Background: Facial nerve preservation remains a challenge during cerebellopontine angle (CPA)
surgery, especially vestibular schwannoma resection. The reliability of using the
blink reflex to guide surgical resection and its prognostic role in facial nerve outcomes
remains uncertain. In this study, we assess the feasibility of using blink reflex
for intraoperative facial nerve monitoring in patients undergoing translabyrinthine
resection of vestibular schwannomas.
Methods: Twelve patients with presumed vestibular schwannomas who underwent translabyrinthine
resection were included in the study. A standardized facial nerve monitoring setup
that has been tested and refined by our neuromonitoring team was used. The first stimulating
electrode was placed over the supraorbital foramen, the second stimulating electrode
was placed ~1 cm superior to it. Two recording electrodes were used. An active recording
electrode was placed subdermally in the orbicularis oculi muscle directly below the
pupil and a reference electrode was placed next to the lateral canthus. The early
component of evoked action potential responses (R1), which represents the disynaptic
pathway between the main sensory nucleus of the trigeminal nerve and the ipsilateral
facial nucleus, was recorded at baseline and throughout the surgery for each patient
for both the ipsilateral and contralateral sides of the surgery. A stimulus train
of 4 to 7 individual pulses was used to elicit a response with a pulse duration of
200 to 400 ms, inter-pulse duration of 0.5 to 2 ms, and intensity of 40 to 80 mA.
Facial nerve function was assessed with the House-Brackmann (HB) grading system postoperatively.
Results: Baseline R1 responses were reliably obtained in 11/12 patients for the ipsilateral
side of the surgery and 9/12 for the contralateral side. In the remaining patients,
the R1 response was present but variable. In 8/12 patients, ipsilateral R1 response
remained stable throughout the case, in 3 patients there was an ipsilateral decrease
in R1 response amplitude at the end of the case, while the contralateral side was
stable. In one patient the amplitude decreased bilaterally, this was considered a
technical issue as facial nerve function was stable postoperatively. Postoperative
HB score was stable in 8/12 patients. Four patients had postoperative deficits (HB
score of II in three patients, HB of V in one patient). In patients with postoperative
facial nerve deficit, three had decreased ipsilateral R1 response amplitude at the
end of the case. The remaining patient had a stable R1 response bilaterally and a
facial nerve triggered EMG response of 300 μV at an intensity of 0.05 mA at the end
of the case.
Conclusions: The study demonstrates the feasibility of recording blink reflex reliably intraoperatively
and opens the potential for neuromonitoring of blink reflex during vestibular schwannoma
surgery. Blink reflex can provide feedback without interruption of surgery as opposed
to direct stimulation. Further studies are needed to determine the prognostic role
of the blink reflex in predicting postoperative facial nerve function, especially
as compared with triggered EMG.
