When the tumor encases or displaces the abducens nerve: anatomically based strategies to prevent its injury in the retrosigmoid route.

Abstract

Lesions of the cerebellopontine angle (CPA) and petroclival region represent a challenging surgical target due to the complex anatomy of the involved neurovascular structures. In this scenario, cranial nerve (CN) VI is particularly exposed to potential injuries due to its deep-seated location and absence of a bony foramen that serves as a reference of its most distal cisternal point, especially when it is encased or displaced by large lesions. This study aimed to provide reliable operative guidance for preventing injuries to CN VI during the retrosigmoid approach to address CPA and petrotentorial lesions. Four formalin-fixed, latex-injected anatomical specimens were dissected to highlight and investigate the relevant anatomy of the CPA and petroclival region during the retrosigmoid approach. Additionally, 50 sides of noninjected formalin-fixed specimens were dissected for morphometric evaluation. Correlations between the petrotentorial junction (PTJ), porus acusticus (PA), and trigeminal impression (TI) with the entry point of CN VI into Dorello's canal were evaluated. An illustrative clinical case and a 3D anatomical model generated through the photogrammetry scanning technique were described. In the sagittal plane, CN VI entrance into Dorello's canal was found in a trajectory parallel to the PTJ, passing through the inferior aspect of the PA and 21.5 ± 1.3 mm anteriorly. In the coronal plane, the entry point of CN VI into Dorello's canal was estimated at 6.2 ± 1.2 mm from the anterior edge of the TI in a trajectory perpendicular to the PTJ in 47 (81%) specimens. The obtained results demonstrated two surgical strategies to locate Dorello's canal within the retrosigmoid route: 1) approximately 20 mm anterior along the inferior edge of the PA parallel to the PTJ; and 2) approximately 6 mm inferior to the anterior edge of the TI, perpendicular to the PTJ. The defined operative strategies provide reliable anatomical guidance to locate the entrance of CN VI into Dorello's canal within the retrosigmoid route, potentially reducing the risk of abducens nerve palsy and improving patient outcomes.