11.1 Patient Selection

As in any operation, careful patient selection is paramount for best outcomes. A multidisciplinary approach including neurology, neurosurgery, and neuropsychology at a minimum is essential to success. Under current recommendations, patients should have had symptoms of Parkinson Disease (PD) for a minimum of 5 years. Candidates for surgery have symptoms that impede their quality of life despite optimal medical management. ⁴ The EARLY STIM trial compared DBS versus medical therapy in a cohort of early stage patients with PD (mean age 53 years with an eight year disease duration) and found significant improvement in quality of life, motor disability, and activities of daily living (ADL) subscales on the Unified Parkinson Disease Rating Scale (UPDRS). ⁵ There are no absolute guidelines in terms of age. In our practice, we have treated PD patients as young as 16 and as old as 80 years. In the aged population, comorbidities and complex medical conditions must be considered. Mostly important, the surgery should make a meaningful improvement in the patient’s life that outweighs the risk of surgery.

Cardinal symptoms, including rigidity, bradykinesia, ON/OFF fluctuations and tremor, improve to the greatest extent. Postural instability and freezing of gait, which occur despite medication, are notoriously difficult to treat with DBS. Dementia and inability to participate in the process of DBS programming are contraindications to surgery. Up to 30% of DBS failures to relieve symptoms have been related to inappropriate indications for surgery. ⁶

Pre-operative work up includes medical clearance, Core Assessment Program for Surgical Interventional Therapies in Parkinson’s Disease (CAPSIT-PD) testing, a neuropsychological evaluation, and an MRI with fine cuts through the basal ganglia that are devoid of significant motion artifact. The CAPSIT includes UPDRS testing. The UPDRS-3, that is the motor section of the exam, is performed ON medications and OFF medications. ⁷ In general, good candidates for surgery have a greater than 30% improvement in UPDRS score with levodopa challenge; however, patients with isolated tremor symptoms or asymmetric disease may not have this 30% improvement and still have meaningful outcomes.

The neuropsychological battery generally includes the Mattis Dementia (MDRS) Rating Scale, Dementia Rating Scale (DRS), Stroop test (ST), Trail Making Test Part A and B (TMT A/B), Wis-consin Card Sorting Test (WCST), and the Parkinson Disease Questionnaire (PDQ-39). Screening for cognitive decline and independence in activities of daily living is vital for pre-operative evaluation because patients with concurrent cognitive decline may have this exacerbated, and the MDRS has been shown to decline over the course of 36 months in patients receiving DBS to the subthalamic nucleus (STN). ³ Psychiatric symptoms such as depression have also been reported to worsen with STN DBS, making it an important pre-operative consideration. ⁸

11.2 Operative Procedure

PD medications are stopped 12 hours prior to the surgery to ensure accurate neurophysiological recording. Anesthesia is usually intravenous sedation combined with local anesthetic in cases where microelectrode recording (MER) will be performed. In MRI based cases, child dystonia cases and other rare indications, anesthesia is used. We prefer to have systolic blood pressure less than 140 mmHg and diastolic blood pressure less than 90 mmHg during the procedure and for 24 hours postoperatively.

Cases may be performed with a traditional frame based procedure (e.g. Leksell, Elekta; Cosman-Roberts-Wells, Integra) or with a “frameless” approach (e.g. Clearpoint, MRI Interventions; StarFix, Pacific Neuroscience). The accuracies between these two methods have been measured with phantoms or with patients undergoing DBS implantation and found to be similar. ^{9 , 10 , 11} Stereotactic planning is performed using MRI. Trajectories should avoid sulcal veins and extreme proximity to the ventricle to reduce risk of bleeding and deflection. Targets may be identified directly when using 3 T MRI or indirectly when using 1.5 T MRI. Indirect targeting is based off the anterior and posterior commissure. The surgeon’s choice between the three possible targets (Vim of the thalamus, GPi, STN) depends largely on clinical characteristics. For patients with dystonia as a primary complaint or patients with neuropsychological issues, the GPi is typically chosen. ³ Several studies have found equivocal post operative effects when comparing the GPi and STN. For patients who have less cognitive reserve and have tremor as a primary complaint, the Vimmay be selected after careful discussion with the patient that the other PD symptoms will not be treated.

A burr hole is then created with the 14 mm perforator drill bit for the insertion of microelectrodes. To minimize brain shift, fibrin sealant is used. Intraoperative electrophysiological via MER are used to reveal characteristic neuronal signals of the specific brain target. ¹² Once the target is selected, the DBS lead is placed and macrostimulation is performed to test for benefit, but especially for stimulation induced side effects that may preclude programming post-operatively. Once tested, the lead is anchored and buried under the skin. At a separate sitting, on average one week later, a second surgery is performed to connect an extension lead from the DBS lead to the implantable pulse generator which is usually placed in the chest. Decisions for which target and whether to perform bilateral or unilateral operations are generally undertaken by the multidisciplinary team during their monthly meeting. Intraoperative MRI may also used by experienced centers in place of MER. ¹³