Incidence, co-occurring psychiatric conditions, and sex differences in young people without intellectual impairment who are autistic, ADHD, or autistic–ADHD: a population-based cross-sectional study in Iceland

Abstract

Background

Population-based studies comparing the incidence and co-occurring psychiatric conditions of young people without intellectual impairment who are autistic, Attention-Deficit/Hyperactivity Disorder (ADHD), or autistic–ADHD are scarce. For autistic, ADHD, and autistic–ADHD youth in Iceland aged 7–18 years without intellectual impairment, we aimed primarily to estimate the age-standardised incidence of these 3 neurotypes, overall and by sex, and secondarily to estimate the prevalence of co-occurring psychiatric conditions and emotional and conduct challenges.

Methods

In this nationwide, population-based cross-sectional study we included young people without intellectual impairment aged 7–18 years who were autistic, ADHD, or autistic–ADHD. Children were referred to the Centre for Child Development and Behaviour in Reykjavik, Iceland, through a structured pre-assessment process during which caregivers completed a validated screening battery on the child's behavioural, emotional, and developmental characteristics. Trained clinicians administered gold-standard clinical assessment procedures to assess autism, ADHD, and co-occurring psychiatric presentations. Caregiver-reported and teacher-reported emotional and conduct challenges were measured with the Strengths and Difficulties Questionnaire. ICD-10 condition classification was determined during consensus meetings with clinical psychologists and a paediatrician. Age-standardised prevalence and incidence rates were calculated.

Findings

Between Feb 11, 2013, and Dec 20, 2021, 2034 children age 7–18 years without intellectual impairment (728 females and 1306 males; mean age 10·93 [SD 2·82]) were recognised as autistic (n=229), ADHD (n=1428), or autistic–ADHD (n=377) in Iceland. Age-standardised incidence rates were 126 per 100 000 person-years (95% CI 116–137) for all autistic young people (ie, autistic and autistic–ADHD) and 374 per 100 000 person-years (357–392) for all young people with ADHD (ie, ADHD and autistic–ADHD). By neurotype groups, the incidence per 100 000 person-years was 48 (95% CI 42–54) for autism, 78 (71–87) for autism–ADHD, and 295 (280–311) for ADHD. Incidence was lower in females than males for all three neurotypes: incidence rate ratio 0·53 (95% CI 0·40–0·69) for autistic young people, 0·43 (0·35–0·54) for autistic–ADHD young people, and 0·64 (0·57–0·71) for ADHD young people.

Interpretation

This study provides robust, population-based estimates of the incidence of autistic, ADHD, and autistic–ADHD young people without intellectual impairment. The higher incidence of autistic–ADHD young people compared with autistic alone underscores the common co-occurrence of ADHD in autistic young people—a pattern that might have been underrepresented in previous literature.

Funding

None.

Translation

For the Icelandic translation of the abstract see Supplementary Materials section.