Controversies Surrounding the Pathophysiology of Tics

Correlation of repetitive behaviors in deer mice with striatal mRNA expression of endogenous opioids and mu, delta, kappa, and dopamine receptors

Deer mice provide a valuable naturally occurring animal model for investigating pathophysiological mechanisms underlying repetitive behaviors. Prior investigations using this model have identified abnormalities in the cortico-basal ganglia circuitry, including alterations within the indirect pathway and levels of endogenous opioids in the frontal cortex. In this study, the behaviors of n = 7 mice were quantified, and their brains were sectioned. Using analysis of in-situ mRNA hybridization (RNAscope®) in four striatal regions (dorsomedial, dorsolateral, ventromedial and ventrolateral), mRNA levels of dopamine receptors (DRD-1 [D1] and DRD-2 [D2]), endogenous opioids (prodynorphin [p-dyn] and proenkephalin [p-enk]), and opioid receptor expression (Oprm1 [mu], Oprd1 [delta], Oprk1 [kappa]) were quantified. Statistical analyses were performed using Spearman's rank correlation coefficients with a False Discovery Rate (FDR) procedure to address the small sample size. Results identified a positive correlation between total activity and DRD-2 mRNA in the dorsolateral striatum. The discussion reviews the potential effect of dopaminergic alterations on several cortical-basal ganglia-thalamo-cortical pathways, the possible role of other brain regions and neurotransmitters, and suggestions for future investigative approaches.

When the interoceptive and conceptual clash: The case of oppositional phenomenal self-modelling in Tourette syndrome

Tourette syndrome (TS) has been associated with a rich set of symptoms that are said to be uncomfortable, unwilled, and effortful to manage. Furthermore, tics, the canonical characteristic of TS, are multifaceted, and their onset and maintenance is complex. A formal account that integrates these features of TS symptomatology within a plausible theoretical framework is currently absent from the field. In this paper, we assess the explanatory power of hierarchical generative modelling in accounting for TS symptomatology from the perspective of active inference. We propose a fourfold analysis of sensory, motor, and cognitive phenomena associated with TS. In Section 1, we characterise tics as a form of action aimed at sensory attenuation. In Section 2, we introduce the notion of epistemic ticcing and describe such behaviour as the search for evidence that there is an agent (i.e., self) at the heart of the generative hierarchy. In Section 3, we characterise both epistemic (sensation-free) and nonepistemic (sensational) tics as habitual behaviour. Finally, in Section 4, we propose that ticcing behaviour involves an inevitable conflict between distinguishable aspects of selfhood; namely, between the minimal phenomenal sense of self-which is putatively underwritten by interoceptive inference-and the explicit preferences that constitute the individual's conceptual sense of self. In sum, we aim to provide an empirically informed analysis of TS symptomatology under active inference, revealing a continuity between covert and overt features of the condition.

Diffusion indices alteration in major white matter tracts of children with tic disorder using TRACULA

BACKGROUND

Tic disorder is a neuropsychiatric disorder characterized by involuntary movements or vocalizations. Previous studies utilizing diffusion-weighted imaging to explore white-matter alterations in tic disorders have reported inconsistent results regarding the affected tracts. We aimed to address this gap by employing a novel tractography technique for more detailed analysis.

METHODS

We analyzed MRI data from 23 children with tic disorders and 23 healthy controls using TRActs Constrained by UnderLying Anatomy (TRACULA), an advanced automated probabilistic tractography method. We examined fractional anisotropy (FA), radial diffusivity (RD), axial diffusivity, and mean diffusivity in 42 specific significant white matter tracts.

RESULTS

Our findings revealed notable differences in the children with tic disorders compared to the control group. Specifically, there was a significant reduction in FA in the parietal part and splenium of the corpus callosum and the left corticospinal tract. Increased RD was observed in the temporal and splenium areas of the corpus callosum, the left corticospinal tract, and the left acoustic radiation. A higher mean diffusivity was also noted in the left middle longitudinal fasciculus. A significant correlation emerged between the severity of motor symptoms, measured by the Yale Global Tic Severity Scale, and FA in the parietal part of the corpus callosum, as well as RD in the left acoustic radiation.

CONCLUSION

These results indicate a pattern of reduced interhemispheric connectivity in the corpus callosum, aligning with previous studies and novel findings in the diffusion indices changes in the left corticospinal tract, left acoustic radiation, and left middle longitudinal fasciculus. Tic disorders might involve structural abnormalities in key white matter tracts, offering new insights into their pathogenesis.

Letting Tourette's be: The importance of understanding lived experience in research and the clinic

The most common clinical research question regarding Tourette syndrome focuses on tic reduction, which follows from classical 'lack of inhibition' models. Rooted in views about brain deficits, this model suggests that with higher severity and frequency, tics are necessarily disruptive and should therefore be inhibited. However, emerging calls from people with lived experience of Tourette syndrome suggest that this is too narrow a definition. This narrative literature review analyses issues with brain deficit views and qualitative research on tic context and feelings of compulsion. The results suggest the need for a more positive and encompassing theoretical and ethical position on Tourette's. The article puts forward an enactive analytical approach of 'letting be', that is, approaching a phenomenon without forcing preconceived reference structures onto it. We suggest using the identity-first term 'Tourettic'. Prioritizing the perspective of the 'Tourettic patient', it urges attentiveness to the everyday issues diagnosed people encounter and how these are embedded in further life. This approach highlights the strong relationship between the Tourettic persons' felt impairment, their adoption of an outsider's perspective, and feeling under constant scrutiny. It suggests that this felt impairment of tics can be reduced by creating a physical and social environment in which the person is 'let be' but not 'let go of'. WHAT THIS PAPER ADDS: Its theoretical position allows a more holistic view of Tourette's, integrating tics with oft-overlooked complex compulsions. The 'letting be' position also allows us to view why the presentation of Tourette's is likely to vary with gender and age of onset. 'Letting be' is a promising approach to improve a clinical understanding of Tourettic well-being over and above tic severity and frequency. It integrates quantitative research on Tourette syndrome in the neurosciences with qualitative neurodiversity literature in the medical humanities. It integrates ethical frameworks as developed in enactive philosophy with the conceptualization and treatment of tics and compulsions.

CANNA-TICS: Efficacy and safety of oral treatment with nabiximols in adults with chronic tic disorders - Results of a prospective, multicenter, randomized, double-blind, placebo controlled, phase IIIb superiority study

Preliminary data suggest that cannabis-based medicines might be a promising new treatment for patients with Tourette syndrome (TS)/chronic tic disorders (CTD) resulting in an improvement of tics, comorbidities, and quality of life. This randomized, multicenter, placebo-controlled, phase IIIb study aimed to examine efficacy and safety of the cannabis extract nabiximols in adults with TS/CTD (n = 97, randomized 2:1 to nabiximols:placebo). The primary efficacy endpoint was defined as a tic reduction of ≥ 25% according to the Total Tic Score of the Yale Global Tic Severity Scale after 13 weeks of treatment. Although a much larger number of patients in the nabiximols compared to the placebo group (14/64 (21·9%) vs. 3/33 (9·1%)) met the responder criterion, superiority of nabiximols could formally not be demonstrated. In secondary analyses, substantial trends for improvements of tics, depression, and quality of life were observed. Additionally exploratory subgroup analyses revealed an improvement of tics in particular in males, patients with more severe tics, and patients with comorbid attention deficit/hyperactivity disorder suggesting that these subgroups may benefit better from treatment with cannabis-based medication. There were no relevant safety issues. Our data further support the role of cannabinoids in the treatment of patients with chronic tic disorders.

Fractality of tics as a quantitative assessment tool for Tourette syndrome

Tics manifest as brief, purposeless and unintentional movements or noises that, for many individuals, can be suppressed temporarily with effort. Previous work has hypothesized that the chaotic temporal nature of tics could possess an inherent fractality, that is, have neighbour-to-neighbour correlation at all levels of timescale. However, demonstrating this phenomenon has eluded researchers for more than two decades, primarily because of the challenges associated with estimating the scale-invariant, power law exponent—called the fractal dimension D_f—from fractional Brownian noise. Here, we confirm this hypothesis and establish the fractality of tics by examining two tic time series datasets collected 6–12 months apart in children with tics, using random walk models and directional statistics. We find that D_f is correlated with tic severity as measured by the YGTTS total tic score, and that D_f is a sensitive parameter in examining the effect of several tic suppression conditions on the tic time series. Our findings pave the way for using the fractal nature of tics as a robust quantitative tool for estimating tic severity and treatment effectiveness, as well as a possible marker for differentiating typical from functional tics.

The Transdiagnostic Relevance of Self-Other Distinction to Psychiatry Spans Emotional, Cognitive and Motor Domains

Self-other distinction refers to the ability to distinguish between our own and other people's physical and mental states (actions, perceptions, emotions etc.). Both the right temporo-parietal junction and brain areas associated with the human mirror neuron system are likely to critically influence self-other distinction, given their respective contributions to theory of mind and embodied empathy. The degree of appropriate self-other distinction will vary according to the exact social situation, and how helpful it is to feel into, or remain detached from, another person's mental state. Indeed, the emotional resonance that we can share with others affords the gift of empathy, but over-sharing may pose a downside, leading to a range of difficulties from personal distress to paranoia, and perhaps even motor tics and compulsions. The aim of this perspective paper is to consider how evidence from behavioral and neurophysiological studies supports a role for problems with self-other distinction in a range of psychiatric symptoms spanning the emotional, cognitive and motor domains. The various signs and symptoms associated with problematic self-other distinction comprise both maladaptive and adaptive (compensatory) responses to dysfunction within a common underlying neuropsychological mechanism, compelling the adoption of more holistic transdiagnostic therapeutic approaches within Psychiatry.

The Role of the Cerebellum in Repetitive Behavior Across Species: Childhood Stereotypies and Deer Mice

Recent studies suggest that the cerebellum may have a significant role in repetitive behaviors. In primary complex motor stereotypies, typically developing children have repetitive movements usually involving rhythmic flapping/waving arm/hand movements. Similarly, the deer mouse animal model exhibits inherited repetitive behaviors, with increased frequencies of spontaneous jumping and rearing. In this study, data from both children with motor stereotypies and deer mice were used to investigate the role of the cerebellum in repetitive behaviors. The 3.0-T MRI volumetric imaging of the cerebellum was obtained in 20 children with primary complex motor stereotypies and 20 healthy controls. In deer mice, cerebellar volume (n = 7/group) and cell counts (n = 9/group) were compared between high- and low-activity animals. Levels of cerebellar neurotransmitters were also determined via HPLC (n = 10/group). In children with stereotypies, (a) there were a statistically significant reduction (compared to controls) in the white matter volume of the posterior cerebellar lobule VI-VII that negatively correlated with motor control and (b) an 8% increase in the anterior vermis gray matter that positively correlated with motor Stereotypy Severity Scores (SSS). In deer mice, (a) there was a significant increase in the volume of the anterior vermal granular cell layer that was associated with higher activity and (b) dentate nucleus cell counts were higher in high activity animals. Similar increases in volume were observed in anterior vermis in children with stereotypies and a deer mouse model of repetitive behaviors. These preliminary findings support the need for further investigation of the cerebellum in repetitive behaviors.

Enrichment and Correlation Analysis of Serum miRNAs in Comorbidity Between Arnold-Chiari and Tourette Syndrome Contribute to Clarify Their Molecular Bases

Due to its rarity, coupled to a multifactorial and very heterogeneous nature, the molecular etiology of Arnold-Chiari (AC) syndrome remains almost totally unknown. Its relationship with other neuropsychiatric disorders such as Tourette syndrome (TS) is also undetermined. The rare comorbid status between both disorders (ACTS) complicates the framework of diagnosis and negatively affects the patients' quality of life. In this exploratory study, we aimed to identify serum microRNA expression profiles as molecular fingerprints for AC, TS, and ACTS, by using a high-throughput approach. For this aim, 10 AC patients, 11 ACTS patients, 6 TS patients, and 8 unaffected controls (NC) were recruited. Nine miRNAs resulted significantly differentially expressed (DE): let-7b-5p (upregulated in ACTS vs. TS); miR-21-5p (upregulated in ACTS vs. AC; downregulated in AC vs. TS); miR-23a-3p (upregulated in TS vs. NCs; downregulated in AC vs. TS); miR-25-3p (upregulated in AC vs. TS and NCs; downregulated in ACTS vs. AC); miR-93-5p (upregulated in AC vs. TS); miR-130a-3p (downregulated in ACTS and TS vs. NCs); miR-144-3p (downregulated in ACTS vs. AC; upregulated in AC vs. TS); miR-222-3p (upregulated in ACTS vs. NCs); miR-451a (upregulated in AC vs. TS and NCs; in ACTS vs. NCs). Altered expression of miRNAs was statistically correlated to neuroimaging and neuropsychological anomalies. Furthermore, computational analyses indicated that DE miRNAs are involved in AC and TS pathomechanisms. Finally, we propose the dysregulation of the miRNA set as a potential molecular tool for supporting the current diagnosis of AC, TS, and ACTS by using liquid biopsies, in an unbiased and non-invasive way.

Tourette syndrome research highlights from 2019

This is the sixth yearly article in the Tourette Syndrome Research Highlights series, summarizing research from 2019 relevant to Tourette syndrome and other tic disorders. The highlights from 2020 is being drafted on the Authorea online authoring platform; readers are encouraged to add references or give feedback on our selections comments feature on this page. After the calendar year ends, this article is submitted as the annual update for the Tics collection F1000Research.