BACKGROUND: The membranous structure of vestibular schwannoma is an important 
factor in its surgical treatment. Herein, we report intraoperative and 
microscopic findings relating to an outermost dura-like membrane in cases of 
vestibular schwannoma and the importance of these findings.
METHODS: Intraoperative findings 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 findings 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 significantly 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 findings proved its continuity with the dura mater. 
In one case, the facial nerve was damaged before it was identified 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.