Executive Functions in Preschool Children with ADHD and DBD: Assessment, Development and Role of Environment

Impairments in executive functions (EF) are consistently associated with attention deficit hyperactivity disorder (ADHD) and to a lesser extent, with disruptive behavior disorder (DBD), i.e., oppositional defiant disorder or conduct disorder, in school-aged children. Recently, larger numbers of children with these disorders are diagnosed earlier in development, yet knowledge about impairments in clinically diagnosed preschool children and the role of co-morbidity is limited. The aims of this dissertation were threefold. We aimed to study EF, the development of EF, and the role of the caregiving environment on the development of EF in preschool children with ADHD and/or DBD. First, we focused on the assessment of EF in these groups by (a) using newly developed age-appropriate EF tasks and examining the EF factor structure; (b) conducting a meta-analysis regarding the relation between EF and externalizing behavior problems in preschool children; (c) conducting a cross-sectional study to examine differences in EF performance in preschool children diagnosed with ADHD and/or DBD. Second, the development of EF in these children was examined in a 1½ year longitudinal study. Third, the role of the caregiving environment on the development of EF was investigated. Method: To examine the aims (except for conducting the meta-analysis) we investigated clinically diagnosed preschool children, 61 diagnosed with ADHD, 33 with DBD, 52 with ADHD+DBD and 58 typically developing children. Five EF tasks were administered at baseline, at nine-months follow-up and at eighteen months follow-up. Caregiving environment characteristics (parenting practices and stress, depression and ADHD symptoms of the caregiver) were evaluated at baseline. Results: In the meta-analysis 22 studies were reviewed with three to six year old children with symptoms of ADHD and/ or DBD from community and referred samples. A medium effect size was obtained for overall EF and for inhibition, whereas a small effect size was found for working memory and for cognitive flexibility. Moderator analyses revealed a stronger effect for older preschoolers compared to younger preschoolers, and for children from referred samples compared to community samples. In our own study, we found a two factor structure of EF: an inhibition and a working memory factor. Clinically diagnosed preschool children with ADHD showed robust inhibition deficits, whereas preschool children with DBD showed impaired inhibition especially where motivational incentives were prominent. Severity of inhibition impairment in the co-morbid group was similar to the ADHD group.For working memory, few group differences were found. Regarding the second aim, inhibition and working memory performance increased over time for children with ADHD and/or DBD, especially in the early preschool period. Working memory impairments became more apparent with age in children with ADHD. Regarding the third aim, even over a relatively brief period of 18 months caregiving characteristics such as maternal stress were associated with young children’s developmental changes in executive functions, also in children with ADHD and DBD. Conclusion: Preschoolchildrenwith ADHD, and to a lesser extent children with DBD, were impaired on inhibition performance. Overall, we found more similarities than differences in EF performance between children with ADHD and DBD.