Usefulness of intraoperative electromyographic monitoring of oculomotor and abducens nerves during skull base surgery

Neurophysiologic Monitoring of Oculomotor Nerves During Transorbital Surgery: Proof of Concept and Anatomic Demonstration

BACKGROUND AND OBJECTIVES

Transorbital neuroendoscopic surgery (TONES) is continuously evolving and gaining terrain in approaching different skull base pathologies. The objective of this study was to present our methodology for introducing recording electrodes, which includes a new transconjunctival pathway, to monitor the extraocular muscle function during TONES.

METHODS

A translational observational study was performed from an anatomic demonstration focused on the transconjunctival electrode placement technique to a descriptive analysis in our series of 6 patients operated using TONES in association with intraoperative neurophysiologic monitoring of the oculomotor nerves from 2017 to 2023. The stepwise anatomic demonstration for the electrode placement and correct positioning in the target muscle was realized through cadaveric dissection. The descriptive analysis evaluated viability (obtention of the electromyography in each cranial nerve [CN] monitored), security (complications), and compatibility (interference with TONES).

RESULTS

In our series of 6 patients, 16 CNs were correctly monitored: 6 (100%) CNs III, 5 (83.3%) CNs VI, and 5 (83.3%) CNs IV. Spontaneous electromyography was registered correctly, and compound muscle action potential using triggered electromyography was obtained for anatomic confirmation of structures (1 CN III and VI). No complications nor interference with the surgical procedure were detected.

CONCLUSION

The methodology for introducing the recording electrodes was viable, secure, and compatible with TONES.

Monitoring surgery around the cranial nerves

Intraoperative neurophysiologic monitoring (IONM) of cranial nerve (CN) function is an essential component in multimodality monitoring of surgical procedures where CNs are at risk for injury. In most cases, IONM consists of localizing and mapping CNs and their pathways, and monitoring of CN motor function during surgery. However, CN VIII, which has no motor function, and is at risk for injury in many surgical procedures, can be easily and accurately monitored using brainstem auditory evoked potentials. For motor CNs, the literature is clear that function can be safely and adequately performed using basic electromyographic (EMG) techniques, such as recording of continuous EMG activity and electrically evoked compound muscle actions potentials. Newer techniques, such as corticobulbar motor evoked potentials and reflex studies, show good potential for a greater degree of functional assessment but require further study to determine their clinical utility. EMG remains the basic clinical neurophysiologic technique with the greatest clinical research supporting its utility in IONM of motor CN function and should be used as part of a comprehensive multimodality IONM protocol. Understanding the physiologic basis of EMG and the changes associated with altered motor function will allow the practitioner to alter surgical course to prevent injury and improve patient safety.

Intraoperative lateral rectus electromyographic recordings optimized by deep intraorbital needle electrodes

OBJECTIVE

We demonstrate the advantages and safety of long, intraorbitally-placed needle electrodes, compared to standard-length subdermal electrodes, when recording lateral rectus electromyography (EMG) during intracranial surgeries.

METHODS

Insulated 25 mm and uninsulated 13 mm needle electrodes, aimed at the lateral rectus muscle, were placed in parallel during 10 intracranial surgeries, examining spontaneous and stimulation-induced EMG activities. Postoperative complications in these patients were reviewed, alongside additional patients who underwent long electrode placement in the lateral rectus.

RESULTS

In 40 stimulation-induced recordings from 10 patients, the 25 mm electrodes recorded 6- to 26-fold greater amplitude EMG waveforms than the 13 mm electrodes. The 13 mm electrodes detected greater unwanted volume conduction upon facial nerve stimulation, typically exceeding the amplitude of abducens nerve stimulation. Except for one case with lateral canthus ecchymosis, no clinical or radiographic complications occurred in 36 patients (41 lateral rectus muscles) following needle placement.

CONCLUSIONS

Intramuscular recordings from long electrode in the lateral rectus offers more reliable EMG monitoring than 13 mm needles, with excellent discrimination between abducens and facial nerve stimulations, and without significant complications from needle placement.

SIGNIFICANCE

Long intramuscular electrode within the orbit for lateral rectus EMG recording is practical and reliable for abducens nerve monitoring.

Resection of Oculomotor Nerve Lesions Using Continuous Stimulation of the Oculomotor Nerve Proximal to the Lesion: A Technical Report

BACKGROUND

We describe a continuous monitoring method aimed at preserving nerve function during biopsy of lesions on the oculomotor nerve using stimulation of the oculomotor nerve proximal to the lesion.

CASE DESCRIPTION

A 5-year-old girl with a recurrent left oculomotor nerve palsy and contrast-enhancing left oculomotor nerve mass on magnetic resonance imaging underwent a biopsy of the lesion to aid in its diagnosis. At the time of surgery, needle electrodes were inserted into the superior and inferior rectus muscles percutaneously, and cotton-covered electrodes were implanted into the oculomotor nerve proximal to the lesion. Compound muscle action potentials of the oculomotor nerve were measured continuously by monopolar stimulation. The lesion was mapped by direct stimulation, and the unresponsive area was excised. The amplitude of the compound muscle action potentials decreased during the resection but recovered postoperatively. After resection of the lesion, the compound muscle action potentials remained the same as they were preoperatively. No obvious postoperative oculomotor nerve palsy was observed.

CONCLUSIONS

This method of continuous monitoring of the function of the oculomotor nerve is simple to use and is suitable for lesions in close proximity to the oculomotor nerve.

A novel needle electrode for intraoperative fourth cranial nerve neurophysiological mapping

OBJECTIVES

Trochlear nerve (CN-IV) mapping method has not been confirmed to date. The compound muscle action potential (CMAP) of CN-IV cannot be recorded because of the low mapping sensitivity and anatomical characteristics of the superior oblique muscle (SOM). The aim of this study was to evaluate the effectiveness of a novel needle electrode (NNE), for the intraoperative mapping of CN-IV.

MATERIALS AND METHODS

The NNEs were inserted in the target extraocular muscles in 19 patients. We compared the CMAP amplitude of the NNE with that of the conventional needle electrode (CNE). Furthermore, we investigated the dissimilarity between the CMAP of the CN-IV and other extraocular cranial nerves (ECNs) and the correlation between the readings of the CN-IV mapping and its postoperative functional outcome.

RESULTS

The CMAP of CN-IV has been measured in nine patients (47.4%). The CMAP of CN-IV was distinguishable from other ECNs. The CMAP of the NNE was found to be three times higher than that of the CNE. Although the NNE has shown the potential to record the CN-IV's CMAP, 4 cases ended up having a CN-IV postoperative dysfunction.

CONCLUSIONS

For the first time, we confirmed the possibility of intraoperative mapping the CN-IV using an NNE inserted into the SOM. The NNE can also be useful for other neurophysiological monitoring methods.

Anatomical location of the abducens nerves (VI) in the ventral approach of clival tumors

The aim of this work was to determine reliable anatomical landmarks for locating and preserving the abducens nerves (6th cranial nerves) during trans-facial or trans-nasal endoscopic approaches of skull base tumors involving the clivus and the petrous apex. In order to describe this specific anatomy, we carefully dissected 10 cadaveric heads under optic magnification. Several measurements were taken between the two petro-sphénoidal foramina, from the bottom of the sella and the dorsum sellae. The close relationship between the nerves and the internal carotid artery were taken into account. We defined a trapezoid area that allowed drilling the clivus safely, preserving the 6th cranial nerve while being attentive to the internal carotid artery. The caudal part of this trapezium is, on average, 20 mm long at mi-distance between the two petro-sphenoidal foramina. The cranial part is at the sella level, a line between both paraclival internal carotid arteries. Oblique lateral edges between the cranial and caudal parts completed the trapezium.

In Vivo Nerve-Specificity of Rhodamines and Si-rhodamines

Accidental nerve damage or transection of vital nerve structures remains an unfortunate reality that is often associated with surgery. Despite the existence of nerve-sparing techniques, the success of such procedures is not only complicated by anatomical variance across patients but is also highly dependent on a surgeon's first-hand experience that is acquired over numerous procedures through trial and error, often with highly variable success rates. Fluorescent small molecules, such as rhodamines and fluoresceins have proven incredibly useful for biological imaging in the life sciences, and they appeared to have potential in illuminating vital nerve structures during surgical procedures. In order to make use of the current clinically relevant imaging systems and to provide surgeons with fluorescent contrast largely free from the interference of hemoglobin and water, it was first necessary to spectrally tune known fluorescent scaffolds towards near infrared (NIR) wavelengths. To determine whether the well-documented Si-substitution strategy could be applied towards developing a NIR fluorophore that retained nerve-specific properties of candidate molecules, an in vivo comparison was made between two compounds previously shown to highlight nervous structures - TMR and Rhodamine B - and their Si-substituted derivatives.

Investigation of Oxazine and Rhodamine Derivatives as Peripheral Nerve Tissue Targeting Contrast Agent for In Vivo Fluorescence Imaging

Accidental nerve transection or injury is a significant morbidity associated with many surgical interventions, resulting in persistent postsurgical numbness, chronic pain, and/or paralysis. Nerve-sparing can be a difficult task due to patient-to-patient variability and the difficulty of nerve visualization in the operating room. Fluorescence image-guided surgery to aid in the precise visualization of vital nerve structures in real time during surgery could greatly improve patient outcomes. To date, all nerve-specific contrast agents emit in the visible range. Developing a near-infrared (NIR) nerve-specific fluorophore is poised to be a challenging task, as a NIR fluorophore must have enough "double-bonds" to reach the NIR imaging window, contradicting the requirement that a nerve-specific agent must have a relatively low molecular weight to cross the blood-nerve-barrier (BNB). Herein we report our efforts to investigate the molecular characteristics for the nerve-specific oxazine fluorophores, as well as their structurally analogous rhodamine fluorophores. Specifically, optical properties, physicochemical properties and their in vivo nerve specificity were evaluated herein.

Novel method of intraoperative ocular movement monitoring using a piezoelectric device: experimental study of ocular motor nerve activating piezoelectric potentials (OMNAPP) and clinical application for skull base surgeries

Intraoperative monitoring systems that utilize various evoked potentials for the detection and/or preservation of cranial nerves have become increasingly common due to recent technical and commercial developments, particularly during skull base surgeries. We established a novel system for the intraoperative monitoring of the extraocular motor nerves (eOMNs) using a piezoelectric device capable of detecting imperceptible vibrations induced by ocular movement, with sensors placed on the eyelids alone. We first evaluated the efficacy and reliability of this device for the intraoperative monitoring of eOMNs in two Beagle dogs. Based on the results, we then determined the appropriate stimulation parameters for use in human surgical cases involving removal of various skull base tumors. Animal experiments revealed that a 0.4 mA monopolar electrical stimulation was required to elicit significant responses and that these responses were not inferior to those obtained via the electrooculogram/electromyogram. Significant responses were also detected in preliminary clinical investigations in human patients, following both direct and indirect monopolar electrical stimulation of the oculomotor and abducens nerves, although obtaining responses from the trochlear nerve was difficult. Intraoperative monitoring using a piezoelectric device provides a simple and reliable method for detecting eOMNs, especially the oculomotor and abducens nerves. This monitoring system can be adapted to various surgeries for skull base tumor.