External Auditory Canal Transillumination-Guided Middle Fossa Approaches: An Anatomical Feasibility Study

The middle fossa approaches are tremendously versatile for treating small vestibular schwannomas, selected petroclival meningiomas, midbasilar trunk aneurysms, and lesions of the petrous bone. Our aim was to localize the internal acoustic canal and safely drill the petrous apex with these approaches. This study demonstrates a new method to locate the internal acoustic canal during surgery in the middle fossa.

The microsurgical anatomy of the middle fossa floor was studied in 11 formalin-fixed and silicone-injected cadaveric heads. Extradural dissection of the skull base was completed from the posterior to the anterior side. A zero-degree rigid endoscope was inserted perpendicularly into the external auditory canal. The light beam was first directed through the tympanic membrane, avoiding injury to the tympanic membrane. The room lights were dimmed to provide a clearer view of the transilluminated bony area. Drilling was performed with transillumination guidance.

The transilluminated area included the tympanic and mastoid tegmen up to the arcuate eminence. The nonilluminated area was bounded posteriorly by the arcuate eminence, laterally by the greater superficial petrosal nerve, and posteromedially by the petrous ridge. In all specimens, drilling the transition line between the Kawase triangle and the transilluminated area unroofed the internal auditory canal (IAC). No transillumination of the carotid canal was seen after anterior petrosectomy in any of the specimens. The entire contents of the IAC were preserved in both anterior petrosectomy and unroofing of the IAC.

In this anatomical study, transillumination of the external auditory canal proved to be feasible, accurate, and safe in guiding the middle fossa approaches. The ease of implementation and cost-effectiveness of the technique may suggest a possible application in operative scenarios.