Intraoperative Blink Reflex Neurophysiological Monitoring in Posterior Skull Base Neurosurgery

Michael WY Lee^*Shu-wah Chau

• Department of Neurosurgery, Pamela Youde Nethersole Eastern Hospital, Hong Kong, Hong Kong, SAR China

Objectives: To enhance surgical safety with frequent intraoperative neuro-monitoring (IONM) of the blink reflex (BR) in posterior skull base neurosurgery Background: There are reports stating the potential of facial nerve function preservation using BR IONM but none stating that it helps to protect corneal sensation and vision.Methods: A prospective cohort of 42 consecutive patients with lesions in proximity to the brainstem was operated between Jan 2021 and Apr 2024. BR, which is considered the electronic equivalent of the corneal reflex, is elicited by electrical stimulation of the supraorbital nerves. Recording electrodes for R1 responses are placed over orbicularis oculi muscle at lower eyelid. BR signal loss or an amplitude drop by half or more in R1 from the baseline is regarded as a significant change in BR response. Preoperative tumour volume and maximal midline shift (MLS) of brainstem are measured, using readily identifiable anatomical landmarks, namely cerebral aqueduct, median sulcus of 4th ventricle, and mid-point of interpeduncular fossa. Patients' intraoperative findings and postoperative clinical outcomes are correlated and reported.Results: BR IONM has a very high accuracy (96.7%) in predicting postoperative corneal complications, but can only be successfully done in 30 out of 42 patients (71.4%). There are 17 female and 13 male patients, with the age ranging from 41 to 81 (mean 62.0). Ten underwent microvascular decompression and 20 had excision of tumours of size ranging from 1.1 to 81.6 cm3 (mean 21.6 cm 3 ). For the tumour cases, Mann Whitney U Test for unpaired data showed statistically significant difference in tumour volume (p=0.0016; 37.8 vs 13.0 cm 3 ) and maximal MLS of brainstem (p=0.0271; 8.1 vs 4.8 mm) regarding the significant change in BR response. This suggests that BR change is a good sentinel indicator of how much brainstem neural tissue is mechanically stretched during excision of the tumour.Conclusions: Frequent IONM of BR is feasible, safe and useful in preservation of corneal sensation, especially for patients with large posterior fossa tumours and distorted brainstem. For patients with the significant change in BR response, prompt referral to Ophthalmologist is useful for prevention of keratitis and corneal ulceration.