Location, location, location: Validating the position of deep brain stimulation electrodes

The success of DBS, much like with real estate, is very dependent on the location of the implanted electrode. The correlation of electrode position with clinical outcome is a vital tool for evaluating the efficacy of the therapy in different disease entities. It is thus crucial that we have the technology to accurately determine electrode location. Factors to consider when validating electrode position should include safety, accuracy, availability, and cost. There are various techniques to verify DBS lead location. The gold standard is likely intraoperative or postoperative high-field MRI; however, this is not without its own potential drawbacks. DBS leads have a signature artefact; there are concerns about lead heating and migration in high-strength magnetic fields, even though at least one manufacturer has recently received U.S. Food and Drug Administration approval for postoperative cranial (and whole body) MRI. MRI is expensive and not ubiquitously available. Alternatively, a high-resolution postoperative CT can be fused with the preoperative MRI. Aside from the concern of exposing the patient to radiation, this technique may also have limitations, as in the first few days following surgery there may well be some intracranial air and attendant brain shift such that the superimposed fused image may not accurately reflect electrode position. Furthermore, image fusion is only as accurate as the original images and the software that performs the fusion. The novel tool reported in this issue by Walter and colleagues describes the fusion of postoperative real-time transcranial sonography (TCS) images with preoperative MRI in an outpatient setting. The procedure was safe and, in the authors’ hands, quick and easy to perform. The system described requires expertise in TCS; however, the nuances of the TCS technique for this purpose may not involve a steep learning curve, especially if TCS is already performed for other indications. TCS therefore meets several of the criteria necessary for a system of validation. The really crucial question that needs to be addressed is how precisely electrode position can be determined using this technique. Modern stereotactic surgery should realistically be able to reach submillimeter accuracy; a method of determining electrode position should therefore be able to match the same level of resolution. Similar with a CT, the fusion of TCS images to the preoperative MRI may involve some inherent inaccuracy associated with suboptimal fusion software and intracranial air. The images provided by the authors suggest that the electrode artefact on the fused image is similar to that seen on an MRI. Ongoing experience with this technique in larger patient cohorts and the correlation of electrode position with patient outcomes may afford a comparison to the accuracy already provided by existing techniques. It is quite amazing that after more than 20 years of experience with subthalamic nucleus DBS, the optimal spot for stimulation has not yet been entirely resolved. Different targets within the subthalamic nucleus may even improve different symptoms to different extents. The ability to accurately determine electrode location, both anatomically and physiologically, will play an important role as we continue to grapple with these questions. TCS technology may be a step forward in this direction.