Objective: There is considerable epidemiological and neuropsychological evidence that attention deficit hyperactivity disorder (ADHD) is best considered dimensionally, lying at the extreme end of a continuous distribution of symptoms and underlying cognitive impairments. The authors investigated whether cortical brain development in typically developing children with symptoms of hyperactivity and impulsivity resembles that found in the syndrome of ADHD. Specifically, they examined whether a slower rate of cortical thinning during late childhood and adolescence, which they previously found in ADHD, is also linked to the severity of symptoms of hyperactivity and impulsivity in typically developing children.
Method: In a longitudinal analysis, a total of 193 typically developing children with 389 neuroanatomic magnetic resonance images and varying levels of symptoms of hyperactivity and impulsivity (measured with the Conners' Parent Rating Scale) were contrasted with 197 children with ADHD with 337 imaging scans. The relationship between the rates of regional cortical thinning and severity of symptoms of hyperactivity/impulsivity was determined.
Results: Youth with higher levels of hyperactivity/impulsivity had a slower rate of cortical thinning, predominantly in prefrontal cortical regions, bilaterally in the middle frontal/premotor gyri, extending down the medial prefrontal wall to the anterior cingulate; the orbitofrontal cortex; and the right inferior frontal gyrus. For each increase of one point in the hyperactivity/impulsivity score, there was a decrease in the rate of regional cortical thinning of 0.0054 mm/year (SE=0.0019 mm/year). Children with ADHD had the slowest rate of cortical thinning.
Conclusions: Slower cortical thinning during adolescence characterizes the presence of both the symptoms and syndrome of ADHD, providing neurobiological evidence for dimensionality of the disorder.