Hearing Preservation during Anterior Petrosectomy: The “Cochlear Safety Line”

Background: The localization of cochlea during anterior petrosectomy is important to prevent inadvertent surgical damage and thus, hearing loss. Till date, no optimal method to determine the position of the cochlea relative to the Kawase quadrangle has been devised to ensure the protection of the cochlea while performing the Kawase approach.

Objectives: To delineate landmarks accessible during anterior petrosectomy and devise a method to locate the cochlea using these landmarks, to prevent cochlear damage while performing the Kawase approach.

Methods: The Kawase approach was performed in eleven specimens. Foramen Ovale and Foramen Spinosum were identified after a subtemporal craniotomy. Anterior petrosectomy was performed and the transitional dural fold lying between the roof and the anterior wall of the internal acoustic canal was identified. This dural fold was termed as the upper dural transitional fold (UDTF). Next two virtual lines, projecting from foramen spinosum (Line A), and the lateral rim of the foramen ovale (Line B), were drawn intersect the UDTF at a right angle (perpendicularly). The cochlea was exposed and the distances between Lines A and B and the closest point of the outer rim and the membranous part of the cochlea were measured, recorded, and analyzed.

Results: The mean distances between Line A to the membranous and bony margins of the anteromedial cochlea were -0.62 ± 1.38 mm and 0.38 ± 1.63 mm, respectively. The mean distances between Line B to the membranous and bony margins of the cochlea were 1.82 ± 0.99 mm and 2.78 ± 1.29 mm, respectively. Line B (designated as the cochlear safety line) never transected the cochlea.

Conclusion: The cochlear safety line is a dependable landmark to locate and protect the cochlea during the Kawase approach. It can be used to delineate the posterior limit of anterior petrosectomy to avoid transgressing the cochlea and thus, preventing hearing loss.

 

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