Anatomical Variations of the Intrapetrous Portion of the Facial Nerve

The intrapetrous facial nerve has the second longest intraosseous course of all cranial nerves, after the mandibular nerve. But it is by far the most complex considering the anatomical structures closely related to it. The auditory and vestibular portions of the inner ear, the dura of the middle fossa and posterior fossa, the sigmoid sinus and jugular bulb, and the internal carotid artery are close enough to merit attention. This article includes an anatomical study on 100 temporal bones with anatomical references as seen from the middle fossa and from the transmastoid approaches that may help identifying the facial nerve and protecting surrounding structures. Anatomical variability was present and noteworthy when considering the venous drainage system through the temporal bone and the mastoid pneumatization. The distance from the geniculate ganglion to the hiatus falopii offered the highest variability with a range of 0 to 7.75 mm and a mean of 3.30 mm. The geniculate ganglion was dehiscent in 20.8% of the specimens and the superior semicircular canal was spontaneously blue-lined in 27% of the cases. Through the transmastoid approach, the highest variability was found regarding the distance between the vertical portion of the facial nerve and the jugular bulb (range from 1.5 to 10.0 mm), the sigmoid sinus (range from 0 to 13.25 mm) and the internal carotid artery (range from 6.0 to 15.0 mm). This study highlights the importance of the relative variability of the facial nerve to other surrounding structures within the petrous portion of the temporal bone. Anat Rec, 302:588-598, 2019. © 2018 Wiley Periodicals, Inc.