Cortical structure in provisional and chronic tic disorders

Background: Structural MRI studies have found thinning of various cerebral cortical regions in people with Tourette syndrome (TS) compared to tic-free controls (TFC). Whether this thinning precedes or follows the first tic is unknown; cortical structure has not been studied in the first year after tic onset (Provisional Tic Disorder, PTD).

Objectives: Compare regional cortical thickness and surface area in PTD, TS and TFC.

Methods: T1-weighted brain MRI was acquired in 391 children 5-10 years old: 83 with PTD, 103 with TS or chronic motor tic disorder, and 205 TFC. Over half of scans used prospective motion correction to minimize error due to head movement. FreeSurfer measured regional cortical thickness and surface area. ANCOVA models compared these measurements by group, controlling for age, sex, handedness, scanner and brain volume. Significance was corrected for multiple comparisons.

Results: Brain volume and mean cortical thickness were lower in TS than TFC, with PTD intermediate (TS<PTD<TFC), as were most cortical regions. The typical developmental cortical thinning over this age range occurred twice as fast in children with tics. By contrast, bilateral insula surface area followed the pattern PTD<TS<TFC. Insula appears to be smaller at tic onset.

Conclusion: These data suggest that the thinner cortex in TS develops after tics begin, and thus may represent a consequence than a cause of tics. It may reflect earlier maturation of inhibitory circuits developed with extended experience suppressing tics. By contrast, the smaller insula may contribute to causing tics, perhaps via its role in the action-mode network.