The effect of calendar month on tic severity in children and youth with Tourette syndrome

Background Known influences on tic severity include medical, biological and contextual factors. Aims We aimed to further understanding of contextual factors by exploring if tic severity is influenced by calendar month. Method This study used data from the Calgary Child Tic Registry. Children are extensively clinically phenotyped at their first visit and followed prospectively until adulthood. We evaluated the mean Yale Global Tic Severity Scale-Revised (YGTSS-R) total tic severity score based on the calendar month. Multivariable linear regression models were fit to assess the individual months adjusted for age, gender, comorbidity and tic treatment variables. Results The study included 370 participants, with 549 assessments of tic severity performed. In the univariable analysis based on calendar month, August had the lowest tic severity, with a mean YGTSS-R total tic severity score of 15.68 (95% CI 13.41–17.95). This was significantly lower than the month with the highest tic severity, February, with a mean score of 20.41 (95% CI 18.19–22.63). In multivariable models adjusted for age, gender, comorbidity and treatment for tics, the omnibus test for whether month contributes to a better fit were not significant (YGTSS-R total tic score P -value: 0.495). The only significant predictors of increased tic severity were treatment for tics ( P < 0.0001), diagnosis of depression ( P = 0.003) and diagnosis of obsessive–compulsive disorder ( P = 0.02). Conclusions While our univariate analysis of tic severity by calendar month supported significantly lower tic severity in August compared with February, this association was no longer statistically significant when controlling for other variables known to impact tic severity.