Stylomastoid foramen !

The stylomastoid foramen is a small opening behind the ear, between two bony points, where the facial nerve leaves the skull for the muscles of the face.

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What it is

A small opening between the styloid and mastoid processes.

What it does

Where the facial nerve leaves the skull, on its way through the parotid gland to the muscles of the face.

Key facts

Where: Between the styloid and mastoid processes
Function: Transmits the facial nerve (VII) and stylomastoid artery

Relations

Contents: Facial nerve (VII) leaving the skull, and the stylomastoid artery, usually from the posterior auricular artery, entering to supply the facial canal and middle ear
Course: Just outside the foramen the nerve gives the posterior auricular nerve and twigs to the posterior belly of the digastric and the stylohyoid, then turns forward over the styloid process into the parotid gland
In front: Styloid process, from which the stylohyoid muscle and the styloglossus arise
Behind: Mastoid process, with the posterior belly of the digastric arising from the mastoid notch just medial to it
Medial: Jugular foramen with the internal jugular vein, a short distance inward

How to find it

It cannot be felt. It lies deep, about level with the ear lobe, between the mastoid tip and the back of the mandibular ramus. In parotid surgery it is reached by following the tympanomastoid suture, which leads down to it; the upper border of the posterior belly of the digastric marks the same depth.

Examination

Topographic testing: normal tear production (Schirmer test), stapedial reflex and taste on the front of the tongue with complete facial weakness place the lesion at or below the foramen

On imaging

High-resolution temporal bone CT shows it between the styloid and mastoid processes, where the mastoid segment of the facial canal ends. On axial CT or T1 MRI it is normally filled by a small pad of fat; loss of that fat or a widened foramen suggests a parotid cancer spreading up the facial nerve. In facial palsy after a temporal bone fracture, CT follows the fracture toward the canal.

Clinical notes

Parotid tumours and surgery, deep cuts behind the angle of the jaw and mastoid surgery can injure the trunk just after it leaves the skull
Electroneurography stimulates the trunk just below the foramen and compares the facial muscle response on the two sides to estimate how many fibres have degenerated
In parotidectomy the trunk is identified as it leaves the foramen and then traced forward to protect its branches

Development

At birth there is almost no mastoid process and the foramen opens on the side of the skull; as the mastoid grows over the first years it comes to lie deeper and lower, shielded by the process.

Bell palsy

Test yourself

Think of the answer, then tap the question to see it.

Which nerve leaves the skull through the stylomastoid foramen?

The facial nerve (VII), with the stylomastoid artery entering alongside.

What does a facial nerve lesion at the stylomastoid foramen spare, and why?

Tears, taste and the stapedius reflex, because their branches leave higher up; all facial expression muscles on that side are paralysed.

Why is the facial nerve at the stylomastoid foramen vulnerable in newborns?

The mastoid process has not grown, so the foramen is superficial; forceps delivery or an incision behind the ear can injure the nerve.

Which skull base infection often affects the facial nerve first at the stylomastoid foramen?

Necrotising (malignant) otitis externa, mostly in older people with diabetes.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): foramen stylomastoideum TA2 684
Official Spanish (DeCS): Agujero estilomastoideo
English (TA2): Stylomastoid foramen

Facial nerve (VII)

Sources: Terminologia Anatomica (TA2); Facial Nerve Anatomy and Clinical Applications; Anatomy, Head and Neck, Skull Foramen.

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Representative model, for educational purposes only.