Objective To determine whether hospital admission for autoimmune disease is associated with an elevated risk of future admission for dementia.

Methods Retrospective, record-linkage cohort study using national hospital care and mortality administrative data, 1999–2012. Cohorts of people admitted to hospital with a range of autoimmune diseases were constructed, along with a control cohort, and followed forward in time to see if they developed dementia. 1 833 827 people were admitted to hospital with an autoimmune disease; the number of people in cohorts for each autoimmune disease ranged from 1019 people in the Goodpasture's syndrome cohort, to 316 043 people in the rheumatoid arthritis cohort.

Results The rate ratio for dementia after admission for an autoimmune disease, compared with the control cohort, was 1.20 (95% CI 1.19 to 1.21). Where dementia type was specified, the rate ratio was 1.06 (1.04 to 1.08) for Alzheimer's disease and 1.28 (1.26 to 1.31) for vascular dementia. Of 25 autoimmune diseases studied, 18 showed significant positive associations with dementia at p<0.05 (with 14 significant at p<0.001) including Addison's disease (1.48, 1.34 to 1.64), multiple sclerosis (1.97, 1.88 to 2.07), psoriasis (1.29, 1.25 to 1.34) and systemic lupus erythematosus (1.46, 1.32 to 1.61).

Conclusions The associations with vascular dementia may be one component of a broader association between autoimmune diseases and vascular damage. Though findings were significant, effect sizes were small. Clinicians should be aware of the possible coexistence of autoimmune disease and dementia in individuals. Further studies are needed to confirm or refute our findings and to explore possible mechanisms mediating any elevation of risk.