between TS and OCD, the data are less clear with regard to the relationships between TS and ADHD and between OCD and ADHD4–6, 12, 18–31. For example, family studies have shown that ADHD alone is not increased in TS families; when present, it is usually as a co-occurring condition with TS, suggesting that these disorders do not share genetic susceptibility factors (see O’Roarke, et al for review)32. Similarly, there is some evidence for an increased rate of ADHD in individuals and families with OCD but without tics30, 31, 33. Familial risk studies have suggested that rather than TS and OCD being genetically related, OCD and ADHD may share susceptibility genes, and that the increased rates of ADHD in TS-affected individuals may be mediated via OCD26, 30, 31, 34, 35. Similarly, the relationship of OCB to TS is unclear. Some studies have reported a prevalence of up to 80% in TS-affected individuals, suggesting that OCB may actually represent an integral part of the TS phenotype, and as such may be more akin to complex tics than to true obsessive/compulsive phenomena36–38.
