Environmental thermal exposure and disability worsening in multiple sclerosis

Background

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system affecting over 2.9 million people worldwide. It usually progresses from relapses with partially reversible neurological symptoms to long-term disability. Increasing evidence suggests that heat exposure may increase relapse risk, but its role in long‑term disability progression remains unclear. This study assesses the association between meteorological factors and disability worsening in MS.

Methods

We used data from the French CONFISEP-OFSEP cohort, including 1,189 patients with relapsing-remitting MS followed up between 2018 and 2021. Disability progression was assessed using the Expanded Disability Status Scale (EDSS). Outdoor environmental thermal exposures during the 6‑month periods preceding each visit were estimated at the patients’ residential addresses using meteorological data from Météo‑France. Disability worsening (i.e. an increase in EDSS score) was modelled according to environmental thermal exposures using continuous‑time Markov multi-state models.

Results

A total of 3,720 EDSS assessments were conducted, with scores ranging from 0 (no disability) to 7 (severe disability requiring wheelchair use). 402 patients experienced at least one episode of disability worsening. Preliminary analyses suggest that the risk of disability worsening was higher in summer–spring than in winter–autumn for EDSS < 2 (hazard ratio (HR) = 1.60–1.70; 95% CI = 1.09–2.44). Higher minimum temperature was also associated with disability worsening for EDSS < 3 (HR = 1.10–1.12; 95% CI = 1.03–1.25).

Conclusions

With rising global temperatures, heat exposure may increasingly influence MS activity. Identifying modifiable environmental risk factors could inform preventive strategies and improve long‑term patient outcomes.

Funding

This research was supported by IReSP (IReSP-AAP-2023-BLANC-331593) and by Assistance Publique–Hôpitaux de Paris (Fonds d’intéressement à la recherche).

Key messages

• This study provides longitudinal evidence of a link between heat exposure and worsening of disability in people with multiple sclerosis.

• This work supports the development of strategies aimed at preventing the impact of rising temperatures on people with multiple sclerosis.