[This corrects the article DOI: 10.3389/fonc.2021.774462.].

OBJECTIVE: To explore the role of neuroendoscope assistance during surgical resection of the intracanalicular portion of vestibular schwannomas via the retrosigmoid approach and the subsequent early facial nerve outcomes.
METHODS: Patients of vestibular schwannoma with intracanalicular extensions undergoing retrosigmoid dissection at a single institution were retrospectively analyzed in this study. Several surgical techniques were applied to ensure maximal and safe removal of tumors. Tumors extending less than 10 mm into the internal acoustic canal (IAC) were classified as Grade A, while those extending over 10 mm into IAC were taken as Grade B. Neuroendoscope was applied at the end of microscopic phase to search for potential remnants for Grade B tumors. Absolute tumor extension was defined and measured. House and Brackmann (HB) scale was used to evaluate immediate CN VII outcomes.
RESULTS: Of the 61 patients, there were 38 females and 23 males. A total of 18 (29.51%) cases were Koos Grade II, 12 (19.67%) cases Koos Grade III, and 31 (50.82%) cases Koos Grade IV. There were 38 cases (62.30%) of Grade A and 23 cases (37.70%) of Grade B. Gross total resection was achieved in 60 cases (98.36%). Four cases of intracanalicular remnants were detected and completely removed under endoscopic visualizations. There was a significantly higher proportion (17%, p = 0.02) of intracanalicular remnants in Grade B than Grade A. CN VII and VIII were anatomically preserved in all cases. A total of 55 cases (90.16%) retained good (HB Grades 1 and 2) facial nerve outcomes.
CONCLUSIONS: In Grade B vestibular schwannomas, after maximal microsurgical removal, endoscopic evaluation of the intracanalicular portion revealed residual tumors in 17% of the patients. Hence endoscopic evaluation of the potential intracanalicular remnants for tumor extending over 10 mm within IAC (Grade B) is recommended.

BACKGROUND: Identification and protection of the cochlea during anterior petrosectomy is key to prevent hearing loss. Currently, there is no optimal method to infer the position of the cochlea in relation to the Kawase quadrangle; therefore, damage to the cochlea during anterior petrosectomy remains a substantial risk.
OBJECTIVE: To identify and define landmarks available during anterior petrosectomy to locate the cochlea and prevent its damage.
METHODS: The Kawase approach was simulated in 11 cadaveric specimens. After a subtemporal craniotomy, foramen spinosum and ovale were identified. Anterior petrosectomy was performed, and the upper dural transitional fold (UDTF) was identified. Two virtual lines, from foramen spinosum (line A), and the lateral rim of the foramen ovale (line B), were projected to intersect the UDTF perpendicularly. The cochlea was exposed, and the distances between lines A and B and the closest point of the outer rim and membranous part of the cochlea were measured.
RESULTS: The average distance between line A to the bony and membranous edges of the anteromedial cochlea was -0.62 ± 1.38 mm and 0.38 ± 1.63 mm, respectively. The average distance between line B to the bony and membranous edges of the cochlea was 1.82 ± 0.99 mm and 2.78 ± 1.29 mm, respectively. Line B (cochlear safety line) never intersected the cochlea.
CONCLUSIONS: The cochlear safety line is a reliable landmark to avoid the cochlea during the Kawase approach. When expanding the anterior petrosectomy posteriorly, the cochlear safety line can be used as a reliable landmark to prevent exposure of the cochlea, thus preventing hearing loss.