Multiple sclerosis and risk of Parkinson's disease: a Danish nationwide cohort study

Background and purpose Case reports have observed a co‐occurrence of multiple sclerosis ( MS ) and Parkinson's disease ( PD ) and it has been hypothesized that MS lesions could affect dopaminergic pathways causing parkinsonism. Our aim was to examine the association between MS and PD in a historically prospective cohort study using Danish nationwide register data. Methods Multiple sclerosis patients identified in the Multiple Sclerosis Registry were followed for PD from 1977 to 2011 in the National Patient Register. As measures of relative risk, ratios of observed to expected incidence rates of first hospitalization for PD amongst persons with MS were used, i.e. standardized incidence ratios ( SIR s) with 95% confidence intervals ( CI s). Results Amongst 15 557 MS patients 26 cases of PD were observed versus 26.51 expected, reflecting no overall increased risk of PD ( SIR 0.98, 95% CI 0.67–1.44). Similar estimates were seen for female ( SIR 0.99, 95% CI 0.58–1.67) and male MS patients ( SIR 0.97, 95% CI 0.55–1.72). Likewise, no increased risk of PD amongst MS patients was observed in a robustness analysis backdating the date of diagnosis of PD by 5 years to account for the time lag between disease onset and first hospital contact with PD ( SIR 0.57, 95% CI 0.32–1.00). Conclusion Our data do not suggest an increased risk of PD amongst patients with MS .