Direct Optic Tract Stimulation in Deep Brain Stimulation for Dystonia: A Case Report

Abstract

Deep brain stimulation (DBS) of the globus pallidus internus (GPi) is an effective therapeutic option for patients with medically refractory dystonia. However, accurate electrode placement is critical, particularly when the trajectory lies close to eloquent structures such as the optic radiations. Intraoperative neurophysiological monitoring using visual evoked potentials (VEPs) can aid in functional localization of the optic tract and enhance targeting accuracy. We report the case of a 62-year-old female with severe oromandibular dystonia and feeding impairment who underwent bilateral GPi DBS under general anesthesia with intraoperative VEP guidance. Cortical VEPs were first obtained using photic stimulation to confirm signal integrity and guide anesthetic titration. Direct optic tract stimulation was then performed using a 2-mm active-tip DBS electrode, with optic tract VEPs (oVEP) recorded to identify proximity to the optic tract. Microelectrode recordings and macrostimulation were used to identify dystonic firing patterns and confirm safe distance from the internal capsule. Final lead placement was guided by the site of maximal oVEP amplitude. Anesthetic depth was maintained at a bispectral index of 70 to 80 using dexmedetomidine, propofol, and desflurane, with careful opioid titration to preserve neurophysiological signals. The patient recovered without complications or awareness and remained neurologically stable postoperatively. This case highlights the feasibility of performing DBS under general anesthesia with intraoperative VEP guidance and emphasizes the importance of individualized anesthetic management and multidisciplinary coordination in complex movement disorder surgeries.

Keywords

dystonia - deep brain stimulation - globus pallidus internus - visual evoked potential - microelectrode recording

Introduction

Deep brain stimulation (DBS) of the globus pallidus internus (GPi) is an effective treatment for patients with medically refractory dystonia, offering substantial improvement in motor symptoms and quality of life. Accurate targeting is essential, especially near critical structures like the optic radiations.[1] [2] Visual evoked potentials (VEPs) can aid in identifying the optic tract during such procedures. Although DBS is typically performed awake, patients with severe dystonia and involuntary movements may require general anesthesia, complicating intraoperative neurophysiological monitoring. We present a case of bilateral GPi DBS under general anesthesia with intraoperative VEP monitoring, highlighting key anesthetic and neurophysiological strategies.

Case Report

A 62-year-old female with a 1.5-year history of medically refractory oromandibular dystonia, dependent on a Ryle's tube for feeding, was scheduled for bilateral GPi DBS. Electrode placement was planned under general anesthesia with intraoperative VEP monitoring to ensure accurate targeting.

The patient was counseled regarding the anesthetic plan, including the possibility of reducing anesthetic depth to facilitate VEP acquisition, and the associated risk of intraoperative awareness. Reassurance was provided that analgesia and comfort would be prioritized. Antidystonic medications were continued up to the day of surgery. On the morning of surgery, general anesthesia was induced with fentanyl (2 mcg/kg), propofol (2 mg/kg), and cisatracurium (0.3 mg/kg). Following tracheal intubation, a computed tomography scan was obtained after Leksell frame placement under pin-site infiltration with local anesthetic. Anesthesia was subsequently maintained with dexmedetomidine infusion, desflurane (minimum alveolar concentration (MAC) < 0.4), and a propofol infusion (50–100 µg/kg/min), titrated to maintain a bispectral index (BIS) value of 40 to 60. No additional neuromuscular blockade was given, allowing assessment of motor side effects during internal capsule stimulation.

For VEP monitoring, bilateral light-emitting diode (LED) goggles delivered photic stimulation, and electrodes were placed at O1, O2, and Oz, with Fz, A1, and A2 as references. As the stereotactic frame precluded goggle straps, the LED goggles were secured with adhesive tapes. A cloth cover was additionally applied as a precaution against any possible ambient light interference. Cortical VEPs (cVEPs) were recorded using the NIM-Eclipse system at 3 Hz with 50 ms stimulation, generating the N75–P100 waveform after averaging. Optimal amplitudes were obtained when BIS was between 70 and 80, necessitating adjustment of anesthetic depth while providing intermittent fentanyl boluses for analgesia.

A 2-mm active-tip DBS electrode was advanced along a trajectory extending from 4 mm above to 2 mm below the planned GPi target and used for direct optic tract stimulation. Stimulation parameters (3 Hz, 50 ms pulses, 4–5 mA) reliably evoked optic tract VEPs (oVEPs), which were recorded from the same scalp electrodes while photic stimulation was suspended. This was followed by microelectrode recordings (MERs), which demonstrated characteristic dystonic neuronal firing. Subsequent macroelectrode stimulation (MES) was performed with stepwise current increments (1–7 mA) to elicit contralateral motor responses and further refine electrode positioning with respect to the internal capsule. Final lead placement was achieved at a site 1 mm from the point of maximal oVEP amplitude and 2 mm lateral to the capsule. Correct positioning was confirmed intraoperatively with O-Arm imaging, after which the implantable pulse generator was successfully inserted.

The patient was extubated uneventfully and transferred to the intensive care unit. She remained neurologically stable and had no recall of intraoperative events at 24-hour follow-up.

Discussion

DBS of the GPi is an established treatment for medically refractory dystonia, with proven efficacy in improving motor symptoms and reducing disability. Precise targeting is crucial to optimize outcomes and avoid MES-related side effects, especially given the GPi's proximity to the internal capsule.[3] However, stimulation responses may originate from adjacent structures such as the putamen, globus pallidus externus, or ansa lenticularis, limiting their value for confirming accurate localization.[2] While modern neuronavigation systems assist in targeting, optic tract stimulation, due to its close anatomical relationship just above the GPi, offers a reliable functional marker to further refine lead placement and prevent off-target positioning.

In this case, both cVEP and oVEPs were employed to enhance the safety and precision of electrode placement near the optic tracts. Although oVEPs provides more targeted information, initial cVEPs obtained via photic stimulation offered key preparatory benefits. First, they served as a functional check of the recording setup, signal integrity, and the baseline functional integrity of the visual pathways. Finally, cVEPs helped optimize anesthetic depth, allowing the team to adjust agents to maintain BIS values (70–80) that preserved evoked responses without compromising immobility. These preparatory steps ensured reliable intraoperative monitoring and contributed to the successful application of oVEPs during targeting. VEPs are near-field potentials and their amplitude reflects the proximity between the target site and the optic tract. In our case, VEP amplitude increased progressively as the stimulating electrode approached the optic tract, from 4 to 1 mm ([Fig. 1]).

Fig. 1 Direct optic tract stimulation (optic tract visual evoked potential [oVEP]) potentials of right optic tract. (A) oVEP waveforms at depth of 4 mm from optic tract. (B) oVEP waveforms at the depth of 1 mm. Note the increase in amplitude p100 waveform in O2-A2 and Oz-A2 on the right side.

Both MER and VEP are highly sensitive to anesthetic agents. Thus, the primary anesthetic challenge is to maintain an optimal depth that preserves neurophysiological signals while ensuring patient immobility. In this case, we chose a combination of low-dose desflurane with titrated propofol infusion rather than propofol alone. This strategy allowed greater flexibility in titrating anesthetic depth during prolonged neurophysiological monitoring, reduced cumulative propofol requirements, and facilitated smoother and faster postoperative recovery. Although we maintained a BIS of 70 to 80, which is slightly lighter than the typical surgical depth,[3] [4] adequate immobility was achieved using intermittent fentanyl boluses. To further suppress respiratory efforts and prevent patient movement, mild hypocarbia was maintained. This strategy was planned and appropriate consent regarding the possibility of intraoperative awareness was obtained. However, no awareness was reported postoperatively in this case. Timing of opioid administration is crucial, as bolus dosing can transiently suppress neurophysiological signals.[5] A low-dose fentanyl infusion offers more stable effects. Although remifentanil, with its rapid onset and neurophysiological compatibility, would have been ideal, it was unavailable. Dexmedetomidine was used instead as it provided analgesia, lowered the need for additional anesthetics, and had minimal effect on MER.

Finally, successful execution of such a procedure relies not only on tailored anesthetic strategies but also on seamless multidisciplinary collaboration. Close coordination among the neuroanesthetist, neurophysiologist, neurologist, and neurosurgeon was essential by contributing their expertise to address the unique challenges of DBS in dystonia.

Conflict of Interest

None declared.

• References

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• 2 Yokoyama T, Sugiyama K, Nishizawa S. et al. Visual evoked potential guidance for posteroventral pallidotomy in Parkinson's disease. Neurol Med Chir (Tokyo) 1997; 37 (03) 257-263 , discussion 263–264
• 3 Chakrabarti R, Ghazanwy M, Tewari A. Anesthetic challenges for deep brain stimulation: a systematic approach. N Am J Med Sci 2014; 6 (08) 359-369
• 4 Elias WJ, Durieux ME, Huss D, Frysinger RC. Dexmedetomidine and arousal affect subthalamic neurons. Mov Disord 2008; 23 (09) 1317-1320
• 5 Samra SK, Dy EA, Welch KB, Lovely LK, Graziano GP. Remifentanil- and fentanyl-based anesthesia for intraoperative monitoring of somatosensory evoked potentials. Anesth Analg 2001; 92 (06) 1510-1515

Publication History

Article published online:
12 December 2025