Exposure of the lateral extremity of the internal auditory canal through the retrosigmoid approach: a radioanatomic study.

The recent trend toward earlier diagnosis of acoustic neuroma has substantially increased the number of candidates suitable for surgery with an attempt at hearing preservation. Although the retrosigmoid approach affords the possibility of saving hearing in selected cases, it is associated with a somewhat greater morbidity that other approaches, in terms of persistent headache, cerebrospinal fluid leakage, and cerebellar dysfunction. For this reason, it is best used selectively, when the probability of success in hearing conservation is high. Only a portion of the internal auditory canal can be exposed through the retrosigmoid approach without violating the inner ear, a maneuver that greatly reduces the chance of preserving residual hearing. Substantial variability exists between individuals as to just how far laterally the internal auditory canal may be opened without compromising labyrinthine integrity. To assess the magnitude of this variability, measurements were obtained from 60 high-resolution temporal bone computed tomography scans with a schema intended to model the surgical angle of view used during the retrosigmoid procedure. Intraoperative measurements in a series of cases established that the actual surgical point of view is situated along a line that passes approximately 1.5 cm behind the sigmoid sinus. In this typical surgical position, these data predict that an average of 3.0 mm (32% of the internal auditory canal length) must be left unexposed to avoid labyrinthine injury, with a range between 1.1 mm and 5.3 mm (9% to 58% of the internal auditory canal). Each additional 1-cm retraction on the cerebellum beyond that customarily used affords approximately 1 mm (10% of the internal auditory canal) further exposure of the canal. When considering the retrosigmoid approach to an acoustic neuroma, the clinician is urged to evaluate each patient individually to estimate the amount of internal auditory canal accessible without the removal of a portion of the inner ear. This can be ascertained from an axially oriented, gadolinium-enhanced magnetic resonance imaging scan in the internal auditory canal plane by drawing a line that originates 1.5 cm behind the posterior margin of the sigmoid sinus and passes tangential to the most medial extent of the labyrinth. If this line intersects the posterior margin of the internal auditory canal at least 2 mm lateral to the deepest point of tumor penetration, then adequate exposure with preservation of the labyrinth is likely an achievable goal.