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|LETTERS TO EDITOR
|Year : 2019 | Volume
| Issue : 2 | Page : 600-601
Globus pallidus internum targeted deep brain stimulation placement using optic tract stimulated visual evoked potentials and corticospinal tract stimulation in a case of severe dystonia
Nitin Manohar1, Keta Thakkar2, Anandh Balasubramaniam1, Astha Palan1
1 Department of Neurosurgery, Yashoda Hospitals, Secunderabad, Telangana, India
2 Department of Anaesthesiology, Christian Medical College, Vellore, Tamil Nadu, India
|Date of Web Publication||13-May-2019|
Dr. Keta Thakkar
Christian Medical College, Vellore, Tamil Nadu - 632004
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Manohar N, Thakkar K, Balasubramaniam A, Palan A. Globus pallidus internum targeted deep brain stimulation placement using optic tract stimulated visual evoked potentials and corticospinal tract stimulation in a case of severe dystonia. Neurol India 2019;67:600-1
|How to cite this URL:|
Manohar N, Thakkar K, Balasubramaniam A, Palan A. Globus pallidus internum targeted deep brain stimulation placement using optic tract stimulated visual evoked potentials and corticospinal tract stimulation in a case of severe dystonia. Neurol India [serial online] 2019 [cited 2020 May 28];67:600-1. Available from: http://www.neurologyindia.com/text.asp?2019/67/2/600/258040
A 25-year old lady with complaints of severe dystonia was planned for deep brain stimulation (DBS) placement into the globus pallidus internum (GPi). Awake DBS placement was not possible due to the non-cooperative nature of the patient who was suffering from severe ophisthotonic paroxysms every 15–20 min. All drugs for dystonia were administered and the patient was shifted to the operation room (OR) where baseline visual evoked potentials (VEPs) were recorded with light emitting diode (LED) goggles. The LED goggles stimulation was administered with a protocol of 2-Hz single pulses, a relative intensity of 90% (3 ′ 1000 millicandella [mcd] LEDs). After a smooth intravenous induction with Inj. fentanyl (2 mg/kg), Inj. propofol (2 mg/kg), and Inj. atracurium (0.5 mg/kg), the patient was intubated and maintained with entropy-guided TIVA (total intravenous anaesthesia) with propofol (50 mg/kg/min) and dexmedetomidine (0.5 mg/kg/h). Under steady-state balanced anesthesia, sufficiently good recordings of cortical visual evoked potential [VEP] (CVEP) signals (P100) were obtained [Figure 1]. Bilateral scalp block with pin site infiltration was done to decrease the requirement of anesthetics, after which the DBS frame was attached and the patient was shifted for a computed tomography scan. After returning to the operating theatre [OR], the entropy-guidance equipment, LED goggles, and VEP electrodes were reattached at the O1, O2, Oz sites, with FpZ as the site of reference electrode placement. A corticospinal tract monitoring (NIM-ECLIPSE monitor; Medtronics, Jacksonville) was also performend for which dual-needle electrodes were placed in four muscle groups – mentalis, deltoid, abductor pollicis, and tibialis anterior. Anesthesia was maintained with TIVA with avoidance of muscle relaxant with the entropy values maintained between 40 and 60, and TOF (train of four) monitoring was done to rule out any muscle relaxant effect. Neuronavigation and C-arm X-ray imaging were also used for the precise location of the target.
|Figure 1: (a) The patient with DBS frame, LED goggles for OT CVEP, O1, O2, Oz, FpZ corkscrew electrodes for cortical VEP. (b) IOMRI revealing the appropriate placement of microelectrodes|
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| » Methodology|| |
Optic tract (OT) stimulation was done directly with a 2-mm active tip electrode (low frequency 3 Hz, 50 ms duration, square monophasic wave with maximum current up to 4 mA). To measure the OT-generated CVEP wave's latency and amplitude, several stimuli (50 stimuli) were averaged every 100 ms, as we expected latencies from N40-P70. Stimulation was started 4 mm above the target till 2 mm below the target. OT-stimulated VEPs were recorded using the same cortical electrodes used for CVEPs. During this OT stimulation, photic stimulation from the goggles was turned off. Also, subcortical cathodal with biphasic train stimulation of 5 mA current were used for checking responses from the nearby corticospinal tracts and for guiding the placement of the electrode. The final DBS electrode placement was done at 1 mm distance away from where the OT VEP amplitudes generated from the occipital cortex electrodes were maximum in the latency N40-P70 ms and also away from the corticospinal tracts. Hence, where we obtained maximal VEP amplitude, we checked for corticospinal tract stimulation by keeping the needle electrodes in the upper and lower limbs. Hence, our main aim was not to have any corticospinal tract stimulation at the point of maximal VEP amplitude. The patient was then shifted for an intraoperative magnetic imaging resonance (MRI) after removing all the electrodes. Electrode placement was confirmed with an intraoperative MRI. The patient was then shifted back to the OR and the battery was placed. The patient was extubated and found to be asymptomatic in the postoperative period. At follow-up visit, the patient had a good relief and improvement in symptoms.
Written and informed consent was obtained from the patient for writing this report. DBS interventions in the GPi are done for the treatment of Parkinson's disease or primary dystonia and require precise placement of electrodes after identification of the somatosensory portion of GPi. The medial border of GPi is located above the midpoint of the OT, the stimulation of which can be used for identifying its anatomical location. Microelectrode recordings (MERs) with electrical stimulation, although a gold standard for intraoperative neurophysiological monitoring for this procedure, are cumbersome, expensive, and require special expertise and an awake patient. CVEP is quick, safe, continuous, less expensive, reliable, easy to comprehend, and can be done in an anesthetized patient, which makes it an attractive alternative for intraoperative neuromonitoring. OT CVEPs with MER and electrode stimulation have also been previously used for postoventral pallidotomy in Parkinson's disease with favorable results.
Challenges faced in such GPi-targeted DBS placements under anesthesia are manifold, and careful planning with team work is important in such cases. Anesthesiologists need to incorporate the knowledge of the neurological disease, neuroanatomical structures, surgical procedure, and special neuromonitoring modalities in the placement of microelectrodes in defined cerebral target areas. OT CVEPs with corticospinal tract monitoring are reliable alternatives to MER to detail the optimal target in GPi surgery and should be considered as routine monitoring techniques for this procedure. Furthermore, the use of OT CVEPs with corticospinal monitoring becomes essential in non-cooperative and severely symptomatic patients who require general anesthesia.
We would like to acknowledge Mr Chaitanya Srinivas Lanka V, Senior clinical application specialist, Medtronics India for his contribution in preparing the manuscript.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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