[Structural magnetic resonance imaging findings as an aid to explain the neurobiology of attention deficit hyperactivity disorder: towards the identification of a neuroanatomical phenotype].

INTRODUCTION: Attention deficit hyperactivity disorder (ADHD) is a highly prevalent mental disorder in childhood. From the nineties, structural MRI has been used in the study of the neurobiological bases of ADHD. AIMS: To summarize and to analyze the main findings reported in the structural MRI studies of ADHD. DEVELOPMENT: A Medline search of MRI studies examining ADHD neuroanatomy from 1990 until 2006 was conducted. Most of the reviewed articles carried out morphometric analysis of selected regions of interest. The more thoroughly examined regions of interest were the frontal lobes and the caudate nuclei. The most replicated positive finding was the decreased volume of the right prefrontal region (5 studies). The most replicated negative finding (5 studies) was the normal volume of the left prefrontal region. The right caudate nucleus was reported to be diminished in almost all the studies (4 out of 5) with a sample size equal or greater than 30. Among the regions of interest not pertaining to the fronto-striatal circuits, the total brain volume (9 out of 17) and morphometric variables associated to corpus callosum and cerebellum encompass most of the positive findings. CONCLUSION: These findings support the involvement of the right fronto-striatal circuit in the neurobiology of ADHD. The diminished right caudate nucleus volume could be a fenotypical trait of pediatric ADHD. The intrinsic heterogeneity of ADHD and methodological issues are the main factors accounting for the observed discrepancies.