Predictors of mental illness onset in adolescents and adults with intellectual disability: A retrospective cohort study in New South Wales, Australia.

Background: People with intellectual disability are disproportionately affected by mental illness, including serious mental illness. While the prevalence of mental illness in this population is well-documented, the factors associated with the onset of any mental illness and serious mental illness lack comprehensive investigation. This study aims to identify demographic, service-related and disability-related factors associated with the onset of any mental illness and serious mental illness in people with intellectual disability using a large, linked dataset in New South Wales, Australia.

Methods: A retrospective cohort study was conducted using linked administrative data for 47,330 individuals with intellectual disability aged 13-80 years. Data from 2004 to 2018 were used to track first recorded contact with mental health services for any mental illness and serious mental illness. Flexible parametric survival analysis was employed to account for time-varying factors and estimate hazard ratios for the risk of developing any mental illness or serious mental illness.

Results: Nearly half of the cohort (48.9%) experienced any mental illness, and 11.7% experienced serious mental illness. Factors associated with any mental illness included attention-deficit/hyperactivity disorder, learning disorders, physical comorbidities, and living in areas of greater socioeconomic disadvantage. Serious mental illness onset was associated with living in outer regional, remote or very remote areas, attention-deficit/hyperactivity disorder, learning disorders, male sex, and a history of any mental illness.

Conclusion: This study identified factors associated with the onset of any mental illness and serious mental illness in people with intellectual disability. These findings emphasise the need for early identification and targeted interventions to improve mental health outcomes in this high-risk population.