Keywords
vestibular schwannoma - acoustic neuroma - translabyrinthine approach - retrosigmoid
approach - middle fossa approach - microsurgery
Introduction
Vestibular schwannomas are benign neoplasms of the vestibular nerve occurring in the
internal auditory canal (IAC) and cerebellopontine angle. Their incidence has been
estimated at 2 per 100,000 individuals.[1] Options for the management of vestibular schwannomas include observation, stereotactic
radiation, and microsurgical resection. While the average growth rate for vestibular
schwannomas is approximately 1.2 mm per year,[2] individual patient characteristics, including tumor growth pattern, degree of hearing
loss and vertigo, and patient age, guide optimal management.[3]
The three most common surgical approaches for the resection of vestibular schwannomas
are the translabyrinthine, middle fossa, and retrosigmoid approaches. While hearing
cannot be preserved through the translabyrinthine approach, the direct access to the
IAC and potential benefits in the preservation of facial nerve continuity makes it
a favorable proposition for tumors exceeding 3 cm and for smaller tumors with significantly
impaired hearing.[4] In addition, the translabyrinthine approach largely avoids cerebellar retraction.
The middle fossa and retrosigmoid approaches, on the other hand, carry advantages:
the former allows excellent exposure of the IAC for the resection of intracanalicular
tumors or tumors with limited cerebellopontine angle (CPA) involvement,[5]
[6] whereas the latter can provide a clear view of the facial nerve at the root entry
zone as well as the fundus and may be more suited for tumors with a larger CPA component.[7] Ultimately, the decision for surgical approach lies with surgeon preference.
Complications of surgery for vestibular schwannomas include facial nerve paresis or
paralysis, hearing loss, cerebrospinal fluid (CSF) leak, and tumor recurrence.[8] Management of recurrent tumors following microsurgical resection depends on symptomatology
and documented growth rate, but strong consideration is given to stereotactic radiation
and revision surgery.[9] Not surprisingly, revision surgery has been associated with increased complication
rates. In this study, we seek to describe our institution's experience with revision
surgery for vestibular schwannomas, with particular emphasis placed on the choice
of surgical approach and perioperative complications.
Materials and Methods
Patients
This study received Institutional Review Board approval at St. Vincent Medical Center,
Los Angeles, California, United States. An internal database of patients undergoing
surgery for vestibular schwannomas between January 1985 and June 2015 was reviewed.
Inclusion criteria were as follows: surgery performed on the same anatomical side
following a history of prior surgery for vestibular schwannoma with an original pathological
diagnosis of vestibular schwannoma, at our institution or at any other institution,
with or without a diagnosis of neurofibromatosis type 2 (NF2), with or without a history
of radiation therapy. Indication for surgery was a growing residual tumor in all cases.
Patients with NF2 who underwent one surgery on each side did not meet inclusion criteria
for revision surgery and were specifically excluded. Demographic and clinical data
were aggregated.
Tumor dimensions were obtained from cross-sectional imaging, and the largest dimension
from measurements in three planes—craniocaudal, anteroposterior, and transverse, including
any IAC component—was taken as the tumor size.
Factors behind the choice of surgical approach for the revision surgery varied. Patients
who had previously undergone a translabyrinthine approach usually underwent a second
translabyrinthine approach for the revision surgery. Similarly, patients with no serviceable
hearing following the first surgery often underwent a translabyrinthine approach for
the revision surgery. However, no strict criteria were used when choosing the specific
surgical approach, and the decision for approach was made on a case-by-case basis.
The decision for the extent of resection was generally taken intraoperatively; severe
adherence to the brainstem or the facial nerve or lack of a clean plane of dissection
led to the intraoperative decision for a partial resection. However, if the tumor
was documented at the time of previous surgery to be adherent to the brainstem or
the facial nerve, a partial resection was planned preoperatively.
Statistical Analysis
Mann–Whitney U and Kruskal–Wallis tests were employed to compare facial nerve outcomes
with patients stratified by various characteristics. The Dunn–Bonferroni test was
used as posthoc testing following any significant differences noted with the Kruskal–Wallis
test. Statistical analysis was performed with SPSS (IBM, Armonk, New York, United
States).
Results
In total, 231 unique patients, 102 males and 129 females, who underwent 250 revision
surgeries within the study period were identified, with a mean length to follow-up
of 30 days (range: 4 days to 1.75 years). The mean age was 43 years (range: 12–88
years; [Table 1]).
Table 1
Patient characteristics
|
Number
|
Percentage
|
|
Gender
|
103 males
|
44
|
|
131 females
|
56
|
|
Laterality
|
125 right
|
50
|
|
124 left
|
50
|
|
Location of prior surgery
|
Own institution: 36
|
14
|
|
Other institution: 197
|
86
|
|
Preoperative irradiation
|
22
|
10
|
|
Neurofibromatosis type 2
|
86
|
37
|
|
Surgical approach
|
Translabyrinthine: 217
|
87
|
|
Transcochlear: 14
|
6
|
|
Middle fossa: 13
|
5
|
|
Retrosigmoid: 6
|
2
|
|
Extent of resection
|
Gross total: 212
|
85
|
|
Subtotal: 33
|
13
|
|
Decompression only: 4
|
2
|
|
Intraoperative transfusion
|
30
|
14
|
|
Cerebrospinal fluid leak
|
21
|
8.4
|
|
Ventriculoperitoneal shunt
|
2
|
0.8
|
|
Mean
|
Range
|
|
Age (years)
|
42.9
|
12–88
|
|
Length to follow-up
|
30 d
|
4 d to 1.8 y
|
|
Number of prior surgeries
|
1.26
|
1–4
|
|
Size of tumor at surgery (cm)
|
2.6
|
0.5–6.9
|
|
Operating time (hours)
|
4.6
|
1–12
|
|
Estimated blood loss (mL)
|
342
|
40–3,000
|
Note: Reported percentages are calculated using number of patients for whom data were
available.
The mean number of prior surgeries was 1.26 (range: 1–4). Thirty-six surgeries followed
a prior resection at our institution; 197 followed surgeries performed at other institutions.
Twenty-two (10%) patients had undergone stereotactic radiation or other radiotherapy
before the present surgery. Of the 15 patients who underwent more than one revision
surgery at our institution, 9 carried a diagnosis of NF2 whereas 6 did not.
Surgical approaches included the translabyrinthine approach (n = 217, 87%), the transcochlear extension of the translabyrinthine approach (n = 14, 6%), the middle fossa approach (n = 13, 5%), and the retrosigmoid approach (n = 6, 2%). Gross total resection was achieved in 212 surgeries (85%); preoperative
planned partial resection was performed in 11 surgeries (4%), whereas an intraoperative
decision for partial resection was made in 22 surgeries (9%).
Surgeries lasted an average of 4.6 hours (range: 1–12 hours). Mean tumor size at the
time of surgery as 2.6 cm (range: 0.5–6.9 cm). Mean surgical time was 4.6 hours (range:
1–12 hours). Estimated blood loss, on average, was 342 mL (range: 40–3,000 mL). Intraoperative
transfusion was performed during 30 surgeries (14%), with a mean transfusion of 1.4
units.
Facial nerve function was analyzed separately for patients with and without a diagnosis
of NF2. For non-NF2 patients, the mean preoperative facial nerve function, as graded
on the House–Brackmann scale,[10] was 2.7 (range: 1–6). The mean immediate postoperative facial nerve function (defined
as facial nerve function within the first 24 hours following surgery) was 3.2 (range:
1–6), and the mean facial nerve function at last follow-up was 3.8 (range: 1–6). For
patients with NF2, the mean preoperative facial nerve function was 2.7 (range: 1–6).
The mean immediate postoperative facial nerve function was 3.8 (range: 1–6), and the
mean facial nerve function at last follow-up was 3.9 (range: 1–6; [Fig. 1] and [Table 2]). When comparing facial nerve function between non-NF2 and NF2 groups, there was
no significant difference when comparing preoperative values (p = 0.694; Mann–Whitney U), immediate postoperative values (p = 0.094; Mann–Whitney U), and values at last follow-up (p = 0.625; Mann–Whitney U).
Table 2
Facial nerve function for all patients preoperatively, immediate postoperatively,
and at last follow-up (mean duration, 30 days), as graded on the House–Brackmann scale
|
Preoperative (%)
|
Immediate postoperative (%)
|
Last follow-up (%)
|
|
I
|
108 (43%)
|
63 (30%)
|
52 (27%)
|
|
II
|
29 (12%)
|
23 (11%)
|
14 (7%)
|
|
III
|
36 (15%)
|
30 (14%)
|
24 (12%)
|
|
IV
|
25 (10%)
|
17 (8%)
|
12 (6%)
|
|
V
|
9 (4%)
|
17 (8%)
|
22 (11%)
|
|
VI
|
40 (16%)
|
60 (29%)
|
69 (36%)
|
Fig. 1 Distribution of facial nerve function (A) preoperatively, (B) immediate postoperatively, and (C) at last follow-up (mean duration, 30 days), as graded on the House–Brackmann scale.
Facial nerve function was further analyzed separately for patients who had a history
of radiation therapy preceding the revision surgery and those who did not. There was
no significant difference between radiation and nonradiation groups with respect to
preoperative facial nerve function (Mann–Whitney U; p = 0.075) as well as postoperative facial nerve function (p = 0.148).
Preoperative facial nerve function did not depend on the extent of resection (p = 0.195; Kruskal–Wallis), but postoperative facial nerve function was significantly
different among the extent of resection (p = 0.016). However, posthoc testing (Dunn–Bonferroni) revealed no significant pairwise
differences between groups when comparing the extent of resection (p > 0.05 for all).
Preoperative facial function was significantly different among different surgical
approaches (p = 0.000; Kruskal–Wallis). Pairwise comparisons revealed that patients undergoing
the transcochlear approach had worse preoperative facial nerve function than patients
undergoing the middle fossa, retrosigmoid, and translabyrinthine approaches (p = 0.000 for all; Dunn–Bonferroni). Postoperative facial function was also significantly
different among different surgical approaches (p = 0.000, Kruskal–Wallis). Pairwise comparisons revealed that patients undergoing
the transcochlear approach had worse postoperative facial nerve function than patients
undergoing middle fossa and retrosigmoid approaches (p = 0.000 and p = 0.015, respectively, Dunn-Bonferroni), but there was no significant difference
between translabyrinthine and transcochlear approaches (p = 0.224; Dunn–Bonferroni).
Twenty-one CSF leaks were encountered (8%); of these, six cases required reoperation.
Two (1%) patients required eventual placement of a ventriculoperitoneal shunt. No
perioperative mortality was noted.
Pathological analysis of the resection specimen confirmed a diagnosis of vestibular
schwannoma in 233 cases (93%); six specimens from revision surgeries were reported
as meningiomas (2.4%), whereas pathology was unavailable in 11 cases.
Discussion
While gross total resection is often performed in primary microsurgery for vestibular
schwannomas, near-total and subtotal resections may be electively performed if involvement
of adjacent structures, such as the facial nerve, is present. While the choice of
surgical approach may be correlated with the extent of tumor removal,[11]
[12] the extent of resection has been shown to be unrelated to tumor recurrence, which
has been estimated at approximately 9% following primary microsurgery.[12] Revision surgery is generally indicated in patients with growing residual or recurrent
disease, and planar measurements have been shown to be adequate in trending growth
of residual or recurrent tumors.[13]
Previous series on revision surgeries for vestibular schwannomas emphasize the relatively
higher rate of complications, including CSF leaks, new cranial nerve deficits, cerebrovascular
accidents, and hematomas.[14] Patients with residual or recurrent disease after a prior retrosigmoid approach
may be successfully treated with a translabyrinthine or transcochlear approach.[11]
In this descriptive series, a majority of patients (87%) underwent revision surgery
through the translabyrinthine approach; a transcochlear approach was employed in an
additional 6% of patients. In our experience, the translabyrinthine approach allows
for excellent exposure and minimizes cerebellar retraction, and the transcochlear
approach, often performed in a canal-wall-down fashion with blind sac closure of the
external auditory canal, can provide additional access, particularly to the anterior
extent of the tumor.
Unsurprisingly, preoperative and postoperative facial nerve functions were generally
poorer than those encountered with primary surgeries, including when compared with
a series of translabyrinthine surgeries performed at our own institution.[15] Non-NF2 and NF2 patients did not differ significantly with respect to preoperative
facial function, immediate postoperative facial function, or facial function at last
follow-up. However, the mean length to follow-up was 30 days, with significant variation,
and long-term facial nerve outcomes in this population are difficult to infer.
In this series, the CSF leak rate was 8%. This is comparable to, or even slightly
exceeds, published CSF leak rates for all vestibular schwannoma surgeries, which have
been reported to range from 0 to 17%,[16]
[17]
[18]
[19]
[20] with most series reporting leak rates of approximately 5 to 6%.[15]
[21]
[22] In our practice, both nasal and incisional CSF leaks following translabyrinthine
or transcochlear craniotomy are treated with revision craniotomy, replacement of fat
graft packing, and blind sac closure of the external auditory canal. For CSF leaks
following middle fossa or retrosigmoid approach, a lumbar drain is the first line
of therapy, and ventriculoperitoneal shunts are reserved for patients demonstrating
dependence on a lumbar drain to abate the CSF leak.
A significant limitation of this study was the relatively short duration of follow-up
for several patients, many of whom were evaluated in person only immediately preceding
and several weeks following surgery. Long-term follow-up, particularly of facial nerve
function, was therefore not available for many patients, and postoperative facial
nerve function, as noted here, may underestimate the rates of recovery of function.
Furthermore, neurologic complications and hearing status were not comprehensively
documented.
Conclusion
Revision surgery for vestibular schwannomas, where indicated, is feasible and safe,
and complication rates, including CSF leak rates, approximate those encountered with
primary surgery. Our preferred approach is the translabyrinthine craniotomy, which
can be expanded to include the transcochlear approach for greater exposure.
