔT_re of 0.2°C was similar (MS: 28 ± 15 min, CON: 32 ± 18 min; P = 0.51). For MS participants, completing 60-min of exercise in Warm, ΔT_re (P = 0.13), ΔT_sk (P = 0.45), LSR_back (P = 0.69), and LSR_arm (P = 0.54) was similar to CON, but ΔT_b (body temperature) (MS: 0.16 ± 0.13°C, CON: 0.07 ± 0.06°C; P = 0.02) and onset time (MS: 16 ± 10 min, CON: 8 ± 5 min; P = 0.02) for sweating were greater in MS. Similarly, in Hot, ΔT_re (P = 0.52), ΔT_sk (P = 0.06), LSR_back (P = 0.59), and LSR_arm (P = 0.08) were similar, but ΔT_b (MS: 0.19 ± 0.16°C, CON: 0.06 ± 0.04°C; P = 0.04) and onset time (MS: 13 ± 7 min, CON: 6 ± 3 min; P = 0.02) for sweating were greater in MS. Even at 35°C, a delayed sweating onset did not alter heat loss to sufficiently affect exercise-induced rises in core temperature. Heat intolerance with MS does not seem attributable to thermoregulatory impairments.