Facial Nerve Anatomy and Paralysis

EMBRYOLOGY AND ANATOMY

What branchial arch does the facial nerve innervate?

The second (hyoid) arch.

What cartilage is associated with the second branchial arch?

Reichert’s cartilage.

How many muscles does the facial nerve innervate?

23 muscles per side:

• Occipitofrontalis

• Procerus

• Nasalis muscle

• Depressor septi nasi

• Orbicularis oculi

• Corrugator supercilii

• Depressor supercilii

• Auricular muscles (anterior, superior, posterior)

• Orbicularis oris

• Depressor anguli oris

• Risorius

• Zygomaticus major

• Zygomaticus minor

• Levator labii superioris

• Levator labii superioris alaeque nasi

• Depressor labii inferioris

• Levator anguli oris

• Buccinator

• Mentalis

The platysma in the neck is innervated by the facial nerve.

The stylohyoid muscle, stapedius and posterior belly of the digastric muscle are also innervated by the facial nerve, but are not considered muscles of facial expression.

Through which foramen does the facial nerve exit the skull?

Stylomastoid.

What term is used to describe the network of anastomoses of the extratemporal facial nerve?

The pes anserinus, and arborization.

What does pes anserinus mean?

The foot of a goose.

What are the three major landmarks used to identify the facial nerve as it exits the stylomastoid foramen?

1. The tympanomastoid fissure: 6 to 8 mm inferior to the drop-off of the fissure.

2. The tragal pointer: 1 cm anterior and 1 cm inferior to the point.

3. Retrograde dissection of the posterior belly of the digastric muscle.

The facial nerve is almost invariably found at a point where the tip of the mastoid, cartilaginous process of the auditory canal, and superior border of the posterior belly of the digastrics muscle meet.

Where does the posterior belly of the digastric muscle insert on the skull?

The medial aspect of the mastoid portion of the temporal bone.

What are the extratemporal branches of the facial nerve?

Temporal, Zygomatic, Buccal, Marginal Mandibular, Cervical (Mnemonic: To Zanzibar By Motor Car). Some patients have a branch to the postauricular musculature, but this is considered vestigial.

What functional fiber types comprise the facial nerve?

SVA: taste from anterior two-thirds of tongue via chorda tympani to the nucleus solitarius.

SVE: motor branches to muscles of facial expression, digastric, stapedius, and stylohyoid from the motor nucleus in the pons.

GSA: sensation from the conchal bowl and part of the external auditory canal. Possibly some V3 contribution to middle ear.

GVE: parasympathetic stimulation of lacrimal, submandibular, sublingual gland, and minor salivary glands (all glands in the head but the parotid).

What is the blood supply to the facial nerve?

Stylomastoid artery branch of posterior auricular artery. Greater superficial petrosal artery from the middle meningeal artery.

What is SMAS?

SMAS is an abbreviation for the submuscular aponeurotic system. The SMAS is a layer of tissue which covers the deeper structures of the cheek area and is linked to the superficial muscle covering the lower face and neck, called the platysma. Some techniques for facelift surgery, lift and reposition the SMAS along with the skin. In doing so, the jowls are lifted, the neck is tightened, and the cheeks are elevated.

Also known as the superficial musculo aponeurotic system, it is an inelastic fibrous tissue immediately below the skin and subcutaneous fat of the face into which the facial muscles insert. It is contiguous with the platysma inferiorly and the superficial temporal fascia and the galea superiorly.

Where does the facial nerve course in the parotid region?

Through the substance of the gland deep to the parotid fascia and SMAS.

Where do the nerve branches exit the parotid fascia to course superficially?

Branches exit at the anterior limit of the parotid gland, continuing deep to the SMAS.

What is the most commonly permanently injured branch of the facial nerve during rhytidectomy?

The temporal branch at the zygomatic arch.

Where does the temporal branch cross the zygomatic arch?

Halfway between the lateral canthus and the root of the auricular helix. Near McGregor’s patch (zygomatic ligaments anchoring cheek).

What is Pitanguy’s line?

It is a theoretical line reflecting the course of the temporal branch of the facial nerve. It is drawn from a point 0.5 cm below the tragus to a point 1.5 cm lateral to the superior orbital rim.

Between which fascial layers does the temporal branch of the nerve course as it crosses the zygomatic arch?

The branch courses deep to the SMAS and immediately superficial to the superficial layer of deep temporal fascia.

A stroke patient may maintain function of the frontalis muscle even though all of the other facial muscles on the ipsilateral side are paralyzed. Why?

Because the nerve fibers that project to the part of the facial nucleus in the pons that innervates the forehead project bilaterally, but those that project to the part of the nucleus that innervates the remaining facial muscles project only contralaterally.

What fascial layer does the marginal mandibular nerve lay deep to as it crosses the mandible?

The nerve lies immediately beneath the superficial layer of the deep cervical fascia.

What branch of the facial nerve innervates the buccinator muscle?

Buccal branch of the facial nerve.

At what site is the primary action of botulinum toxin?

Botulinum toxins block acetylcholine release at the presynaptic cell surface, causing a chemical denervation. The light chain, which has zinc metalloprotease activity, cleaves SNAP-25, which is involved in vesicle fusion and release of acetylcholine.

What are the botulinum toxin subtypes?

There are seven serologically distinct subtypes of botulinum toxin A through G. Only botulinum toxin type A is FDA approved for cosmetic use.

FACIAL NERVE PATHOLOGY

Why does a lesion proximal to the motor nucleus of VII produce paralysis of only the lower face, whereas a lesion below the motor nucleus produce complete facial paralysis?