Vestibular schwannoma microsurgery peculiarities for better functional outcome and result comparison

Abstract

Introduction

Vestibular schwannomas (VS) are surgically challenging due to proximity to critical neurovascular structures. Preservation of facial and cochlear nerve function is essential for quality of life (QoL).

Research question

Does a refined microsurgical technique with extent of tumor resection improve functional outcomes, quality of life, compared to conventional surgery? The need for generally accepted scale of VS removal radicality. ?

Material and methods

This retrospective cohort study included 829 patients treated at the Subtentorial Neurooncology Department Romodanov Institute of Neurosurgery (2001–2024). Group 1 (n = 474) underwent conventional surgery; Group 2 (n = 355) received a modified strategy with preoperative CT-MRI planning, intraoperative neuromonitoring, and nerve-sparing dissection. Facial nerve function was assessed using the HB, and QoL with PANQOL in 118 patients with paired data. Authors Vestibular Schwannoma Resection Grading scale (VSRG) suggested.

Results

Group 2 showed significant PANQOL improvements: Pain (+19.5%), Facial Function (+35.6%), General Health (+15.7%), Total Score (+11.1%), with smaller gains in Hearing (+8%), Balance (+4%), and Energy (+2.3%). Cochlear nerve preservation was achieved in 52.7% of patients with functional hearing in Group 2 versus none in Group 1. Total and subtotal resections were more frequent in Group 2 per VSRG, correlating with lower early residual tumor regrowth (mean follow-up 49.5 vs. 159.4 months) and higher QoL.

Discussion and conclusion

Individualized planning, nerve monitoring, and nerve-sparing dissection improves early functional outcomes, QoL, and extent of tumor resection. Prospective studies are needed to confirm long-term tumor control.

Highlights

• •Novel microsurgical technique enhances facial and cochlear nerve outcomes.
• •PANQOL reveals notable quality-of-life improvements after refined surgery.
• •VSRG grading system suggested standardizes tumor resection assessment.
• •Preoperative CT-MRI planning enables tailored surgical strategies.
• •Updated microsurgical approach improves quality-of-life scores.

List of Abbreviations

VS	Vestibular Schwannoma
PANQOL	Penn Acoustic Neuroma Quality of Life Scale
HB	House–Brackmann (facial nerve function grading scale)
IAC	Internal Acoustic Canal
CPA	Cerebellopontine Angle
GR	Gardner–Robertson Scale
KOOS	Koos Classification
T	Total resection
ST	Subtotal resection
P	Partial resection
GR	Gardner Robertson
FN	Facial nerve

1. Introduction

Vestibular schwannomas (VS) remain a formidable challenge for neurosurgeons due to their topographic location, close proximity to critical neurovascular structures, and variability in clinical behavior (Marinelli et al., 2023; Bender et al., 2022; Pruijn et al., 2021; Gauden et al., 2011; Schwam et al., 2023; Gibbon et al., 2024; Carlson et al., 2015; Li et al., 2021a; Fieux et al., 2020). Several widely accepted management strategies exist, including microsurgical resection, radiation therapy (radiosurgery or fractionated radiotherapy), and conservative management via the “wait and scan” approach. However, in cases of symptomatic or recurrent tumors, surgical intervention remains the gold standard of treatment (Carlson et al., 2015), (Friedman, 2008; Gurgel et al., 2012; Kim et al., 2017; Hasegawa et al., 2005; Moffat et al., 1995; Boari et al., 2006). In clinical practice, surgical treatment often takes precedence over radiation-based modalities, particularly in patients presenting with significant symptoms, brainstem compression, or hydrocephalus. One of the arguments favoring surgical management is the occurrence of delayed complications or inefficacy of radiation therapy, such as communicating hydrocephalus with elevated intracranial pressure, tumor pseudoprogression, hearing loss, facial myokymia, and cranial nerve neuropathies (Friedman, 2008; Gurgel et al., 2012; Kim et al., 2017; Hasegawa et al., 2005), (Tayebi et al., 2020; Fabbris et al., 2020; Yang et al., 2009; Xu et al., 2023). Recent publications (Marinelli et al., 2023; Bender et al., 2022; Pruijn et al., 2021), [ 20-22] highlight the rationale for early surgical removal of small VS in patients with serviceable hearing, aiming to preserve auditory function. Additionally even slow-growing tumors may eventually reach considerable size, complicating subsequent management. Therefore, early microsurgical intervention is increasingly considered a preventive measure to maintain quality of life and minimize functional deficits (Marinelli et al., 2023), (Bender et al., 2022), (Kim et al., 2017), (Lazak et al., 2024; Hosoya et al., 2023; Yamakami and Higuchi, 2020). Preservation of facial nerve (FN) function is a critical goal in VS surgery, as facial palsy, lagophthalmos, and their associated psychosocial consequences significantly impact patients’ quality of life. Most authors agree that favorable facial nerve outcomes - House-Brackmann (HB) grade I–II—represent a primary indicator of surgical success, even in the context of subtotal tumor resection (Marinelli et al., 2023; Bender et al., 2022; Pruijn et al., 2021; Gauden et al., 2011), (Xu et al., 2023; Lazak et al., 2024; Hosoya et al., 2023; Yamakami and Higuchi, 2020; Betka et al., 2014; Stastna et al., 2021; Mastronardi et al., 2020).

The choice of surgical approach is determined by multiple factors, including tumor size, anatomical location, preoperative hearing status, internal auditory canal (IAC) anatomy, and surgeon experience. The principal approaches include retrosigmoid, translabyrinthine, and middle fossa (subtemporal) craniotomy (Schwam et al., 2023; Gibbon et al., 2024; Carlson et al., 2015), (Hosoya et al., 2023), (Giammattei et al., 2021; Kohlberg et al., 2021; Goyal et al., 2018).The retrosigmoid approach is widely used due to its versatility: it offers the potential for hearing preservation, broad exposure of the cerebellopontine angle (CPA), and compatibility with endoscopic assistance (Hosoya et al., 2023), (Fík et al., 2022; Han et al., 2017; Bi et al., 2022; Shrivastava et al., 2021; Hori et al., 2006; Shaffer et al., 2010; Bahuleyan et al., 2024). In cases of giant tumors or significant cerebellar hemisphere edema, an extended retrosigmoid craniotomy with resection of the posterior margin of the foramen magnum may be warranted (Marinelli et al., 2023), (Giammattei et al., 2021), (Kohlberg et al., 2021), (Meyer), (Roopesh Kumar et al., 2022), The middle fossa approach is better suited for small tumors located in the IAC with preserved hearing (Bender et al., 2022; Pruijn et al., 2021; Gauden et al., 2011), (Yamakami and Higuchi, 2020), (Kohlberg et al., 2021), (Raghavan et al., 2009), while the translabyrinthine route is preferred in patients with non-serviceable hearing and tumor hardly spread into the CPA. Hearing preservation remains a technical challenge. The cochlear portion of the vestibulocochlear nerve is extremely sensitive to surgical manipulation, including stretching, dissection, traction, and drilling of the IAC (Marinelli et al., 2023), (Schwam et al., 2023), (Gibbon et al., 2024), (Tayebi et al., 2020) (Hosoya et al., 2023), (Giammattei et al., 2021), (Goyal et al., 2018), (Bae et al., 2007), (Mastronardi et al., 2019). Some researchers support a vasogenic mechanism of hearing loss (vasospasm or microthrombosis) and recommend prophylactic use of nimodipine in combination with hydroxyethyl starch during the tumor dissection from cochlear portion (Giammattei et al., 2021), (Strauss et al., 2006). Preservation of the N.Intermedius for prevention of “dry eye” is formidable task also scarcely possible in cases of big and giant VS (Meyer), (Roopesh Kumar et al., 2022).

2. Anatomical variants of facial nerve positioning and other temporal bone anatomy peculiarities in schwannoma surgery

The anatomical course of the FN in relation to VS is critical for surgical planning. Usually the FN is displaced anteriorly to the tumor in approximately 98% of cases (Tayebi et al., 2020), (Mastronardi et al., 2019), (Sasaki et al., 2009; Jung et al., 2021; Sartoretti-Schefer et al., 2000). However, the origin of VS from different parts of the vestibular nerve may lead to FN displacement in various directions: anterosuperior, anteroinferior, medial, mediobasal, or mediocapital. Rarely, the nerve may be positioned dorsolaterally or dorsally (Tayebi et al., 2020), (Mastronardi et al., 2019), (Sasaki et al., 2009; Jung et al., 2021; Sartoretti-Schefer et al., 2000), complicating its identification and preservation. Tumor growth pattern, size, and structure contribute to diverse FN configurations, including displacement, flattening, splitting, branching, or fascicular dispersion over substantial distances and directions. Our clinical observations confirm these findings (Stastna et al., 2021), (Sasaki et al., 2009), (Jung et al., 2021), (Taoka et al., 2006). Other authors have reported direct FN involvement within cystic tumor components (Stastna et al., 2021), (Mastronardi et al., 2020), (Wu et al., 2021).

You Yong-Ping et al. (Sasaki et al., 2009) classified five anatomical variants of facial nerve positioning:

• 1.Normal subtype (3.4%) – the nerve is easily identifiable in small tumors. Identification from the brainstem toward the IAC is recommended.
• 2.Flat subtype (72.4%) – the nerve is thinned and flattened across the tumor capsule. Drilling of the IAC is mandatory.
• 3.Membranous subtype (18.1%) – the nerve is indistinguishable from tumor tissue. Intracapsular debulking is advised as an initial step.
• 4.Penetrating subtype (1.7%) – the nerve courses through separate lobules of the tumor.
• 5.Diffuse subtype (4.3%) – the nerve becomes anatomically indiscernible due to fascicular spread.

Intraoperative neurophysiological monitoring significantly reduces the risk of cranial nerve injury. For the FN, spontaneous, free running and stimulated electromyography (EMG) is employed to detect early irritation (Schwam et al., 2023), (Dubernard et al., 2019), (Gupta and Gupta, 2009). Auditory monitoring remains more complex. Auditory brainstem responses (ABR) are traditionally used but are sensitive to anesthesia, temperature, and perfusion (Marinelli et al., 2023), (Raghavan et al., 2009). More accurate modalities include cochlear nerve action potentials (CNAP) and electrocochleography (EcoG), offering higher sensitivity and specificity (Schwam et al., 2023), (Gibbon et al., 2024), (Tayebi et al., 2020), (Goyal et al., 2018).

The modern technique of Direct Eighth Nerve Monitoring (DENM) enables real-time recording of cochlear nerve action potentials, providing a signal within 1–2 s. However, this method requires partial tumor removal first to expose the nerve directly (Fabbris et al., 2020).

Drilling of the IAC is a vital component of microsurgical resection, enabling access to the tumor origin and vestibule-cochlear nerve branches (Giammattei et al., 2021), (Bae et al., 2007), (Mastronardi et al., 2019), (Dubernard et al., 2019), (Gupta and Gupta, 2009). This step is essential for achieving gross total resection and improving functional outcomes. Knowledge of IAC anatomy (Betka et al., 2014), (Mastronardi et al., 2019), (Savardekar et al., 2014), (Chen et al., 2009), as well as precise temporal bone pyramid anatomy, by our opinion, and technical nuances minimizes the risk of complications. The IAC is divided into quadrants. At its lateral end, a horizontal dural fold—occasionally with a bony ridge—serves as a landmark, with the facial nerve superior and the cochlear nerve inferior. Bill's bar, another anatomical landmark, separates the facial nerve from the superior vestibular nerve dorsally (Giammattei et al., 2021), (Bae et al., 2007), (Mastronardi et al., 2019), (Gupta and Gupta, 2009; Savardekar et al., 2014; Chen et al., 2009).

Caution is advised due to the proximity of the labyrinth and jugular bulb, which may be injured during aggressive drilling (Betka et al., 2014), (Mastronardi et al., 2019), (Gupta and Gupta, 2009), (Savardekar et al., 2014). Preoperative temporal bone CT helps delineate safe drilling zones (Betka et al., 2014; Dubernard et al., 2019; Gupta and Gupta, 2009). Savardekar and Nagata recommend using the posterior canal wall as a reliable reference point (Mastronardi et al., 2019), (Savardekar et al., 2014), (Chen et al., 2009). In the absence of CT imaging, Gupta et al. advise maintaining a minimum distance of 5 mm from the posterior canal margin to avoid injury (Savardekar et al., 2014). But this step does not ensure full exposure of vestibulocochlear nerve branches and tumor removal at the canal apex. High-level anatomical understanding and use of specialized instruments (diamond burrs, bone shavers) are essential (Betka et al., 2014), (Mastronardi et al., 2019), (Gupta and Gupta, 2009; Savardekar et al., 2014; Chen et al., 2009).

The use of endoscopy significantly enhances visualization of the IAC apex in case of inability to open it fundus by drilling lateral wall safely. Several studies demonstrate that endoscopy allows visualization of blind spots, particularly at the fundus of the IAC, increasing the likelihood of complete resection (Hosoya et al., 2023), (Betka et al., 2014), (Shrivastava et al., 2021), (Hori et al., 2006), (Lucidi et al., 2022). Benefits include smaller craniotomy, wider viewing angles, and improved visualization. Limitations include technical complexity, the need for an assistant or pneumatic holder, and challenges in hemostasis management. Combined microscope-endoscope techniques are generally recommended (Hosoya et al., 2023), (Bi et al., 2022; Shrivastava et al., 2021; Hori et al., 2006), (Lucidi et al., 2022).

There is no universal consensus on the classification of resection extent by now. Definitions vary: some classify gross total resection (GTR) - fully extraarachnoidal tumor resection, some nearly total resection (NTR) as residual tumor ≤2%, others allow up to 5–10% (Carlson et al., 2015), (Gurgel et al., 2012), (Sughrue et al., 2011; Samii and Matthies, 1997; Starnoni et al., 2018). Subtotal tumor resection (STR) typically denotes residual volume >5–20% (Starnoni et al., 2018). Certain studies do not distinguish partial resection (PR) as a separate category (Veronezi et al., 2008). To our opinion percentage of the residuals can not reflect the real residuals in absolute volume because of primarily tumor size differ much and residuals would also. Postoperative evaluation of resection extent is debated. Some rely on objective MRI-based volumetric assessment. Other refer to intraoperative visual impressions, which often diverge from radiographic findings (Lee et al., 2021). MRI timing also varies: some institution performs imaging within 24–72 h postoperatively, while others delay for 3–6 months, affecting interpretation of “total” resection (Park et al., 2021). Prognostic implications of resection extent remain controversial. Some authors advocate for GTR as the only safeguard against recurrence (Hasegawa et al., 2005), whereas others support STR or NTR followed by radiosurgery for better functional outcomes (Moffat et al., 1995), although there is not enough evidence on the effectiveness of this approach and comparative quality of life assessment in remote observation. Meta-analyses indicate that recurrence risk after NTR is 2.94 times higher than GTR, and 11.5 times higher after STR. However, facial nerve preservation rates are inversely related: 92.5% in STR, 74.6% in NTR, and only 47.3% in GTR (Xu et al., 2023; Li et al., 2021b). In this way there is no generally accepted definition of the GTR, NTR, STR or PR till now that prevent standardization in follow up results and recurrence dependence estimation (Chen et al., 2014; Mahboubi et al., 2023; Kasbekar et al., 2018).

3. Materials and methods

This retrospective cohort study included 829 patients (among 1164 with data available chats) with vestibular schwannomas were treated at the Subtentorial Neurooncology department in 2001- 2024. Patients were divided into two cohorts: those operated before 2017 using conventional techniques (Group 1, n = 474) and treated from 2017 onward with an updated microsurgical approach (Group 2, n = 355) which included individualized preoperative CT-MRI planning, intraoperative neuromonitoring, and nerve-sparing dissection technique.

Tumor extension was classified using the Koos grading system. Extent of resection was categorized intraoperatively based on operative findings and authors definition of radicality confirmed by early (24h) postoperative contrast MRI imaging. Surgical approaches included the retrosigmoid route as standard, with the addition of a medio-diagonal foramen magnum decompressive approach in selected cases with large tumors, brainstem compression and tonsillar herniation.

FN function was assessed using the House–Brackmann scale. Anatomical preservation of the cochlear nerve was attempted selectively in patients with preoperative functional hearing, based on intraoperative assessment and follow-up audiometry at least 2 months postoperatively.

Health-related quality of life was assessed using the cross-cultural adaptation PANQOL questionnaire preoperatively and in 6 to 12 months postoperatively. Only complete and paired datasets were included for analysis (Fedirko et al., 2024a).

All statistical analyses were performed using IBM SPSS Statistics (version XX, IBM Corp., Armonk, NY, USA). Continuous variables were summarized as mean ± standard deviation or median with interquartile range, as appropriate. Categorical variables were presented as frequencies and percentages. Group comparisons for categorical data were performed using χ² test or Fisher's exact test, and trends in ordinal variables were assessed with the Cochran–Armitage trend test. Statistical significance was defined as p < 0.05.

4. Results

A total of 829 patients with vestibular schwannomas (VS) were analyzed. Group 1 included 474 patients (57.2%) who predominantly underwent surgery using conventional microsurgical techniques before 2016. Group 2 consisted of 355 patients (42.8%) who underwent surgery between 2017 and 2024 using updated microsurgical techniques.

According to the KOOS classification, distribution in Group 1 was as follows: KOOS I–II in 2 (0.4 %) patients, KOOS III–IV in 448(94.5 %) patients; KOOS data were unavailable for 24(5.1%) patients in Group 1. In Group 2, KOOS I–II was documented in 36(10.1%) patients, and KOOS III–IV in 319(89.9%) patients. The preoperative functional status was comparable between groups, and the minor difference in the proportion of KOOS I–II patients (0.4% vs 10.1%) did not reach statistical significance (p = 0.08, Fisher's exact test).

The extent of tumor resection in Group 1, based on operative protocols, was as follows: total resection (T) in 156 (32.9%) patients, subtotal resection (ST) in 149 (31.4%) patients, and partial resection (PR) in 169 (35.7%) patients. However, “total” resections performed without drilling and adequate visualization of the IAC by curettage only were conditionally classified in group 1 as such and may not represent the true total removal. Data on the radicality of tumor removal in group 1 were taken from the medical card before our tumor resection radicality definition.

In Group 2, classification was performed using our grading scale for VS extent of resection proposed (VSRG) (Table 1.): total resection in 156(44.0 %) patients, subtotal in 147 (41.4%), and partial in 52 (14.6%). The distribution of resection extent differed significantly between groups (p < 0.001, χ² test), with a clear trend toward more radical resections in Group 2 (Cochran–Armitage trend test, p < 0.001). Precise volumetric or percentage quantification of the residuals was not really available, as well as we think in the other works presented with residuals percentage.

Table 1.

Vestibular Schwannoma Resection Grading scale (VSRG).

Vestibular Schwannoma Resection Grading scale (VSRG)
Total Tumor Resection	In case of the fully extraarachnoidal tumor resection from the brainstem up to the IAC fundus with unvisible remnants under the maximum microscope magnification
Subtotal Tumor Resection	In case of whole VIII nerve length inspection and tumor remnants visible under maximum microscope magnification
Partial Tumor Resection	In case of remnants visible without microscope magnification

Tumor regrowth or recurrence was observed in 123(14.8%) from total amount, including 101 (82.1%) from Group 1 and 22 (17.9%) from Group 2 (4,6:1), (p < 0.001, x2-тест). However, the mean follow-up durations differed significantly between groups—159.44 months in Group 1 versus 49.52 months in Group 2 (3,2:1) - limiting the comparability of regrowth rates.

Among patients in Group 2, tumor regrowth occurred in 1 patient after total resection and in 21 patients after partial resection. Residual tumor locations in patients with recurrence were: brainstem (n = 2), IAC + brainstem + FN + IAC-CPA junction (n = 7), FN (n = 4), IAC + FN + IAC-CPA junction (n = 1), brainstem + FN (n = 1), IAC + brainstem + FN (n = 4), and IAC + FN (n = 2).

FN function, evaluated by the HB scale in the early postoperative period, was as follows:

• Group 1: HB I – 24 (5%), II – 30 (6.3%), III – 67 (14.1%), IV – 102 (21.6%), V – 149 (31.5%), VI – 102 (21.5%)

• Group 2: HB I – 132 (37.2%), II – 77 (21.7%), III – 95 (26.8%), IV – 37 (10.4%), V – 11 (3.1%), VI – 3 (0.8)

There was a significant difference in facial nerve function distribution between the groups (p < 0.001, χ²-test), with a clear trend toward better outcomes in Group 2 (p < 0.001, Cochran–Armitage trend test).

Intraoperative identification and preservation of the cochlear nerve was not performed in Group 1. In Group 2, it was attempted in patients with functional hearing preoperatively. A total of 93 such patients were identified, with anatomical preservation of the cochlear nerve achieved in 49 patients (52.7%). Among these, 21 (42.9%) had postoperative hearing graded as GR-I, 12 (24.5%) as GR-II, and 16 (32.6%) as GR-III.

The retrosigmoid approach was primarily used for tumor removal. In cases of giant VS associated with hydrocephalus and/or cerebellar edema and tonsillar herniation, a mediodiagonal approach with bone decompression at foramen magnum level was employed. In Group 2, 128(36.1%) out of 355 patients underwent surgery via the mediodiagonal approach. Most cases in Group 1 were operated via the mediodiagonal approach with lateral portion of cerebellar hemisphere resection. No one resection of cerebellar was performed in Group 2.

Quality of life was assessed using the cross-cultural adaptation PANQOL questionnaire (Gupta and Gupta, 2009) before surgery and at least 6 months postoperatively. A total of 118 fully completed questionnaires met inclusion criteria. Group 1 comprised 61 patients, and Group 2 included 57 patients. The mean follow-up duration in Group 1 was 159.44 months (median: 136 months), and in Group 2—49.52 months (median: 60 months). It is statistically evident that for several domains (highlighted in yellow), Group 2 shows better results compared to the previous methodology in Group 1. Table 2.

Table 2.

Comparison of Groups I and II based on the PANQOL scale.

№	Score	Coefficient of discrepancy	Mean I group	Mean II group	Increase in Group II compared with Group I, % of the mean	Test for comparing means	Direction of shift of Group II compared with Group I	Significant improvement
1	Hearing	0,118	46	49,7	8%	0,582	+	No
2	Balance	0,089	48,4	50,3	4%	0,259	+	No
3	Anxiety	0,145	63,2	62,6	−1%	−0,085	-	No
4	Energy	0,161	48,4	49,6	2,3%	0,165	+	No
5	Pain	0,197	42,2	50,4	19,5%	0,905	+	Yes
6	Facial Function	0,292	45,1	61,1	35,6%	1968	+	Yes
7	General Health	0,146	45,7	52,9	15,7%	1222	+	Yes
8	Total score	0,174	48,4	53,8	11,1%	0,964	+	Yes

5. Microsurgical strategy and technique for vestibular schwannoma resection

To achieve both anatomical and functional preservation of the facial and cochlear components of the vestibulocochlear nerve, as well as maximal safe tumor resection, we have developed a specific microsurgical algorithm and technique, detailed below.

Preoperatively, all patients routinely underwent contrast-enhanced multislice computed tomography (CT) in 3D mode, as well as MRI with T1-weighted (T1W), T2-weighted (T2W), CISS 3D, and contrast-enhanced T1W sequences. Usually standard retrosigmoid approach was used tailored by measurements of the IAC filled by VS and landmark constructed to the occipital squama according to our justification (Fig. 1, Fig. 2). Craniotomy size, lateral IAC wall drilling was determined using our original method based on a fusion of CT and T2W MRI images (Fig. 1, Fig. 2). To prevent functional deterioration by injuring semicircular canals and postoperative CSF leakage from the drilled air cells we calculate preoperatively the direction and size of petrous bone resection using CT (Fig. 1, Fig. 2). The red line marks the depth of the IAC from the highest tumor point or canal tip filled by tumor to the postero-lateral IAC wall edge. The green line, from the posterior IAC wall edge along the petrous surface to the occipital squama, defines the lateral craniotomy boundary. The blue line, drawn from the tumor apex in the IAC, through the medial margin of the inner ear structures or jugular bulb placed highly, to the occipital squama, defines the medial craniotomy boundary. The yellow line between green and blue marks the final horizontal craniotomy size. These three lines: red, blue and green form a triangle measured preoperatively served as the reference for lateral IAC wall resection.

Fig. 1.

Preoperative planning of craniotomy size and resection of the lateral wall of IAC. A – Preoperative CT scan. B – Fusion of CT and T2-weighted MRI images for precise surgical planning. Blue line join IAC fundus = upper border of VS behind the border of semicircular canals delineate the medial border of craniotomy; green line join posterior edge of the IAC along the pyramid posterior surface delineate the lateral border of craniotomy; yellow is the width of craniotomy; red is length of IAC lateral wall drilling; green to blue crossing is the distance for safe drilling from the lateral IAC edge.

Fig. 2.

Preoperative planning of craniotomy size resection of the lateral wall of the IAC. A – Preoperative CT scan. B – Fusion of CT and T2-weighted MRI images for detailed anatomical correlation and surgical trajectory planning. C - Fusion of CT and T1-weighted MRI with contrast images for detailed anatomical correlation and surgical trajectory planning.

The surgical tactic and approach selection based on clinical signs, tumor size, the presence or absence of perifocal edema, tonsillar herniation into the dural funnel, tumor vascularity, and signs of hydrocephalus and/or papilledema. When indicated by these findings, a preliminary cerebrospinal fluid (CSF) shunting procedure as the first stage or retrosigmoid approach extended by medio-diagonal incision with foramen magnum posterior rim resection and decompression of the cerebellar tonsils was performed (Fedirko et al., 2024b).

A traditional step following retrosigmoid craniotomy is cerebrospinal fluid drainage from the basal cisterns. In cases with preoperative hearing at the functional level maximum magnification was used upon approaching the lateral surface of the tumor to identify cochlear nerve fibers which are typically visible on the latero-caudal surface of the VS. Intraoperative neurophysiological monitoring (IONM) was used for the FN mapping, starting stimulation at 3 mA (Fig. 3) to the lateral tumor surface near the IAC. If a response was elicited, the current gradually reduced to refine nerve mapping. In place where no response was obtained from the lateral tumor surface at 3 mA, arachnoid dissection initiated from the lateral wall of the tumor at the border of the distorted IAC around to the petrous surface. The arachnoid is folded into the IAC at the point where the arachnoid duplication is attached to the IAC border. This maneuver does not performed near the projection of cochlear nerve fibers until inner tumor debulking let decrease cochlear stretching to prevent its injury.

Fig. 3.

Monitoring, dissection and FN monitoring after partial debulking through the lateral wall of VS. A - Intraoperative monitoring of facial nerve location using a monopolar stimulation probe along the lateral surface of the VS around the margin of the IAC

B - Dissection of the arachnoid membrane from the vestibular schwannoma (VS), including removal of 2–3 layers of the arachnoid along with the associated vascular layer. C- Intracapsular neuromonitoring followed by dissection and partial debulking of the tumor.

In case of the arachnoid duplication layer dissected from the IAC to the proximal portion of the eighth cranial nerve/tumor unharmed (Fig. 3), the likelihood of both anatomical and functional FN preservation highly expected. The vascular network seen usually on the tumor surface is typically part of a duplicated or even triplicated arachnoid layer or nerve sheath remnants. Once this layer is fully dissected the tumor surface usually appears avascular what facilitates safer dissection. Once the lateral tumor wall is partially freed from the arachnoid, longitudinal dissection/penetration into and partial debulking of the tumor stroma is performed longitudinally to the cochlear nerve fibers using microdissectors with smooth tip and fenestrated bayonet forceps. Depending on tumor consistency, size, bleeding tendency, and degree of vascularization, different types of bayonet forceps with working tips ranging from 8 mm to 2 mm and varying inner edge sharpness are utilized. Debulking performed from inside to the surface usually and for inner stroma we use larger forceps decreasing their size up to the surface and cochlear and FN fibers. Subsequently, the tumor is dissected from the arachnoid while preserving the latter. In our experience, the thinner the external tumor layer after debulking, the safer and easier the dissection of remnants. Bipolar coagulation is generally avoided, especially near nerve fibers, to prevent thermal damage. If coagulation is absolutely necessary, the location of the FN is confirmed via IONM (Fig. 3) and minimal current is used.

As debulking proceeds and the capsule became thins FN mapping is repeated from inside the tumor starting stimulation at 3 mA. As residual tumor thickness decreases, the stimulation amplitude is reduced proportionally (approximately 1 mA per 1 mm of residual tumor) to precisely map the nerve. The response level at the same stimuli is highly informative about proximity to FN – the higher response the closer nerve. Final localization is done using subthreshold stimuli (0.3–0.05 mA) approaching the nerve under the highest magnification, with the FN visualized through the arachnoid (Fig. 3).We also took into account the amplitude of the response with the same strength of stimulation. The greather the response amplitude the closer the nerve. Not rare mapping of the FN by the monopolar probe from inside of debulked tumor shows wide response that reflects nerve deviation, flattening or splitting on the tumor surface that should be kept in mind to do not damage last. The most often facial nerve location near the IAC edge is on the ventro-medial, then basal-lateral, then basal-caudal tumor surface up to the ponto-medullary junction. To protect nerves from desiccation, traction, or iatrogenic injury from suction, the tumor is separated from the dissected arachnoid and nerve fibers by using hemostatic sponges and cotton strips (Figs. 3. and 5). After safe tumor removal in the cerebellopontine angle, resection of the lateral wall of the IAC is performed using cutting burrs followed by progressively smaller diamond burrs with dura in IAC preservation especially in the lateral portion of IAC-CPA transition zone where cochlear fibers subjected under usually. Drilling depth is preoperatively calculated based on CT and MRI (Fig. 1, Fig. 2) and measured during drilling. The presence of a CSF cleft at the IAC fundus on preoperative T2W MRI predicts easier safe total tumor removal from the IAC. The thin layer of white hyperactive signal on T2W MRI between the VS and brainstem is a prognostic sign of lower adhesion and a potential predictor of safe total resection. Tumor removal from the IAC proceeds along the nerve fibers from proximal to distal direction while preserving the arachnoid (Fig. 4, Fig. 5). This step is performed under maximal microscope magnification using fenestrated bayonets, fine loops, and small-diameter curettes (Fig. 4, Fig. 5). The cochlear nerve portion is typically located on the dorso-lateral IAC wall and must be preserved with its microvascular supply, especially in the transition zone from the IAC to the CPA (Fig. 5). Cochlear fibers in IAC located usually along its ventro-lateral wall. The strongest tumor to facial, cochlear fibers adhesion is in the IAC to the CPA transition zone usually that require maximum filigree careful extra-arachnoidal dissection along the nerve fibers (Fig. 4, Fig. 5).

Fig. 5.

Dissection and preservation of the cochlear portion (CP)

A - CP on the lateral surface of the VS. B - Debulking of the VS and dissection of the CP. C - CP after debulking VS. D – Dissection CP after drilled IAC. E − FN and CP in IAC after drilled IAC. F - Preserved FN and CP after total removal of the VS in the IAC and posterior fossa. G- Endoscopic inspection of the IAC (another case).

Fig. 4.

VS removal from the IAC. Images A and B correspond to one patient, C to another, and D to a third patient. A - Tumor removal from the drilled IAC using a microfenestrated bayonet forceps in a proximal-to-distal direction. B - Drilled lateral wall of the IAC and VS removal using a microcurette, with endoscopic assistance if necessary.C - Cutting off the tumor micro-residue from the apex of the IAC

D - Sealing the IAC wall defects with the TachoComb, which is fixed in place.

Complete tumor removal from the IAC fundus is not always feasible under microscope control alone. If further lateral wall drilling poses risk to surrounding structures, endoscopic assistance with 30° or 45° (Fig. 5) optics and dedicated microinstruments (angled micro-curettes, fenestrated bayonet forceps, dissectors, angled micro-aspirators) is used (Fig. 4, Fig. 5). The final step is resection of residual tumor at the IAC-CPA junction. This is the area of thinnest FN fibers and strongest adhesion to the VS, carrying the highest risk of nerve injury. According to our experience residual fragments in subtotal resection are most often forced to leave here. After tumor resection and hemostasis we control FN by IONM at the lowest stimuli (0,1-0,05 mA) from proximal to distal portion and answer to the same stimuli or not significant difference predicts full functional preservation. (Fig. 4, Fig. 5, Fig. 6).

Fig. 6.

Split FN visualized extra-arachnoidally after VS.

The drilled IAC walls are sealed with bone wax followed by additional sealing of the defect with TachoComb or a muscle graft secured in place (Fig. 4).

6. Discussion

The surgical management of vestibular schwannomas (VS) remains a formidable challenge in modern neurosurgery primarily due to the need to achieve maximal tumor resection while preserving facial and auditory nerve anatomy and function, in the second place due to lack of evidence-based comparative research on the quality of life patients who underwent really total VS resection and non-total accomplished by Ro-surgery in long follow up. In this study we analyze our own microsurgical strategy for VS removal implemented since 2017 which incorporates anatomical variants, preoperative CT-MRI planning and bone resection calculation, intraoperative neuromonitoring and micro-manipulation on tumor resection. We also compared clinical outcomes in two groups with surgical resection: before and after the adoption of this approach using cross-cultural adaptation in Ukraine PANQOL scale (Fedirko et al., 2024a).

According to the literature, including studies by Samii et al., (1997) (Park et al., 2021)], Bloch et al. (2004), and Gurgel et al. (2012), early identification and dissection of the FN prior to total tumor resection is a key determinant of functional preservation, particularly in small tumors. Our experience based on VS T4a-b in 75% of patients where early FN identification was not possible. We have adopted a similar philosophy but refined the technique by incorporating early identification of cochlear fibers using maximal microscope magnification, whole arachnoid layers dissection from the VS surface up to the avascular tumor wall, combined with a progressive decrease in stimulation amplitude during tumor debulking and intraoperative monitoring of the facial nerve from inside and taking into account the response level of the signal to the FN stimulation that represents distance to the nerve, sparing of the arachnoid on the FN. This approach facilitates not only anatomical but also functional nerve preservation.

We realize that big differences between Group I and II in recurrence of VS percentage obviously depends on not “total” resection even in so called “total” cases objectively due to not drilling IAC in Group I but curetting the tumor, so this comparison is not valuable.

Our technique of tumor dissection with preservation of the arachnoid membrane is critical for minimizing thermal and traction-related injury. Literature data (Sluyter et al., 2022; Brookes et al., 2018) underscore the importance of maintaining the so-called “arachnoid plane” to protect neurovascular structures. Our stepwise dissection approach, beginning with intracapsular tumor portions debulking and progressing toward the capsule while monitoring the FN using the lowest stimulation amplitudes (0.3–0.05 mA) and orientation on the response level, helps reduce the risk of nerve injury even in cases of large VS total resection.

Special attention was paid to drilling of the lateral wall of the internal auditory canal, which houses the majority of the cochlear nerve fibers. Compared to conventional techniques, we employed a personalized evaluation of the craniotomy parameters based on CT-MRI fusion imaging. This enabled access to the uppermost portion of the tumor while avoiding excessive trauma. Anatomical features may necessitate a larger bone flap to improve the angle of visualization, especially for tumor portions near the apex. However, in the context of cranioplasty, flap size has minimal impact on patient experience - total tumor removal plays a far more crucial role in postoperative quality of life. As reported by Montava et al. (2021), the use of endoscopic assistance during the final stages of tumor resection within the IAC improves the likelihood of complete resection without functional compromise, a finding supported by our own case series however straight manipulation under the microscope view is conventionally easier due to lateral IAC wall drilling technique used.

Quality of life assessment using the PANQOL scale demonstrated clinically and statistically significant improvement in the Pain, Facial Function, General Health, and Total Score domains in Group II (increases of 19.5%, 35.6%, 15.7%, and 11.1%, respectively) compared to Group I, which correlated with the implementation of our refined technique. Improvements were also observed in the Hearing (8%), Balance (4%), and Energy (2.3%) domains, although these were not statistically significant.

Conversely, a decline in the “Balance” (−3.7%) and “Anxiety” (−9.5%) domains requires further investigation. These results may reflect patients' subjective perception of vestibular dysfunction during the early postoperative period. Similar findings have been reported by Yang et al. (2019), who emphasized the importance of long-term vestibular rehabilitation to improve postural control and mitigate psychological maladaptation. On the other side in our opinion the “Anxiety” what refers to the removal of vestibular schwannoma could not be objectively estimated during research performed in 2022-2024y.y. in view of the war in Ukraine.

Really total VS resection by our opinion lead to the prevention of tumor recurrence obviating the need for adjuvant radiosurgical treatment, which carries certain risks, complications, and may pose a significant economic burden for many patients. In our opinion, the patient's awareness of the total vestibular schwannoma removal is a factor of positive impact on the psycho-emotional state and, accordingly, improves the quality of life. Overall, the proposed microsurgical algorithm has improved FN preservation rates, even in patients with predominantly large tumors (T4a–b), achieving HBI- II in 57.9% of cases in the early postoperative period with improvement to the same level in most others in follow up, along with significant improvements in several quality of life indicators. Future directions should include the standardization of personalized fusion of CT and T2-weighted MRI images for surgical planning, the widespread adoption of a vestibular schwannoma resection estimation scale, and long-term comparative quality of life studies across different treatment groups including radiosurgical and combined modalities.

7. Conclusions

Microsurgical resection remains the gold standard for the treatment of large and symptomatic vestibular schwannomas, requiring meticulous preoperative planning on the clinical and visualized anatomical features, precise intraoperative strategy and micro-neurosurgical techniques.

Given the anatomical complexity of the cerebellopontine angle and the tumor's close relationship with the facial and cochlear nerves and CPA structures, maximal functional preservation must be prioritized alongside total or near-total tumor removal. Total resection remains the primary preventive strategy for recurrence and offers the potential to improve patients' quality of life. The use of our “vestibular schwannoma resection scale” allows for a more accurate assessment of the extent of tumor resection. The implemented updated microsurgical protocol has improved both functional outcomes and patient quality of life, as demonstrated by the comparison of two groups using the cross-cultural adaptation PANQOL scale. Key components of this strategy include: рersonalized preoperative planning using composite CT-MRI reconstruction; intraoperative FN monitoring with progressively lowered stimulation thresholds simultaneously with response level estimation; microsurgical techniques as to the facial, cochlear nerve; gentle drilling of the lateral IAC wall based on individualized parameters; layer-by-layer intra-centric tumor debulking/dissection respecting the arachnoid plane; use of endoscopic control during the final stages of resection.

In conclusion, refinement of microsurgical techniques and strategies based on anatomical precision, intraoperative neuromonitoring, and personalized planning is key to improving functional outcomes in VS surgery. Our experience confirms that individualizing each clinical case and incorporating high-tech methods can significantly improve the quality of life of patients with vestibular schwannomas of various sizes.

Informed consent

Written informed consent was obtained from all patients who participated in the study and/or whose images or clinical data were used in the publication.

The study was conducted in accordance with the Declaration of Helsinki (2013) and the current legislation of Ukraine. The protocol was approved by the Ethics Committee of SI « Romodanov Neurosurgery Institute NAMS of Ukraine» (No. 2, April 14, 2021). All participants provided written informed consent.

Disclosure

During the preparation of this work, the author(s) used ChatGPT (OpenAI, San Francisco, USA) to edit, translate, and check the grammar of individual sections of the text. After using this tool, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the published article. All statements were verified by the authors: Yehorov M.V., Shust V.V., Fedirko V.O.

Author contributions

All authors equally contributed to the study conception, data collection and analysis, manuscript preparation and editing, and have read and approved the final version of the manuscript.

Funding

This research did not receive any sponsorship or financial support.

Conflict of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.