Incidence of neurological and psychiatric comorbidity over time: a population-based cohort study in Ontario, Canada

INTRODUCTION: Comprehensive, population-based investigations of the extent and temporality of associations between common neurological and psychiatric disorders are scarce. METHODS: This retrospective cohort study used linked health administrative data for Ontarians aged 40-85 years on 1 April 2002, to estimate the adjusted rate of incident dementia, Parkinson's disease (PD), stroke or mood/anxiety disorder (over 14 years) according to the presence and time since diagnosis of a prior disorder. Sex differences in the cumulative incidence of a later disorder were also examined. RESULTS: The cohort included 5,283,546 Ontarians (mean age 56.2 ± 12.1 years, 52% female). The rate of dementia was significantly higher for those with prior PD (adjusted hazard ratio adjHR 4.05, 95% confidence interval CI 3.99-4.11); stroke (adjHR 2.49, CI 2.47-2.52) and psychiatric disorder (adjHR 1.79, CI 1.78-1.80). The rate of PD was significantly higher for those with prior dementia (adjHR 2.23, CI 2.17-2.30) and psychiatric disorder (adjHR 1.77, CI 1.74-1.81). The rate of stroke was significantly higher among those with prior dementia (adjHR 1.56, CI 1.53-1.58). Prior dementia (adjHR 2.36, CI 2.33-2.39), PD (adjHR 1.80, CI 1.75-1.85) and stroke (adjHR 1.47, CI 1.45-1.49) were associated with a higher rate of an incident psychiatric disorder. Generally, associations were strongest in the 6 months following a prior diagnosis and demonstrated a J-shape relationship over time. Significant sex differences were evident in the absolute risks for several disorders. CONCLUSIONS: The observed nature of bidirectional associations between these neurological and psychiatric disorders indicates opportunities for earlier diagnosis and interventions to improve patient care.