Intraoperative Neurophysiological Monitoring during Microvascular Decompression Surgery for Hemifacial Spasm

Abstract

Hemifacial spasm (HFS) is due to the vascular compression of the facial nerve at its root exit zone (REZ). Microvascular decompression (MVD) of the facial nerve near the REZ is an effective treatment for HFS. In MVD for HFS, intraoperative neurophysiological monitoring (INM) has two purposes. The first purpose is to prevent injury to neural structures such as the vestibulocochlear nerve and facial nerve during MVD surgery, which is possible through INM of brainstem auditory evoked potential and facial nerve electromyography (EMG). The second purpose is the unique feature of MVD for HFS, which is to assess and optimize the effectiveness of the vascular decompression. The purpose is achieved mainly through monitoring of abnormal facial nerve EMG that is called as lateral spread response (LSR) and is also partially possible through Z-L response, facial F-wave, and facial motor evoked potentials. Based on the information regarding INM mentioned above, MVD for HFS can be considered as a more safe and effective treatment.

Keywords: Hemifacial spasm; Intraoperative neurophysiological monitoring; Microvascular decompression surgery.

Fig. 1.

Example of consecutive INM of the BAEP on left side using a stimulation rate of 10 Hz/sec and 1000 averaging times (green line : baseline BAEP; black line : obtained BAEP). A : First BAEP showing minimal wave V change. B : Second BAEP showing a slight change in wave V (the latency of wave V was delayed by 1.36 ms with a minimal decrease in the amplitude. C : Third BAEP showing a critical change in wave V (loss of wave V). INM : intraoperative neurophysiological monitoring, BAEP : brainstem auditory evoked potential.

Fig. 2.

A representative case showing only latency prolongation (≥1 ms) without a significant change in amplitude (the latency of wave V was delayed by 2.00 ms from 6.30 ms to 8.30 ms with a minimal decrease in the amplitude). The patient in this example did not experience postoperative hearing loss (the average pre-PTA threshold was 22.5 dB, and the average post-PTA threshold was 6.25 dB). A : The INM of BAEP during MVD surgery (blue arrow, blue box : baseline of wave V; red arrow, red box : wave V showing maximal prolongation in latency). B : Pure tone audiometry of the patients obtained prior to surgery and 7 days postoperatively. PTA : pure tone audiometry, INM : intraoperative neurophysiological monitoring, BAEP : brainstem auditory evoked potential, MVD : microvascular decompression.

Fig. 3.

A : Facial nerve mapping performed preoperatively. The locations of maximal LSR elicitation were divided into three regions. B : The direction of stimulation in the conventional and the new methods. In the conventional method, electrodes are placed in the temporal or zygomatic branch of the facial nerve, about 3 cm lateral to the lateral margin of the orbit, and centripetal impulses are transmitted towards the brainstem with the cathode positioned proximally. In contrast, electrodes were inserted intradermally with the anode located proximally over the area just anterior to the mandibular fossa and the cathode located distally in the temporal branch of the facial nerve in the new method. The direction of stimulation was centrifugal outwards from the brainstem. F : the direction towards the frontalis muscle, which was almost vertical with respect to the anode, O : the direction towards the orbicularis oculi muscle, F-O : in between F and O, LSR : lateral spread response.