The most popular method for determining where to place electrodes during deep brain stimulation may not be the most effective or the least risky. The vast majority of DBS centers today continue to offer traditional awake DBS using microelectrode recording (MER) to place leads, a technique that was developed a quarter of a century ago.
Many of these centers are clinging to this method in the false belief of “if it’s not broken, don’t fix it.” Indeed, results of DBS using this method have proven to be superior over the use of medications alone in controlling symptoms of Parkinson’s disease, improving quality of life, and lengthening lifespan.
However, it is important that we recognize the lack of evidence supporting this technique in achieving the best lead placement accuracy possible and that we also recognize the advancements made in recent years, including Asleep DBS and robotic-assisted DBS, and their potential of achieving greater results.
Lack of Evidence Supporting Current DBS Methodology
One of the most popular measurement tools for assessing DBS lead placement accuracy is the use of microelectrode recording (MER). MER measures small electrical potentials from one or a few neurons at the tip of a fine wire electrode as it is passed through the brain.
Done with the patient awake in the operating room, the surgeon measures variations of these potentials with voluntary and passive movement by the patient, as well as sensory input, to confirm whether the location being probed is the ideal target. As awake surgery was the dominate methodology at the time of initial submission to the FDA in 2002, DBS leads by Medtronic are only considered “on-label” when used where the lead is clinically tested prior to the generator being implanted.
The drawbacks for this methodology are that tests can only be performed while the patient is laying down and relies upon patient feedback. This feedback is prone to vary with fatigue as the procedure can take many hours. While tests of tremor and rigidity improvements are relatively easy, gait testing is impossible.
Even though MER is the most popular measurement tool being used today, there is no Class-I or Class-II evidence that MER improves the accuracy or efficacy of the DBS implantation. Indeed, a 309-patient trial reported in 2014 found that only 64% of leads placed utilizing MER hit the ideal target within the subthalamic nucleus.
In addition to the limitations in accuracy, MER also poses potential risks. During MER, microelectrodes are inserted into the target area, then taken it out and reinserted, sometimes multiple times, to try to find the right spot. As highlighted in a 2012 study in the Journal of Neurosurgery, this practice can cause potential bleeding, stroke, and brain damage.
Another less obvious limitation is that there is a fundamental limit to the accuracy of MER testing. Each lead is placed by a cannula which is typically 1.8 mm in outer diameter. It is not possible in practice to move a lead to another trajectory closer than approximately 2 mm from a previous trajectory as the cannula and lead will tend to shift into the prior tract. This limits the accuracy of clinical lead testing to about 2 mm.
Advancements in Lead Placement Accuracy
Two recent advancements in DBS are showing the potential of greater lead accuracy than the traditional MER method. Those advancements are Asleep DBS and robotic-assisted DBS. Utilizing a portable CT machine (CereTom®) during Asleep DBS, we have been able to place leads within 1 millimeter of the target, compared to 1-2 millimeters with the traditional Awake DBS procedure. Going even further, we have introduced the use of robotic guidance during DBS, which has allowed us to to get even closer to the target with less than 1 millimeter deviation.
With the lack of a common system to measure the placement of leads—the most critical aspect of DBS efficacy—I have analyzed the results and provide the foundation for future research in a 2015 white paper, Accuracy of Robotic Guided Subthalamic Nucleus