The purpose of this randomized, sham-controlled study is to evaluate the effectiveness of DBS therapy in individuals suffering from severe OCD and to investigate DBS treatment with functional [18F]FDG-Brain-PET.

The overall planned study duration per subject is 36 weeks, whereby inclusion is timepoint zero and implantation of DBS will be conducted during the first four study weeks. Patients will then undergo an 8-week open-label active DBS treatment phase followed by a 12-week double blind active or sham treatment and finally a 12-week crossover period with the inverse (active/sham) treatment with at least biweekly study visits. Patients as well as patient handling study psychiatrists will be blinded to active/sham. In case of unbearable aggravation of the symptoms during sham, the sham-period will be shortened to a tolerable length. After maximal 38 weeks all study procedures will be completed, and active DBS treatment will be maintained as long as clinically necessary.

Inclusion Criteria:

• a score of 25 or higher on the Yale-Brown Obsessive Compulsive Scale
• previous failure to respond to at least two medication trials with serotonin reuptake inhibitors at or, if tolerated, beyond the FDA maximum recommended dose for a minimum duration over at least ten weeks each.
• at least one trial with tricyclic medication at or, if tolerated, beyond the FDA maximum recommended dose for a minimum duration over at least ten weeks each.
• at least one trial of augmentation with antipsychotic medication, lithium, a benzodiazepine or buspirone
• at least on trial of psychotherapy (cognitive behavioral therapy or comparable techniques) for at least 20 sessions
• ability to provide written informed consent

Exclusion Criteria:

• any history of current or past psychotic disorder
• a manic episode within the preceding three years
• any current clinically significant medical or neurological disorder, that is a contraindication against DBS
• any disease that could lead to an altered glucose reactivity (e.g. diabetes)
• any clinically significant preoperative MRI abnormality or inability to undergo presurgical MRI
• current or unstable remitted substance abuse or dependence except nicotine
• pregnancy or high risk of becoming pregnant during study duration (desire to have children) and refusal to utilize a proper method of contraception
• Any current severe personality disorder except comorbid anankastic personality disorder
• Inability to follow the study protocol or adhere to operational requirements
• Current and unstable suicidality