Background: The membranous structure of vestibular schwannoma is an important factor in its surgical treatment. Herein, we report intraoperative and microscopic fndings relating to an outermost "dura-like membrane" in cases of vestibular schwannoma and the importance of these fndings. Methods: Intraoperative fndings of 16 cases of vestibular schwannoma treated with an initial surgery were studied with an aim to determine if the cases had a dura-like membrane. Then we studied microscopic fndings of the dura-like membrane using hematoxylin and eosin, Masson trichrome, and immunohistochemical staining in 2 cases. Results: The dura-like membrane was observed in 8 out of 16 cases. The average tumor size of the cases that had a dura-like membrane was 30 ± 8.1 mm, and Koos grading 4 was in 7 out of 8 cases, and one was grade 3. In cases without a dura-like membrane, these values were signifcantly smaller, with an average tumor size of 12.8 ± 5.2 mm, and Koos grading 4 was only in 1 of 8 cases, grade 3 was in 2 cases, and other 5 cases were grade 2. The outermost dura-like membrane enveloped the vestibular schwannoma around the internal acoustic meatus and was continuous with the dura mater. Reactive angiogenesis was observed in the dura mater. Microscopic fndings proved its continuity with the dura mater. In one case, the facial nerve was damaged before it was identifed during subcapsular dissection. In that case, the dura-like membrane negatively affected our ability to identify the facial nerve. Conclusions: A dura-like membrane sometimes envelops vestibular schwannoma around the internal acoustic meatus. Recognition of this membranous structure is important for the surgical preservation of facial and acoustic nerves.
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