Afferent thermosensory function in relapsing–remitting multiple sclerosis following exercise-induced increases in body temperature

New Findings • What is the central question of this study? Between 60 and 80% of multiple sclerosis (MS) patients experience transient worsening of symptoms with increased body temperature (heat sensitivity). As sensory abnormalities are common in MS, we asked whether afferent thermosensory function is altered in MS following exercise‐induced increases in body temperature. • What is the main finding and its importance? Increases in body temperature of as little as ∼0.4°C were sufficient to decrease cold, but not warm, skin thermosensitivity (∼10%) in MS, across a wider temperature range than in age‐matched healthy individuals. These findings provide new evidence on the impact of heat sensitivity on afferent function in MS, which could be useful for clinical evaluation of this neurological disease. In multiple sclerosis (MS), increases in body temperature result in transient worsening of clinical symptoms (heat sensitivity or Uhthoff's phenomenon). Although the impact of heat sensitivity on efferent physiological function has been investigated, the effects of heat stress on afferent sensory function in MS are unknown. Hence, we quantified afferent thermosensory function in MS following exercise‐induced increases in body temperature with a new quantitative sensory test. Eight relapsing–remitting MS patients (three men and five women; 51.4 ± 9.1 years of age; Expanded Disability Status Scale score 2.8 ± 1.1) and eight age‐matched control (CTR) subjects (five men and three women; 47.4 ± 9.1 years of age) rated the perceived magnitude of two cold (26 and 22°C) and two warm stimuli (34 and 38°C) applied to the dorsum of the hand before and after 30 min cycling in the heat (30°C air; 30% relative humidity). Exercise produced similar increases in mean body temperature in MS [+0.39°C (95% CI: +0.21, +0.53) P = 0.001] and CTR subjects [+0.41°C (95% CI: +0.25, +0.58) P = 0.001]. These changes were sufficient to decrease thermosensitivity significantly to all cold [26°C stimulus, −9.1% (95% CI: −17.0, −1.5), P = 0.006; 22°C stimulus, −10.6% (95% CI: −17.3, −3.7), P = 0.027], but not warm, stimuli in MS. Contrariwise, CTR subjects showed sensitivity reductions to colder stimuli only [22°C stimulus, −9.7% (95% CI: −16.4, −3.1), P = 0.011]. The observation that reductions in thermal sensitivity in MS were confined to the myelinated cold‐sensitive pathway and extended across a wider (including milder and colder) temperature range than what is observed in CTR subjects provides new evidence on the impact of rising body temperature on afferent neural function in MS. Also, our findings support the use of our new approach to investigate afferent sensory function in MS during heat stress.