Brain development and ADHD

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Abstract

Attention-Deficit/Hyperactivity Disorder (ADHD) is characterized by excessive inattention, hyperactivity, and impulsivity, either alone or in combination. Neuropsychological findings suggest that these behaviors result from underlying deficits in response inhibition, delay aversion, and executive functioning which, in turn, are presumed to be linked to dysfunction of frontal–striatal–cerebellar circuits. Over the past decade, magnetic resonance imaging (MRI) has been used to examine anatomic differences in these regions between ADHD and control children. In addition to quantifying differences in total cerebral volume, specific areas of interest have been prefrontal regions, basal ganglia, the corpus callosum, and cerebellum. Differences in gray and white matter have also been examined. The ultimate goal of this research is to determine the underlying neurophysiology of ADHD and how specific phenotypes may be related to alterations in brain structure.

Section snippets

Normal brain development

Over the past 20 years, converging studies have shown that over 90% of a young adult's total brain volume is attained by age 5 (Giedd et al., 1996) and that total cerebral volume (TCV) reaches its maximum volume by early adolescence (Courchesne et al., 2000, Giedd et al., 1999). A recent study of 45 children who received MRI scans 2 years apart between the ages of 5 and 11, showed expansion of the brain to be approximately 1 mm per year, predominantly in the prefrontal cortex (Sowell et al.,

Neuroanatomical correlates of ADHD

As described earlier, MRI studies of children with ADHD must be considered in light of several potential limitations including sample heterogeneity, small sample sizes, and the lack of consistency in MRI methodology (see Table 1). Despite this, there is consistent evidence that the brains of children with ADHD are significantly smaller, on average, than the brains of healthy comparison children throughout childhood and adolescence (Castellanos et al., 2002, Durston et al., 2001, Durston et al.,

Conclusions

Overall, there is now conclusive evidence that ADHD is associated with globally decreased brain volumes relative to age- and sex-matched typically developing controls. This volumetric difference appears to represent a nonprogressive deficit presumably resulting from early genetic and/or environmental factors. These are likely to be equivalent across sex since when patient samples have been recruited to meet equivalent severity criteria (Sharp et al., 1999), results have been statistically

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