Severity of Tourette's syndrome in one large kindred. Implication for determination of disease prevalence rate.

An accurate prevalence rate for Tourette's syndrome (TS) has not been established. To assess severity of illness, a potential source of bias in determining prevalence rate, we administered standardized questionnaires and examinations to 159 members of a large Mennonite kindred showing apparent autosomal dominant transmission of motor and vocal tics (TS) or chronic motor tics (CMTs). Fifty-four family members were diagnosed as having definite or probable TS or CMTs. For these 54 subjects, 30% (n = 16) were unaware of tics noted by the examiners and only 18.5% (n = 10) had sought medical care. Our findings suggest that most cases of TS and CMTs are mild and do not come to medical attention. These tic disorders are probably much more prevalent than generally appreciated.