Prevalence and Cumulative Incidence of Autism Spectrum Disorders and the Patterns of Co-Occurring Neurodevelopmental Disorders in a Total Population Sample of 5-Years-Old Children

Background: Whether there is a true increase in Autism Spectrum Disorder (ASD) frequency or not remains unclear. Additionally, the rates of co-existing neurodevelopmental disorders (NDD) in a total population sample hasn't been fully examined before. Methods: All 5-year-old children in the catchment area in Japan underwent the screening annually from the year 2013 - 2016. Screen-positive children were invited to a comprehensive assessment, including child and parent interview, behavioral observation, and cognitive and motor function testing. All cases were reviewed by a multidisciplinary team. ASD prevalence and cumulative incidence were computed. Outcomes: Caregivers of 3,954 children returned the screening, among which 559 children underwent the assessment with 87 children receiving an ASD diagnosis. Adjusted ASD prevalence was 3.22% (95% Confidence Interval (CI): 2.66 - 3.76). The male to female ratio of the crude prevalence was 2.2:1. The cumulative incidence of ASD up to 5 years of age for the total study years was 1.31 (95% CI: 1.00 - 1.62). A generalized linear model revealed no significant linear trends in 5-year cumulative incidence over the study years. Only 11.5% of children had ASD alone; the remaining 88.5% were found to have at least one co-existing NDD. Interpretation: Our findings demonstrate the stability of the 5-year cumulative incidence of ASD, implying no true rise in ASD incident cases over the 4-year study period in the study catchment area. High rates of co-existing NDDs reflect the importance of investigating broad developmental challenges in ASD children. Funding: Grants-in-Aid for Scientific Research and Hirosaki Institute of Neuroscience. Declaration of Interest: The authors have no financial relationships relevant to this article to disclose. Ethical Approval: This study was approved by the Committee of Medical Ethics of Hirosaki University Graduate School of Medicine. Moreover, the information security policies of the city and committee were followed to protect the personal data of the participants.