Koos grade IV vestibular schwannomas: considerations on a consecutive series of 60 cases—searching for the balance between preservation of function and maximal tumor removal

Abstract

Koos grade IV vestibular schwannomas (VS) (maximum diameter > 3 cm) compress the brainstem and displace the fourth ventricle. Microsurgical resection with attention to the right balance between preservation of function and maximal tumor removal is the treatment of choice. Our series consists of 60 consecutive patients with unilateral VS, operated on from December 2010 to July 2019. All patients underwent microsurgical removal via the retrosigmoid approach. The adherence of VS’ capsule to the surrounding nervous structures and the excessive tendency of tumor to bleed during debulking, because of a redundant vascular architecture, was evaluated by reviewing video records. Microsurgical removal of tumor was classified as total (T), near-total (NT: residue < 5%), subtotal (ST: residue 5–10%), or partial (P: residue > 10%). Maximal mean tumor diameter was 3,97 cm (SD ± 1,13; range 3,1–5,8 cm). Preoperative severely impaired hearing or deafness (AAO-HNS classes C–D) was present in 52 cases (86,7%). Total or NT resection was accomplished in 46 cases (76,7%), 65,8% in cases with, and 95,4% without tight adhesion of capsule to nervous structures (p < 0,001). Endoscopic-assisted microsurgical removal of VS in the IAC was performed in 23 patients: in these cases, a T resection was obtained in 78,3% versus 45,9% of microsurgery only (p < 0,001). The capsule of VS was tightly adherent to nervous structures in 63,3% of patients, whereas hypervascular high-bleeding tumors represented 56,7%. Hearing preservation was possible in 2 out of 8 patients with preoperative class B hearing. At last follow-up, 34 (56,7%) patients had a normal postoperative FN outcome (HBI), 9 (15,0%) were HBII, 8 (13,3%) HBIII, and 9 (15,0%) HBIV. The total NT resection of solid and low-bleeding VS, without tight capsule adhesion, was associated with better FN outcome. Mortality was zero; permanent complications were observed in 2 cases (diplopia, hydrocephalus), transient in 9. Microsurgery of Koos grade IV VS seems to be associated with more than acceptable functional results, with high rate of T and NT removal of tumor. Long-term FN results seem to be worse in patients with cystic Koos grade IV VS, in cases with tight capsule adherences to nervous structures and in high-bleeding tumors.

Introduction

Vestibular schwannomas (VS) are tumors arising from Schwann cells of the superior or inferior vestibular nerve. According to Koos’ classification [25], grade IV are large and giant VS (longitudinal diameter > 3 cm), cause compression on the brainstem, and may displace the fourth ventricle. Since the 1980s, their incidence has progressively declined due to broader and earlier access to MRI imaging [51,52,53,54]: larger VS represented 40% of all tumors in the past and accounted only a few percent during the last decade [40].

These giant tumors adversely affect patients, and the goal of treatment is to achieve oncological control while maintaining quality of life with facial or cochlear nerve preservation [32, 40, 57, 58]. Microsurgical resection offers the best hope of achieving this goal. However, it is still necessary to clarify if their frequent tendency towards hypervascularity and adhesion to neurological structures affects the functional outcome.

In this study, we retrospectively analyze the clinical and surgical data of a consecutive series of 60 patients with Koos IV VS operated on by retrosigmoid approach, highlighting the extent of tumor removal, postoperative nerve functions, and complications.

Materials and methods

We reviewed 60 consecutive cases of Koos grade IV VS out of 225 unilateral VS surgically treated in our department between December 2010 and July 2019. General conditions and preoperative risk were assessed according to the American Society of Anesthesiology (ASA) classification [32]. Two patients with neurofibromatosis type 2 affected by unilateral VS were included. Clinical data such as patients’ age, sex, presenting symptoms, and tumor size were recorded. Preoperative neuroimaging included temporal bone CT and contrast-enhanced MRI in all patients.

Tumor size was categorized according to the international criteria, measuring the volume and the largest extrameatal tumor diameter on post-contrast axial MRI [23].

Preoperative audio-vestibular evaluation included pure tone audiometry and speech audiometry. Hearing level (HL) was assessed according to the American Academy of Otolaryngology–Head and Neck Surgery (AAO-HNS) classification [7]. HL classes A and B were categorized as good hearing, and hearing preservation was only attempted in these patients. Successful hearing preservation corresponded to classes A, B, and C (< 50%).

Microsurgery via retrosigmoid approach was performed in all cases. Location of FN and its adherence to the tumor were evaluated by reviewing radiological images and surgical and video records. The course of the FN was classified according to its position in relation to the tumor: anterior (A), anterior-inferior (AI), anterior-superior (AS), and dorsal (D) [28, 43]. Based on operative reports and videos, tumors were classified as cystic (one or more cysts within the tumor), adherent, and hypervascular. In detail, adherent VS were considered those without a clear perineural dissection plane between nervous structures (in particular, facial nerve, and brainstem) and tumor capsule, in comparison to VS with well recognizable dissection plane (less adherent). Hypervascular VS had an unusual superficial and internal vascular architecture and were complicated by high-rate bleeding during debulking and dissection, because of the rupture of multiple vessels present in the capsular and intracapsular portion of tumor.

The extent of tumor removal was calculated by means of manual segmentation and volumetry from pre- and postoperative MRI scans and was classified as total (T), near total (NT, thin strip of residual tumor; < 5% of preoperative volume), subtotal (ST, strip of residual tumor; 5–10% of preoperative volume), and partial (P, nodular residual tumor; > 10% of preoperative volume).

Since October 2017, at the end of microsurgical resection, a 4-mm flexible video endoscope (4-mm × 65-cm, Karl Storz, GmbH, Tuttlingen, Germany) has been inserted in the surgical cavity, handled by the surgeon [8, 9], in order to detect eventual tumor residues in the internal auditory canal (IAC). Endoscopic-assisted microsurgical removal was performed in 23 patients out of 60.

FN function was assessed according to House-Brackmann [18] (HB) grading system and was evaluated preoperatively, at discharge and at final follow-up (≥ 6 months).

The study was approved by the Internal Ethics Committee of the Hospital. A written consent for scientific treatment of personal data was obtained from any patient before surgery.

Operative technique

The retrosigmoid approach was used in all the surgical procedures, with the patient in lateral position. A 5–6-cm slightly curved incision was made 1–2 cm posterior to the mastoid process, exposing the lateral occipital bone and the superior and inferior nuchal lines. A retrosigmoid suboccipital craniotomy, 3 cm in length and width, was performed, exposing the sigmoid sinus anteriorly and the transverse sinus superiorly. The retrosigmoid dura was incised in a semicircular fashion, and the lateral medullary cistern was opened and the cerebrospinal fluid (CSF) was aspirated to obtain adequate cerebellar decompression. After cutting the dura along the “Tuebingen line” [2], covering the roof of IAC, the canal was opened with a 4-mm extra-coarse diamond burr or by ultrasonic aspirator with dedicated bone tips. The tumor surface was exposed, possibly without retraction, and the rare but possible dorsal displacement of FN was investigated with monopolar stimulation. A V-cut was performed on the dorsal surface of tumor with microscissors or handheld thulium laser, and debulking of tumor was obtained with microscissors, microcurettes, bipolar forceps, handheld laser fiber, and ultrasonic aspirator. The tumor was then dissected from brainstem and cranial nerves during continuous facial nerve monitoring. In some cases of strong tumor adherence to surrounding structures, a millimetric remnant of tumor capsule was left, thus yielding a near-total resection. In few cases with tight tumor capsule adherences to FN and/or brainstem or excessive bleeding of tumor, a ST or P removal was performed. Accurate hemostasis, mastoid cells obliteration, and tight dura closure by pericranial graft, hemostatic materials, and sealants were performed and a fitted titanium mesh—or the bone operculum—was placed on the craniectomy with miniscrews.

Intraoperative neurophysiological monitoring

All the surgical procedures were performed with a FN monitoring system (Nimbus i-Care 100 intraoperative neurophysiologic monitoring; Newmedic Division of Hemodia, Labège, France), with electrodes inserted in orbicularis oris and orbicularis oculi muscles, to detect FN responsivity. The nerve stimulation was performed with monopolar (on tumor surface) or bipolar (close to the nerve) probes to locate the FN and verify its functional state.

We always exposed distal and proximal FN with the feedback of IONM. The stimulation threshold was carefully monitored throughout the resection. If the threshold started to exceed 0,5 mAmp, especially in cases where no clear dissection plane, further tumor removal was not performed. The final threshold was recorded in every case.

Clinical follow-up

Clinical and radiological follow-up was scheduled at 6 months after operation and then once a year. Long-term FN outcome evaluation was performed at minimum 6 months after operation in 5 patients and after more than 1 year in 55 (final follow-up: January 31, 2020) and categorized according to House-Brackmann grades I–VI [18].

Statistical analysis

Statistical analysis was performed by means of chi-square test for categorical variables and Student’s t test for continuous ones, using the software MS Excel (Microsoft Corporation, Redmond, WA, USA). Statistical significance was set at p < 0,05.

Results

Characteristics of the patients and clinical presentation

Among the 60 cases of unilateral VS, 26 were males and 34 females. The mean age was 48,8 ± 15,7 years (range 21–80). The mean maximal tumor diameter was 3,97 cm (SD: ± 1,13; range 3,1–5,8 cm): the VS was large (> 3 cm of maximal diameter) in 25 patients (41,7%) and giant (> 4 cm) in 35 (58,3%). The preoperative mean ASA class was 2,08 (range 1–4). Fourteen tumors were cystic and 46 solid. Severely impaired hearing or deafness (AAO-NS classes C–D) was present in 52 cases (86,7%), ataxia in 18 (30,0%), trigeminal symptoms in 20 (33,3%), and facial palsy HB II to IV in 11 (18,3%). Preoperative patients’ data are summarized in Table 1. Follow-up period ranged between 6 and 113 months (mean 59,3 ± 13,7 months; median 61 months).

Table 1 Characteristics of the patients and clinical presentation

In the cisternal segment, FN was AS in 26 (43,3%) cases, A in 18 (30,0%), and AI in 16 (26,6%), in line with previously published data [28, 43].

Adhesions of tumor capsule, bleeding rate, and extent of resection

Tumor capsule was tightly adherent to nervous structures in 38 patients (63,3%), whereas hypervascular tumors represented 56,7% (34 patient) of cases. Among cystic VS, adherences of capsule to nervous structures were present in 85,7% (12 out of 14) of cases versus 56,5% (26 out of 46) of solid tumors (p < 0.001).

Total or NT resection was accomplished in 46 (76,7%) cases: 65,8% (25 patients) with and 95,4% [21] without tight adhesions of capsule to nervous structures (p < 0,001).

Endoscopic-assisted microsurgical removal of tumor in the IAC was performed on 23 patients: in these cases, a T resection was obtained in 78,3% (18 out of 23) versus 45,9% of those operated on with microsurgery alone (17 out of 37) (p < 0,001); furthermore, T-NT removal was possible in 100% (23 patients) of cases, versus 56,7% (21 out of 37 cases) (p < 0.001). These data are summarized in Table 2.

Table 2 Adhesions of tumor capsule, bleeding rate, and extent of resection

Long-term hearing and facial outcome

Hearing preservation was possible in 2 out of 8 patients with preoperative class B hearing: in both cases, the postoperative class of hearing level was unchanged.

FN anatomical and neurophysiological integrity was preserved in 54 cases (90,0%). In 11 cases, a preoperative HB II–IV FN deficit was present postoperatively. At the last follow-up (Table 3), 34 (56,7%) patients had a normal postoperative FN function (HBI), 9 (15,0%) HBII, 8 (13,3%) HBIII, and 9 (15,0%) HBIV.

Table 3 Preoperative and postoperative facial and cochlear nerve function

As expected, mean FN stimulation threshold at the end of surgery was inversely correlated with the extent of resection. In particular, T-NT/mean stimulation threshold was 0,24 mAmp; ST-P/mean stimulation threshold is 0,57 mAmp (p < 0,001).

Statistical analysis demonstrated a significant correlation between good FN outcomes (HB I–II) and the following conditions (Table 4):

  1. i)

    Solid tumors: 80,4% (37 out of 46) versus 42,9% of cystic (6 out of 14) (p < 0,01)

  2. ii)

    Presence of preoperative trigeminal symptoms: 80,0% (16 out of 20) versus 67,5% (27 out of 40) (p < 0,001)

  3. iii)

    Presence of preoperative ataxia: 77,8% (14 out of 18) versus 69,0% (29 out of 42) (p<0,001)

  4. iv)

    Absence of tight tumor capsule adhesions: 95,4% (21 out of 22) versus 57,9% (22 out of 38) (p < 0,001)

  5. v)

    Low-bleeding VS: 95,1% (25 out of 26) versus 59,2% (18 out of 34) (p < 0,001)

  6. vi)

    Endoscopic-assisted microsurgery: 86,9% (20 out of 23) versus 62,2% with microsurgery only (23 out of 37) (p < 0,001)

  7. vii)

    T-NT tumor removal: 80,4% (37 out of 46) versus 42,9% of ST-partial removal (6 out of 14) (p < 0,01)

Table 4 Long-term facial outcome

Other variables tested (age, sex, course and position of FN in the cisternal portion, large or giant size) had no significant correlations with the long-term FN outcome.

Postoperative complications and long-term follow-up

Mortality rate was zero. One patient had a permanent diplopia for abducens nerve paralysis. One patient developed triventricular hydrocephalus 11 days after surgery, successfully treated with ventriculo-peritoneal shunt.

Postoperative transient complications were observed in 9 patients, without correlation with preoperative ASA class: CSF leak followed by wound infection in 3 (successfully treated with antibiotics), transient diplopia in 2, cerebellar mutism, cerebellar infarction, dysphagia, and pneumonia (resolved with antibiotics) in 1 each.

At a mean follow-up of 59,3 months, a recurrence/re-growth of residue was observed in 8 (13,3%) cases operated on with ST or P resection (5 ASA class 2 and 3): in 2 of them, a second surgery was necessary for large cystic transformation, 2 and 4 years after the first operation, respectively. In the other 6 cases, the initial growth of tumor ceased at MRI controls is performed once a year.

Discussion

Grade IV VS represent a surgical challenge: a risk/benefit ratio must be for obtaining a satisfactory extent of resection against a good postoperative functional outcome, in order to find the right balance between preservation of functions and maximal tumor removal [52].

The rate of total tumor resection, as reported in the literature, ranges between 28,6% [64] and 95,5% [26] with a recent meta-analysis showing an overall T resection rate of 77% [52]. Two case series are reported, wherein total resection could be accomplished in all of the cases [45, 50]. However, the postoperative facial function preservation rate is quite different in the two studies, being 45% in Silva et al [50] and 75% in Samii et al. [45]. The preservation of FN functional state is of paramount importance in VS surgery and it is accomplished, as reported in the literature, in 32,9–83,3% of cases [16, 19, 26, 45, 62, 65], with a meta-analysis demonstrating FN preservation in 60% of cases [52]. The highest value of FN function preservation rate was reported by Zhang et al. [18] in a cohort of patient who underwent subtotal resection instead of radical extirpation.

Although hearing loss is one of the most common signs at presentation (41,7% of cases [1], as far as hearing is present preoperatively, attempts should be made—when possible—to accomplish hearing preservation [11, 52]. As far as large and giant VS are concerned, hearing has been reportedly preserved in 21,4–50% [11, 31, 60, 61] and 66,7% [45] of cases, respectively. Preoperative severely impaired hearing (AAO-HNS hearing class C) was present in 30 cases (50,0%) of our series and hearing preservation was achieved only in 2 out of 8 patients with preoperative AAO-HNS hearing class B (Table 3).

In our patients, the clinical presentation featured signs of hearing impairment, balance disturbances, preoperative FN deficit, and headache; facial paresthesias and other trigeminal symptoms were also noted by 33,3% of patients, which has been occasionally reported in the case of giant tumors [49, 50]. In particular, FN outcome seems to be better (HBI-II) in patients with preoperative trigeminal symptoms, 80,0% versus 20,0% (p < 0,001), and in those with preoperative ataxia, 77,8% versus 60,0% (p < 0,001). The reasons of the better outcome of FN in patients with preoperative trigeminal impairment and ataxia remain unclear.

A relatively large group of cystic tumors (14 out of 60 cases: 23,3%) was also observed among our grade IV VS. According to several authors [12, 15, 16, 30], cystic VS did not show signs of greater adherence intraoperatively nor did they significantly affect the postoperative outcome. However, in the present series, we observed in 12 out of 14 cystic cases (85,7%) tight adherences of capsule to nervous structures, in comparison to 56,5% (26 out of 46) of solid tumors (p < 0,001). In addition, a better FN outcome (HBI-II) was obtained in solid tumors: 80,4% versus 42,9% of cystic (p < 0,01).

Total or NT resection was feasible in 46 of our 60 cases (76,7%), which is in line with other reports in the literature [16, 19, 20, 46, 52]. In particular, T or NT resection was accomplished in 65,8% of cases with and 95,4% without tight adhesions of capsule to nervous structures (p < 0,001). In 23 patients of our series operated on with flexible endoscopic-assisted microsurgical removal of tumor in the IAC, a T resection was obtained in 78,3% (18 out of 23) versus 45,9% of cases with microsurgery only (p < 0,001), whereas, T-NT removal was possible in all the 23 patients operated on with endoscopic-assisted microsurgery versus 56,7% of those operated on with microsurgery only (21 out of 37 cases) (p < 0,001) (Table 2).

Large and bleeding VS: adherence of capsule to nervous structures

The large and vascularized VS represent a unique microsurgical challenge, especially those with the capsule adherent to nervous structures, namely, to brainstem and FN. Many authors reported incomplete surgical removal, with the need of close long-term follow-up for detecting possible recurrences.

According to Peris-Celda et al. [38], large tumors are significantly more frequent among younger patients at diagnosis suggesting a more aggressive tumor biology. In their study, the authors found that more than 17% of 1304 VS patients had a tumor size > 3 cm and more than 7% > 4. On comparing VS > 4 cm with the rest of the cohort, they observed a statistically significant difference in terms of mean age at diagnosis: 52,3 years for VS smaller than 4 cm versus 42,4 years for larger tumors (p < 0,001) [38].

Several mechanisms may underly the hypervascular and adherent nature of large and giant VS and may present targets for future therapy. For the enlargement of any solid tumor, including schwannomas, angiogenesis is essential: vascular endothelial growth factor (VEGF) and matrix metalloproteinases (MMPs) are considered to be potent mediators of tumor angiogenesis and growth pattern [4, 14, 39, 59]. Moller et al. [33] observed that tumor concentration of MMP-9 correlates with VS growth rate. Uesaka et al. [59] performed immunohistochemical studies for VEGF and VEGFR-1 mRNA on 36 VS and confirmed that these proteins, especially in recurrent tumors, are prominently expressed [59]. A relationship among vascularization, adhesions, and tumor size is quite reasonable, probably through MEK/ERK effectors, oncogenic gene miR-21, and mTOR pathways [63]. The adhesion to nerve structures seems to develop through the downstream effectors that induce the synthesis of metalloproteinase of extracellular matrix [33].

Carlson et al. [3] described the genetic alterations observed in a series of sporadic VS, in order to identify if more clinically aggressive variants possess different genetic alterations compared to the more indolent. To date, few studies attempted to profile genome-wide alterations in sporadic VS. Using high-throughput deep sequencing, “two-hit” alterations in the NF2 gene were identified in every tumor and were not present in peripheral blood supporting that all events were somatic [3]. The type of NF2 gene alteration and accessory mutations outside the NF2 locus may predict phenotypic expression and clinical course.

Surgical dissection for larger, vascularized, and adherent VS

The safe resection of VS depends on a thorough understanding of the microanatomy of these lesions. According to Roosli et al. [42], VS may arise anywhere along the course of the axons of cochleo-vestibular nerve from the glial-Schwann sheath junction up until their terminations within auditory and vestibular end organs. This variable origination may play a role in the relation of VS with arachnoidal plan: epiarachnoid or subarachnoid. Epiarachnoid tumors are defined by the absence of an arachnoid membrane on the tumor surface after moving the arachnoid fold (double layers of the arachnoid membrane) towards the brainstem. In contrast, subarachnoid VS are characterized by arachnoid membrane remaining on the tumor surface after moving the arachnoid fold. Kohno et al. [24] used intraoperative views and light and electron microscopy to confirm the existence of an arachnoid membrane after the arachnoid fold had been moved: they observed that the majority of VS are subarachnoid tumors, with epiarachnoid variant being considerably less common. Thus, according to Kohno et al. [24], there are 3 planes for possible tumor dissections: (A) subarachnoid; (B) subperineural (subcapsular); and (C) intracapsular.

According to Sasaki et al., [48] subperineural plane is the ideal cleavage plane for preservation of FN and cochlear nerve functions during VS microsurgery. Therefore, the layers we encounter starting from the surface of tumor are (1) arachnoid membrane, (2) FN and cochlear nerve, and (3) perineurium/nerve fibers of vestibular nerve of origin of VS [14, 33, 41], and bimanual dissection is an essential part of the technique [24, 29, 36, 44].

Functional FN results

Although great emphasis is currently placed on preserving nerve functions after VS resection, FN injury still represents a relatively common postsurgical complication. Even when careful FN dissection is performed, an anatomically intact nerve does not necessarily predict a normal facial function. Despite anatomical knowledge and experience, finding its proximal portion may be difficult especially in the case of large tumors. In our practice, we usually adopt the following steps: (1) intracapsular tumor debulking; (2) identification of FN and, in smaller tumors, cochlear nerve by means of intraoperative neurophysiological monitoring techniques; (3) nerve dissection from the tumor capsule trying to preserve the arachnoid plane; and (4) bimanual dissection of tumor capsule from FN and, in selected cases, cochlear nerve [24].

Analyzing the FN results in detail, we observed better outcome (HBI-II) in 43 (71,7%) cases, especially in the following conditions (Table 4): solid tumors (80,4% versus 42,9%, p < 0,01); presence of preoperative trigeminal symptoms (80,0% versus 20,0%, p < 0,001); presence of preoperative ataxia (77,8% versus 69,0%, p < 0,001); absence of tight tumor capsule adhesions (95,4% versus 57,9%, p < 0,001); low-bleeding tumor (95,1% versus 59,2%, p < 0,001); use of endoscopically assisted microsurgery (86,9% versus 62,2%, p < 0,001); and T-NT tumor removal (80,4% versus 42,9%, p < 0,01).

A recent study, Troude et al. [56] analyzed the predictive factors of short-term and long-term FN function after VS resection by either translabyrinthine or retrosigmoid approach (no inter-approach difference was studied). In the short-term, i.e., at discharge and at 1-month follow up, FN function was best anticipated by preoperative facial function, intraoperative cochlear preservation, and extrameatal tumor diameter. Such association had already been described by Nutik [34], whose review of 108 consecutive cases reported that anatomic preservation of FN was inversely related to tumor size and improved as the series progressed, thus stressing the importance of experience in VS microsurgery. When the nerve was anatomically preserved, satisfactory postoperative outcome was inversely related to tumor size. Chiluwal et al. [6] found that T1–T3a tumor extension (according to Hannover classification) was significantly associated with better functional outcome than T3b at 3 months follow-up. In the long term, according to Troude et al. [56], FN function is best anticipated by postoperative early FN outcome; in the same multivariate analysis, a previous surgery negatively affected facial outcome. Furthermore, in univariate analyses, additional predictive factors were identified as surgeon’s experience and tumor remnant volume. As the extent of resection did not seem to condition FN outcome per se, the authors concluded—and so do we—that maximal safe resection is the best operative strategy for patients harboring large VS.

Increasingly popular is the concept of the planned less-than-total resection performed for FN preservation [52]. According to some authors [22, 56, 66], outcome might be improved in selected cases by a combined surgical and radiosurgical treatment. Zumofen et al. [66] reported HBI-II postoperative rate of 89%, with no need for salvage surgery after Gamma Knife had been administered onto planned tumor residues. However, Iwai et al. [22] found that optimal the FN outcome (HBI-II postoperative rate of 95%) could be jeopardized by the need for salvage surgery after Gamma Knife in case of large tumor remnants (at least 6cm3). Such findings underline that radiosurgery is not an enemy of microsurgery [13, 56], even if surgical removal should be attempted with the objective of maximal safe tumor eradication.

Large tumor size often hinders safe and effective gross total resection [6, 57]. In the series of Tos et al. [53,54,55], HB I–II function was achieved in 97% of medium size, 87% of large, and 66% of giant tumors. Similar findings were reported by Ojemann [37] in his series of 410 patients, although the size categories were different: normal FN function was achieved in 98% of small, 96% of medium size (1–1,9 cm), 75% of medium large (2–2,9 cm), 56% of large (3–4 cm), and 56% of giant tumors (> 4 cm). In their cohort of 45 large VS, Hoshide et al. [16] reported a HBI-II rate of 83% after total or near-total resection in each of their cases. In a recent meta-analysis of large VN, a 60% HBI-II rate was reported [52]. Furthermore, a cystic consistency has been independently associated with poorer FN outcomes [13, 35]. Tumor size being comparable, cystic VS shows a tendency towards poorer early FN results [13]. Our results seem in line with such features, as T or NT resection was achieved in 82,9% and HBI-II was reported in 59% of cases in the whole series.

Complications

Mortality rate did not occur in our series, whereas permanent complications have been observed in 2 cases (abducens nerve paralysis and obstructive hydrocephalus). Nine patients had postoperative transient complications, without correlation with preoperative ASA class.

At a follow-up ranging from 6 to 113 (mean 59,3), a recurrence/re-growth of residue was observed in 8 (13,3%) cases operated on with ST or P resection. In 2 of them, a second surgery was necessary for large cystic transformation, whereas in other 6 cases, the initial growth of tumor ceased at yearly MRI controls.

Similar rates are in line with the literature [1719,20,21, 26, 45, 47, 57, 58, 61, 62] and confirm that the retrosigmoid approach is safe and feasible to remove even giant lesions [20, 26, 45, 47, 50, 57]. The translabyrinthine approach has been traditionally advocated for this kind of tumors, with arguably good results in terms of extent of resection (rates of total resection being approximately 90%), postoperative facial outcome (HBI-III close to 75%), and perioperative complications (CSF leaks present in nearly 2% of cases) [5, 10, 16, 27, 47]. On the other hand, perioperative complication rates as high as 14,3% have also been reported [16]. Even if translabyrinthine approach is a feasible alternative, the results of our series contribute to support the use of the retrosigmoid approach.

Surgical resection represents the ideal treatment for large and giant VS. It significantly and positively impacts on the patients’ quality of life [52, 58] and should be considered even in the case of elderly ones.

Conclusions

Searching for the right balance between preservation of functions and maximal tumor removal of Koos grade IV VS, from the analysis of the international literature and from the data of our personal limited experience, satisfactory results can be obtained adopting the following strategy:

  1. 1.

    T or NT resection should be the primary objective of surgery, especially in ASA class ≤ 2 patients, with encouraging FN outcome.

  2. 2.

    The judicious resection should be preferred in ASA class ≥ 3, in cystic, adherent, and bleeding tumors, especially if the capsule of VS is firmly adherent to FN and/or brainstem.

  3. 3.

    The use of endoscopy for removal of tumor in the IAC is associated with higher rate of T-NT removal of tumor and better FN outcome.

The acceptable rate of postoperative complications confirms that the retrosigmoid approach is safe and feasible to remove even large and giant VS.

Data availability

None.

Code availability

None.

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L.M.: Study design, study conception, data extraction, data analysis, and manuscript writing

A.C.: Data extraction, data analysis, and statistical analysis

F.B.: Data extraction, data analysis, and literature analysis

C.G.S.: Data extraction, data analysis, and radiological measurement

E.C.: Data extraction, data analysis, and radiological measurement

G.C.: Data extraction, data analysis, and literature analysis

R.R.: Data analysis, statistical analysis, and critical review of the manuscript

A.S.: Critical review of the manuscript and study supervision

A.Z.: Critical review of the manuscript and study supervision

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Correspondence to Luciano Mastronardi.

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Mastronardi, L., Campione, A., Boccacci, F. et al. Koos grade IV vestibular schwannomas: considerations on a consecutive series of 60 cases—searching for the balance between preservation of function and maximal tumor removal. Neurosurg Rev (2021). https://doi.org/10.1007/s10143-021-01501-9

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Keywords

  • Vestibular schwannoma
  • Adhesions
  • Cystic
  • Facial nerve
  • Microneurosurgery