What is the role of the timing of tics in the pathophysiology of Tourette syndrome (TS) and other tic disorders?

Updated: May 30, 2019
  • Author: William C Robertson, Jr, MD; Chief Editor: Stephen L Nelson, Jr, MD, PhD, FAACPDM, FAAN, FAAP  more...
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Peterson and Leckman have drawn attention to the timing of tics. [98] In the course of an office visit, tics tend to occur in bouts rather than being distributed evenly. Similarly, viewed over the course of several months, days with worse tics also tend to cluster together.

A consistent temporal pattern when viewed at any of various time scales is a fractal pattern, a typical feature of a chaotic mathematical system. This suggests the possibility of searching for neuronal firing patterns or other physiologic processes that replicate on even smaller time scales the timing of tics as observed over minutes or months.

Several clinical syndromes are distinct from TS but have overlapping features. These include the repetitive, intrusive thoughts or suppressible but eventually irresistible rituals in OCD, and echophenomena or utilization behavior in patients with catatonia or frontal lobe injury. Conceivably, progress in any of these conditions may yield further insights into the pathophysiology of tic disorders.

Additional insights into tics may be gathered by reference to other illnesses with overlapping features. Tics may be classified as a stereotypic movement disorder, in that the movements are often complex and are repetitive rather than random.

Stereotypies are observed in a number of human and animal situations and may bear some relevance to the anatomy and pathophysiology of TS. Animal models include stallions with inherited repetitive movements, grooming rituals, and self-injury; tethered sows or other animals confined to small quarters; Labrador dogs who repeatedly lick their paws to the point of abrasions; rodents given apomorphine or stimulants; and more recently, rodents injected with plasma from patients with TS. The relevance of these animal models has been reviewed.

In people, a spectrum of stereotyped movement severity ranging from normal to problematic may occur. [99] Simple stereotypies are common in infancy and early childhood. Habits and mannerisms are nearly ubiquitous. However, stereotypies become clearly pathologic in autism or Rett syndrome. Determining why tics chronically persist in a few individuals but briefly appear and then wane in others is important.

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