Impedance Monitoring And Stimulation Through A Macroelectrode

Additional intraoperative physiologic confirmation of the brain target may be obtained using the lesioning probe or deep brain stimulator. These

"macroelectrodes" are typically about 1 mm in diameter and thus much too large to record cellular activity, but they can be used for impedance monitoring or electrical stimulation. Because the macroelectrode has a low impedance, the impedance of the surrounding brain tissue contributes significantly to current flow in response to applied voltage. Gray and white matter have different impedances, so it is possible to detect transitions between gray and white matter if the impedance to a test pulse is measured as the macroelectrode descends.

Electrical stimulation through the lesion probe or DBS lead, or "macrostimulation," affects a larger volume of tissue than microstimulation, as the probe tip and the current flow are several orders of magnitude larger. Macrostimulation provides the final intraoperative check on target localization just before lesioning or permanent stimulator placement, and has been used during surgery for movement disorders for many years [65,66]. As with microstimulation, macrostimulation can evoke visual, motor, and cutaneous sensory responses by effects on structures that border the target. In GPi, proximity of the probe to inferior and posteromedial borders is indicated by the current thresholds for activating optic tract and corticobulbar/corticospi-nal tracts, respectively [41]. In motor thalamus, proximity to posterior and lateral borders is indicated by activation of sensor thalamus and cortico-bulbar tract, respectively. In STN, proximity to posteromedial and lateral borders may be indicated by activation of lemniscal fibers and corticobulbar tract, respectively [4,67]. During STN surgery, macrostimulation may also evoke oculomotor effects [67] or mood changes [68], although the exact structures or pathways responsible for these effects are not yet clear.

For patients with tremor, intraoperative tremor suppression provides another indication of adequacy of probe placement. This has long been known for thalamic surgery [65,69-71], but appears to be true for surgery of GPi and STN as well [72]. In STN, intraoperative suppression of rigidity has also been used as a guide to placement of chronic STN stimulators [4,73].

Caution is advised, however, when using acute stimulation to predict the effects of lesioning or chronic stimulation. Symptoms other than tremor may not respond immediately to acute intraoperative test stimulation. During GPi pallidotomy, for example, bradykinesia often fails to improve with acute test stimulation through the lesion probe, but this does not predict a poor long-term effect of lesioning for bradykinesia [41]. After implantation of a DBS lead into GPi, symptomatic benefit may require time (from hours up to several weeks) to appear [74], again illustrating the point that failure to improve with brief test stimulation does not necessarily imply an incorrect lead placement. Pallidotomy and chronic pallidal stimulation for dystonia are also associated with long delays (weeks or months) between surgery and symptomatic benefit [75,76]. With the exception of tremor control, therefore, we use intraoperative macrostimulation mainly to confirm appropriate voltages for adverse effects, rather than to assess for clinical benefit.

In any of the surgical targets discussed here, motor symptoms may be partially or completely suppressed by simple placement of the macroelec-trode, even before stimulation or lesioning. This "microlesion effect,'' when observed, provides evidence that the probe is within or has passed through the motor territory of the target nucleus. Unless a permanent lesion is subsequently made deliberately, this effect is generally temporary. For DBS procedures, failure to observe a microlesion effect does not necessarily predict a poor outcome from chronic stimulation.

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