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Objective-Structural brain imaging studies in Obsessive-Compulsive Disorder (OCD) have produced inconsistent findings. This may be partially due to limited statistical power from relatively small samples and clinical heterogeneity related to variation in disease profile and developmental stage.Methods-To address these limitations, we conducted a meta-and mega-analysis of data from OCD sites worldwide. T 1 images from 1,830 OCD patients and 1,759 controls were analyzed, using coordinated and standardized processing, to identify subcortical brain volumes that differ in OCD patients and healthy controls. We additionally examined potential modulating effects of clinical characteristics on morphological differences in OCD patients.Results-The meta-analysis indicated that adult patients had significantly smaller hippocampal volumes (Cohen's d=−0.13; p=5.1x10 −3 , % difference −2.80) and larger pallidum volumes (d=0.16; p=1.6x10 −3 , % difference 3.16) compared to adult controls. Both effects were stronger in medicated patients compared to controls (d=−0.29; p=2.4x10 −5 , % difference −4.18 and d=0.29; p=1.2x10 −5 , % difference 4.38, respectively). Unmedicated pediatric patients had larger thalamic volumes (d=0.38, p=2.1x10 −3 ) compared to pediatric controls. None of these findings were mediated by sample characteristics such as mean age or field strength. Overall the mega-analysis yielded similar results. Conclusion-Our study indicates a different pattern of subcortical abnormalities in pediatric versus adult OCD patients. The pallidum and hippocampus seem to be of importance in adult OCD, whereas the thalamus seems to be key in pediatric OCD. This highlights the potential importance of neurodevelopmental alterations in OCD, and suggests that further research on neuroplasticity in OCD may be useful. IntroductionObsessive-compulsive disorder (OCD) is a neurodevelopmental disorder that affects 1-3% of the population (1; 2). In more than 50% of all OCD cases, symptoms emerge during Location of work and address for reprints: Premika S.W. Boedhoe, M.Sc.,

The parietal cortex was consistently implicated in both adults and children with OCD. More widespread cortical thickness abnormalities were found in medicated adult OCD patients, and more pronounced surface area deficits (mainly in frontal regions) were found in medicated pediatric OCD patients. These cortical measures represent distinct morphological features and may be differentially affected during different stages of development and illness, and possibly moderated by disease profile and medication.

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