Nail biting (onychophagia) is very common in unselected children, but has also been included in descriptions of complex tics. A report from Taiwan examines prevalence of nail biting in over 2000 children, including 765 with a primary tic disorder, finding that nail biting is very common in TS (56.6%) and provisional tic disorder (27.4%)—much more common than in controls (15.0%)—and begins prior to onset of definite tics \cite{Hsueh2022}.
\cite{Cui_2022a} reported about emotional and behavioral profile of children with GTS in China and compare this profile with sex-matched health controls, ADHD, OCD and depression groups. To assess for behavioral and emotional problems in all groups the Child Behavior Checklist (CBCL) was implemented. No association between the eight factors of the CBCL and motor tics, vocal tics or tic severity assessed by the YGTSS was found. Nevertheless, there was a positive association between the impairment scale of the YGTSS and thought problems as well as rule-breaking behavior as assessed by the YGTSS. Contrary to expected, children with GTS showed similar profile of CBCL to the children with depression, but not ADHD and OCD. 
\cite{Li2022} reported about children with refractory GTS and have found that pediatric refractory GTS is characterized by earlier age of onset, longer disease duration, lower IQ, higher prevalence of PU and higher prevalence of psychiatric comorbidities.
\cite{Baizabal-Carvallo2022c} published a study about self-injurious behaviors in GTS.  The authors included 201 patients with GTS and 34 (16.9%) of them had comorbid SIB. Majority of patients experienced self-inflicted damage (11.4%), while only 3.5% of participants also experienced aggression towards the others and only 2% had what the authors denominated as tic-related SIB. In this study, the authors compared in detail the distribution of different tics in patient with SIB and without in univariable model and concluded that individuals with SIB are more inclined to have tics involving shoulder, trunk, arm, as well as dystonic tics; complex motor tics, copropraxia, complex phonic tics, higher number of phonic tics, coprolalia and OCD. In multivariable analysis SIB was found to be associated with complex motor tics, OCS and greater tic severity. Interestingly enough, patients with SIB have also been selected more frequently for the DBS procedure. 

Etiology

Genetics

Epigenetics

Environmental risk factors

In a Taiwan birth cohort of 309,376 singleton live births at term gestations showed an increased risk of tic disorders (6-52 weeks after birth) after exposure to particular matter with an aerodynamic diameter less than 2.5 μm  \cite{Chang2022}.
\citet{Rönö2022} compared more than 5 million singletons born after the use of assisted reproductive technology (ART) with almost 5 million singletons born without the use of ART. There was not found a difference in risk of tic disorders between the groups.
In a nationwide cohort of 14,024 children and adolescents who were hospitalized with a bacterial infection with different pathogens, an increased risk of tic disorders was found when compared to controls without a bacterial infection \cite{Hsu2022}. Future studies are needed to replicate these findings.

Pathophysiology

Animal models

Pathological studies

Electrophysiology

An EEG study assessing inhibitory control of frontal lobe regions,  which are important for motor inhibition in chronic tic disorders, was conducted using a stop signal task \cite{Zea2022}. Right superior frontal gyrus gamma event-related desynchronization (ERD) was elevated in patients with chronic tic disorder during stop preparation. Elevated right superior frontal gyrus gamma ERD correlated with decreased tic severity, suggesting that right superior frontal gyrus gamma ERD may reflect a mechanism of tic suppression. Using magnetoencephalography, error-related negativity, an event-related potential component and an index of performance monitoring processes during simple stimulus-response tasks, was examined for changes in the performance monitoring system in TS \cite{Metzlaff2022}. The results suggest that increased motor control induced by conflict between high target task performance and tic suppression in TS patients may influence early error-related processing, and that TS patients may initially tend to process all responses as error responses. In the future, it is envisioned that electroencephalography will be useful as a biomarker in TS and in understanding the pathophysiology of tics.
Electrophysiology has also been discussed as a biomarker for tic disorders. TMS was also discussed including reduced short-interval intracortical inhibition at rest, which suggests a correlation with motor tic severity, shortened cortical silent period duration, increased intracortical facilitation, and decreased motor evoked-potential amplitude \cite{Jannati2022}. Using EEG as a biomarker for comprehensive behavioral intervention (CBIT), a randomized controlled trial was conducted to determine whether EEG coherence during Go/NoGo tasks correlated with CBIT outcomes \cite{Morand-Beaulieu2022}. No association was found between EEG coherence during the Go/NoGo task and changes in tic severity, suggesting that brain processes in the inhibition of motor responses do not play any role in CBIT.