Newly Described Anatomical Variations of the Stylomastoid Artery and An Accessory Foramen: Implications for Facial Nerve Preservation in the Transcochlear Approach

Background:The transcochlear approach remains an important option for extensive tumors involving the petrous bone and ventral brainstem. Postoperative facial nerve palsy remains common and has been attributed to disruption of facial nerve vascular supply. We describe a previously unreported arterial variant entering the fallopian canal through a distinct osseous channel separate from the stylomastoid foramen (SMF). We characterize the prevalence and course of this vessel and discuss its implications for facial nerve rerouting.Methods: Fifty-seven dry skulls were examined for accessory foramina adjacent to the SMF. Seven latex-injected cadaveric specimens underwent dissection of the mastoid, fallopian canal, and facial nerve arterial supply. An illustrative case is used to demonstrate preoperative identification using diagnostic cerebral angiography.Results: Four SMF configurations were identified in 114 sides: 17.5% had a singular SMF; 6.1% had a SMF with a bony spur; 22% had a SMF with an internal septum; and 54.4% had a distinct accessory foramen adjacent to the SMF. Three vascular configurations supplying the fallopian canal were identified: classical, variant, and dual classical-combined. The most common was the variant, in which a previously undescribed arterial branch from the stylomastoid, occipital, or posterior auricular artery entered the mastoid through an accessory foramen and coursed within a discrete osseous canal anterior to the fallopian canal before joining it and supplying the facial nerve.Conclusion:This variant artery represents a previously undescribed source of facial nerve vascular supply. Its concealed osseous course may increase vulnerability during facial nerve transposition. Preoperative angiography may facilitate surgical approach selection and vascular preservation.