Neuroimaging of tic disorders with co-existing attention-deficit/hyperactivity disorder

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Abstract
Background Tourette syndrome (TS) and Attention-Deficit/Hyperactivity Disorder (ADHD) are common and debilitating neuropsychiatric illnesses that typically onset in the preschool years. Recently, both conditions have been subject to neuroimaging studies, with the aim of understanding their underlying neurobiological correlates. Objective The relation of TS and ADHD is discussed against the background of findings from previous Magnetic Resonance Imaging (MRI) studies. Methods We review the designs and major findings of previous studies that have examined TS with comorbid ADHD, and we briefly contrast these findings with those in ADHD without comorbid tic disorders. Results The frequent comorbidity of TS and ADHD may reflect a common underlying neurobiological substrate, and studies confirm the hypothesized involvement of fronto-striatal circuits in both TS and ADHD. However, poor inhibitory control and volumetric reductions in fronto-striatal circuits appear to be core features of ADHD, whereas reduced volumes of the caudate nucleus, together with activation and hypertrophy of prefrontal regions that likely help to suppress tics, seem to be core features of TS. Conclusion The etiological relationship between TS and ADHD must be clarified further with cross-sectional and, if possible, longitudinal imaging studies that examine samples of substantial size, including subgroups with pure TS and ADHD, as well as with comorbid conditions.