Intraoperative neurophysiological monitoring of trigeminal nerve function during giant vestibular schwannoma resection: blink reflex monitoring

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Introduction

Intraoperative neurophysiological monitoring (IONM) during vestibular schwannoma (VS) resection includes triggered and free-run electromyography (EMG), corticobulbar motor evoked potential (CoMEP) monitoring, and less frequently, brainstem auditory evoked potentials (BAEPs) and somatosensory evoked potentials (SSEPs). Large and giant VSs, classified as Koos stage III–IV and Samii stage T4b, compress the brainstem and adjacent nerves, primarily damaging the V, VII, and VIII cranial nerves (CNs) [1, 2]. IONM is an essential part of the surgical treatment stage, the goal of which is to preserve CN function. Triggered and free-run EMG, as well as CoMEP monitoring, have demonstrated high sensitivity and specificity in identifying CNs and intraoperatively monitoring their function during VS removal [1–8]. Additional modalities include monitoring of brainstem reflexes (trigemino-trigeminal, trigemino-hypoglossal, trigemino-cervical, blink reflex, etc.), among which the most studied is the blink reflex (BR) [9, 10]. The intraoperative use of the BR during VS resections may become an additional effective IONM method.

Study aim: To analyze the intraoperative reproducibility of the BR, assess its relationship with transcranial CoMEPs and EMG monitoring data during VS removal surgeries, and determine its correlation with postoperative trigeminal nerve (TN) function.

Materials and methods

The treatment results for patients who underwent surgical treatment for VS in the Neurosurgery Department of the State Autonomous Healthcare Institution Interregional Clinical and Diagnostic Center, Kazan, from January 2024 to January 2025 were analyzed.

All patients underwent preoperative studies of brainstem auditory evoked potentials and trigeminal evoked potentials, as well as BR testing with assessment of the early/first (R1) and second (R2) response components. Multimodal IONM included monitoring of CoMEP, triggered and free-run EMG of muscles innervated by the facial nerve and the TN. The warning criterion for CoMEP was a decrease in amplitude of more than 50% [11]. Free-run EMG recorded the responses of the facial muscles, masseter, and temporalis muscles elicited by irritation of the VII or V CNs. An A-train was considered critical, but the neurosurgeon was informed of all patterns recorded during free-run EMG. For triggered EMG, a Raabe probe (Inomed Medizintechnik) was used for continuous monitoring at a current intensity of 0.1–3.0 mA. Facial nerve paresis was assessed using the House-Brackmann (HB) scale.

Results

From January 2024 to January 2025, 17 VS removal surgeries were performed using IONM for BR monitoring. Two patients were operated on after previous radiosurgical treatment for VS, and 1 patient underwent surgery after prior surgical removal of VS (due to continued growth). Fourteen patients reported anacusis on the affected side. Twelve patients complained of sensory disturbances in the face ipsilateral to the lesion. Five patients had preoperative facial palsy (1 patient — HB grade 5, 1 patient — HB grade 4, 3 patients — HB grade 3).

Brainstem auditory evoked potentials were severely abnormal in all patients: in 11 patients, responses from the affected side were not recorded; in 6, increased latency of peaks I, III, and V was detected. During BR testing, 10 patients had increased latency of R1 responses, 2 patients had no R1 response, 2 patients had no R1 or R2 responses, and 3 patients had BR within normal limits (see Table). During trigeminal evoked potential EP recording, responses from the maxillary and mandibular branches were not recorded in 2 patients; in 3 patients, there were no responses from one of the TN branches; in 10 patients, latency was increased in both or one of the TN branches; and in 2 patients, trigeminal EPs were within normative values.

 

Characteristics of BR, CoMEP, and sensory and motor functions of the face

Patient	Sex	Side	Paresis of the face before surgery	Sensitivity before surgery	Paresis of the face after surgery	Sensitivity after surgery	BR before surgery	BR intraoperative, latency, µV	CoMEP
1	F	R	N	Hypesthesia	1	Hypesthesia	↑ R1 latency	17.8 stable	Stable
2	F	R	N	Burning sensation of the tongue	1	N	N	17.7 stable	Stable
3	М	L	3	Hypesthesia	5–6	Hypesthesia	No R1 and contralateral R2	18.5 disappeared abruptly	Stable
4	F	L	N	N	1		N	18.3 stable	Stable
5	F	R	N	N	2	Hypesthesia	N	15.4 disappeared abruptly	↓Amplitude by 40%
6	F	R	2–3	Hypesthesia	3	Hypesthesia	No R1	Not recordable	Stable
7	F	L	5	Hypesthesia	5	Hypesthesia	No R1 and contralateral R2	Not recordable	Low-amplitude, unstable
8	М	L	N	N	2	N	↑ R1 latency	16.9 stable	↓Amplitude by 40%
9	М	R	N	N	4–2	N	↑ R1 latency	15.8 stable	↓Amplitude by 30%
10	F	L	N	Hypesthesia	2	Hypesthesia	↑ R1 latency	16.2 stable	Stable
11	М	L	N	N	2–1	N	↑ R1 latency	13.9 stable	Stable
12	М	R	N	N	2–1	N	↑ R1 latency	11.6 stable	Stable
13	F	L	2–3	Hypesthesia	3–2	Hypesthesia	No R1	Not recordable	↓Amplitude by 30%
14	F	R	N	Hypesthesia	2	Hypesthesia	↑ R1 latency	16.9 stable	↓Amplitude by 70%, with recovery
15	F	R	N	Hypesthesia of the gums	2	Hypesthesia	↑ R1 latency	12.2 stable	↓Amplitude by 70%
16	М	L	4	Hypesthesia	3–4	Hypesthesia	↑ R1 latency	18.9 stable	↓Amplitude by 30%
17	F	L	N	Hypesthesia	4	Hypesthesia	↑Latency R1	19.1 stable	Stable

Note: M — male; F — female; R — right; L — left; N — normal; ↑ — increase; ↓ — decrease.

 

Intraoperatively, prior to tumor removal, BR was reliably recorded in 14 (82%) patients; in 3 (18%) patients, BR was not recorded intraoperatively, and preoperative testing had already shown severely abnormal response (in 1 patient — absence of the R1 response; in 2 patients — absence of both R1 and R2 responses). In 3 (8%) patients, BR disappeared abruptly during monitoring. In one of these patients, the CoMEP amplitude also decreased critically; in one female patient, it decreased by 40%; and in one patient, the CoMEP remained stable (see Table 1).

Case Report 1

Patient B., 45 years old male, presented with complaints of headache, dizziness, double vision, facial asymmetry, unsteadiness when walking, periodic drooping of the left upper eyelid, and left-sided anacusis. Neurological examination revealed grade 3 facial palsy on the HB scale, hypesthesia in the left half of the face, horizontal nystagmus, positive Romberg test, and an unsteady gait. Brain MRI revealed an extra-axial mass in the left cerebellopontine angle with clear contours, heterogeneous structure, associated with the acoustic nerve (30 × 44 × 35 mm), with pronounced peritumoral edema and significant compression of the brainstem, the fourth ventricle, and the cerebellar peduncle (Fig. 1).

 

Fig. 1. T2-weighted brain MRI of patient B.

 

A subtotal resection of the VS was performed. IONM included monitoring of CoMEPs from the left orbicularis oris muscle and mentalis muscle, bilateral BR monitoring, control of free-run EMG of the facial muscles (orbicularis oris muscle, mentalis muscle, orbicularis oculi muscle), from the masseter muscle and temporalis muscle, and continuous monopolar stimulation. The latency of the CoMEP from the mentalis muscle was 18.2 msec. The BR on the left had a latency of 18.5 ms. During the surgery, abnormal prolonged (more than 10 s) A-trains from the facial muscles and the masseter muscle were recorded (Fig. 2), after which CoMEPs and BR disappeared (Fig. 3). Irrigation with warm saline was performed, but the CoMEPs and BR did not recover by the end of monitoring. According to postoperative brain MRI data, subtotal removal of the VS was performed (Fig. 4).

 

Fig. 2. Free-run EMG of patient B. Prolonged A-trains were recorded on the left in the facial muscles and the masseter muscle.

 

Fig. 3. EMG of patient B. A — BR on the left; B — CoMEP from the mentalis muscle.

 

 

Fig. 4. Brain MRI of patient B. 3 months after surgery. Subtotal VS resection on the left.

 

In the early postoperative period, patient B. developed grade 5 facial palsy according to the HB scale, persistent hypoesthesia of the left half of the face, horizontal nystagmus, and unsteady gait, which did not resolve in the late postoperative period (after 6 months).

Case Report 2

Patient A., 57 years old female, with right-sided anacusis, positive Romberg test, and unsteady gait on neurological examination, without signs of facial nerve or trigeminal nerve damage. Brain MRI demonstrated a cystic-solid mass (31 × 22 × 27 mm) with calcifications in the right cerebellopontine angle, filling the entire right internal auditory canal, with perifocal edema and intense contrast enhancement. The mass compressed the middle cerebellar peduncle, the right cerebellar hemisphere, and the pons (Fig. 5).

 

Fig. 5. MRI of patient A. in T2 mode.

 

A subtotal resection of the VS was performed. IONM included monitoring of CoMEPs from the right orbicularis oris muscle and mentalis muscle, BR monitoring on the right, control of free-run EMG of the facial muscles (orbicularis oris muscle, mentalis muscle, orbicularis oculi muscle), from the masseter muscle and temporalis muscle innervated by the trigeminal nerve, and continuous monopolar stimulation. The CoMEP latency was 20.2 msec. The right BR had a latency of 15.4 ms. During the surgery, abnormal A-trains were recorded from the masseter muscle (Fig. 6), followed by the BR disappearance (Fig. 7, A). At that moment, the CoMEP amplitude decreased by 40% (Fig. 7, B).

 

Fig. 6. Free-run EMG of patient A. Prolonged A-trains were recorded from the masseter muscle (seventh trace).

 

Fig. 7. EMG of patient A. A — left BR: abrupt disappearance was recorded; B — CoMEP from the mentalis muscle, the arrow indicates the decrease in CoMEP amplitude.

 

The neurosurgeon was informed of the changes and paused for 5–7 minutes. After the pause, the BR did not recover, and the CoMEP amplitude remained unchanged. The surgery proceeded. The CoMEP amplitude subsequently remained stable, with no further decline. The BR did not recover by the end of the monitoring session (Fig. 8). According to postoperative brain MRI data, subtotal removal of the VS on the right was achieved (Fig. 9).

 

Fig. 8. EMG of patient A. CoMEP from the mentalis muscle.

 

Fig. 9. Brain MRI of patient A. 3 months after surgery. Subtotal VS resection on the right.

 

In the postoperative period, the patient complained of unpleasant sensations in the right half of her face. Concurrently, mild asymmetry of the palpebral fissures was noted, D > S, and the nasolabial fold was slightly asymmetric on the right during speech, corresponding to grade 2 facial palsy on the HB scale. Six months after surgery, the patient had persistent hypesthesia in the areas innervated by the 2nd and 3rd branches of the right TN, and the facial palsy corresponded to grade 1 on the HB scale.

Discussion

Monitoring the sensory portion of the TN, particularly its first branch, is possible with intraoperative BR monitoring, first described by E. Kugelberg in 1952 [12–15]. This is an oligosynaptic brainstem reflex arc between the TN afferents, the TN sensory nucleus, the motor nucleus of the facial nerve, and the efferents of the facial nerve. An early R1 component, which is the response of the afferent reflex part (the first TN branch), and a late R2 component, which is bilateral and manifests as blinking, i.e., movement of the muscles that close the eye, are distinguished. Recording the BR allows assessing the sensory part of the first TN branch and the motor part of the facial nerve, as well as the brainstem where the trigeminofacial pathways pass [15, 16]. For a long time, it was believed that the BR could not be recorded in anesthetized patients. The method of using BR intraoperatively was proposed by V. Deletis et al. in 2009 [16]. One solution to the issue is a short train of high-frequency stimuli, which can partially overcome the depressive effect of anesthetics. A double train with an inter-train interval of 20–40 ms is also recommended. In this case, only the R1 component is recorded intraoperatively, without the R2 component.

In a 2014 study, I. I. Fernández-Conejero et al. noted that 3 out of 8 patients, who showed a critical decrease in the CoMEP amplitude and BR during surgery, developed significant facial palsy after the procedure. When CoMEPs and BR were preserved, no facial nerve palsy was noted in the postoperative period [17]. In a 2021 study, I. Fernández-Conejero et al. observed postoperative TN impairment in patients whose BR disappeared intraoperatively [18].

In a study by E.I. Aydinlar, published in 2022, BR was monitored in 32 patients during VS resections [10]. It was noted that the disappearance of BR and CoMEP in patients was followed postoperatively by severe falsy (HB grade 6). With a critical decrease in CoMEP amplitude and preserved BR, significant facial nerve palsy (HB 5) was observed in the early postoperative period, but in the late postoperative period (after 2 months), the palsy regressed to HB grade 2. Also, in several patients, an earlier change in BR response was noted compared to CoMEP [19].

J. Liu et al. intraoperatively recorded BR in 103 (93%) of 110 patients during cerebellopontine angle tumor resections [20]. CoMEP from the orbicularis oculi muscle was recorded in 43 (39%) patients, demonstrating the high reproducibility of BR. Furthermore, BR significantly correlated with postoperative facial palsy. A decrease in BR amplitude together with a decrease in CoMEP amplitude predicted postoperative facial palsy, particularly impairment of eye closure: a decrease in BR amplitude of ≥ 51.0% increased the risk of the eye-closing muscle paresis [20].

A distinctive feature is that BR recording can be performed without interrupting the neurosurgeon’s work, as stimulation is performed with a current pulse duration of no more than 40 ms and movements of the orbicularis oculi muscle are imperceptible. In contrast, transcranial stimulation for recording CoMEP can cause noticeable movements of the facial muscles, which is inconvenient for the surgeon during the procedure.

In our patient cohort, the masseter reflex was recorded in 14 out of 17 patients with VS. In patients with preoperative facial nerve and TN impairment, the BR was intraoperatively detected in only 1 (25%) out of 4 such patients. Monitoring the masseter reflex allowed assessing the sensory function of the fifth cranial nerve, which cannot be evaluated by any other method of intraoperative neurophysiological monitoring.

A decrease in the BR amplitude during VS resection may indicate impairment of the sensory portion of the TN and the motor portion of the facial nerve. In one clinical case, an abnormal A-train indicated compromise of the facial and trigeminal nerves, which was demonstrated by a sharp disappearance of the BR and CoMEPs. The facial palsy, which was present in the patient preoperatively, significantly worsened in the postoperative period.

In the second clinical case, an abnormal A-train was observed in the TN-innervated masseter muscle, which was accompanied by a sharp decrease in the BR in this female patient. In the early and late postoperative periods, the patient reported sensory changes in her face. At the same time, CoMEPs remained stable, and facial motor function was practically unaffected by the surgical treatment.

Conclusion

The stability of the BR and CoMEPs throughout the entire surgical procedure determines the degree of trigeminal and facial nerve impairment in the postoperative period. Decreased BR and CoMEPs indicated intraoperative injury to these CNs. The BR is suggested to support CoMEP monitoring and intraoperative EMG, allowing for the prediction of postoperative facial sensory and motor function status.

About the authors

Diliara Z. Miftakhova

Kazan State Medical Academy — Branch of the Russian Medical Academy of Continuous Professional Education; Interregional Clinical Diagnostic Center

Author for correspondence.
Email: dmiftakhova@yandex.ru
ORCID iD: 0000-0001-8748-7138

assistant professor, Functional diagnostics department; physician, Functional diagnostics department No. 2

Russian Federation, Kazan; Kazan

Arseniy A. Pichugin

Interregional Clinical Diagnostic Center; Kazan State Medical University

Email: arsenicus@yandex.ru
ORCID iD: 0000-0002-0134-1005

Cand. Sci. (Med.), Head, Neurosurgery department; physician, Neurosurgery department

Russian Federation, Kazan; Kazan

Roman V. Ivanov

Kazan State Medical University

Email: ivanovrv@yandex.ru
ORCID iD: 0009-0006-5311-2577

resident in neurology, Neurosurgery department

Russian Federation, Kazan

Daniia K. Mukhamadieva

Interregional Clinical Diagnostic Center

Email: daniakamil@mail.ru
ORCID iD: 0009-0006-7435-5463

Head, Functional diagnostics department No. 2

Russian Federation, Kazan

References

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