Cognitieve ontwikkeling van kleuters met een risico op dyslexie en ADHD

Dyslexia and ADHD are among the most prevalent neurodevelopmental disorders, each affecting at least 5% of the population. These two disorders often co-occur, as about 15-40% of children with one diagnosis also have symptoms of the other. However, research has primarily focused on children at risk for either dyslexia or ADHD, excluding those at risk for co-occurring dyslexia and ADHD. Consequently, the unique cognitive profile of children at risk for co-occurring dyslexia and ADHD remains poorly understood, leaving a crucial gap in the literature. Focusing on the cognitive characteristics of neurodevelopmental disorders is essential, not only to advance our fundamental knowledge about these disorders, but also to find direction for effective interventions. To date, research on cognitive characteristics of co-occurring dyslexia and ADHD has only been done in older children with diagnoses. As such, much less is known about early signs and the cognitive development of co-occurring dyslexia and ADHD. Yet, this is vital for accurately identifying vulnerable children, refining aetiological models, and targeting specific areas that may require intervention. This doctoral dissertation aims to address this critical gap in the existing research by conducting a longitudinal study, tracking the cognitive development of 186 children at risk for dyslexia, ADHD, co-occurring dyslexia and ADHD, or without risk. In Chapter 2, we examined the cognitive profiles of these preschoolers at age 5. Post hoc group differences revealed two distinct groups: one including children without risk or at risk for ADHD who performed significantly higher on almost all measures than the other group, including children at risk for dyslexia or at risk for co-occurring dyslexia and ADHD. In summary, ADHD risk did not impact the cognitive profile of these children. Depending on the analytical sample, the group of children at risk for co-occurring dyslexia and ADHD showed lower performance on a composite EF score. Chapter 3 explored the EF development of the entire sample of children. Results revealed that a one-factor model best explained EF across all risk groups and across all age groups, specifically ages 5, 6, and 8. Further analysis showed that children at risk for dyslexia or co-occurring dyslexia and ADHD had lower starting points in EF compared to children without risk. Children in dyslexia risk groups (regardless of ADHD status) showed significantly steeper growth rates compared to those not at risk, but they were never able to catch up fully. In the last study, Chapter 4, we regrouped children based on diagnostic status at age 8. All children showed improvement over time. There were both shared deficits in processing and unique cognitive deficits among the clinical groups, particularly in the co-occurring dyslexia and ADHD group. This doctoral project shed light on the cognitive vulnerability associated with co-occurring dyslexia and ADHD, both for at-risk and diagnosed children. These findings have important implications for early diagnosis and intervention, potentially leading to better outcomes for children who face the dual challenges of dyslexia and ADHD as they transition from preschool to primary school.