Videotape Tic Counts in the Assessment of Tourette's Syndrome: Stability, Reliability, and Validity

OBJECTIVE: We examined the short- and long-term temporal stability of tic counts to estimate the minimum length of videotape needed for a reliable index of overall tic activity and determined the interrater reliability and validity of tic counts based on prolonged videotape segments (> 10 minutes). METHOD: Motor and phonic tic counts and clinician ratings were performed on 43 patients with Tourette's syndrome (TS), aged 7 to 50 years. Short-term stability was estimated by determining the mean interval-to-interval correlation of sequential equal-length segments from 30-minute videotape recordings of 20 subjects. Long-term stability was determined by correlating tic counts at 1-week (N = 14) and 2-week intervals (N = 11). In addition, tic counts were correlated with the most widely used clinical ratings of TS. RESULTS: The short-term stability data indicated that estimates of motor and phonic tic frequencies should be based on videotape counts of at least 5 minutes' duration. Tic counts also were highly reliable and were significantly correlated with clinical ratings with the Yale Global Tic Severity Scale and the Clinical Global Impression Scale for Tourette Syndrome. CONCLUSIONS: Standardized videotape tic counts can provide highly reliable, stable measures of tic frequencies that are moderately correlated with selected global ratings of tic severity.