Incidence of unipolar and bipolar depression, and mania in adults with intellectual disabilities: prospective cohort study

Background Incidence and determinants of affective disorders among adults with intellectual disabilities are unknown. Aims To investigate affective disorder incidence, and determinants of unipolar depression, compared with general population reports. Method Prospective cohort study measuring mental ill health of adults with mild to profound intellectual disabilities living within a defined community, over 2 years. Results There was 70% cohort retention (n = 651). Despite high mood stabiliser use (22.4%), 2-year incident mania at 1.1% is higher than the general population; 0.3% for first episode (standardised incident ratio (SIR) = 41.5, or 52.7 excluding Down syndrome). For any bipolar episode the SIR was 2.0 (or 2.5 excluding Down syndrome). Depression incidence at 7.2% is similar to the general population (SIR = 1.2), suggesting more enduring/undertreatment given the higher prevalence. Problem behaviours (odds ratio (OR) = 2.3) and life events (OR = 1.3) predict incident unipolar depression. Conclusions Depression needs improved treatment. Mania has received remarkably little attention in this population, despite high prevalence and incidence (similar to schizophrenia), and given the importance of clinician awareness for accurate differential diagnosis from attention-deficit hyperactivity disorder and problem behaviours.