Comorbidity of tic disorders & ADHD - Conceptual and methodological considerations

Background The causes and pathophysiological mechanisms of the common comorbidity of tic disorders and Attention-deficit/Hyperactivity disorder (TD + ADHD; about 50% of TD, about 20% of ADHD) still remain unclear. Studies with a 2 x 2 design comparing groups of children with TD, ADHD, TD + ADHD and healthy controls are in need and may disentangle the influences of TD versus ADHD in the case of comorbidity. Unfortunately, conceptual and methodological problems can restrain possible conclusions from these studies. Method A review of the literature on artifactual and non-artifactual models of comorbidity in general and specially for TD + ADHD was conducted. Results The first section describes various possible models of comorbidity and their corresponding hypotheses concerning expected patterns of findings comparing groups of children with TD, ADHD, TD + ADHD and healthy controls. In the second part re- search results concerning psychopathological, neuropsychological, neurophysiological, structural and functional imaging, as well as genetic characteristics are summarized. In the third section possible conclusions and their limitations due to conceptual and methodological problems possibly contributing to the ambiguous results are discussed. Finally, future research strategies and the need for full causal models are outlined. Conclusion Some components of the etiological pathways of TD + ADHD may well be shared with the 'pure' conditions while others may be unique.