facial nerve anatomy for medical students and ent postgraduates

DR. CH.B. PRATHYUSHA PG ENTNARAYANA MEDICAL COLLEGE 25TH AUGUST 2015

FACIAL NERVE ANATOMY

“ Otology could be a dull way of life with out the facial nerve arrogantly swerving through the temporal bone to the muscles of facial expression”

John Groves M D

(Co author of Scott and Brown)

EMBRYOLOGY

FACIAL NERVE NUCLEI

COURSE OF THE NERVE

SUMMARY OF THE BRANCHES

BLOOD SUPPLY

APPLIED ANATOMY

EMBRYOLOGY

Embryology

Pons develops from metencephalon

3rd week

Facioaccoustic primordium develops giving raise to 7th and 8th cranial nerves

FIRST distinguishable feature of facial nerve

4th week Facioacoustic primordium differentiates into 7th and 8th cranial nerves

Chorda tympani and main trunk can be seen seperately

Chorda tympani joins the mandibular arch

Main trunk joins the hyoid arch

5th week

Geniculate ganglion (separate origin from that of facial nerve)

Nervus intermedius

Greater superficial petrosal nerve

6th and 7th week

Muscles of facial expression develop

Middle ear develops and facial nerve can be seen along the middle ear

8th week Terminal branches can be seen

Extensive branching due to rapid caudal movement of 1st branchial arch

Facial nerve is distorted forming 1st and 2nd genu with GSPN as the anchor

10th to 12th week

Facial nerve makes 2nd genu

Peripheral branches are completely developed

At term

Almost to that of adult

More superficial as the mastoid process is absent

Age 1 to 3

Mastoid process develops

Nerve is displaced medially and inferiorly

Applied anatomy

Ritchers cartilage forms the bones of 2nd pharyngeal arch ( stapes, styloid process, cornua of hyoid bone )

Any abnormality should prompt nerve damage

facial canal is derived from ritchers cartilage

Congenital atresia is associated with facial nerve palsy in 50% of cases

Malformations of 1st and 2nd arches

Treacher Collins Syndrome Goldenhar syndrome

Mobius syndrome

Agenesis of 7th nerve

Agenesis of 6th nerve

Normal intellegence

Skeletal abnormalities

Dull facial expression

Diff between adults and childrenchild adult

1. Absent mastoid process and incomplete tympanic ring

2. Chorda tympani exits through stylomastoid foramen

3. Second genu is very acute and lateral

4. When exits from stylomastoid foramen is more anterior

5. Nerve superficial over angle of the mandible

1. Matoid process and ring is complete

2. Chorda tympani exits proximal to stylomastoid foramen

3. Less acute and medial

4. Due to parotid it is less anterior

5. Less superficial

FACIAL NERVE NUCLEI

Facial nerve nuclei components

Branchiomotor (main motor)

Visceromotor (supra salivatory nucleus)

Special sensory ( tractus solitarius)

General sensory (upper part of spinal nucleus of trigeminal nerve)

motor nucleus

Lies in the lower part of the pons

Lateral to the 6th CN and medial to the 8th nerve

Supplies the facial muscles

Superior salivatory nucleus Lies in the pons

Medial to motor nucleus

supplies the secretomotor parasympathetic fibres

Nucleus solitarius(special sensory ) A column of grey matter embedded in the MO lateral to vagus nerve

Rostral deals with taste

Caudal part deals with GI and cardio respiratory function.

Dorsolateral to the facial motor nucleus

Recieves taste sensation from the anterior 2/3 rds of the tongue

Upper part of spinal trigeminal nucleus(general sensory)

Upper part of trigeminal spinal nucleus

Recieves sensations from concha and auricle through vagus nerve

Bipolar neurons with their cell bodies in the geniculate ganglion

Motor component forms the largest component of facial nerve nuclei

The other 3 components form a distinct facial sheath called nervus intermedius

Remember!!

The sensory fibres have their cell bodies in the geniculate ganglion

They are bipolar

One arm extending to periphery

Other arm extending to the pons

COURSE OF THE FACIAL NERVE

Course of the facial nerve

Has six segments Intracranial segment Meatal segment Labrynthine segment Tympanic segment Mastoid segment Extratemporal segment

Intracranial segment (23 to 24mm)

From pons to internal acoustic meatus

Motor fibres loop over the abducens nerve forming facial colliculus in the floor of the fourth ventricle

Joined by the nervus intermedius

Together with 8th nerve cross CP angle Lies ventral to 8th nerve

Applied anatomy

Intracranial portion lacks epineurium

Regained once it enters facial canal

surgery within the CP angle (schwannoma) makes the nerve vulnerable for iatrogenic injury

Meatal segment (8 to 10mm)

IAC to meatal foramen

Located anterosuperior to vestibulo cochlear nerve

Superior to crista transversa and anterior to crista verticalis ( bills bar)

NO branches

Labrynthine segment (3 to 5 mm)

Shortest division

From entry of facial canal up to the genu

Susceptible to vascular injury

Enters the facial canal between cochlea and vestibule and runs posteriorly

Applied anatomy

The periosteum is thicker here than the entire facial canal

This should be cut if decompression to be performed

NO anastomosing collaterals here making it vulnerable to ischemia

( bottle neck anatomical nature)

In the facial canal Longest bony canal of any nerve

Occupies 73% of the bony canal

Nerve makes an acute turn of 40 to 80 degree

Applied anatomy First genu being formed due to the pushing of the otic capsule (app anatomy)

3 branches

Greater superficial petrosal nerve

Lesser petrosal nerve

External petrosal nerve

Arises from geniculate ganglion

Joins deep petrosal nerve

Forms vidian nerve or nerve of pterygoid canal

Travels in pterygoid canal

Joins pterygo palatine ganglion in pterygopalatine fossa

Other branches

Lesser petrosal nerve

Joins the otic ganglion

External petrosal nerve

Joins the sympathetic plexus around the middle meningeal artery

Tympanic segment ( 8 to 11 mm)

NO branches

Lies beneath the LCC in the medial wall of the middle ear

Passes behind the oval window and the promontory

Passes posterior to the cochleariform process , tensor tympani, and oval window

Just distal to pyrimidal eminence it makes a second turn ( second genu) passing vertically downward as the mastoid segment

Applied anatomy

Nerve may prolapse against the arch of stapes

Bifurcate around stapes

Course below the oval window

More acute turn, susceptible to injury in antrotomy

Bony wall of the tympanic segment is dehiscent in 35 to 55% of cases

ASOM in children and neonates present with facial nerve neuropraxia

Mastoid segment (10 to 14 mm)

Extends to the stylomastoid foramen with 3 branches

Nerve to stapedius

Chorda tympani

Nerve from the auricular branch of the vagus nerve ( pain fibres from the posterior part of the external acoustic meatus

Applied anatomy

Normal function of stapedius in congenital facial palsy

Animal studies show separate neurons other than main motor nucleus

Applied anatomy

Referred otalgia in bells palsy, vesicular eruption in herpes zoster due to sensory function in ear

Chorda tympani nerve

Arises 6 mm above stylomastoid foramen

Perforates the posterior wall of the tympanic cavity

Passes on the medial surface of the tympanic membrane crossing the handle of the malleus

Comes out through petrotympanic fissure to infratemporal fossa

Joins the lingual nerve

Through lingual nerve it supplies secretomotor fibres to submandibular ganglion

Taste fibres from anterior 2/3 of the tongue

Extra temporal segment Posterior auricular nerve supplies auricularis posterior and occipital belly of occipitofrontalis

Digastric branch posterior belly of digastric muscle

Stylohoid branch to stylohyoid muscle

Afferent sensory fibres

Sensation from Ear lobe EAC Tympanic membrane

Extra temporal segment

Passes between posterior belly of digaastric and stylohyoid muscles and enters the parotid gland

Lies between superficial and deep lobes of the gland

From the anterior border of the gland 5 branches emerge

Terminal branches Temporal Zygomatic Buccal Marginal mandibular cervical

Temporal Runs along the lower border of the manddible

acts as the efferent limb of the corneal reflex

Zygomatic

3.Buccal (largest of all terminal branches)

Mandibular ( marginal )

Cervical

Applied anatomy Mandibular branch in 20% 2cm below mandible in submandibular area can lead to paralysis of mouth depressors

Temporal branch is superficial to aponeurotic system over the zygomatic arch, (hence at risk during surgery ) hence repairs to be made deep

SUMMARY OF THE FACIAL NERVE BRANCHES

CENTRAL CONNECTIONS

Motor circuit

Secretomotor circuit

Surgical anatomy

Intratemporal part of the facial nerve

Cochleariform process: tympanic segment is located deep to this

Lateral semicircular canal: second genu lies inferior to this

Digastric ridge: stylomastoid foramen is located anterior to it

Extratemporal part of the facial nerve

Tragal pointer: nerve is identified 1 cm inferior and deep to this

Posterior belly of digastric muscle : at its insertion to mastoid process nerve exits stylomastoid foramina anterior to it

BLOOD SUPPLY

Blood supply of facial nerve 4 vessels

Labrynthine artery a branch of anteroinferior cerebellar artery

Superficial petrosal artery branch of middle meningeal

Stylomastoid artery

Posterior auricular artery distal to stylomastoid foramen

Petrosal artery

Stylomastoid artery:

Ascends stylomastoid foramen and supplies upto 2nd genu

Petrosal artery: arises from middle meningeal artery

anastomoses with stylomastoid artery

reaches as far as stylomastoid foramen

Labrynthine artery Arises from anterior inferior cerebellar artery

Supplies the intra cranial part except the genu

Applied anatomy

The Labrynthine portion does not have any overlap

Petrosal artery alone

More vulnerable for ischemia

Applied anatomy Recurrent paralysis may be due to sudden compressiion and decompression by a tumor like vestibular schwannoma

In vestibular schwannomas only 10% of facial neurons are required for normal facial function

Vestibular schwannomas rarely present with facial weakness

Presence of facial weakness facial schwannoma to be ruled out

UMN AND LMN LESIONS OF THE FACIAL NERVE

UMN LMNLower part of the face is involved

Both lower and upper part of the face is involved

No bells phenomenon Bells phenomenon is seenTaste is NOT effected Taste is effectedNo hyperacusus Hyperacusis may be present if

nerve to stapedius is involved

Usually associated with hemiplegia

Usually not associated unless any pontine lesion is present causing crossed hemiplegia

Site of the lesion is above facial nucleus usually in the internal capsule

Usually in the nucleus or distal to the nucleus

No wasting or atrophy Wasting or atrophy may be present

Bibliography Clinically oriented Anatomy, 6th edition, Keith L

Moore, lippincott, Williams and Wilkins publications

Grays Anatomy, 40th edition The Anatomical basis of Clinical practice, Susan Standring, Churchill Livingstone Elsivier publications

Clinical Neuroanatomy, 7th edition, Richard S. Snell, Lippincott Williams and Wilkins publications

K.J. Lee’s Essential Otolaryngeology, Head and Neck Surgery 10th edition McGrawHill publications

Scott-Browns Otorhinolaryngeology, Head and Neck Surgery, 8th edition, Miachel Gleeson etal CRC press publication

Thank you