Many people with GTS report hypersensitivity to subtle sensory inputs. Marked discomfort from the tag in the neck of a T-shirt, or from the quiet buzz of fluorescent lamps, are classic examples. Careful studies have shown that sensory hypersensitivity (SH) is explained by differences in central processing, e.g. attention, not by the peripheral nervous system (___). \citet{37026772} gathered all reported cases of SH following a stroke (N=8), which had pointed to lesions of the insula. In 3 newly identified cases, the lesions overlapped in the right anterior insula, the claustrum, and the Rolandic operculum.
Transient effects of environment on tic severity
A fascinating study from Israel examined the timing of tics moment to moment while children with tic disorders watched a movie and played a video game \citep{37946628}. This study involved substantial labor on the part of the researchers. Tics did not occur randomly over time but rather were more or less common across participants during specific moments of the movie clip and when reward was expected in the video game. One interesting hypothesis presented for future study was that "the portrayal of motor actions in movies elicits" an urge to tic. Similarly, one would be very interested in whether movie or game conditions eliciting higher tic rates correspond to greater release of striatal dopamine in people with and without tics \citep{9607763}.
Functional tic-like behaviors
Various authors have identified differences in the frequency or character of premonitory phenomena (premonitory urges, PUs) as a potential feature that can discriminate functional tic-like behaviors (FTLB) from primary tic disorders \citep{36362696,Martino2023}. The frequency of PUs in those prior reports differed to a clinically important degree from the frequency in typical tic patients at a similar disease duration \citep{37224324}. However, prospectively comparing 83 patients with typical tics and 40 with FTLB from the Calgary tic registry, Szejko and colleagues found no significant differences in premonitory urge severity (PUTS scale total score) nor in any of the individual PUTS items \citep{Szejko_2023}. The authors noted that their results are supported by other reasonably large case series \citep{36203825,34824091}, and provide a brief but compelling discussion of potential implications.
Clinical differences between functional tics and neurodevelopmental tics were confirmed in a study by \citet{Cavanna2023}. In this study, 105 consecutive patients who had developed functional tics in the period April 2020 to March 2023 were examined with a neuropsychiatric assessment. Besides the (sub)acute onset and high frequency of complex movements and vocalizations, it was shown that 23% had a pre-existing tic disorder, 70% had comorbid anxiety, 40% had a comorbid affective disorder, and 41% had at least one other functional neurological disorder. The same group directly "compared the clinical features of patients who developed functional tics during the COVID-19 pandemic (N = 83) to patients with Tourette syndrome matched for age and gender (N = 83)" \citep{37421881}. This comparison identified many variables previously reported to differ between the two groups, but the statistically strongest indicators were "tic-related obsessive-compulsive behaviors" and a family history of tics, both of which were much more common in typical TS.
As noted above, patients with TS are not immune from also developing functional tic-like behaviors (FTLB). This association is not surprising, as for example pseudoseizures are more common in people with epilepsy. \citet{M_ller_Vahl_2023} present data on 71 TS patients whom they also diagnosed with FTLB. A majority (56%-79%) had psychological features common in people with other functional symptoms, and about a third of them had a history of other medically unexplained symptoms; these findings suggest that the cause of FTLBs is likely similar to that of other functional neurological symptoms. The authors comment that their ability to identify a fairly large sample of TS+FTLB suggests that clinicians faced with treatment-resistant symptoms in TS should consider whether the symptoms are FTLB rather than tics.
Diagnostic agreement in assessing FTLB was examined by asking eight experts in diagnosing and treating patients with tics to evaluate videos from 24 adults and diagnose them with either functional tics, primary tics or both \citep{Rigas2023}. The diagnostic agreement was fair based on phenomenology alone, and increased to moderate when additional clinical information was provided. The diagnostic distinction between primary and functional tics is shown to be difficult in the absence of clinical information.