Intraoperative Imaging-Based Lead Implantation

4 Intraoperative Imaging-Based Lead Implantation

R. Mark Richardson

Abstract

This chapter outlines the evolution of intraoperative imaging-based deep brain stimulation (DBS) lead implantation. Current outcome data following DBS lead placement via intraoperative computed tomography and intraoperative or interventional magnetic resonance imaging (iMRI) are reviewed. A practical description of the workflow for iMRI-DBS is included along with remarks on the potential future evolution of this technique.

Keywords: deep brain stimulation, movement disorders, interventional MRI, intraoperative MRI, intraoperative CT, stereotactic neurosurgery

4.1 Introduction

Multiple randomized controlled studies have established deep brain stimulation (DBS) as the current standard of care for Parkinson’s disease (PD) with motor fluctuations due to increase in off-time, improvements in quality of life, and reduction in medication compared to medical management alone.1,2,3 DBS for PD, therefore, has provided a model indication for the development of intraoperative imaging-based lead implantation in patients under general anesthesia as an alternative to neurophysiology-guided placement. The primary question is no longer whether DBS helps patients with PD, but rather to what extent the method of lead implantation affects the efficacy of DBS. The use of intraoperative imaging for DBS lead implantation in dystonia and essential tremor is evolving in similar fashion. This chapter discusses the evolution of the subfield of intraoperative imaging-based DBS lead implantation.

4.2 Evolution of Lead Implantation in the Asleep Patient

The fundamental basis for the recent shift to “asleep” DBS, i.e., DBS under general anesthesia in the absence of microelectrode recording (MER), is the cumulative experience of the field in verifying effective DBS lead locations using postoperative magnetic resonance imaging (MRI). Other important factors include the desire of some patients to avoid an awake brain-mapping procedure and the desire of patients with severe symptoms to avoid potential discomfort that would come with awake surgery. Two general methods currently predominate for DBS lead implantation in patients under general anesthesia: intraoperative MRI (iMRI)-based DBS with real-time imaging, and intraoperative computed tomography (iCT)-based DBS with immediate postimplantation imaging. Prior to the advent of these techniques, however, some centers had already adopted DBS without MER on a regular basis, often under general anesthesia, using immediate postoperative MRI for lead location confirmation.

In the early 2000s, Gill and colleagues developed a method for implanting guide tubes and radiopaque stylets into the STN and verifying target accuracy with MRI, prior to lead implantation.4 This method can be carried out with the patient under general anesthesia, although this group initially used macro-stimulation through the DBS lead to evaluate the need for trajectory adjustment,⁵ reporting a 61% improvement in off-medication Unified Parkinson’s Disease Rating Scale (UPDRS) III scores at 1 year.

For many years, Hariz and colleagues have advocated that MER is not necessary for successful DBS implantation in PD, relying instead on dynamic impedance monitoring, including patients under general anesthesia.⁶ In this method, a smooth-tip radiofrequency electrode is advanced to the target, prior to inserting the DBS lead. Immediately following implantation of the DBS leads, all patients undergo a stereotactic MRI scan to confirm the lead positions before implantation of the pulse generator. The surgery is not considered to be complete until acceptable lead placement is confirmed. This approach was shown to be both safe and efficacious with 52% improvement in off-medication UPDRS III scores at 1 year.

It should be noted that Machado and colleagues have reported an approach to MRI-verified DBS that is specific to iMRI. This group studied 33 patients with movement disorders (64 total leads, 27 PD patients). All the patients underwent implantation with standard frame-based techniques under general anesthesia and without MER.⁷ MR images were acquired immediately after lead implantation and fused to the preoperative plan to verify accuracy. The authors reported 27 iMRI globus pallidus interna (GPi) implantations for PD with an average reduction of 46% in UPDRS III scores.

4.3 Intraoperative-CT-verified DBS

Ponce and colleagues 8,9,10 have pioneered the use of iCT to perform immediate verification of DBS lead locations (iCT-DBS) in patients under general anesthesia. Using this method, asleep Vim DBS for essential tremor without intraoperative test stimulation was reported to be as safe and effective as awake lead implantation (N=17).⁹ In a prospective follow-up study, outcomes also did not differ between subjects implanted while awake (16) versus asleep (40).¹⁰ This is quite notable, given that the Vim cannot be visualized at 1.5 T or 3 T, requiring a type of direct targeting based solely on other landmarks.

For PD DBS, 6-month outcome data was reported from a study of 78 GPi (16 awake, 62 asleep) and 55 STN (14 awake, 41 asleep) subjects.⁸ UPDRS-III score improvement with stimulation did not differ between awake and asleep groups for GPi (awake, 38.5%; asleep, 37.5%) or STN (awake, 40.3%; asleep, 48.8%) targets. A separate study by Burchiel and colleagues¹¹ similarly showed no difference in UPDRS II or III score improvements between subjects who underwent DBS asleep (N = 30) versus awake (N=39), although the awake cohort represented a historical rather than prospective control. Interestingly, outcomes from asleep DBS in that study were superior with regard to speech fluency and quality of life.

On comparing DBS lead implantation awake versus iCT-DBS, no significant differences in complications, length of stay, and 30-day readmissions have been reported,12 while iCT-DBS performed asleep may be associated with a lower cost variation relative to awake procedures.¹³

4.4 Intraoperative-MRI or Interventional-MRI-guided DBS

iMRI-guided DBS lead implantation (iMRI-DBS) relies on real-time confirmation of accurate trajectory alignment and lead placement. The benefit of iMRI over other anatomic verification approaches lies in the fact that it allows the surgeon to correct for inaccuracy in trajectory planning prior to electrode placement, almost always resulting in a single brain penetration for electrode placement. The precision achieved by functional verification of electrode location with MER enabled the iMRI-DBS field. Imaging electrode locations after MER-guided placement has demonstrated that the sensorimotor territory lies in the dorsolateral portion of the STN,14,15 and within the ventral posterior lateral portion of the GPi,¹⁶ allowing the identification of the functional territory of these nuclei by direct visualization on MRI.

Currently, the ClearPoint system is the sole Food and Drug Administration (FDA)-approved platform for iMRI-based stereo-logical procedures. The platform, pioneered by the group at the University of California San Francisco (UCSF), is based on the concept of prospective stereotaxy, the alignment of a skull-mounted trajectory guide within an MRI system.¹⁷ This approach provides immediate detection of complications, eliminates the need for microelectrode mapping, and reduces brain penetrations. The key features of this strategy are: (1) patient positioning supine on the MRI gantry under general anesthesia; (2) integration of planning, insertion, and real-time MRI confirmation of DBS lead placement during a single procedure; (3) trajectory alignment and DBS lead insertion via a burr hole-mounted trajectory guide in place of a traditional stereotactic frame and arc system; (4) definition of target coordinates with respect to the MRI isocenter rather than to a separate stereotactic space using fiducial markers. Depending on preference for how the dura is opened, acquisition of target images also can occur after burr hole creation and intracranial air entry, to account for brain shift.

The first validation of the accuracy of the ClearPoint system occurred in nonhuman primates,¹⁸ followed by workflow simulation in the postmortem human brain,¹⁹ both of which demonstrated an average targeting error of less than 1 mm. Subsequent to FDA approval in 2012, the UCSF group reported 1-year outcomes following iMRI-DBS for PD of 40% UPDRS III score improvement.²⁰ Other groups have reported various outcome measures of iMRI-DBS for PD, all of which are similar to those reported from outcome studies of MER-based-DBS for PD ( Table 4.1), with similar low complication rates. A classic argument against asleep DBS lead placement has been that a lack of functional verification of lead location increases the risk for side effects from stimulation. On the contrary, our retrospective study of a contemporaneous cohort of 45 consecutive patients who underwent either iMRI- or MER-guided DBS lead implantation showed that side effect thresholds during initial programming were slightly lower in the MER group, with similar thresholds for clinical benefit and no significant difference in the reduction of symptoms or levodopa equivalent doses.²¹ These findings bolstered previous work indicating that iMRIDBS lead implantation occurs with greater anatomic accuracy, in locations demonstrated to be the appropriate functional region of the STN by the production of equivalent clinical outcomes.^7,20,22,23

Importantly, in a 10-year study period, the UCSF group reported 272 electrode implantations in 164 iMRI-guided surgical procedures, with an overall infection rate of 3.6%.²⁴ A modification of sterile practice occurred after the first 10 patients, reducing the infection rate to 2.6%, all of which occurred at the internal pulse generator (IPG) site. This author has experienced one scalp infection in 70 iMRI-DBS cases, which was successfully treated with intravenous plus oral antibiotics without hardware removal.