Transient environmental effects on tic severity
A study of 45 children with TS supported the typical antecedent–behavior–consequence behavioral psychology model \citep{27044052}. Specifically, consequences of tics, "such as receiving accommodations or attention from others," explained significantly more variance in tic severity than did the child's level of separation anxiety, though the latter was also a significant factor. This study provides supportive evidence for the approach taken by "CBIT-Jr," a behavior therapy designed for younger children with TS \citep{RN9681}.
Other
Non-tic symptoms in TS are reviewed by \citet{Martino2017}. \citet{28364427} used Taiwan’s National Health Insurance Research Database to compare 1124 newly diagnosed TS patients to controls in a 1:3 match. Sleep disorders were twice as common in TS, and remained significantly higher in TS after accounting for anxiety disorders, which were the comorbid conditions associated with the highest risk. In a sample of 811 TS subjects recruited for a genetics study, hair pulling (3.8%) and skin picking (13.0%) disorders by DSM-5 were surprisingly common \citep{29098466}.
Autism spectrum disorders (ASD) comprise an underexplored comorbidity of TD. There are few epidemiological studies on the subject but the prevalence of ASD in children with tic disorers is estimated at 20% \citep*{17075357}. In a large study including patients with TD (n=535) and their family members (n=234), \citet{28647013a} used the Social Responsiveness Scale Second Edition (SRS) to characterize ASD symptoms, and compared them to historical ASD samples. SRS scores in participants with TD were similar to those observed in other clinical samples but lower than in ASD samples. This is mostly but not entirely explained by elevations in the RRB (restricted interests and repetitive behaviors) subscale, which may be indicating tics rather than other stereotypic movements. The presence of OCD was associated with higher scores on the social cognition and RRB subscales. Complex tics and OCD symptoms (repetitive behaviors) can also be hard to discriminate from core ASD symptoms, especially those related to social communication.
Etiology
A national database study found that parents of children with chronic tics had significantly higher rates of psychiatric illness: more than twice as high in mothers and 39% higher in fathers \citep{28335873}. The maternal risk, which included a range of psychopathology, was significantly higher than the paternal risk, which comprised primarily OCD and anxiety disorders. Further work will be needed to clarify whether results reflect maternal-specific environmental risks, genetic risks, factors related to parental care-seeking, or (to a modest extent, given the typical ages of onset for various parental disorders identified) parental stress.
Genetics
A large mixed genetic sample yielded two heritable collections of symptoms that cross diagnostic boundaries, here named symmetry (including some other obsessions and compulsions) and disinhibition (including complex verbal tics) \citep{27809572}. Whole exome sequencing from over 500 trios identified a clear excess of likely gene-disrupting de novo mutations, and 4 risk genes that were altered by different mutations in multiple probands \citep{Willsey2017}. Another report described whole exome sequencing in a 3-generation family with TS, using induced pluripotent stem cells converted into neuronal cell types and assessing protein expression levels \citep{28894297}. The PNKD gene product was expressed at a lower rate in family members with TS or OCD.