Intraoperative EMG-Based Prediction of Pyramidal Tract Side Effects During Deep Brain Stimulation for Essential Tremor Under General Anesthesia

Abstract

Deep brain stimulation (DBS) for essential tremor (ET) poses challenges for both neurosurgeons and neuroanesthetists. ET with prominent postural and action tremors may respond better to DBS targeting the caudal zona incerta (cZI) than the traditional ventral intermediate nucleus. A key intraoperative concern is identifying the pyramidal tract side effect (PTSE) threshold, as a low threshold limits the therapeutic window.

During awake DBS, macrostimulation helps assess benefits and side effects, including PTSE. Under general anesthesia (GA), however, patient feedback is unavailable. In such cases, intraoperative electromyography (EMG) can detect PTSE responses.

We report a case of medication-refractory ET, treated with cZI-targeted DBS under GA. EMG monitoring successfully detected stimulation-induced PTSE, allowing estimation of the threshold without patient input. This highlights the value of EMG in asleep DBS for optimizing lead placement and minimizing side effects when awake testing is not possible.

 

MissingLinkText

Keywords

essential tremor - DBS under general anesthesia - caudal zona incerta (cZI) - pyramidal tract side effects

Introduction

Deep brain stimulation (DBS) of caudal zona incerta (cZI) is an effective treatment for medication-refractory essential tremor (ET), especially when traditional targets like the ventral intermediate nucleus (VIM) are not effective. Recent studies suggest that targeting subthalamic nucleus (STN), globus pallidus interna (GPi), and/or cZI may provide better outcomes for patients with prominent postural and action tremors.[1] However, DBS surgery for ET under general anesthesia (GA) presents challenges, as patient feedback—crucial for identifying side effects like pyramidal tract side effects (PTSEs)—is unavailable. In awake DBS, macrostimulation allows real-time assessment of both benefits and side effects, including PTSE. Under GA, without patient input, the PTSE threshold becomes difficult to determine. Intraoperative electromyography (EMG) can detect PTSE responses, providing a way to monitor and estimate the threshold for safe and effective stimulation. This case report highlights the value of EMG monitoring in DBS for ET under GA, emphasizing its role in optimizing lead placement and minimizing side effects when awake testing is not an option.

 

MissingLinkText

Case Report

A 58-year-old male with a history of hypertension presented with tremors involving the head, jaw, trunk, and upper limbs, sparing the lower limbs. The tremors were of resting, postural, and action types and had been present since the age of 18. A prior diagnosis of Parkinson's disease was considered, and a levodopa trial was initiated, but symptoms were refractory. Based on clinical features—predominantly postural and action tremors—a diagnosis of ET was made, and DBS targeting the cZI was planned under GA.

Preoperative 3T magnetic resonance imaging was performed 4 days before surgery. The patient was induced with propofol and remifentanil (target-controlled infusions, Eleveld model). After securing the compact head ring and arc adapter plate with a computed tomography (CT) localizer frame, a contrast-enhanced CT brain was obtained. Imaging was fused to determine the cZI target coordinates, and a Cosman-Roberts-Wells frame was applied ([Fig. 1A, B]).

Fig. 1 (A) Cosman-Roberts-Wells (CRW) frame-guided electrode stimulation. (B) Neuroimage guiding coordinate measurements.

Intraoperative EMG was recorded from the facial muscles, upper limbs (biceps brachii, brachioradialis), and lower limbs (tibialis anterior, extensor digitorum brevis). Stimulation was delivered using a Boston Scientific Vercise 8-contact lead (185 Hz, 60 µs pulse width), with amplitude titrated from 0.5 mA. EMG response in the limb muscles was noted at 4.5 mA ([Fig. 2A, B]) left, marking the threshold for PTSEs. The patient was extubated uneventfully. At 2-week follow-up, stimulation at 4.5 mA reproduced hand twitching, confirming the PTSE threshold.

Fig. 2 (A) Stimulation settings (Vercise 8-contact lead [185 Hz, 60 µs pulse width], with amplitude titrated from 0.5 mA). (B) Electromyography (EMG) recordings of left limb muscles noted at 4.5 mA indicating the threshold for pyramidal tract side effect (PTSE).

 

MissingLinkText

Discussion

ET is one of the most common movement disorders, affecting approximately 0.9% of the general population and up to 4 to 5% of individuals over the age of 65.[2] While medical therapy remains the first line of treatment, a significant number of patients become refractory to medications and experience disabling tremor. In such cases, DBS has proven to be an effective therapeutic option. Traditionally, the VIM nucleus of the thalamus has been the primary DBS target for ET. However, recent studies have highlighted the cZI[3] as a promising alternative target due to its close proximity to the dentato-rubro-thalamic tract, enabling superior tremor suppression, especially for postural and action tremors. Compared with VIM, STN, or GPi, cZI offers a wider therapeutic window and fewer stimulation-induced side effects. Importantly, cZI stimulation is associated with fewer cognitive, speech, or gait-related adverse effects, making it a safer and more focused target.

When targeting the cZI during DBS for ET, careful monitoring for PTSEs is crucial. The cZI lies in close anatomical proximity to the internal capsule, which contains corticospinal (pyramidal) fibers.[4] Inadvertent stimulation spread to these fibers can lead to side effects such as muscle contractions, dysarthria, or facial pulling—indicating stimulation-induced motor pathway involvement. Early intraoperative detection of PTSEs helps refine electrode placement and optimize programming to achieve maximal tremor suppression while minimizing unwanted side effects.[5] This is particularly important under GA, recent studies, including the GALAXY randomized controlled trial, have shown that DBS surgery performed under GA (asleep DBS) offers clinical outcomes comparable to the traditional awake approach, particularly in terms of motor improvement and electrode accuracy.[6] Asleep DBS has emerged as a safe and effective alternative, with the added benefit of being less physically and emotionally taxing for patients, but traditional patient feedback is unavailable, making objective electrophysiological monitoring even more essential for safe and effective targeting.

Intraoperative EMG monitoring is a critical adjunct during DBS surgery targeting the cZI to minimize stimulation-induced motor side effects and optimize electrode placement. EMG electrodes are strategically placed on muscles representing corticospinal and corticobulbar innervation, commonly including facial muscles, as well as upper limb and lower limb muscles. High-frequency stimulation (185 Hz) was delivered sequentially to each contact and directional segment, starting at 0.5 mA and gradually increasing up to 5 mA or until side effects are observed; higher frequencies can be used for chronic therapy but low frequency is better for intraoperative testing in awake patients.[7] The presence of time-locked EMG responses to stimulation pulses indicates spread of current to motor pathways, establishing threshold amplitudes for motor activation. Contacts eliciting EMG activity at lower stimulation amplitudes are avoided, while those that do not provoke muscle activation within therapeutic ranges are preferred for chronic therapy. Directional programming further refines stimulation by steering current away from structures inducing motor side effects. This systematic EMG-guided approach enhances intraoperative decision-making, facilitating precise lead positioning and improving clinical outcomes in cZI DBS procedures.

In our case, EMG monitoring successfully detected stimulation-induced PTSE, allowing estimation of the threshold for PTSE without patient input. This highlights the value of EMG in asleep DBS for optimizing lead placement and minimizing side effects when awake testing is not possible. In this case, evoked potentials such as somatosensory-evoked potentials or motor-evoked potentials were not utilized because the primary intraoperative objective was to detect PTSE related to spread of stimulation, rather than monitor the integrity of long tract conduction pathways. Unlike evoked potentials, EMG allows real-time, contact-specific evaluation of motor side effects during stimulation mapping as patient is under GA.

 

MissingLinkText

Conclusion

This case underscores the importance of accurately identifying the PTSE threshold during DBS targeting the cZI for ET. Due to the close proximity of the cZI to the internal capsule, inadvertent stimulation can easily spread to corticospinal fibers, causing unwanted motor side effects. Intraoperative EMG monitoring played a pivotal role in objectively detecting the PTSE threshold in this asleep DBS procedure, enabling precise electrode placement without relying on patient feedback. Recognizing this threshold is essential to maximize tremor suppression while minimizing stimulation-induced muscle contractions or other motor complications. This case reinforces that EMG-guided detection of PTSE thresholds is a critical safety measure in asleep cZI DBS, helping to optimize therapeutic outcomes and reduce adverse effects.

 

MissingLinkText

 

MissingLinkText

Conflict of Interest

None declared.

• Reference

• 1 Chandra V, Hilliard JD, Foote KD. Deep brain stimulation for the treatment of tremor. J Neurol Sci 2022; 435: 120190
• 2 Louis ED, Ferreira JJ. How common is the most common adult movement disorder? Update on the worldwide prevalence of essential tremor. Mov Disord 2010; 25 (05) 534-541
• 3 Philipson J, Blomstedt P, Hariz M, Jahanshahi M. Deep brain stimulation in the caudal zona incerta in patients with essential tremor: effects on cognition 1 year after surgery. J Neurosurg 2019; 134 (01) 208-215
• 4 Plaha P, Ben-Shlomo Y, Patel NK, Gill SS. Stimulation of the caudal zona incerta is superior to stimulation of the subthalamic nucleus in improving contralateral parkinsonism. Brain 2006; 129 (Pt 7): 1732-1747
• 5 Leung LWL, Lau KYC, Kan KYP. et al. Prediction of pyramidal tract side effect threshold by intra-operative electromyography in subthalamic nucleus deep brain stimulation for patients with Parkinson's disease under general anaesthesia. Front Surg 2024; 11: 1465840
• 6 Holewijn RA, Verbaan D, van den Munckhof PM. et al. General anesthesia vs local anesthesia in microelectrode recording-guided deep-brain stimulation for parkinson disease: the GALAXY randomized clinical trial. JAMA Neurol 2021; 78 (10) 1212-1219
• 7 Neumann WJ, Köhler RM, Kühn AA. A practical guide to invasive neurophysiology in patients with deep brain stimulation. Clin Neurophysiol 2022; 140: 171-180

Publication History

Article published online:
15 September 2025