Fractality of tics as a quantitative assessment tool for Tourette syndrome.

Payton Beeler,Deanna J Greene,Nicholas O Jensen,Bradley L Schlaggar,Kevin J Black,Soyoung Kim,Rajan K Chakrabarty,Amy Robichaux-Viehoever

doi:10.1098/rsif.2021.0742

Abstract

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 Df—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 Df is correlated with tic severity as measured by the YGTTS total tic score, and that Df 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.

Highlights

Tics are brief, purposeless, unintentional behaviours appearing as repeated movements of skeletal or vocal musculature, affecting more than 20% of all children [1,2]
Interface 19: 20210742 series is correlated with a standard clinical measure of tic severity (TTS), suggesting that Df of the tic time series can be used as an objective measure of tic severity
We measure for the first time the effect of tic suppression on the temporal dynamics of tic occurrence by quantification of Df

Summary

Introduction

Purposeless, unintentional behaviours appearing as repeated movements of skeletal or vocal musculature, affecting more than 20% of all children [1,2]. 0.5% of children have Tourette syndrome (TS), which is diagnosed when both motor and vocal tics occur over a period of a year or longer [3]. The COVID-19 pandemic has resulted in increased symptoms in children diagnosed with tic disorders, as well as an influx of patients with sudden onset of severe tics and tic-like attacks, the latter of which are commonly believed to be functional tic-like movements [6,7]. The similarity of tics and functional tic-like movements, and the variability of symptoms over time in tic disorders make diagnosis of functional tic-like movements a historically challenging task. Current best practice requires extensive expertise with tic disorders, thorough elicitation of historical information from the patient and family, and careful observation of the patient

Methods

Results

Conclusion