# Toward isolating perceptual and physiological contributors to heat sensitivity in multiple sclerosis: insights from a new experimental model

## TL;DR

This study explores whether cooling the skin can reduce heat-related vision problems in multiple sclerosis patients, finding that it has limited effectiveness.

## Contribution

A new experimental model is introduced to distinguish perceptual and physiological effects of heat on visual performance in MS.

## Key findings

- Local skin cooling during heating only modestly reduced whole-body thermal sensation.
- Visual performance decrements due to heat were not mitigated by skin cooling.
- Heating reduced visual evoked potentials amplitude in the affected eye similarly across conditions.

## Abstract

To determine if reductions in whole-body thermal sensation (WBTS) with localised skin cooling mitigate heat-induced visual performance decrements in people with multiple sclerosis (MS), optic neuritis, and heat-sensitive visual symptoms, independent of core temperature increases.

Thirteen participants (7 relapsing–remitting MS (MS) patients with unilateral (left) optic neuritis and heat-sensitive visual symptoms; 6 controls) underwent visual performance testing on each eye at baseline and during passive heating (0.6℃ rise in gastrointestinal temperature (ΔTGI) via a hot water-perfused suit) under two counterbalanced crossover ordered conditions: 1) cold packs (0℃—CLD) or 2) hot packs (50℃—HOT) applied to the lower back. WBTS, visual symptoms, multifocal visual evoked potentials (mf-VEPs) amplitude/latency, and contrast sensitivity were assessed.

ΔTGI was consistent across trials (p = 0.213; ηp2 = 0.21). WBTS was only marginally lower (p = 0.017; ηp2 = 0.42) in CLD than HOT for MS (CLD: 5.8 ± 0.9 a.u.; HOT: 6.4 ± 0.7 a.u.) and controls (CLD: 5.0 ± 0.9 a.u.; HOT: 5.9 ± 0.7 a.u.). Passive heating worsened (p = 0.027; ηp2 = 0.59) visual symptoms in the affected eye similarly (p = 0.356; ηp2 = 0.14) for HOT and CLD conditions. Heating reduced mf-VEPs amplitude in the left (affected) eye (p = 0.007; ηp2 = 0.50) similarly (p = 0.332; ηp2 = 0.09) across groups and conditions. For the unaffected (right) eye, reductions in mf-VEPs amplitude were greater in MS than controls (p = 0.031; ηp2 = 0.36), with no difference between conditions (p = 0.339; ηp2 = 0.08). mf-VEPs latency and contrast sensitivity were unaffected by heating.

Localised skin cooling during passive heating to a moderate core temperature produces only a modest reduction in WBTS and does not mitigate heat-induced visual performance decrements. The limited perceptual difference achieved suggests the localised skin cooling was insufficient to meaningfully isolate the effects of skin temperature from core temperature.