Erwin B. Montgomery Jr. MD
Vol. 2, No. 7
July 25, 2016
We recently learned that some intraoperative neurophysiologists and neurosurgeons utilize the large indifferent or reference electrical contact for macrostimulation during targeting of the Deep Brain Stimulation (DBS) lead. Such macrostimulation may be useful to infer the regional anatomy and physiology of the electrode location and to, perhaps, anticipate post-operative clinical response. Those interested in a fuller discussion of these benefits might consult Montgomery Jr. E.B., Intraoperative Neurophysiological Monitoring for Deep Brain Stimulation: Principles, Practice and Cases, Oxford University Press, 2015.
There is a problem with some microelectrodes in that the sharp recording tip may extend many millimeters beyond the indifferent or reference electrode on the outer cannula of the microelectrode. Thus, advancing the indifferent or reference electrode to the desired depth also means advancing the extended sharp tip, perhaps to a depth that could injure important structures.
Apparently, some intraoperative neurophysiologists and neurosurgeons will withdraw or retract the sharp tip into the outer cannula and then continue to advance the microelectrode until the indifferent or reference electrical contact is at the desired depth. However, this results in the open end of the cannula moving through brain tissue. The concern is that this will result in the “coring out” of tissue similar to what occurs during procedures such as stereotactic biopsies. Advancing the electrode with the tip withdrawn could lead to an increased risk of intracranial hemorrhage.
We examined this possibility by passing a standard DBS microelectrode through fresh cow brain. The sharp tip was extended and then the electrode was inserted. Once the tip of the outer cannula was well into the brain, the sharp tip was retracted and the electrode moved through the brain until the electrode emerged. The tip of the electrode was inspected for brain tissue fragments and none were found. When the sharp tip was extended, tissue fragments were extruded, indicating these fragments were within the outer cannula. These results were repeated on a second pass.
While these observations warrant further examination, neurosurgeons and intraoperative neurophysiologists must at least consider the potential operative risk associated with advancing the electrode with the tip withdrawn. Both this incremental risk and the benefits referenced above must be included in the risk-to-benefit analysis of the procedure. It is important to note that this issue is not evidence of a design or manufacturing problem. Rather, the risk is only attendant on the use of these types of microelectrodes in this particular manner.
At least in the United States, if an FDA-approved microelectrode in used in a manner that is not warranted by the FDA-approved manufacturer labeling or manual, it is considered an “off-label” use of an FDA-approved device. Advancing the electrode with the tip withdrawn could be considered an example of this. Physician discretion in such “off-label” use is well respected by the FDA; however, the FDA expects the physician to use due diligence.1
Unfortunately, there is no data currently available to determine the magnitude of the incremental risk associated with advancing a DBS microelectrode with the tip withdrawn. As discussed in the last professional newsletter (“Is It a Sin If One Does Not Get Caught?”, Vol. 2 No. 6, June 25, 2016) the risk is already so low as to make detection of even a doubling or more of the risk unlikely. Nevertheless, this is not license and does not obviate the responsibility of physicians to be reasonable and responsible.