The Future of Microelectrode Recording

KIM J. BURCHIEL

To say that intraoperative microelectrode recording (MER) for localization is at a crossroads might be clichéd, but nevertheless, it is true. The technique enjoys a resurgent popularity unparalleled since its first applications in the 1950s and 1960s. In many ways MER is now considered tantamount to an imprimatur of quality the logic being that any center that can mount the effort to conduct MER has surpassed a minimum level of effort and commitment to functional stereotactic neurosurgery.

Indeed, any center that assembles the requisite team and equipment to perform respectable MER must have, at a minimum, a vision of itself as a center of excellence for movement disorder surgery. Most neurosurgeons and neurologists are not trained neurophysiologists, requiring that centers either have ready access to this expertise or hire a neurophysiologist. The recording systems can be “homegrown,” at modest expense, but increasingly, turnkey MER systems complete with Food and Drug Administration (FDA) approval are dominating this small market. This is an expensive proposition, given that many commercial MER systems cost in the neighborhood of $100,000, a full-time neurophysiologist can cost $50,000 to $75,000 a year, and expendables such as the MER electrodes may run several hundred dollars per procedure. Microelectrode recording clearly adds substantially to the operative time, a valuable commodity in most hospitals, and one that the patient or insurer will be charged for: all this time and expense for an aspect of functional neurosurgery that, with regard to professional fees, is at best underreimbursed and, at worst, unreimbursed. In considering my colleagues throughout the world of functional neurosurgery, it is remarkable to me (with appropriate apologies to Winston Churchill), that “so many have done so much for so little.”

The end of the twentieth century saw a wave of enthusiasm for movement disorder surgery, which carried with it a reawakening of interest in MER. In the early 1990s, few centers had any substantial experience in movement disorder surgery or MER. Now, more than a decade later, arguably over 100 centers in North America alone can lay claim to a significant body of experience in the surgical management of movement disorders. Many, if not most, of these centers now routinely employ MER as an aid to surgical localization. It is perhaps now that the more gimlet-eyed among us can begin to address the important questions for the use of MER in the future. These questions can be summarized as follows:

1. Does MER add demonstrable value to movement disorder surgery?
   
2. Does MER add risk to movement disorder surgery, and if so, what is the risk/benefit ratio?
   
3. If MER is proven to add value to movement disorder surgical procedures, what can be done to simplify, or at least facilitate, the recording procedures and their analysis?

Each of these questions will be discussed.

Given this projection, it is likely that the continued use of MER will be based on class II or III evidence. This means that the best we can hope for is a prospective randomized trial from a single center that will address the question of MER morbidity only. Previous meta-analyses comparing the results of pallidotomy performed using MER or macrostimulation, although showing that MER was associated with a small but significantly higher rate of symptomatic intracranial hemorrhage, failed to show a significant difference in outcome with regard to dyskinesia or bradykinesia. Unless the VA/NIH study indicates that MER is associated with a substantial improvement in outcome, a small study is unlikely to show a significant difference.

Although this assessment may seem pessimistic, it is, in effect, a product of the proliferation of the surgery for Parkinson’s disease and other movement disorders. Simply put, no single center will have enough patients to answer definitively the outcome questions related to MER. What is likely is that, over the next decade, the relevance of MER for stereotactic neurosurgery will be at least partially eclipsed by two developments: new techniques for movement disorder surgery and improvements in imaging.

Advances in neurobiology will unquestionably improve movement disorder surgery. The development of intraparenchymal trophic factor infusions and the engineering of useful stem or progenitor cell lines may diminish the need to exquisitely map brain nuclei. Targeting in these instances may be more biological (e.g., receptor specific) than anatomical. As imaging continues to improve, our dependence on physiological confirmation of stereotactic targets may also diminish. High-field MRI already promises improved direct targeting of tracts and nuclei, and further enhancement of image processing and image guidance may distance us from cumbersome intraoperative MER technology.

The value of MER today is questionable. Unless it is proven essential or can be made more straightforward in its implementation and objective in its end points, it will be, once again, abandoned in favor of simpler and faster methods of brain mapping.

Microelectrode recording technology has not evolved dramatically since the 1960s. Computer-controlled stepmotor microdrives add ease and precision, digital amplifiers reduce noise and interference, multichannel recordings allow wide neuronal sampling, and signal processing and template matching allow unprecedented capacity for intraoperative analysis. Despite these advancements, MER still can be reduced to a high-impedance microelectrode being driven through the brain by an experienced neurophysiologist upon whose shoulders rests the responsibility of separating signal from noise, and one pattern of discharge from another. This process can be streamlined, and can be made more objective, but first, we need to decide what information we wish to derive from MER. We need to temporarily put aside our innate curiosity and research interests and ask ourselves what is the unique contribution of MER to the outcome of the surgical procedure. How can we obtain this information with minimum risk to the patient, and the most efficient expenditure of time and resources?

Ultimately, I would submit that the future of MER will hinge on a simple equation: Value = Quality/Price. In this case, price is the combination of risk to the patient and expenditure of resources. Quality is the excellent outcome we all seek. If we can reduce morbidity and resource utilization, then even a modest improvement in quality can be justified, and MER will remain a valued part of our neurosurgical armamentarium. Without improvement in these components of the method, it is likely that the current renaissance of MER will again be largely reduced to a historical footnote.