Is the Entire Fundus of the Internal Auditory Canal Visible During the Middle Fossa Approach for Acoustic Neuroma?

Abstract

Hypothesis 

To determine the degree to which the fundus of the internal auditory canal (IAC) can be visualized during the middle fossa approach (MFA).

Background 

Conventional wisdom states that the MFA provides excellent access to the IAC from the porus acusticus to the fundus. On the basis of observations derived from a substantial surgical experience, it became obvious that a variable fraction of the fundus lies obscure from the surgeon's line of sight during the MFA because of (1) the overhand of the transverse crest and/or (2) the immobility of the facial nerve at its entry into the fallopian canal.

Methods 

Intraoperative measurements were performed in ten cases to determine the typical angle of view to the fundus of the IAC in the MFA. This angle of view was projected onto coronal computed tomography scans of 40 temporal bones. Measurements of the IAC were made to determine the amount of fundus that could not be directly visualized during a MF exposure.

Results 

On the basis of a surgical line of sight, the fraction of the inferior compartment of the canal that could not be directly visualized because of overhand of the transverse crest ranged from 14% to 34% (median 25%).

Conclusions 

Complete resection of IAC tumors involving the fundus via the MFA requires some degree of blind dissection. Specialized tools and techniques are required to minimize the risk of neural injury during this indirect dissection. Inspection of the fundus with either mirror or endoscope is often necessary to exclude the possibility of retained tumor fragments.