In contrast, the prevalence estimates for CT Narrow (0.5%) and CT Intermediate (1.1%) were somewhat lower than the rates of 1.3% to 3.7% reported in prior population-based studies.9-11 Both CT definitions had lower male-to-female ratios and rates of co-occurring OCD and ADHD compared with the TS groups (Table 3), a finding that is consistent with the one previous population-based study that examined rates of co-occurring conditions in both TS and CT in the same cohort.4 In that study, most of the CT-associated OCD and ADHD arose from subjects with chronic vocal tics (CVT) (8% with OCD, 33% with ADHD) rather than subjects with chronic motor tics (CMT) (0% with OCD and 12% with ADHD). Although a trend toward higher rates of OCD was observed in the ALSPAC sample in CVT relative to CMT, the small sample size of these subgroups, particularly in the Narrow definitions, limit the interpretability of these results (Table S3, available online).
