FACIAL NERVE ANATOMY

FACIAL NERVE ANATOMY DR. GIRISH. S

EMBRYOLOGY - BRANCHIAL ARCHANATOMY OF FACIAL NERVECOURSE AND BRANCHES ARTERIAL SUPPLY OF FACIAL NERVESURGICAL LANDMARKS

FACIAL NERVE7th cranial nerve , mixed Unique longest course in a bony canalNerve of second archNerve of facial expressionApprox 7000 10000 fibres

FACIAL NERVE EMBRYONIC DEVELOPMENTFacial nerve course, branching pattern, and anatomical relationships are established during the first 3 months of prenatal life

The nerve is not fully developed until about 4 years of age

The first identifiable FN tissue is seen at the third week of Gestation- FACIOACOUSTIC PRIMORDIUM

5-Neural crest cells appear dorsolateral to the still open rhombencephelon/hindbrain and just rostral to the otic placode.-The collection of neural tissue gives rise to both the 7th and 8th cranial nerves.

Facial nerve develops in the 2nd pharyngeal arch

1st branchial arch/groove & internal pouch External ear, external acoustic meatus & middle ear

Hence anomalies of these are associated with anomalies of facial nerve

By the third week of gestation, the FACIOACOUSTIC PRIMORDIUM gives rise to CN VII and VIII.

During the fourth week, the CHORDA TYMPANI can be discerned from the main branch. The former courses ventrally into the first branchial arch and terminates near a branch of the trigeminal nerve that eventually becomes the lingual nerve.

The main trunk courses into the mesenchyme, approaching the EPIBRANCHIAL PLACODE.

34

The geniculate ganglion, nervus intermedius, and greater superficial petrosal nerve are visible by the fifth week.

The second branchial arch gives rise to the muscles of facial expression in the seventh and eighth week.

To innervate these muscles, FACIAL NERVE courses across the region that eventually becomes the middle ear.

By the eleventh week, the facial nerve has arborized extensively.

57-811

TIME DURING GESTATION WHEN ANATOMIC STRUCTURES APPEAR

WEEK OF GESTATION STRUCTURES NOTED 3rd Collection of neural crest cells, anlage of VII cranial nerve nucleus (CNN) 5th Chorda tympani, greater petrosal, VII motor nucleus 6th External genu, postauricular branch, branch to posterior belly of digastric 7th Geniculate ganglion, nervus intermedius 8th Stapedius nerve, temporofacial and cervicofacial part of extracranial facial n End of 8th Terminal branches of VII 7th to 8th Myoblasts that will form the facial muscles 12th All facial muscles

ANATOMY OF FACIAL NERVE

NERVE FIBER COMPONENTSEPINEURIUM nerve sheath; vasa nervorum

PERINEURIUM surrounds endoneural tubules; tensile strength, protects against infection

ENDONEURIUM surrounds axons, adherent to Schwann layer, endoneural tubules regeneration

Approx 10,000 motor, sensory and parasympathetic fibres with 7000 special visceral efferent to facial muscles.

Pathways of the facial nerve are variable

Knowledge of the key landmarks of facial nerve is essential for accurate physical diagnosis and safe and effective surgical intervention in the head and neck.

COURSE3 segments

SUPRANUCLEARNUCLEARINFRANUCLEARINTRA TEMPORALEXTRA TEMPORAL

SUPRANUCLEAR SEGMENTMOTOR FACIAL AREA PRECENTRAL GYRI OF CEREBRAL CORTEX

Forehead uppermost , Midface nose and lips sequentially located more inferiorly

PRECENTRAL GYRUS OF THE FRONTAL CEREBRAL CORTEXCORTICOBULBAR TRACTGENU OF INTERNAL CAPSULECEREBRAL PEDUNCLESMOTOR NUCLEUS

Windows User (WU) - Muscles of the upper face (frontalis, orbicularis oculi, and corrugator muscles) RECEIVES CORTICOBULBAR FIBERS FROM BOTH RIGHT & LEFT PRECENTRAL MOTOR CORTICES

Supranuclear tracts innervating the LOWER FACE are CROSSED fibres from the other hemisphere ONLY..

NUCLEAR SEGMENT

NUCLEAR SEGMENT

2 ROOTSMAIN MOTOR ROOTSENSORY ROOT [ NERVE OF WRISBERG ]

MAIN MOTOR ROOT arises from facial nerve nucleus in the lateral portion of anterior pons.

Winds around 6th nerve nucleus FACIAL COLLICULUS [ floor of 4th ventricle]

INTERNAL GENU

SENSORY ROOT

NERVUS INTERMEDIUS / NERVE OF WRISBERG / GLOSSOPALATINE NERVENTS [medulla] SUPERIOR SALIVATORY NUCLEUSSPINAL TRIGEMINAL NUCLEUS

OTHER CONNECTIONSAfferent inputs fromTrigeminal nerve n nucleus trigemino-facial reflexes [ eg corneal reflex]SUPERIOR OLIVE stapedial reflexSUPERIOR COLLICULUS optic reflex centre Blinking of eyes light or touch on cornea contraction of stapedius in response to sound

NERVE TO STAPEDIUS arise from region outside the main nucleus. [SUPERIOR OLIVE]

In congenital 7th nerve palsy , stapedial reflex is normal.

In brainstem lesions there can be normal 7th nerve with abnormal stapedial reflex.

INFRANUCLEAR SEGMENT

INTRACRANIAL PORTIONExtends from the brainstem to opening of IAM [12-19mm]

Motor and sensory root exit from the brainstem At the posterior border of pons between olive and inferior cerebellar peduncle. [CP Angle]

Pass laterally with 8th cranial nerve

8TH nerve lies behind n below 7TH motor root and forms an anterosuperior concave gutter.

Nervus Intermedius is sandwiched between them.

The nerves are in contact with the lateral recess of fourth ventricle and cerebellum.

They pass laterally and slightly upwards to enter the Internal Acoustic Meatus.

Before entering the IAM, AICA may pass in front/ behind/ loop around/ pass between them.

Internal auditory br of AICA/ basilar artery accompanies the nerves into the meatus.

Acousticofacial bundle is close to the posterior / lateral border of the meatus.

Anterior part of the canal at the porus appears to contain no neural structures.As the neural bundle passes laterally towards the fundus , FN sweeps more anteriorly

FN n NI moves close together within a common investment of piamater.They are fused as one at the fundus.

IAMBony channel from postr fossa to labyrinth8-10 mm length, 3.4mm diameterFN is located antero-superiorly.

FUNDUS lateral wall of IAM

CRISTA FALCIFORMIS separates it from inferior vestibular nerve and cochlear nerve.BILLS BAR separates it from superior vestibular nerve [ named after DR. WILLIAM HOUSE ]

IMPORTANT LANDMARK IN MIDDLE FOSSA n TRANSLABYRINTHINE APPROACH.

INTRATEMPORAL SEGMENTFUNDUS OF IAM to STYLOMASTOID FORAMEN.

FALLOPIAN CANAL [28-30mm] INTRALABYRINTHINE PARTTYMPANIC SEGMENT [ HORIZONTAL ]MASTOID SEGMENT [ VERTICAL ]

GABRIEL FALLOPIUS

FALLOPIAN CANAL No other nerve in the body covers such a long distance through a bony canal.

Remarkable for the Z shape of its intratemporal portion

It has also a ganglion

LABYRINTHINE SEGMENTTerm is derived from the location of this part immediately posterior to cochlea [ LESS THAN 0.5 mm to cochlea]

The nerve is anteromedial to the ampullated ends of the horizontal and superior SCC

It rests on the anterior part of the vestibule in this segment

45

LABYRINTHINE SEGMENTThe nerve takes a turn posteriorly forming the 1ST GENU

From FUNDUS OF IAM to distal portion of GENICULATE GANGLION [to a point immediately short of the processus cochleariformis ]

NARROWEST AT PROXIMAL PART, at the entrance [0.68 mm]

SHORTEST SEGMENT OF THE NERVE only 4 mm

Here facial nerve fibres are loosely arranged, without an epineural covering

Narrowest at the entrance BOTTLE NECK CONCEPT

Predispose the nerve to strangulation in cases of oedematous swelling

UGO FISCH BOTTLE NECK CONCEPT

GENU causes a horizontal prominence on the medial wall of middle ear

Ie. Immediately above the fenestra vestibuli and posterior to the processus cochleariformis

IMPORTANCETHICK PERIOSTEUM needs to be cut during decompression

ABSENCE OF ARTERIAL ARCADES [end arteries ]BOTTLE NECK DEFORMITY

TEMPORAL BONE FRACTURES involve this segment due to traction or compression by fracture fragments.

During trans-labyrinthine approach , this segment is at risk when drilling near superior SCC

VULNERABLE TO ISCHAEMIA

COGRIDGE OF BONE THAT PROJECTS INFERIORLY FROM TEGMEN TYMPANI IN THE EPITYMPANUM

Lies superiorly & posterior to the cochleariform process & anterior to head of malleus

The nerve takes a turn posteriorly forming the 1ST GENU GSPNLSPNESPN

GSPNRuns in the groove in superior surface of the petrous bone to the Foramen Lacerum

HIATUS FOR GREATER PETROSAL NERVE to the middle cranial fossa [ described by GABRIELE FALLOPIUS ]

This nerve passes deep to the GASSERIAN GANGLION [trigeminal ganglion ]

GSPN Carries touch, pain, temperature sensations from soft palate [ gva ]

Parasympathetic supply to the lacrimal gland [gve ]

Carries taste from the hard and soft palate [sva]

HIATUS FOR GREATER PETROSAL NERVEfor the passage of theGSPNand thePetrosal branch of MMA.

Joined by DEEP PETROSAL NERVE [ sympathetic from ICA ]

NERVE OF PTERYGOID CANAL [ VIDIAN NERVE ]

SPHENOPALATINE GANGLION

LACRIMAL, NASAL & PALATAL GLANDS

Afferent impulse from palate are routed in the reverse fashion.

SOFT PALATE is supplied by GSPN through LESSER PALATINE NERVE

LSPNCarries secretory fibre to the ParotidCarries parasympathetic contributions from the TYMPANIC PLEXUS [IX] n NI

LSPNcarries parasympathetic (secretory) fibers to parotid glandTYMPANIC PLEXUS [arises fromCN IX via Jacobsons nerve]NERVUS INTERMEDIUS

Pass forwards through its own canal back into the MCF.

Here it is between the two layers of thedura mater

Then pass forwards to exit theskull via FORAMEN OVALEto eventually join theOTIC GANGLION

ESPNMinute inconstant branch

Joins the sympathetic plexus on the Middle Meningeal Artery

TYMPANIC SEGMENTHORIZONTAL SEGMENT

From geniculate ganglion [1st genu ] to 2nd genu in posterior wall of tympanum

After 1st genu , the nerve runs sharply backward around the promontory. [ at the medial wall of epitympanic recess ]

Lies against the medial wall of cavum tympani, above and posterior to the oval window.

TYMPANIC SEGMENT8-11mm , slightly inclined inferiorly .

Runs between Ampullary end of lateral SCC n oval window. Line of junction of the facial canal with the SCC is demarcated by a prominent mucosal vein.

**** bone covering this region is thin n easily fracturedSo dehiscence occurs frequently Uncovered nerve prolapse into the oval window.

MASTOID SEGMENT VERTICAL SEGMENT ~13 mm length.

At the posterior end of oval window , nerve turns downwards

Runs into the substance of bony posterior wall of middle ear , passing with a slight lateral inclination beneath the fossa incudis and behind the pyramid.

In this part the nerve may come into direct relationship posteriorly with mastoid antrum & air cells.

Immediately behind the pyramid , NERVE TO STAPEDIUS emerge, which passes through the pyramid to supply the stapedius muscle.

Nerve continues vertically downwards to enter stylomastoid canal

2mm behind the entrance , it gives off CHORDA TYMPANI nerve.

It comes out through the stylomastoid foramen , lies lateral to styloid process and medial to tympanic plate.

In the mastoid portion just prior to entering the stylomastoid foramen, the nerve takes a distinct obtuse angled turn forward towards the foramen deviating from its vertical course.

IMPORTANT LANDMARK FOR THE STYLOMASTOID FORAMEN

THIRD GENU

Here it is separated from the tip of mastoid process by posterior belly of digastric muscle.

Prone to injury in young children in whom mastoid is incompletely developed.

CHORDA TYMPANIit arises about 6mm above the stylomastoid foramenit passes in the cranial direction parallel to the facial nerve in posterior wall of middle ear POSTERIOR CANALICULUS [accompanied by a branch of stylomastoid artery ] it emerge through a Small aperture between the base of pyramid and bony tympanic annulus. It then crosses upper part of middle ear, passing between the long process of incus and handle of malleusLeaves the middle ear through ANTERIOR CANALICULUS [ petrotympanic fissure ] [ accompanied by anterior tympanic branch of maxillary artery ]

CHORDA TYMPANITraverses through CANAL OF HUGIER Emerges from the skull in the medial surface of spine of sphenoid.Infratemporal fossa , lying medial to the lateral pterygoid muscle.

Incorporated into the lingual nerve Taste buds of anterior 2/3rd tongue

Also to the submandibular ganglion Secretomotor postganglionic fibres to submandibular n sublingual glands.

FACIAL RECESSGroup of cells seen lateral to 2nd genu of FN & in the upper part of vertical segment of FN.

Rarely exceeds 2-3mm

Superiorly FOSSA INCUDISLaterally CHORDA TYMPANIMedially proximal part of vertical segment of FNAnteriorly ANNULUS

INDICATION Cochlear implant insertion FN decompression

SENSORY AURICULAR NERVEAnother intra temporal branch of facial nerve. Just below the chorda tympaniCommunicates with auricular branch of vagusSupply skin of external auditory canal

EXTRA TEMPORAL PORTION

After exiting the stylomastoid foramen , it pass forward.

Lies in relation posteriorly with styloid process , laterally with posterior belly of digastric muscle, and anteriorly with the parotid gland.

It almost immediately disappears into the parotid gland .

Before doing it so, it gives off the following branches

POSTERIOR AURICULAR NERVE NERVE TO POSTERIOR BELLY OF DIGASTRICNERVE TO STYLOHYOIDANSA HALLER [ inconstant] COMMUNICATING BRANCHLINGUAL BRANCH

POSTERIOR AURICULAR NERVE1-2mm below stylomastoid foramenWinds around digastric muscle & extends posteriorly upon the anterior surface of mastoid

Courses inferior to auricle posterior auricular musclesAscends superficially on mastoid occipital belly of occipetofrontalis

Also communicates with great auricular nerve, lesser occipital nerve n auricular branch of vagus

ANSA HALLERInconstantArises immediately below the stylomastoid foramenAnastomose with IXth CR N while passing lateral to the jugular vein.

BRANCHES IN PAROTIDEnters the parotid and divides into 2 main branches TEMPOROFACIAL BRANCH CERVICOFACIAL BRANCH

Finally within the parotid substance , it divides into 5 main branches.These are superficial n lateral to ECA and posterior facial vein/retromandibular vein

**** Parotid superficial to the nerve network may be dissected off with limited nerve damage.

TEMPORALZYGOMATIC [ INFRA ORBITAL ] BUCCAL MANDIBULAR [ SUPRAMANDIBULAR ] CERVICAL [ INFRAMANDIBULAR ]

During submandibular gland excision , dissection should be done in a plane deep to the fascia overlying submandibular gland, to save the marginal mandibular nerve.

ANATOMIC VARIATIONS KATZ AND CATALANO CLASSIFICATION

TYPE I 25% a] splitting of zygomatic branch and reunion b] splitting of mandibular branch and reunion TYPE II 14% buccal branch fuse distally with zygomatic branchTYPE III 44% communication between buccal and other branchesTYPE IV 14% anastamotic branching before major divisionsTYPE V 4% 7th nerve leaves the skull as more than one trunk

TYPE ITYPE II

FACIAL NERVE SHEATHOuter periosteal layerVascular plane of loose connective tissueFirm , fibrous layer in contact with perineurium.

Proximally it blends with dural covering of the nerve at IAM. Distally at stylomastoid foramen it fuses with periosteum n adjacent facial layers.

BLOOD SUPPLYDerived from 4 main blood vessels: MCA, MMA, AICA n posterior auricular arteries

CORTICAL MOTOR AREA rolandic br of MCAN in CP Angle AICAN in IAC labryrinthine artery of AICAGENICULATE GANGLION superficial petrosal artery [ MMA ]MASTOID SEGMENT stylomastoid artery of posterior auricular arteryEXTRATEMPORAL PART posterior auricular artery

Venous drainage parallels the arterial blood supply. No lymphatics in the nerve itself , but they course in the epineural layers.

ANATOMICAL LANDMARKS OF FACIAL NERVE

MIDDLE EAR AND MASTOID SURGERY1. PROCESSUS COCHLEARIFORMIS It demarcates the geniculate ganglion which lies just anterior to it.

2. COG bony ridge , which hangs from the tegmen anterior to the head of malleus, useful in identifying the first genu .

3. OVAL WINDOW The facial nerve runs above the oval window. Posterior segment of horizontal portion of nerve.

4. LATERAL SEMICIRCULAR CANAL : Lies posterior-superior to tympanic segment5. PYRAMID. Nerve runs behind the pyramid. 2nd genu is lateral and posterior to the pyramidal process.

6. RETROFACIAL AIR CELLS : helps in delineating the medial aspect of vertical segment of facial nerve

7. TYMPANOMASTOID SUTURE. In vertical or mastoid segment, nerve runs behind this suture.8. DIGASTRIC RIDGE. The nerve leaves the mastoid at the anterior end of digastric ridge.

PAROTID SURGERY 1. CARTILAGINOUS POINTER [CONLEY ] The nerve lies 1 cm deep and sightly anterior and inferior to the pointer. Cartilaginous pointer is a sharp triangular piece of cartilage of the pinna and "points" to the nerve.

2. TYMPANOMASTOID SUTURE Nerve lies 6-8 mm deep to this suture.

3. STYLOID PROCESS The nerve crosses lateral to styloid process.

4. POSTERIOR BELLY OF DIGASTRIC If posterior belly of digastric muscle is traced backwards along its upper border to its attachment to the digastric groove, nerve is found to lie between it and the styloid process.

TERMINAL BRANCHESRAMUS FRONTALIS line from tragus to lateral canthus

RAMUS BUCCALIS - line from tragus towards the alae of nose parallel the zygoma , but 1cms below

RAMUS MANDIBULARIS near the angle of mandible at a point 4-4.5cms away from the attachment of lobule of pinna .

DIFFERENCE IN ANATOMICAL RELATIONSHIP OF 7TH NERVE IN ADULTS & CHILDREN CHILDADULTABSENT MASTOID PROCESS & INCOMPLETE TYMPANIC RING ; CHORDA TYMPANI MAY EXIT THROUGH THE SM FORAMENMASTOID PROCESS PRESENT & TYPANIC RING COMPLETE BY ADOLESCENCESECOND GENU IS MORE ACUTE & LATERAL SECOND GENU IS LESS ACUTE & MEDIAL NERVE AFTER EXIT FROM SM FORAMEN IS MORE ANTERIOR & LATERAL NERVE TRUNK IS LESS ANTERIOR & DEEPERNERVE IS VERY SUPERFICIAL OVER ANGLE OF MANDIBLENERVE SUPERFICIAL OVER ANGLE OF MANDIBLE.

Thank you.

SEGMENT LOCATION LENGTH mm

Supranuclear Cerebral cortex NA

Brainstem Motor nucleus of facial nerve, superior salivatory nucleus of tractus solitarius NA

Meatal segment Brainstem to internal auditory canal (IAC) 13-15

Labyrinthine segment Fundus of IAC to facial hiatus 3-4

Tympanic segment Geniculate ganglion to pyramidal eminence 8-11

Mastoid segment Pyramidal process to stylomastoid foramen 10-14

Extratemporal segment Stylomastoid foramen to pes anserinus 15-20

4 COMPONENTSBRANCHIOMOTOR [SVE]Largest portionM of facial exprsn, digastric postr belly, stylohyoid, stapediusVISCEROMOTOR [GVE]Parasymp innervtn of lacrimal, submandibular,sublingualSPECIAL SENSORY Taste senstn from antr 2/3 rd of tongue n soft palateGENERAL SENSORY [GSA]Skin of conchae