Location and Anatomy
The stylomastoid foramen is located on the external cranial base (basis cranii externa), viewed from the inferior (basal) aspect of the skull. Specifically, it lies on the inferior surface of the petrous part (pars petrosa) of the temporal bone.
It is situated directly between two prominent bony landmarks:
- Styloid process (processus styloideus) — positioned anterior to the foramen.
- Mastoid process (processus mastoideus) — positioned posterior and lateral to the foramen.
This foramen represents the terminal point (exit aperture) of the mastoid (third, descending) segment of the facial canal (canalis nervi facialis). Thus, it serves as a critical conduit connecting the interior of the facial canal to the exterior skull base.
Contents of the Stylomastoid Foramen
The foramen serves as an essential passageway linking the cranial structures to the face and scalp. Its primary contents include:
- Facial nerve (n. facialis) — transmits the motor root of the facial nerve. Within the bony canal, the nerve occupies approximately 70% of the cross-sectional area. Immediately upon exiting the skull, and before entering the parotid gland, the nerve gives off several branches: the posterior auricular nerve (n. auricularis posterior), the digastric branch (r. digastricus), and the stylohyoid branch (r. stylohyoideus). Distally, it innervates all muscles derived from the second pharyngeal arch (muscles of facial expression, platysma, stapedius, etc.).
- Stylomastoid artery (a. stylomastoidea) — a branch of the posterior auricular artery (itself a branch of the external carotid artery). The artery enters the foramen alongside the facial nerve and gives off the posterior tympanic artery (a. tympanica posterior) to supply the tympanic cavity.
Topography and Adjacent Structures
The stylomastoid foramen is an important topographical landmark. If a probe is inserted into the foramen and directed superiorly and anteriorly, it points toward the origin of the canaliculus for the chorda tympani (canaliculus chordae tympani). This tiny canaliculus branches off the facial canal near the end of its descending segment before traversing the tympanic cavity.
Other notable structures nearby on the inferior surface of the petrous temporal bone include the jugular notch (incisura jugularis), jugular foramen (foramen jugulare), jugular fossa (fossa jugularis), and the external aperture of the carotid canal (apertura externa canalis carotici).
Clinical Significance: Facial Nerve Lesions
The level of the stylomastoid foramen is critical in the topical diagnosis of facial nerve (CN VII) lesions.
If nerve injury occurs at or distal to the stylomastoid foramen, the patient develops an isolated peripheral facial palsy (Bell's palsy presentation) on the affected side.
Other functions remain intact: taste on the anterior two-thirds of the tongue, salivation, lacrimation, and hearing are normal. This is because the fibers responsible for these functions (greater petrosal nerve, nerve to stapedius, and chorda tympani) branch off the main trunk of the facial nerve more proximally—within the bony canal of the petrous bone, well before reaching the stylomastoid foramen.