Clinical neurophysiology of tics

Clinical neurophysiology constitutes a relevant tool for clinicians in movement disorders since it expands their characterization beyond clinically observable features and supports their diagnosis. Although the field has yet to identify a specific neurophysiologic marker for the diagnosis of tics, neurophysiology serves as a valuable tool for clinicians in differentiating tics from other hyperkinetic disorders. Beside diagnosis, recent years have witnessed the emergence of two primary domains in which neurophysiology plays a pivotal role in the realm of tic disorders: investigating the pathophysiologic foundations of tics, in an attempt to unravel the neural mechanisms that give rise to this complex movement disorder, and the application of noninvasive brain stimulation (NIBS) techniques as an avenue for therapeutic intervention. The two domains are inherently connected, as the development of noninvasive treatment methods is grounded in the mechanisms believed to underlie tics. Although the precise neurobiology of tic disorders remains incompletely understood, growing evidence implicates dysfunction within cortical-striatal-thalamic-cortical circuits as a key contributor to their pathophysiology. In this context, NIBS has been explored as a potential therapeutic approach aimed at modulating pathologic cortical hyperexcitability in specific regions, thereby enhancing inhibitory circuit engagement and ultimately alleviating tics. An intriguing alternative to transcranial stimulation is peripheral electrical nerve stimulation, which offers a distinct yet complementary mechanism of action. For instance, studies have demonstrated that rhythmic stimulation of the median nerve within the alpha frequency band can entrain neuronal oscillations in the sensorimotor cortex, leading to a measurable reduction in tic frequency and intensity. In this chapter, we will first present a comprehensive overview of clinical neurophysiology studies that have significantly contributed to our understanding of the pathophysiology of tics, and then we will offer a brief synopsis of the application of noninvasive stimulation for the treatment of tics, illustrating that clinical neurophysiology is a powerful tool that provides valuable insights into the neural underpinnings of tics and holds potential as adjunct therapy for tic management.