clinicaltrials.gov Identifier: NCT00932204.

Background: Deficits in cortical inhibitory processes have been suggested as underlying pathophysiological mechanisms of obsessivecompulsive disorder (OCD). We examined whether patients with OCD have altered cortical excitability using paired-pulse transcranial magnetic stimulation (TMS). We also tested associations between TMS indices and OCD-related characteristics, including age of onset and response inhibition in the go/no-go paradigm, to examine whether altered cortical excitability contributes to symptom formation and behavioural inhibition deficit in patients with OCD. Methods: We assessed motor cortex excitability using paired-pulse TMS in 51 patients with OCD and 39 age-matched healthy controls. We also assessed clinical symptoms and response inhibition in the go/nogo task. All patients were undergoing treatment with serotonin reuptake inhibitors. We performed repeated-measures multivariate analy sis of covariance to compare TMS indices between patients with OCD and controls. Results: Compared to controls, patients with OCD showed a shorter cortical silent period and decreased intracortical facilitation. However, we found no significant difference between groups for resting motor threshold or short-interval intracortical inhibition. In the OCD group, the shortened cortical silent period was associated with a prompt reaction time in the go/no-go task and with early onset of OCD. Limitations: We could not exclude the influence of medications on motor cortex excitability. Conclusion: These findings suggest abnormal cortical excitability in patients with OCD. The associations between cortical silent period and response inhibition and age of onset further indicate that altered cortical excitability may play an important role in the development of OCD.