Ear through Nose: An Endoscopic Endonasal Approach to IAC and Cochlea—Anatomic Study

Background: Endoscopic endonasal surgery has become a widely accepted and increasingly adopted approach for skull base surgery in the last decades. It has become relatively widely applied for midline to skull base regions, decreasing morbidity and providing direct access for deep tumors which displace neurovascular structures laterally. While surgical limitations along the sagittal plane have been well-described, coronal/lateral limitations are not as well defined, other than avoiding crossing cranial nerves. The aim of this study was to understand the lateral-most petrous structures accessible endonasally, namely the IAC and cochlea, which might become relevant in level IV–V surgeries for tumors expanding into the petrous bone.

Methods: The IAC and cochlear anatomy were evaluated relative to neighboring structures through an endoscopic endonasal transclival, infrapetrous, and contralateral transmaxillary (CTM) approach in 10 colored silicon-injected human head specimens (20 sides). Semicircular canals and otic capsule were excluded from this anatomical description, since their location lateral to the IAC obviates their access from an endonasal perspective. The relationship between the IAC/cochlea and trigeminal nerve, Gasserian ganglion, foramen lacerum, posterior genu and petrous portion of ICA and GSPN were evaluated with the aid of image guidance ([Fig. 1]).

Results: The trigeminal nerve at the porus trigeminus remained one of the most important landmarks: it was identifiable (i.e., visible or barely visible) through its dural covering in 19/20 sides ([Fig. 2]). The IAC was always located above (left, 8.63 ± 0.97 mm; right, 8.36 ± 0.93 mm), a plane passing through the foramen lacerum, near or at the level of the lateral margin of the porus segment of the trigeminal nerve (left, 2.55 ± 0.384 mm; right, 2.11 ± 0.335 mm). However, in their anterior course, the trigeminal and facial-cochlear complex separate, creating an angle (left trigeminofacial angle, 56.9 ± 6.0 degrees; right trigeminofacial angle, 57.8 ± 12.5 degrees) where, anteriorly, lies the basal turns of the cochlea. The mean distance from the lateral porus segment of the trigeminal nerve and the cochlea was 10.45 ± 1.651 mm and 10.5 ± 1.547 mm on the left and right side respectively. In all of our specimens the basal turn of the cochlea was always located posterior or posterolateral to the posterior genu of the petrous ICA: in all of our dissections, the cochlea was not reachable through a CTM alone without unsafe manipulation of the paraclival ICA ([Fig. 3]).

Conclusion: The cochlea and IAC remain the most lateral structures which can be visualized from an endoscopic endonasal perspective, and as a result of our dissection, they are unlikely to be damaged. These are not surgical targets, but rather anatomical structures at the lateral limit of access for endoscopic endonasal and CTM approaches to the petrous bone.

Fig. 1

Fig. 2

Fig. 3