Influence of comorbid diabetes mellitus on outcomes in multiple sclerosis: an English population-based matched cohort study

Importance: As new treatments increase quality and length of life in people with multiple sclerosis (MS), effective prevention and management of common comorbidities, including Diabetes Mellitus (DM), is increasingly important. Objective: To compare incidence of DM and its associations with hospitalisation and mortality in adults with MS and matched controls. Design: Using English primary care data from the Clinical Practice Research Datalink (CPRD), linked to Hospital Episode Statistics and national mortality records, we matched adults with MS diagnosed between 2000 and 2023, with up to ten controls without MS by age, sex, and practice. We excluded individuals with preexisting DM, defined using diagnostic and management codes. Outcomes included all-cause hospitalisation (number and duration) and mortality. We used Poisson, negative binomial, linear, and Cox proportional hazards models, adjusting for demographic and socioeconomic factors, adding interaction terms to examine if ethnicity, deprivation, and urbanity were associated with outcomes. Results: We included 9,010 individuals with MS and 78,121 matched controls. Over a mean follow-up of 13.2 years, people with MS had over twice the incidence of DM compared with controls (adjusted incidence rate ratio [aIRR]=2.26, 95% CI: 1.96 to 2.61, p<0.001). Among people with MS, incident DM was associated with higher hospitalisation rates (aIRR=1.82, 95%CI: 1.47 to 2.28, p<0.001), longer hospitalisation duration (median 18 vs 4 days, adjusted beta;=0.53, 95%CI: 0.41 to 0.65, p<0.001), and increased all-cause mortality when incident DM was modelled as a time-varying exposure (adjusted hazard ratio=1.46, 95%CI: 1.17 to 1.82, p<0.001), compared to those who did not develop DM. Similar patterns were observed among controls (hospitalisation rates: aIRR = 2.96, 95% CI 2.63 to 3.23, p<0.001; hospitalisation duration: adjusted {beta} = 0.93, 95% CI: 0.86 to 0.99, p<0.001; mortality [time-varying]: HR = 1.50, 95% CI: 1.27 to 1.77, p<0.001). The relationship between DM and increased hospitalisation was stronger in rural areas among those with MS and stronger in White groups among controls. Conclusions: People with MS are more likely to be diagnosed with DM, resulting in greater all-cause hospitalisation and all-cause mortality. This highlights the importance of equitable screening, prevention, and management of DM in people living with MS, with particular attention to geographical health inequalities.