Altered cortical excitability in obsessive-compulsive disorder

Objective: To assess cortical inhibitory and excitatory mechanisms in obses sive-compulsive disorder (OCD). Background: Transcranial magnetic stimulati on (TMS) studies have found decreased neuronal inhibition and a reduced cor tical silent period in the primary motor area in Tourette's syndrome, focal dystonia, and other disorders believed to involve dysfunction of subcortic al structures, including the basal ganglia. Dysfunction of the basal gangli a and linked regions also has been implicated in OCD, which has significant clinical and familial overlap with tic disorders. Methods: We applied the TMS techniques previously used in Tourette's syndrome to a group of 16 OCD patients (seven unmedicated) and II age-matched healthy volunteers extensiv ely screened for psychopathology. Measures of motor cortex excitability inc luded resting and active motor threshold, cortical silent period duration, and intracortical inhibition and facilitation using a paired-pulse TMS tech nique with a subthreshold conditioning stimulus. Results: Similar to recent findings in Tourette's syndrome and focal dystonia, this study reports sig nificantly decreased intracortical inhibition (ICI) relative to the volunte ers at interstimulus intervals from 2 to 5 msec. We also found decreased ac tive and resting motor evoked potential threshold in the OCD patients, anot her indication of increased cortical excitability. Neither abnormality appe ared medication related. The decreases in ICI and motor threshold were grea test in OCD patients with comorbid ties, but remained significant in patien ts without ties. Conclusions: The data suggest abnormal cortical excitabili ty in obsessive-compulsive disorder. These findings are congruent with the hypothesis that Tourette's syndrome and obsessive compulsive disorder (OCD) are analogous disorders with overlapping dysfunction in corticobasal circu its. Patients with tic-related OCD may have more abnormal motor cortex exci tability than OCD patients without tics.