The routine use of MR guided stereotactically placed neuro-surgical lesions and the increasing availability of the gamma knife offer more precise procedures with objectively quantifi-able lesions. High resolution structural MRI studies are being used to track volumetric changes local and distal to the lesions (Rauch et al., 2000) and PET studies are providing informa-tion about blood flow and regional metabolism that may serve as predictors of response (Rauch et al., 2001). Recordings in implicated brain structures such as the cingulum (similar to intracortical recordings and microdialysis in the temporal lobe in surgical epilepsy patients) may provide useful additional in-formation. Strategies such as magnetic resonance spectroscopy (MRS) and newer methods such as diffusion tensor imaging (DT-MRI) can provide information about regional neurochem-istry and white matter tract integrity, respectively. They offer the exciting possibility of collecting valuable neurobiological data, which may aid in monitoring changes as well as predict-ing response.
Recently, several new experimental strategies have emerged and are being investigated in refractory OCD and MDD. These are invasive modalities currently being offered only in the context of research studies and thus have not been proven or approved for treatment. They are included in this discussion for the sake of completion.
Deep brain stimulation (DBS) involves stimulation of target regions via stereotactically placed electrodes that can be pro-grammed to deliver low voltage electrical stimulation to modulate transmission of specific brain pathways. While DBS of different regions has been experimentally investigated for treatment-refrac-tory pain syndromes, movement disorders and seizure disorders, only DBS for treatment-refractory Parkinson’s disease or essential tremor is currently FDA approved in the USA. Recently following a preliminary report of deep brain stimulation in the anterior limb of the internal capsule in four patients with treatment-refractory OCD (Nuttin et al., 1999), a few specialized centers in the world (including the USA) are conducting ongoing controlled studies of DBS in treatment-refractory OCD patients. Nuttin and colleagues described four patients with treatment-refractory OCD who un-derwent DBS with bilateral stereotactically implanted quadripolar electrodes in the anterior limb of the internal capsule (instead of an-terior capsulotomy) and reported “beneficial effects” in three of the four patients. It is important to note that this remains an experimen-tal procedure at this stage for treatment-refractory OCD. Neverthe-less these developments offer the exciting possibility of reversible “lesions” as well as ethically justifiable sham procedures (controls can be implanted but the stimulation not turned on or at lower volt-age undetectable by the patient who can subsequently receive the benefit of the treatment) (Greenberg, personal communication).
Vagal nerve stimulation, used formerly only as an FDA approved treatment strategy for some patients with drug-resistant, partial-onset epilepsy due to its seizure attenuating effects, has been ap-proved for use in treatment of depression.