NASBS

North American Skull Base Society

2025 Proffered Presentations

2025 Proffered Presentations

 

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S367: NEUROPHYSIOLOGICAL MONITORING IN ENDOSCOPIC MIDDLE SKULL BASE SURGERY: A INITIAL CLINICAL EXPERIENCE
Simona Serioli, MD1; Ginevra Federica D'Onofrio, MD2; Michele Di Domenico, BS2; Federico Valeri, MD2; Laura Broglio, MD1; Ludovico Agostini, MD2; Marco Maria Fontanella, MD1; Alessandro Olivi, MD2; Francesco Doglietto, MD, PhD2; 1University of Brescia; 2Università del Sacro Cuore, Fondazione Policlinico Gemelli, Rome, Italy

Introduction: The exponential development of endoscopic surgery has highlighted the necessity of applying intraoperative monitoring of cranial nerves (CNs) to reduce the potential risk of post-operative deficits. Several methods of CNs mapping are described, including electrooculography (EOG), free-run electromyography (frEMG), and trigger electromyography (tEMG). The use of t-EMG seems to be the most promising method in comparison to f-EMG, although there is no statistically significant confirmation that it reduces the incidence of postoperative complications; this may depend on a lack of solid statistical analysis and limited data available in the current literature.

Objectives: The aim is to provide an analysis of the different IONM techniques and their predictive role in reducing the risk of post-operative complications, using a modified dual subdermal needle and the corticobulbar motor evoked potentials (CbMEPs) for extraocular pathways monitoring. Furthermore, a series of retrospective cases of patients affected by skull base tumors undergoing surgical treatment with the use of three methods (EOG with surface adhesive pad electrodes and subdermal needle, tEMG, fEMG and CbMEPs with curved needles and hookwire needles) was evaluated to understand the differences between the different methods.

Method: Twenty-five patients underwent EEA surgery with IONM for skull base tumors by the same surgeon at the Department of Neurosurgery, Fondazione Policlinico Gemelli (Italy) and Spedali Civili of Brescia (Italy) from 2017 to 2024. Classic twisted dual subdermal needle (27G, 13 mm in length) was appropriately modified for our purpose. Extraocular muscles activation was studied using direct (EMG) and indirect (EOG) techniques. A 90°-fold was made at the end of the coated part of the needle to obtain an easier, safer, and more stable fixation. In addition, corticobulbar motor evoked potentials (CbMEPs) were also performed to monitor extraocular motor pathways.

Results: Preoperative CN deficits were identified in 8 cases (30.8%): 3 patients presented CN III palsy (two partial deficits and one complete), 5 patients had CN VI palsy, and hypoesthesia in V1 territory was referred by two patients. A total of 78 cranial nerves were monitored: CN III was investigated bilaterally in 13 patients and monolaterally in 13 patients. CN IV was studied in 2 patients, and CN VI was studied bilaterally in 13 patients, and monolaterally in 9 patients. tEMG via the hookwire needles showed positive stimulation in 6 out of ten cases without clearly identifying the stimulated nerve, and in four cases, the stimulation did not achieve any specific muscular activation with unstable impedance values. tEMG using a 90°-folded subdermal needle, a clear and distinguished muscular activation with an impedance value less than 1kΩ was registered in all the patients. Monitoring of the cranial nerves using CbMEP (90°-folded needle - bipolar recording) offered excellent signal quality, clear, and reproducible responses in 6 patients. Contrariwise, the artefactual component in EOG made it impossible to obtain a clear response for CbMEPs monitoring.

A, Signal of muscular activation recorded by hookwire electrodes: the stimulated OMN nerve is not identifiable only on the neurophysiological data, but it is supposed to be the oculomotor nerve based on the anatomy. B, Signal of muscular activation recorded by subdermal needle: example of intraoperative signal with a clear activation of the right inferior rectus muscle. C-D, Acquisition of CbMEPs using different techniques. C, a clear and stable response from EMG electrodes, D, the artefactual component in EOG require amplification about 20-25 mV that make impossible recognize clear responses except for (in this case) for R VN-REFN. E, Raw EMG signals recorded from all EMG electrodes compared with EOG electrodes (below). The artefactual component recorded from EOG channels without any surgical stimulus applied is evident.

Needle for intraoperative monitoring and their positioning A, Image of the modified classic twisted dual subdermal needle. B, Placement of the needles on patient before endoscopic transnasal surgery.

F-G, Example of right CN III direct stimulation recorded by EMG channels with a clear muscular activation. G, Example of right CN III direct stimulation recorded by EOG without a clearly recognizable response. H, Example of right CN VI activation recorded by EMG channels that show a selective response in the RT VI channel. I, Example of right CN VI activation recorded by EOG channels: a diffused response is observed. In this case, the EOG gave back a quantitative response identifying a muscular activation but isn't qualitative: based on the EOG response only is not possible a clear nerve identification. J, Example of left CN VI activation recorded by EMG channels with an activation of both left CN III and CN VI; K, Example of left CN VI activation recorded by EOG channels with a diffused activation with a wider amplitude on left horizontal channels (both adhesive and needle electrodes). The nerve was recognized using stimulation and anatomical observation.

Conclusion: The use of modified dual subdermal needles and the corticobulbar motor evoked potentials (CbMEPs) for the monitoring of extraocular pathways reduced the number of adverse events recorded during the surgical procedure in comparison to the standard method.

 

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