Surgical Treatment of Movement Disorders

Deep Brain Stimulation Procedure

Proper patient selection for DBS surgery is a critical first step, and it requires a team composed of movement disorder neurologists, neuropsychologists, and psychiatrists. Severe fluctuations in motor symptoms and the presence of medication-induced dyskinesias are the primary indications for DBS in patients with PD. Disabling tremor, refractory to medical treatment, is the primary indication for DBS for ET. Screening involves measuring symptoms when patients are on and off medication to assess the degree of improvement. MRI of the brain is also required to assess for any anatomic impediments to the DBS lead placement. Neuropsychological testing is performed to evaluate the patient's cognitive status and ability to understand the expected outcomes of the procedure. A multidisciplinary approach is essential to selecting the proper patients and selecting the proper target. PD patients typically have leads placed into the STN or, less commonly, the GPi, whereas the ventral intermediate (VIM) nucleus of the thalamus is typically the target in patients with ET. There is no clear evidence demonstrating superiority with implanting electrodes in the STN over the GPi for PD; however, results have shown that patients with electrodes in the STN often are able to reduce their medications after DBS surgery, whereas those with electrodes in the GPi may have greater improvement in control of dyskinesia. Bilateral placement of DBS leads is commonly done, although in patients older than 65 years of age, the procedures are done as separate operations because these patients seem to have more postoperative side effects after bilateral lead placement in a single surgical procedure.

The DBS procedure is often staged, with the intracranial lead(s) placed during the first stage and the implantable pulse generator (IPG) or battery placed at a later date. Critical to the procedure is proper placement of the electrode into the appropriate intracranial location. This requires precise imaging localization with a guiding device to ensure that the proper trajectory is followed. In many centers, physiologic monitoring is used to ensure proper lead placement.

The lead placement procedure begins with the application of a guiding device, or stereotactic frame, to the patient's head ( Fig. 37.1A ). Four pins are placed in the outer table of the skull, holding the frame securely in place. CT showing elements of the frame called fiducial markers is obtained and then fused via software to previously acquired MRI. This process provides the spatial accuracy of the CT with the anatomic resolution of the MRI. Using these fused data, the computer creates a three-dimensional reconstruction of the brain. Based on well-recognized intracranial structures (anterior and posterior commissures), the location of the STN, the GPi, and the VIM can be localized using standard anatomic atlases (see Fig. 37.1B ). The target site for the DBS lead is selected on MRI, and the computer calculates coordinates in the x, y, and z planes. The trajectory of the lead placement is demonstrated on MRI, clearly depicting the anatomic structures and blood vessels to avoid. Selection of the proper trajectory not only includes the target point but also the angle of the lead within the target nucleus, allowing for maximum contact of the leads with the target.

In the operating room, the patient is placed in the supine position, and the localizing frame is attached to the operating room table. The patient is mildly sedated for the incision and creation of the burr hole. The computer-generated target coordinates and angles are set on the guiding device, and the entry point is selected on the scalp. After creation of a 1-cm burr hole, a microelectrode used for recording is then precisely and slowly passed to the target point (see Fig. 37.1C ). Neuronal activity is mapped, confirming the proper target using physiologic parameters. More than one pass is required to properly localize the target 60% of the time. Macrostimulation is then performed through the electrode, looking for adverse reactions, such as paresthesias, motor tract stimulation, speech impairment, and eye movements. The patient is awake and cooperates during this part of the procedure, providing real-time feedback for any subjective symptoms. The efficacy of the DBS electrode also can be assessed during the procedure, with immediate resolution of tremor and rigidity seen in many patients. Once the optimal location has been determined, the permanent lead is placed in the same location, and the distal part of the lead is tucked under the scalp in the retroauricular region ( Fig. 37.2 ). The incision is closed, the guiding frame is removed, and the patient is admitted overnight to the hospital.

Seven to ten days later, the patient returns for placement of an extension lead that connects the intracranial electrode that had been placed under the scalp to the IPG, which is placed in a pocket on the ipsilateral anterior chest wall (see Fig. 37.1D ). This part of the procedure is performed with general anesthesia. The patient is discharged home on the same day, returning for programming within the next week.

DBS programming is an iterative process that often is completed in several outpatient sessions. The IPG is capable of creating current between the contacts at the end of the electrode implanted in the brain. The current can vary in pulse width, frequency, and amplitude, and the programmer tests many combinations of these parameters, looking for maximum benefit with minimal adverse effects.