Obsessive-Compulsive Disorder (OCD) is among the most disabling psychiatric disorders as more than 40% of patients are resistant to the standard pharmacological and psychotherapy approaches and about 10% show severe disability and require institutionalization. These resistant patients may benefit from new surgical therapeutic approaches such as Deep Brain Stimulation (DBS) using high frequency stimulation of specific cerebral regions to modulate neural networks. Although promising, these results need nevertheless to be replicated and confirmed within a larger cohort of patients and considering a different main objective, instead of clinical improvement only. Indeed, despite a positive treatment response, adaptive functioning and quality of life may continue to be negatively impacted in OCD. Thus beyond symptom reduction, health-related quality of life (QoL) represents a more important objective of a treatment, as it includes both the individual's functional status and the individual's subjective perception of the impact of the illness on the patient's life. STN DBS induces significant clinical improvement, which may not be proportional to the QoL gain. Consequently, QoL appears to be a better outcome to target in the coming studies than clinical improvement alone. THe investigators thus propose a prospective study assessing the QoL changes of resistant OCD patients under STN DBS+BMT versus Best Medical Treatment (BMT) at 12 months, in order to assess the DBS induced gain in QoL in BMT-managed patients versus BMT alone.

The study will focus on an innovative therapeutic strategy (DBS) and on an original objective, quality of life, which is considered to better reflect the impact of a therapeutic strategy. Moreover, the study will help to define the predictive biomarkers /biosignatures of response to STN DBS in OCD.

Inclusion Criteria:

• OCD for > 5 years
• YBOCS> 25 and/or YBOCS sub-scale >15
• GAF< 45
• 3 or more documented SRI trials, including clomipramine (10-12 weeks at adequate dose)
• SRI augmentation for > 4 weeks with at least one antipsychotic and with one of the following: lithium, clonazepam
• Adequate trial of CBT (Exposure Therapy and Response Prevention) (intolerance or >15 sessions)
• Ability to provide informed consent

Exclusion Criteria:

• Hoarding (if the only OCD symptom)
• OCD with poor insight (BABS score > 12)
• Lifetime diagnosis of psychosis or bipolar disorder;
• Substance abuse or dependence within the previous six months;
• Baseline Montgomery and Asberg (MADRS) suicidality item (item 10) score >2;
• Current DSM-5 personality disorder of Cluster A (e.g., paranoid or schizotypal personality disorder) or B (e.g., borderline or antisocial personality disorder);
• Brain pathology, such as moderate or marked cerebral atrophy, stroke, tumor or previous neurosurgical procedures (i.e. capsulotomy etc), history of cognitive impairment and cognitive deterioration (Addenbrooke's Cognitive Examination ACE score of < 80).
• Contra-indications to surgery, anaesthesia, or MRI
• compulsory hospitalization/ care; pregnant or nursing patients