Prevalence and Case Validity of Autism Spectrum Disorders in the Stockholm Youth Cohort

Background: The prevalence of autism spectrum disorders (ASD) has risen sharply over the past two decades and is now estimated at approximately 1% to as high as 2.6% in recent large-scale studies. Recent Scandinavian reports are inconsistent, with rates ranging from 0.1 to 1%. Although changes in diagnostic practices and wider recognition may explain part of the observed rise, a true increase in incidence of ASDs has not been ruled out. Therefore, there is a need of large, prospective, population-based studies exploring modifiable risk factors.

 Objectives: To describe the study design of the Stockholm Youth Cohort (SYC), a register-based total population study with extensive and prospectively recorded information on potential determinants and consequences of ASD. Furthermore, to determine the prevalence of ASD with and without comorbid intellectual disability and explore the validity of registry-based diagnoses.

Methods: The Stockholm Youth Cohort is a record-linkage study comprising all individuals aged 0-17 years, ever resident in Stockholm County in 2001-2007 (N=589,114). Cases (N=4,952) were identified using a multisource approach, involving registers covering all pathways to ASD diagnosis and care, and categorized according to presence of co-morbid intellectual disability. Clinical notes for 177 randomly selected cases were reviewed in detail to ascertain case validity.

Results: The 2007 year prevalence of ASD in all youths was 1.1%, presenting with intellectual disability in 42% of cases. Prevalence varied across birth cohorts, peaking at 1.4% for children aged 12-16 years. Clinical notes for 177 randomly selected cases were reviewed, and ASD diagnosis confirmed in 96%. 

Conclusions: Findings from this contemporary total population cohort, set in Stockholm County, is in accordance with recently reported estimates from Western countries at around 1%, based on valid case ascertainment. The Stockholm Youth Cohort, in light of the availability of extensive information from Sweden’s comprehensive registers, constitutes an important resource for ASD research. Ongoing work, including collection of biological samples, will enrich the study further over coming years.