What have we learned about ADHD from the structural neuroimaging studies?

Attention-deficit/hyperactivity disorder (ADHD) belongs to the most common behavioural disorders of childhood. ADHD can be conceptualised as a diverse developmental disorder characterised by a variable clinical expression, which is underlain by heterogeneity leading to the neural system dysfunction. Neuroimaging for childhood psychiatric disorders has the potential to increase our understanding of the pathophysiology of childhood mental disorders. In recent years, neuroimaging techniques have been used with increasing frequency in attempts to identify structural and functional abnormalities in the brains of children with ADHD. Structural imaging methods have localized abnormalities in key brain regions and neural networks associated with cognition and behaviour consistent with the clinical picture of ADHD. Structural imaging studies suggest that the ADHD pathophysiology would be conditioned by the dysfunction in frontosubcortical pathways. Currently increasing is the evidence that other brain regions such as the cerebellum, the parietal lobes and temporal lobes may also have an important role in this condition. The findings generally suggest deficits in the brain areas mentioned above, with decreased volumes. However, it is also evident that some areas show enlargement as a compensation for the observed deficits. Apart from the issue of reliability, there is a more basic question about how the results of neuroimaging studies are to be interpreted.