Visual evoked potentials during posteroventral pallidotomy for Parkinson's disease

OBJECTIVE: To examine the efficacy of intracerebral recording of visual evoked potentials (VEPs) during posteroventral pallidotomy comparing macro- and microelectrode stimulation. MATERIALS AND METHODS: The optic tract was identified by intracerebral recording of VEPs in 16 patients. Electrical stimulation through a lesioning electrode (macroelectrode stimulation: biphasic wave, 50 Hz, 0.2 ms, 0-6 V) and a microelectrode (microelectrode stimulation: impedance of 1 M Omega at 300 Hz, biphasic pulse, 300 Hz, 0.2 ms, 10-50 mu A) was delivered to assess visual responses. RESULTS: VEPs were recorded in every patient from 18.0 +/- 3.3 mm (n = 14) to 22.8 +/- 3.4 mm (n = 14) in the mediolateral direction. The mean amplitude at this midpoint (57.7 +/- 15.4 mu V, n = 35) was significantly higher than other amplitudes (t test, P < 0.05). Significant increment of amplitudes appeared at 4 mm (64.3 +/- 18.5 mu V, n = 16) below the initial target along trajectories that pass close to the midpoint of the optic tract. The distance from the initial target to the dorsal surface of the optic tract along these trajectories ranged from 3 to 6 mm, with a mean of 5.0 +/- 0.7 mm (n = 16), whereas adjusted distances perpendicular to the intercommissural plane ranged from 1.3 to 4.1 mm, with a mean of 3.0 +/- 0.7 mm (n = 16) below, and the distance from the optic tract to the intercommissural plane ranged from 6.3 to 8.7 mm, with a mean of 7.5 +/- 0.7 mm (n = 16). Macroelectrode stimulation evoked visual responses at various sites in the pallidal region, totaling 27 sites in 11 of 16 patients (68.7%). Microelectrode stimulation evoked responses at only five sites in 3 of 13 patients (23.1 %), within a very limited area 4 to 7 mm below the target where the electrode was very close to the optic tract. Physiological targeting of VEPs necessitated change of the initial target in 13 of 16 patients (81.2%). CONCLUSION: VEPs of prominent oscillatory potentials with high amplitudes indicate the location of the optic tract, which allows easy identification of the optic tract and facilitates neurophysiological targeting of the globus pallidus internus in conjunction with microelectrode recording.