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Auditory and vestibular disorders
Korean Journal of Audiology 2011;15(2):85-89.
Retrosigmoid Approach in the Removal of Vestibular Schwannoma.
Bong Jin Park, Young Jin Lim, Cheol Eon Park, So Yoon Lee, Seung Geun Yeo
1Department of Neurosurgery, School of Medicine, KyungHee University, Seoul, Korea.
2Department of Otorhinolaryngology, School of Medicine, KyungHee University, Seoul, Korea. yeo2park@yahoo.co.kr
Abstract
BACKGROUND AND OBJECTIVES
The use of several approaches, involving different cerebellopontine angles, has enabled vestibular schwannoma removal to be tailored to each patient's pathology and physiological status. The retrosigmoid approach provides simple and direct access to cerebello-pontine angle lesions. SUBJECTS AND METHODS: We retrospectively assessed outcomes in 35 consecutive patients who underwent vestibular schwannoma removal via the retrosigmoid approach.
RESULTS
Of the 35 patients, 12 were men and 23 women; their age was 52.5+/-10.4 years (range, 35-75 years). One tumor was small (< or =1 cm), 18 were medium (1-3 cm), and 16 (45.7%) were large (>3 cm). Symptoms included hearing disturbance (31 patients, 89%), tinnitus (14 patients, 40%), headache (12 patients, 34%), vertigo (11 patients, 31%), and facial palsy (9 patients, 25%). Postoperative complications included facial palsy, intracranial hemorrhage, dysphagia, and disseminated intravascular coagulopathy, with facial palsy remaining permanently. Four patients (11.4%) had tumor regrowth, at a mean of 36.3 months after primary surgery. The mean diameter of regrowing tumors was 20.5+/-4.4 mm (range 14.5-25.0 mm).
CONCLUSIONS
The retrosigmoid approach for vestibular schwannoma removal was associated with higher rates of facial palsy and hearing loss. This approach, however, can minimize injury to the lower cranial nerve.
Keywords: Vestibular schwannoma;Retrosigmoid approach

Address for correspondence : Seung Geun Yeo, MD, Department of Otorhinolaryngology, School of Medicine, KyungHee University, 1 Hoegi-dong, Dongdaemun-gu, Seoul 130-702, Korea 
Tel : +82-2-958-8474, Fax : +82-2-958-8470, E-mail : yeo2park@yahoo.co.kr

Introduction


Vestibular schwannoma is a benign tumor of the vestibular nerve that develops in the internal acoustic meatus. As the tumor increases in size to the cerebellopontine angle (CPA), it compresses the cochlear, vestibular, and facial nerves, as well as the cerebellum and brain stem, giving rise to various symptoms.1) Treatments for vestibular schwannoma include conventional surgery and gamma knife radiosurgery, depending on the size of the tumor, location and growth rate, and each patient's hearing level, age, and general health condition. Surgical approaches can include the middle cranial fossa, translabyrinthine, and retrosigmoid approaches, as well as concomitant surgery. The goals of surgery or radiosurgery include the preservation of hearing and the functions of the facial nerves, as well as complete tumor resection.2)
Of the surgical approaches utilized, the retrosigmoid approach has several advantages. For example, neurosurgeons are familiar with the anatomical structures involved. Moreover, surgery via this approach can be easily performed in patients with large tumors and in patients with concurrent hydrocephalus. Moreover, damage to adjacent structures can be minimized because this approach allows all anatomical structures to be viewed. The approach provides simple and direct access to CPA lesions, with surgical exposure extending from the trigeminal nerve and tentorium superiorly to the foramen magnum and inferiorly to the jugular foramen.2) Thus, compared with other approaches, the retrosigmoid technique provides a better view of the CPA and allows early identification of neurovascular structures, more complete removal of lesions, less morbidity, and avoids the risks of major neurological complications. We therefore assessed the surgical outcomes of patients who underwent vestibular schwannoma surgery using the retrosigmoid approach.

Subjects and Methods

We retrospectively evaluated outcomes in 35 consecutive patients who underwent vestibular schwannoma removal using the retrosigmoid approach between 2000 and 2009 at the KHMC, Korea. Complete follow-up data were available for all patients, including regular imaging through 1 July 2009.
Among the clinical parameters evaluated were symptoms at initial presentation, pre- and postoperative pure-tone audiometry results, tumor size on MRI scans, and pre- and postoperative assessments of facial nerve function. Tumor size was expressed as the magnitude of the longest diameter after measuring both the longest and shortest diameters. Tumors were classified as small (≤1 cm), medium (1-3 cm), and large (>3 cm). Hearing ability was assessed as the average of measurements at 500 Hz, 1,000 Hz, 2,000 Hz and 4,000 Hz on pure-tone audiometry. Facial nerve functions were described using House-Brackmann (HB) grades, and classified as normal (HB I), mild (HB II), moderate (HB III-IV), and severe (HB V-VI).

Results

Analysis of tumor size, gender and directions (Table 1
Of our 35 patients, 12 (34.3%) were men and 23 (65.7%) women; their mean age was 53 years (range, 35-75 years). Tumors were on the right side in 19 patients (54.3%) and on the left side in 16 (45.7%). One tumor (2.9%) was classified as small, 18 (51.4%) as medium, and 16 (45.7%) as large. The smallest tumor was 7 mm and the largest 50 mm in diameter.

Major symptoms (Table 2)
At the time of admission, 31 patients (88.6%) complained of hearing disturbance, 14 (40.0%) of tinnitus, 12 (34.3%) of headache, 11 (31.4%) of dizziness, and 3 (8.6 %) of facial palsy. Five patients complained of decreased hearing ability only, whereas 30 complained of two or more symptoms. 

Pure-tone audiometry (Table 3)
Of the 35 patients, 3 (8.6%) had normal hearing; 3 (8.6%) had hearing loss of 26-40 dB on pure-tone audiometry; 6 (17.1%) had hearing loss of 41-55 dB; 6 (17.1%) had hearing loss of 56-70 dB; 2 (5.7%) had hearing loss of 71-90 dB; and 15 (42.9%) had hearing loss of >90 dB, including 10 of the 16 patients (62.5%) with large (>3 cm) tumors.

Pre- and postoperative facial nerve functions (Table 4)
Preoperative facial nerve function was HB Grade I in 17 patients (48.6%), Grade II in 9 (25.7%), Grade III-IV in 6 (17.1%), and Grade V-VI in 3 (8.6%). Postoperative facial nerve function was Grade I in 11 patients (31.4%), Grade II in 8 (22.9%), Grade III-IV in 10 (28.6%), and Grade V-VI in 6 (17.1%). Two patients had complete facial palsy (H-B grade VI) preoperatively, which persisted postoperatively.

Postoperative complications (Table 5)
Postoperative complications included aggravated or newly developed facial palsy in 10 patients (28.6%), intracranial hemorrhage in 3 (8.6%), dysphagia in 2 (5.7%), and disseminated intravascular coagulopathy (DIC) in 1 (2.9%). In six patients, facial palsy was permanent. DIC developed during surgery, involving extended craniectomy with cerebellar lobectomy, and extra ventricular drainage insertion was performed because of intracranial hemorrhage (ICH) and massive brain swelling. The two patients with postoperative dysphagia completely recovered 3 and 5 months, respectively, after surgery. Of the other two patients who developed ICH, one underwent hematoma evacuation and the other recovered after conservative management. 

Postoperative regrowth
Tumor regrowth was observed in four patients (11.4 %), at a mean interval of 36.3 months after primary surgery. The mean diameter of recurrent tumors was 20.5±4.4 mm (range 14.5-25.0 mm). 

Discussion

Vestibular schwannomas account for 6-8% of intracranial tumors and approximately 80% of CPA tumors.3) Such tumors are managed by observation, stereotactic radiosurgery, or microsurgical resection, with the latter performed using retrosigmoid, translabyrinthine and middle fossa approaches.1)
Gamma knife radiosurgery has been indicated for the removal of tumors of maximum diameter 3 cm. Clinically, however, this method shows a good response when used to remove tumors of maximum diameter 2 cm, with a tumor control rate of approximately 90%, but cystic degeneration may occur following gamma knife radiosurgery. Surgery may be considered for tumors inadequately controlled by the gamma knife. A middle cranial fossa approach can be used to remove tumors restricted to the internal auditory canal, especially to preserve hearing ability. An anatomical approach is technically difficult, however, and carries a risk of development of facial palsy. Although the translabyrinthine approach can also be used, it has several disadvantages, especially for removing large-sized tumors; moreover, most neurosurgeons are unfamiliar with the approach and it is thus associated with prolonged operation time. Prior to surgery, the location of the facial nerves is ascertained, thus reducing the risk of developing surgery-associated facial palsy. The translabyrinthine approach has been recommended, regardless of tumor size, for patients with poor audiological prognosis, based on a threshold of puretone audiometry >50 dB and speech discrimination <50%.
A retrosigmoid surgical approach can be used when neurosurgeons are familiar with the anatomical structures, both for removal of large-sized tumors and in patients with vestibular schwannomas accompanied by hydrocephalus. Because this approach enables the neurosurgeon to perform the operation while viewing all relevant anatomical structures, the technique can minimize damage to adjacent tissues. However, as the location of the facial nerves can be confirmed only when tumors are almost completely removed, this approach is associated with a higher risk of damage to facial nerves. Moreover, the retrosigmoid approach, when performed in association with CSF drainage, increases the postoperative incidence of headache. The approach is used in patients with a threshold of pure-tone audiometry <50 dB and with speech discrimination >50%, as well as to remove more internally located tumors and tumors that did not invade the internal area of the basal part of the internal acoustic meatus.
Surgical approaches vary depending on tumor size, preoperative hearing status, and the individual surgeon's experience and preference. Our use of a retrosigmoid approach was based on several considerations, including better post-operative facial nerve function, avoidance of the risk of major neurological complications, and patient hearing level. 
When we reviewed the major symptoms in our patient cohort, we found that hearing loss was the most common, followed by tinnitus. Our findings are in good agreement with previous results, showing that 94% of patients with vestibular schwannoma complained of hearing disturbance. In contrast, other studies have reported that the major symptom of vestibular schwannoma is tinnitus, followed by sudden hearing loss,4) or that smaller-sized tumors were accompanied most frequently by dizziness.5)
Surgery performed via the retrosigmoid approach has been found to be advantageous in preserving hearing ability in patients with small-sized tumors. In the present study, however, abnormal findings were observed in patients with large tumors, 91.5% of whom had hearing disturbances prior to surgery. We also found that hearing loss was more severe in patients with large than with medium-sized tumors.
Typically, patients are placed in a lateral or sitting/semisitting position for surgery. Over the past 10 years, our institution has placed patients in the lateral position for vestibular schwannoma removal via the retrosigmoid approach. In contrast, many surgeons in Western countries have favored the use of a sitting/semisitting position for this type of surgery.6,7,8,9,10) The semisitting position, however, has been associated with a higher risk of air embolism, despite anesthetic monitoring to prevent this complication.11,12) Moreover, a high incidence of hematoma has been observed following the retrosigmoid removal of cystic tumors with patients in the semisitting position,7) whereas no hematomas occurred when patients were in the lateral position.13) The semisitting position has, however, been found appropriate for the physique of Caucasian patients with long arms. The semisitting position has been associated with reduced intracranial venous pressure, which shrinks the peritumoral vein and prevents troublesome intraoperative bleeding. Thus, this vein may be overlooked unless a postoperative hematoma develops. In the lateral position, peritumoral veins may cause intraoperative bleeding, requiring hemostasis.
We found that, overall, postoperative functions of the facial nerves were poor because preoperative functions were unsatisfactory. We found that only 73% of patients (19 of 26) with good preoperative facial nerve function showed preservation of function after surgery. Thus, 27% of our patients developed postoperative facial palsy, a higher percentage than previously observed.2) These patients were treated with high-dose dexamethasone and physiotherapy, which restored facial nerve function in four patients after 1-6 months. It is not clear whether recovery from paralysis was attributable to steroids or to the natural history of the condition. In the remaining six patients, however, facial paresis was permanent.
Removal of large-sized tumors via a translabyrinthine14,15,16,17) or retrosigmoid7,8,19) approach preserved facial nerve function in 70-80% of patients, with 42-52.6% showing excellent function.2,14,15,16,17) The degree of recovery from facial palsy has been defined as excellent (H-B grade I/II) or intermediate (H-B grade III/IV). Preservation of facial nerve function requires intraoperative monitoring using a facial nerve stimulator-monitor.20)
Among the postoperative complications of the retrosigmoid approach when used to remove vestibular schwannomas are death, hemorrhage in the CPA and cerebellum, brain stem injury, post-operative hydrocephalus, CSF leakage, meningitis, infarcted cerebellum, dysphagia/dysarthria, facial nerve transaction, cerebellar edema, pulmonary embolus, intra-operative cardiac arrest, trigeminal dysfunction, abducens dysfunction, diplopia, urinary tract infection, bronchial infection, and diabetic dysregulation. However, we found that only one patient had DIC. Although the exact etiology of this condition remains unclear, it has been reported to occur when the petrosal vein was removed in areas adjacent to the tumor. Although we attempted to preserve the petrosal vein in our patient, this was not possible because the tumor could not be removed without excising the vein.
Tumor regrowth rate after the total removal of vestibular schwannoma has been found to range from 0.4-2%.21,22) In patients with large tumors, the regrowth rate is reported to be higher because of partial resection. Using regular long-term follow up with MRI, we found that the tumor regrowth rate was higher than in previous studies, perhaps because we attempted to preserve the facial nerves, hearing ability, and major intracranial structures. To reduce the regrowth rate, however, it is necessary to completely remove the tumor. 

Conclusion

We assessed the results of surgery using a retrosigmoid approach in 35 patients with acoustic nerve tumors, 34>1 cm in size, with the major symptom being hearing loss. Some patients developed irreversible facial palsy, whereas 11% showed tumor regrowth. As neurosurgeons are relatively familiar with the retrosigmoid approach for the surgical removal of vestibular schwannoma, use of this technique can minimize injury to the lower cranial nerves and can enable suitable manipulation in unexpected situations. We found, however, that this approach was associated with higher rates of facial palsy and hearing loss. All these factors should be considered when planning surgical removal of vestibular schwannomas. 


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