A factor analysis of tic symptoms in Gilles de la Tourette's syndrome.

OBJECTIVE: Gilles de la Tourette's syndrome is a well-characterized disorder with clear DSM-IV criteria. However, the great deal of clinical variability across patients may represent an underlying etiologic complexity. Issues of phenotypic heterogeneity are particularly critical to current efforts at mapping genes involved in this syndrome.
METHOD: Lifetime tic symptom data were obtained from direct structured interviews of 85 Tourette's disorder probands. Information on 29 tic symptoms was elicited. The probands' tic symptoms were grouped by using agglomerative hierarchical clustering, with no a priori assumptions concerning the relatedness of symptoms. Scores for the probands' symptom clusters were used as variables in a principal-component factor analysis. The relationships of the resulting factor scores to comorbidity in probands and recurrence risks in relatives were examined. In addition, intraclass correlations were calculated for within-family factor scores of 36 families.
RESULTS: Four significant factors were identified: 1) aggressive phenomena (e.g., kicking, temper fits, argumentativeness), 2) purely motor and phonic tic symptoms, 3) compulsive phenomena (e.g., touching of others or objects, repetitive speech, throat clearing), and 4) tapping and absence of grunting. Analysis of the symptom loadings, comorbidity, recurrence risks, and within-family correlations indicate that these factors represent a valid structure with clinical and biological relevance.
CONCLUSIONS: In this symptom-based factor analysis of Tourette's disorder, four factors accounted for 61% of the phenotypic symptom variance in Tourette's disorder probands and their first-degree relatives. Three of these factors may indicate the presence of heritable components of the Tourette's disorder phenotype.