Extreme temperature events and multiple sclerosis relapses: evidence from France

Background

Multiple sclerosis (MS) is a chronic autoimmune disease that affects around 130000 people in France. It is characterized by recurrent relapses corresponding to a newly formed inflammatory lesion in the central nervous system. Lesion development begins several weeks to months before clinical manifestation. While extreme temperatures are suspected activate relapses, epidemiological evidence is limited.

Methods

We collected data from 1189 relapsing MS patients included in the French CONFISEP cohort (2018-2021), including daily mean temperature at geolocated residential addresses, daily occurrence of MS-relapse and clinical characteristics. An individual-level case-time series design was applied. The impact of extreme temperature on the risk of MS-relapses was quantified by conditional logistic regression combined with distributed lag non-linear models. We first assessed the overall temperature-relapse association and then explored the effect across 17 definitions of heatwaves and 15 definitions of cold spells.

Results

573 patients experienced at least one relapse, accounting for 939 relapses overall. Temperatures ≥30 °C where related to higher odds of relapse at lags between 19-24 days, with maximum OR 1.23 (95%CI: 1.05, 1.43). The risk rose with increasing heatwave intensity and duration, with odds ratio at lags 17-26 days ranging from 1.06 (95%CI: 1.01, 1.13) during mild heatwaves to 1.39 (95%CI: 1.01, 1.94) during the most severe events. No associations were observed for extreme cold spells.

Conclusions

Exposure to extreme heat significantly increases the risk of MS-relapse, with the greatest impact observed approximately three weeks after the event, consistent with clinical observations that lesions develops gradually over several weeks. These findings highlight the need for targeted preventive measures, patient guidance and public health policies to protect vulnerable populations in the context of climate change.

Key messages

• Consistent with clinical observations, exposure to extreme heat significantly increases the risk of relapse approximately three weeks after the event.

• These findings highlight the need for targeted preventive measures, patient guidance and public health policies to protect vulnerable populations in the context of climate change.