Deep Brain Stimulation: Surgical Technique




Abstract


Traditionally, deep brain stimulation has been performed by using frame-based stereotaxy in an awake patient. Frame-based stereotaxy has been the technique of choice due to its perceived superior accuracy over “frameless” approaches, while awake testing was deemed essential to both confirm clinical efficacy and ensure a well-tolerated side effect profile. In recent years, the tide has shifted toward the use of targeting devices other than traditional stereotactic frames, as well as toward the use of intraoperative imaging to confirm lead placement and, in many cases, replace the need for intraoperative physiologic testing. Several reports have discussed in detail the target-specific surgical and microelectrode recording techniques for the surgical treatment of movement disorders, This chapter will review the fundamentals of these techniques as practiced by the authors.




Keywords

Deep brain stimulation, Microelectrodes, Movement disorders, Stereotactic frames, Stereotaxy, Surgical navigation

 






  • Outline



  • Introduction 503




    • Stereotactic Frame-Based Approach 503



    • “Frameless” Approach 504



    • Comparison 504



    • Image Acquisition 504



    • Target Localization 504



    • The Surgical Procedure 506



    • Physiologic Confirmation 506




      • MER and Microstimulation 507



      • Electrode Implantation and Fixation 508




    • Intraoperative Image-Based DBS Surgery – “Awake Versus Asleep DBS” 508






  • References 510




Introduction


Traditionally, deep brain stimulation (DBS) has been performed by using frame-based stereotaxy in an awake patient. Frame-based stereotaxy has been the technique of choice due to its perceived superior accuracy over “frameless” approaches, while awake testing was deemed essential to both confirm clinical efficacy and ensure a well-tolerated side effect profile. In recent years, the tide has shifted toward the use of targeting devices other than traditional stereotactic frames, as well as toward the use of intraoperative imaging to confirm lead placement and, in many cases, replace the need for intraoperative physiologic testing. Several reports have discussed in detail the target-specific surgical and microelectrode recording (MER) techniques for the surgical treatment of movement disorders ( ). This chapter will review the fundamentals of these techniques as practiced by the authors.


Stereotactic Frame-Based Approach


There are several stereotactic headframe-and-arc systems available, and the choice depends largely on the surgeon’s experience and preference. Fig. 36.1 shows the Leksell (Elekta, Stockholm) stereotactic headframe-and-arc system fully assembled, as used in DBS implantation surgery. The headframe is placed usually on the morning of the surgical procedure with the patient awake or sedated, and the scalp is anesthetized at each pin site with local anesthetics. The canthal–meathal line can be used as a reference to align the frame as parallel as possible to the anterior commissure–posterior commissure (AC-PC) plane. In addition, care should be taken to avoid any roll, pitch, or yaw with the placement to minimize errors when changing the coordinates for additional electrode penetrations during surgery. After placement of the headframe, computed tomography (CT) and/or magnetic resonance (MR) imaging (MRI) studies are acquired with a localization device attached to the frame. The stereotactic fiducials become part of the imaging data set, and from that moment onward, planning and stereotactic localization will rely on the constant stability of the frame in relation to the patient’s head.




Figure 36.1


Elekta G model headframe and arc, assembled as used in deep brain stimulation surgery. The headframe ( arrow ) is secured to the skull by using sharp pins. The stereotactic images are acquired with the fiducial box loaded on the headframe. In the operating room, the arc is assembled on the headframe and set to the planned stereotactic coordinates.


“Frameless” Approach


“Frameless” stereotactic devices are now approved by the US Food and Drug Administration for use in the United States and are a valid alternative to traditional frame-based stereotaxis. Advantages of these systems include less discomfort associated with the frame placement, as well as suitability for use in patients with severe kyphotic deformities that preclude CT/MRI image acquisition with a frame in place and the rare patient with a head too large to fit in a frame. The reference system used for frameless surgery consists of at least four skull fiducials typically implanted in the office setting under local anesthesia in the day(s) preceding surgery. A stab wound is created with a scalpel and the fiducial’s self-tapping screw is fixed to the skull. The day of surgery, a platform-like device is affixed to the patient’s head, either a custom-made platform that mounts on the screws themselves and does not require image guidance ( ) or a standard one that mounts on the burr hole cap and uses optical tracking ( ).


Comparison


Studies comparing the stereotactic accuracy of frame-based versus frameless systems have indicated that any differences in accuracy between the two methods do not appear to be clinically significant ( ). From an economic standpoint, stereotactic frames typically represent a large one-time capital expense that can be applied repeatedly for several years (with maintenance), while the frameless disposable system adds significant cost to each individual surgery. Additionally, frame-based stereotaxy can be performed with MRI alone, while frameless systems require an additional CT scan to localize the bone fiducials.


Image Acquisition


MRI is the imaging modality of choice in stereotactic targeting and planning, allowing for accurate identification of the AC and PC, standard landmarks used in indirect targeting ( Fig. 36.2 ), and direct identification of targets or landmarks. However, magnetic distortion may hamper the geometry of the stereotactic space. Our preference is to acquire the MRI in the weeks that precede surgery and then fuse the images to a stereotactic CT scan obtained on the day of surgery. The objective is to benefit from the excellent resolution of MR images while relying on CT for stereotactic accuracy. CT-based stereotaxis may be the only option in patients with implantable cardiac pacemakers-defibrillators, in whom indirect AC-PC based targeting (see later) is the primary targeting method.




Figure 36.2


The AC ( arrowhead ) and PC ( arrow ) are shown in (A) axial and (B) sagittal views. The midpoint of the line traced between these two landmarks is the midcommissural point.


Target Localization


The target can be localized by directly identifying the structure in the preoperative MRI images ( Figs. 36.3 and 36.4 ). Indirect targeting relies on coordinates referenced to the midcommissural point (the midpoint of the AC–PC line) or on atlas-based outlines of subcortical structures fused and reformatted to fit the patient’s preoperative images ( Fig. 36.5 ). For example, typical coordinates for targeting the subthalamic nucleus are 11–13 mm lateral to the midline, 3–4 mm posterior to the midcommissural point, and 4–5 mm inferior to the intercommissural plane. In practice, we use a combination of indirect targeting methods and direct visualization of the nucleus to determine the final stereotactic target. In addition, other regional anatomic structures such as the red nucleus ( Fig. 36.3 ), substantia nigra, and optic tract ( Fig. 36.6 ) can be visualized and serve as landmarks for localization ( ). When direct targeting cannot be performed, as in patients with non–MRI-compatible cardiac pacemakers, CT-based indirect targeting is performed. Furthermore, targeting of the ventral intermediate thalamic nucleus (Vim) continues to be performed indirectly by using coordinates that are AC–PC based, as individual thalamic nuclei cannot be reliably distinguished using standard MRI sequences.




Figure 36.3


The subthalamic nucleus can be visualized on an axial T2 view ( arrow ). Note the red nucleus medial and posterior to the STN. The anterior border of the red nucleus serves as an anterior-posterior reference for selecting the STN target ( line ). The images were acquired as volumetric coronal T2 scans and then reformatted to the axial view.



Figure 36.4


The globus pallidus is seen on preoperative IR scans ( arrow ). The genu and posterior limb of the internal capsule serve as a reference for the medial border of the posterior globus pallidus internus. The pallidotomy target is defined in the posterior-ventral region. However, DBS electrodes may be implanted more anteriorly to allow for current spread within the nucleus without affecting the internal capsule fibers medially.



Figure 36.5


The Schaltenbrand and Warren atlas is loaded on commercially available computerized planning stations and can be overlaid on the patient’s MR or CT images. The atlas can then be reformatted to attempt to fit the patient’s anatomy. Current systems allow only for two-dimensional adjustments in height and width. Due to this limitation, it is often not possible to “fit” the atlas contours correctly on all basal ganglia and diencephalic structures. In this sagittal view, a characteristic trajectory to the subthalamic nucleus is seen, crossing the anterior thalamus and then the zona incerta before reaching the STN. Note that although the optimal region for stimulation is likely to the dorsal STN, the target is defined on the ventral posterior aspect of the nucleus. In this fashion, the DBS electrode’s tip is placed at the ventral region while the more proximal contacts flank the dorsal subthalamic region for chronic stimulation.



Figure 36.6


The optic tract ( arrow ) is an important landmark for targeting the globus pallidus internus (GPi). It is located ventral to the nucleus and can be identified during microelectrode recordings. Light stimulation can elicit visually evoked responses and microstimulation may elicit phosphenes in the patient’s visual field. Identification of the optic tract during MER corroborates to the anatomical targeting of the GPi.


Once the target is defined, the entry point and angle of approach are determined. We prefer to use contrast-enhanced volumetric T1 image sets to facilitate visualization of intracranial vessels to be avoided in the planned trajectories. Typically, an entry point is selected at or just anterior to the coronal suture at an angle that is approximately 10–20 degrees from the parasagittal plane for the STN and thalamus and, if possible, 0–10 degrees for the globus pallidus internus (GPi) to allow for a true parasagittal approach. The trajectory is altered in such a way as to avoid sulci, the lateral ventricles, periventricular vessels, or any large vessels highlighted by gadolinium enhancement in order to prevent hemorrhagic complications.


The Surgical Procedure


The patient is positioned supine and the headframe is fixed to the table. In the case of frameless surgery, a modified rigid cervical collar is used to support the patient’s neck and head while maintaining access to the surgical site and skull fiducials. The patient’s feedback is solicited to obtain the most comfortable head/neck position for the case. Sedation with a continuous titratable infusion of dexmedetomidine, supplemented by intermittent infusion of short-acting agents such as propofol, provides the appropriate combination of anesthesia necessary in the vast majority of cases. While the patient does not need to be awake and cooperative for the skin incision and burr hole placement, we prefer to perform the physiologic localization of the subcortical target with minimal sedation to optimize the data from MERs and to obtain the best patient feedback during macrostimulation. Tight blood pressure control (systolic blood pressure <130 mm Hg) is helpful to minimize the risk of intracranial hemorrhage. We prefer to drill the burr holes not perpendicular to the skull but, rather, coaxial to the planned stereotactic trajectory. In this fashion, the inner and outer rims of the burr hole are equally aligned, preventing cannula deflection from the bone edges. The dura mater is opened, and the pia and arachnoid are coagulated and opened to facilitate the penetration of the cannula without resistance. The electrodes are inserted to a predetermined offset dorsal to the final target. We use Gelfoam and/or fibrin glue around the cannula in the burr hole to prevent cerebrospinal fluid loss and subsequent pneumocephalus and brain shift. Fig. 36.7 illustrates the stereotactic assembly of a frame-based procedure. When using a frameless device, the apparatus is assembled on a base that is fixed to skull with self-tapping screws ( Fig. 36.8 ). The burr hole continues to be accessible from the sides, but the working window is reduced ( Fig. 36.9 ).


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Sep 9, 2018 | Posted by in NEUROLOGY | Comments Off on Deep Brain Stimulation: Surgical Technique

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