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

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. 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. 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. 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. None. For the Icelandic translation of the abstract see Supplementary Materials section.