ObjectiveReports of rising prevalence of autism spectrum disorders (ASD), along with their profound personal and societal burden, emphasize the need of methodologically sound studies to explore their causes and consequences. We here present the design of a large intergenerational resource for ASD research, along with population-based prevalence estimates of ASD and their diagnostic validity.MethodThe 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). ASD cases (N = 5,100) were identified using a multisource approach, involving registers covering all pathways to ASD diagnosis and care, and categorized according to co-morbid intellectual disability. Prospectively recorded information on potential determinants and consequences of ASD were retrieved from national and regional health and administrative registers. Case ascertainment was validated through case-note review, and cross validation with co-existing cases in a national twin study.ResultsThe 2007 year prevalence of ASD in all children and young people was 11.5 per 1,000 (95% confidence interval 11.2–11.8), with a co-morbid intellectual disability recorded in 42.6% (41.0–44.2) of cases. We found 96.0% (92.0–98.4) of reviewed case-notes being consistent with a diagnosis of ASD, and confirmed ASD in 85.2% (66.2–95.8) of affected twins.ConclusionsFindings from this contemporary study accords with recently reported prevalence 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 registers, constitutes an important resource for ASD research. On-going work, including collection of biological samples, will enrich the study further.

In a record-linkage study in Stockholm, Sweden, the year 2011 prevalence of diagnosed autism spectrum disorders (ASD) was found to be 0.40, 1.74, 2.46, and 1.76% among 0-5, 6-12, 13-17, and 18-27 year olds, respectively. The corresponding proportion of cases with a recorded diagnosis of intellectual disability was 17.4, 22.1, 26.1 and 29.4%. Between 2001 and 2011, ASD prevalence increased almost 3.5 fold among children aged 2-17 years. The increase was mainly accounted for by an eightfold increase of ASD without intellectual disability (from 0.14 to 1.10 %), while the prevalence of ASD with intellectual disability increased only slightly (from 0.28 to 0.34%). The increase in ASD prevalence is likely contributed to by extrinsic factors such as increased awareness and diagnostics.

In order to define potential subgroups pertaining to the spectrum of‘high‐functioning’pervasive developmental disorders (PDDs) the medical and psychiatric records of 101 children with PDD were reviewed. Ninety‐one children had a diagnosis of Asperger syndrome, nine had a PDD not otherwise specified, and one had‘high‐functioning’autism. Mean age of the children (71 males, 30 females) was 9 years 8 months (age range 5 to 12 years). Apart from the core dysfunctions of the PDD, i.e. deficient social interaction, communication and repertoires, and restricted interests, 95% had attentional problems, 75% had motor difficulties, 86% had problems with regulation of activity level, and 50% had impulsiveness. About three‐quarters had symptoms compatible with mild or severe attention‐deficit‐hyperactivity disorder (ADHD) or had deficits in attention, motor control, and perception (DAMP), indicating a considerable overlap between these disorders and high‐functioning PDD in children of normal or near‐normal intelligence. A combination of affective dysregulation, thought disturbance, and severely restricted social interaction, referred to as a multiple complex developmental disorder (MCDD; a condition possibly related to schizoaffective disorder), was recorded in about 8% of the children. Seventeen percent had another major medical diagnosis or medical syndrome, which highlights the importance of completing a neurological assessment of all children with PDDs.

The diagnostic validity of the new research algorithms of the Autism Diagnostic Interview-Revised and the revised algorithms of the Autism Diagnostic Observation Schedule was examined in a clinical sample of children aged 18-47 months. Validity was determined for each instrument separately and their combination against a clinical consensus diagnosis. A total of N = 268 children (n = 171 with autism spectrum disorder) were assessed. The new Autism Diagnostic Interview-Revised algorithms (research cutoff) gave excellent specificities (91%-96%) but low sensitivities (44%-52%). Applying adjusted cutoffs (lower than recommended based on receiver operating characteristics) yielded a better balance between sensitivity (77%-82%) and specificity (60%-62%). Findings for the Autism Diagnostic Observation Schedule were consistent with previous studies showing high sensitivity (94%-100%) and alongside lower specificity (52%-76%) when using the autism spectrum cutoff, but better balanced sensitivity (81%-94%) and specificity (81%-83%) when using the autism cutoff. A combination of both the Autism Diagnostic Interview-Revised (with adjusted cutoff) and the Autism Diagnostic Observation Schedule (autism spectrum cutoff) yielded balanced sensitivity (77%-80%) and specificity (87%-90%). Results favor a combined usage of the Autism Diagnostic Interview-Revised and Autism Diagnostic Observation Schedule in young children with unclear developmental problems, including suspicion of autism spectrum disorder. Evaluated separately, the Autism Diagnostic Observation Schedule (cutoff for autism) provides a better diagnostic accuracy than the Autism Diagnostic Interview-Revised.

In order to define potential subgroups pertaining to the spectrum of 'high-functioning' pervasive developmental disorders (PDDs) the medical and psychiatric records of 101 children with PDD were reviewed. Ninety-one children had a diagnosis of Asperger syndrome, nine had a PDD not otherwise specified, and one had 'high-functioning' autism. Mean age of the children (71 males, 30 females) was 9 years 8 months (age range 5 to 12 years). Apart from the core dysfunctions of the PDD, i.e. deficient social interaction, communication and repertoires, and restricted interests, 95% had attentional problems, 75% had motor difficulties, 86% had problems with regulation of activity level, and 50% had impulsiveness. About three-quarters had symptoms compatible with mild or severe attention-deficit-hyperactivity disorder (ADHD) or had deficits in attention, motor control, and perception (DAMP), indicating a considerable overlap between these disorders and high-functioning PDD in children of normal or near-normal intelligence. A combination of affective dysregulation, thought disturbance, and severely restricted social interaction, referred to as a multiple complex developmental disorder (MCDD; a condition possibly related to schizoaffective disorder), was recorded in about 8% of the children. Seventeen percent had another major medical diagnosis or medical syndrome, which highlights the importance of completing a neurological assessment of all children with PDDs.