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Microsurgery of Vestibular Schwannoma Post-radiosurgery
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.271243
Keywords: Gamma knife radiosurgery, microsurgery, salvage surgery, stereotactic radiosurgery, vestibular schwannoma
Microsurgery for vestibular schwannoma (VS) has progressed from total excision of the tumour at any cost to the preservation of the nerve function as the primary goal.[1],[2],[3] Stereotactic radiosurgery (SRS) has become an exciting option for small- and medium-sized tumours. Multiple modalities such as Gamma Knife, CyberKnife, X-knife, Novalis, or proton beam therapy are available.[4] Gamma Knife Radiosurgery (GKRS) is part of a standard treatment regimen for VS at multiple neurosurgical units worldwide.[5],[6],[7],[8] GKRS achieves excellent control of the tumour in more than 90% of patients. Therefore, microsurgery for the recurrent tumour post-GKRS is uncommon, and constitutes <5%.[5],[9],[10],[11] However, as and when it needs to be performed, it poses unique challenges to the operating microsurgeon due to structural changes and adhesions with the surrounding structures.[2],[12],[13],[14] As a neurosurgical team which specializes in the management of VS and performs both microsurgery and GKRS, the authors analyze the technical challenges and neurological outcomes associated with microsurgery of recurrent VS following radiosurgery.
A retrospective analysis was performed of the patients who underwent microsurgery at a single institution for recurrent VS post radiosurgery or a radiotherapy procedure between 1997 and 2017. The preoperative data, intraoperative findings, and post-operative outcomes were analyzed. House-Brackmann grading was used to evaluate the preoperative and post-operative facial nerve function.[15] The study comprised 16 patients with VS. [Table 1] There were eight males and eight females, age ranged from 33 to 81 years, with an average age of 52.6 years. Seven of these patients had tumour on the right side, while eight had on the left. One patient had bilateral VS, of which the right side was operated. Four patients had giant tumours (>4 cm), while eleven had large tumours (2.5-4 cm). One patient had a size of 2.3 cm.
Three patients had been operated once first, followed by radiosurgery [Figure 1]. There was one patient who had undergone two surgeries before receiving radiosurgery. Twelve patients had only radiosurgery before the current microsurgery. Of these, one had received Cyber knife therapy, and one had X-knife elsewhere. Nine patients had previous radiosurgery in our institute and seven had elsewhere. The median prescribed dose for Gamma Knife was 13 Gy at 50% isodose. The median interval between radiosurgery and surgical resection was 37 months (range of 3-96 months). The smallest tumour operated had a maximum dimension of 2.3 cm, while the largest was 5.1 cm, with an average diameter of 3.4 cm.
The approach for all the patients was the standard retrosigmoid suboccipital craniotomy under the cover of facial nerve monitoring. Tumour was decompressed internally as much as feasible, decompressing the cerebellum and the brainstem. The internal auditory canal was drilled to about 6-7 mm. Near total, subtotal and partial excision was defined as more than 95%, 90%-95% and <90%. The average follow-up period after surgical resection was 50 months. Follow-up magnetic resonance imaging (MRI) was performed at 3-month post-operative and then every year. Indications of surgery were tumour enlargement with stable symptoms in ten patients and tumour enlargement with new or increased symptoms in six patients.
Pre-operative function All the patients in the series had a non-serviceable hearing at the time of surgery. The symptoms included facial numbness, recurrence or onset of appendicular cerebellar signs, or involvement of lower cranial nerves. The status of the facial nerve before the current surgery was House-Brackmann (HB) grade I/II in 13 patients, grade III in one and grade IV/V in two. Operative findings The tumours found at surgery were firmer, with an avascular core but increased vascularity near the surface. The tumour-arachnoid plane was indistinct with thickened arachnoid and often adhesions with surrounding structures making total excision difficult in 12 patients. Interestingly, a moderately good plane of dissection was observed in four. There were fibrous changes of the tumour mass, cyst formation and brownish-yellow or purple discoloration of the tumour capsule. Severe adhesions between the tumour capsule and cranial nerves, vessels, and the brainstem were observed in eight. Twelve patients underwent gross total or near-total resection. Functional outcome Facial function Post-surgery, nine patients had grade I/II HB facial function, four had grades III, and three had grades IV/V. Of the latter three, two had the same deficit preoperatively. There was no associated mortality or morbidity. There was no significant difference in facial nerve preservation between those with failed radiosurgery, and those with surgery plus failed radiosurgery. At the last follow-up (average 50 months), all patients had a stable facial function. Hearing function: None of the patients in our series had a serviceable hearing at the time of salvage surgery. Tumor control At the follow-up post-salvage surgery, 12 patients had no enhancing lesion on the magnetic resonance imaging (MRI), the residual tumour was sub-centimeter in two, while in another two it was about a centimetre (in one dimension), unchanged over the follow-up period. None of the patients have undergone any other radiosurgery/microsurgery since the last intervention.
VS are benign tumours with a slow growth rate. The patients were commonly present with the classic triad of sensorineural hearing loss, tinnitus and vertigo. Some patients may present with trigeminal symptoms of numbness and lower cranial nerve symptoms.[16] The rate of recurrence after complete excision is 0.5%-9.2%, and increases to about 44% in incomplete removal after a mean follow-up of about 6 years.[16],[17],[18] SRS is an accepted mode of management for small tumours and residual/recurrent tumours following microsurgery with excellent tumour control. Microsurgery and radiosurgery are the treatment options for the rare tumours that tend to grow following radiosurgery.[16],[19] Microsurgery following radiosurgery Indications VS tend to expand in size for variable intervals post-radiosurgery. The duration between the radiosurgery and the salvage surgery thus becomes important. Although more than half of these tumours decrease in the first year, about one-tenth stay large even after 5 years of surgery. The indications of microsurgery in these patients are clinical worsening and cyst formation, both of which tend to aggravate the symptoms.[12],[13],[20],[21],[22] Challenges during microsurgery[21],[22],[23],[24],[25] The biggest challenge for a neurosurgeon while operating on VS, post-radiosurgery, is the fibrosis and adhesions with the surrounding structures, especially with the brainstem and the facial nerve. In delayed surgery, post-radiosurgery, the arachnoid is thickened, and there is a pial breach in the brainstem. The best outcome is therefore achieved by not attempting to remove the last vestige of the tumour, especially when adhesions with critical structures are encountered. In a few patients, however, no adhesions have been reported and there was good plane between the tumour.[13],[14],[21],[22],[23] Pollock et al. operated on 13 patients and took surgeons' opinions about the difficulty level they faced while operating on tumours post-surgery and GKRS. It was found to be difficult in eight patients, similar in four and easy in one.[21] Most surgeons prefer a retrosigmoid approach for primary removal of a VS. In a recurrent or salvage surgery, a different approach like translabyrinthine approach is used by some surgeons to avoid adhesions, scar tissue, and altered surrounding anatomy. The authors of this paper, however, prefer the retrosigmoid approach for both the primary as well as salvage surgery. Facial nerve function In our series, it was found that the preoperative facial function was one of the critical parameters in predicting the post-operative facial nerve outcome. Nine patients had some worsening of the facial function: five had complete excision and four had a small residue. Eight patients in our series had tumours with significant amount of adhesions with the facial nerve, four of which showed a worsening of the facial function in spite of leaving a small residue behind. The hearing was affected in all the patients in our series. Most of the series also consisted of patients with a non-serviceable hearing at the time of salvage surgery. Iwai et al. reported that the facial nerve dysfunction and trigeminal neuropathy recovered in some patients post-salvage surgery, and attributed it to the compression rather than the radiation.[22] Also, it has been reported that the facial nerve outcome is poorer in patients who have undergone microsurgery prior to radiosurgery as compared with those who have undergone only radiosurgery.[13] Most of the series show a moderate degree of facial nerve preservation after salvage surgery. An attempt to achieve gross total removal is associated with a poorer outcome while a conservative approach preserves the facial function better.[2],[12],[13],[14],[19],[20],[22] Iwai et al. reported a facial nerve preservation rate of 88% with deterioration of nerve function in two of the 17 patients with one patient developing facial spasm.[22] Pollock et al. reported a good tumour excision in 11 of their 13 patients with a facial nerve HB function of I/II in three patients and III/IV in another three. Seven patients had grade VI HB function.[21] Samii et al. reported their results with 53 patients who were operated for unilateral VSs following previous intervention. Seventeen of these had received pre-GKRS microsurgery, while the rest had not received any radiosurgery. These were in turn compared to a control group of 30 patients who were not reoperated. They concluded that tumour size and preoperative facial function were good indicators of the post-operative outcome.[13] A comparative data of the studies discussed in the article with respect to the number of involved patients; the facial nerve preservation rates and the extent of tumour resection have been shown in [Table 2].
Patients with VS who fail SRS with either tumour progression or worsening of clinical symptoms will occasionally need microsurgery. At operation, the tumours have a vascular capsule, avascular core and an increased rate of adhesions to the neurovascular structures making the salvage surgery of radiation-failed tumours more difficult. However, adequate tumour resection with an acceptable outcome can be achieved with proper technique and neuromonitoring. Leaving a tiny residue attached to critical structures is the key to optimal outcome. Highlights
Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.
[Figure 1]
[Table 1], [Table 2]
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