Stylomastoid foramen

Where is the stylomastoid foramen located?

It is located on the external cranial base (basis cranii externa), which is viewed from the inferior (basal) norma. The foramen is situated on the inferior surface of the petrous part (pyramid) of the temporal bone.

It lies between two bony prominences:

• Styloid process (processus styloideus) — located anterior to the foramen.
• Mastoid process (processus mastoideus) — located posterior and lateral (bounded by the occipital margin).

The foramen represents the terminal point (exit aperture) of the mastoid (third, descending) part of the facial canal (canalis nervi facialis). In fact, the external cranial base in this region accommodates the exit openings of the main canals of the temporal bone.

What passes through the stylomastoid foramen?

The foramen serves as an important pathway of communication between the cranial cavity and the external structures of the head. The following pass through it:

• Facial nerve (n. facialis) — its motor component exits through the foramen. Within the bony canal itself, the nerve trunk occupies about 70% of the cross-sectional area. Immediately after emerging from the skull, even before piercing the capsule of the parotid gland, the nerve gives off branches: the posterior auricular nerve (n. auricularis posterior), digastric branch (r. digastricus), and stylohyoid branch (r. stylohyoideus). Subsequently, the nerve innervates all muscles derived from the 2nd pharyngeal arch (facial muscles, platysma, stapedius muscle, etc.).
• Stylomastoid artery (a. stylomastoidea) — a branch of the posterior auricular artery (which branches from the external carotid artery). The artery passes through 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 it and directed superiorly and anteriorly, the origin of the canaliculus for the chorda tympani (canaliculus chordae tympani) can be found. This canaliculus branches off from the facial canal near the end of its descending part and then runs through the tympanic cavity.

Nearby on the inferior surface of the pyramid are also the jugular notch (incisura jugularis), jugular foramen (foramen jugulare), jugular fossa (fossa jugularis), and external aperture of the carotid canal (apertura externa canalis carotici).

Clinical significance: facial nerve lesion

The level of the stylomastoid foramen is critically important in the topical diagnosis of facial nerve lesions (cranial nerve VII).

If nerve damage occurs at the level of this foramen or lower (so-called "Level 5"), the patient develops an isolated peripheral palsy of the facial muscles on the affected side.

At the same time, other functions are not impaired: taste on the anterior 2/3 of the tongue, salivation, lacrimation, and hearing remain normal. This is because the fibers responsible for these functions (greater petrosal nerve, stapedius nerve, chorda tympani) branch off from the main trunk of the facial nerve higher up — within the bony canal of the pyramid, before exiting the stylomastoid foramen.

Class and exam questions

What else to study on this topic

• Facial canal (Canalis nervi facialis)
• Facial nerve (Nervus facialis)
• External cranial base (Basis cranii externa)
• Temporal bone (Os temporale)

Other topics in the "Anatomical Structures" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.