Facial Nerve

Traumatic injury and repair

• Gross anatomy• Histology• injury classifications• Degeneration• Regeneration• Surgical nerve repair• recovery

General outline• Nuclei• Main Facial N and

Intermediate N• Internal Auditory Meatus• Geniculate Ganglion• Intratemporal branches

– Greater petrosal N– Stapedial N– Chorda Tympani

• Stylomastoid Foramen• Extratemporal Branches

– Posterior Auricular N– TZBMC

Facial Nerve• Traverses temporal bone in bony

canal – fallopian canal• Begins at fundus of internal

auditory canal (IAC), terminates at stylomastoid foramen

• Lacks fibrous sheath in IAC, surrounded by a thin layer of arachnoid

• Segments:– Labytinthine– Tympanic– Mastoid– Extracranial

Facial Nerve• Labyrinthine segment

– First, shortest, narrowest– Superior to cochlea– Opens into geniculate fossa, located

just deep to supratubal recess • Geniculate Ganglion

– Formed by juncture of nervus inrermedius and facial nerve into a common trunk

– 3 nerves• Greater superficial pertrosal nerve• Lesser petrosal nerve• External petrosal nerve

– Secrotomotor fibers to lacrimal nerve– Lesser petrosal nerve secretory fibers

to parotid

Facial Nerve• Tympanic segment– Occupies medial wall of anterior

attic– Skims over superior aspect of

cochleariform process– Forms superior wall of oval

window niche posteriorly• 2nd Genu– At pyramidal eminence– To mastoid/vertical segment– Anteroinferior to lateral SCC– Anterior to line drawn through

short process of incus

Facial Nerve

• Mastoid / Vertical Segment– Variable course– Chorda tympani branch– Stapedial branch– Aponeurosis of Post belly of

Digastric at stylomastoid foramen

– Tympanomastoid Suture line

Extratemporal Course

• Exits through SM Foramen, near origin of posterior belly of digastric m.

• Branches:– Post Auricular– Temporal, Zygomatic,

Buccal, Marginal Mandibular, Cervical

• Divides into upper temperofacial and lower cervicofacial division

Peripheral branches

• Posterior Auricular Nerve– Passes upwards behind

ear– Supply

• Auricularis Posterior• Occipital belly of frontalis

• Muscular branch– Post belly of digastric– Stylohyoid

Intraparotid Branches…• Plexiform arrangement within parotid

gland – pes anserinus

• Superficial to retromandibular v and ECA

• Temporal branches– Upper border of gland, crosses

zygomatic arch– Supply: auricularis anterior & superior

part of frontalis

• Zygomatic branches– Crosses zygomatic arch and bone, lies

directly on periosteum– Supply: orbicularis oculi

• Buccal branches– Close to parotid duct– Supply: Buccinator, nose,

upper lip

• Marginal mandibular branch– Runs forward along

lower border of mandible

– Passes superficial to facial A and V

– No connection with other branches

• Cervical branches– Supplies platysma

Variations on facial nerve anatomy

Function• Motor

– Facial expression– Post belly digastric, stylohyoid– Stapedius

• Parasympathetic– Chorda tympani: Submandibular

& sublingual gland– Greater Petrosal N: Nasal mucosa

& lacrimal gland via PtPal ganglion

• Corneal Reflex (efferent limb)• Afferent

– Taste from ant 2/3 tongue via chorda tympani to solitary nucleus

– Auricle – Nervus intermedius

The nerve circuit

Histology

Degrees of facial nerve injuries

• Sunderland’s classification (1978)

• Seddon’s classification( 1943)

Neuropraxia (1st degree injury)

• Physiologic neural block is created by increased intraneural pressure

• Nerve will not conduct an impulse across site of compression

• Nerve will respond to electric stimulation applied distal to lesion

• Once compression relieved immediate return of muscle mvt or within 3 weeks

Axonotmesis (2nd degree injury)

• Occurs if compression is not relieved

• Obstruction of venous drainage with increased intraneural pressure

• Results in axon damage• If process is reversed there

will be complete gradual recovery provided there is no loss of endoneural tubes

Neurotmesis (3rd degree injury)

• Intraneural pressure continues and loss of endoneural tubes

• Marked reduction of response to electrical tests

• Spontaneous recovery delayed and incomplete

• Synkinesis

Transection (4th/5th degree injury)

• Partial or complete transection of the nerve

• Spontaneous recovery not expected

• Recovery even under ideal conditions not good

• Need surgical interventions

Degree of injury

Pathology EEMG response % of normal

Neural recovery Clinical recovery begins

1 CompressionDamming of axoplasm

100 No morphologic changes noted

1 – 3 wks

2 Compression persistsIncreased intraneural pressure. Loss of axons

25 Axons grow into intact empty endoneural tubes at rate of 1mm/day

3 weeks to 2 months

3 Loss of endoneural tubes

0 – 10 New axons mix up and split, causing mouth mvts with eye closure and mass mvts (synkinesis)

2 – 4 months

4 Above plus disruption of perineurium

0 Axons blocked by scars, impairing regenerations

4 – 18 months

5 Complete transections 0 Complete disruption with scar filled gap. Barrier to regrowth and muscle innervation

none

Histological changes in nerve injuries

• Changes in axotomy 1st described by Franz Nissl for cells of facial neucleus

• Chromatolysis and retrograde reaction or axonal reaction

Proximal segment

• Degeneration occurs for a varying distance proximal to nerve injury

• Closer the injury to cell body, and more traumatic, the worse the degree of injury proximally

• Earliest regenerative response is seen within 24 hrs (rat)

• Within three days the proximal stump of the nerve begins to enlarge and form axonal sprouts

• Once regeneration begins the rate of recovery can be estimated by measuring the distance between the site of injury and the region of denervated muscle

• Decrease in diameter of axons. NB because it explains spontaneous decompression that occurs following degeneration of nerve

Proximal segment

• Rate of nerve regeneration was stated by TINEL to be 1mm/day or 1in/month

• Seddon’s figure 1.5 + 0.2mm/day

• Tinel line can be traced along a cross facial nerve graft as it advances across the face

Distal segment

• Undergoes Wallerian degeneration

• Schwann cells proliferate and take on a phagocytic role, removing axonal and myelin debris

• Occurs within 5 hours• Basal lamina of schwann

cells provides the chemotactiv scaffolding for the advancing growth cone

Muscle end

• Knowledge still incomplete• Following degeneration muscles atrophy

progressively over a period of weeks to months

• Denervation then stabilises for a varied and perhaps indefinite period of time, depending on completeness of the original lesion and the degree of spontaneous reinnervation

Cell body regeneration

• Cell body begins regenerative process within 7 hours of nerve injury and is believed to persist indefinitely, providing cell body survives the initial injury

• Poor results of late neural repair thus related to inability of the peripheral structures to regenerate after certain critical period following injury

Axon regeneration

• Schwann cells also produce growth factors

• Produce tubes through which new axons travel

• Regenerating axons advance along the distal nerve segment eventually making contact with distal receptors

Axon regeneration

• If regenerating axons do not reach channels provided by bunger bands within period of 3 to 4 months, the channels degenerate and replaced by connective tissue

Muscle

• New NMJ formed after an injury far outnumber the original NMJ

• Preselection mechanism appears to assure that the denervated muscles receive motor unit axons arising from the appropriate cell body

• Eventually complete denervation will lead to atrophy of muscles and at some future time replaced by fibrous tissue

Response to injury

• Increased of levels of cytokines is related to death of cell bodies

• May be partially prevented by early suture repair of the nerve

Surgical repair• Surgeon suturing proximal to distal segment is suturing

a proximal segment containing bundles of regenerating units to distal segment composed of schwaan cells

• Grafting provides segment of nerve undergoing active wallerian degeneration with all the benefits of schwaan cells, without potential influence of distal receptors

Graft material

• Usually cutaneous nerve segments from various body parts– Median antebrachial

cutaneous nerve– Sural nerve– Greater auricular nerve– Cervical cutaneous nerve

• Futuristic materials – Eg conduit repair

Principles of nerve repair

• Microsurgical technique, including magnification and microsurgical instruments

• A primary epineurial nerve repair with 9-0 or 10-0 nylon sutures to minimise reactions

• Nerve repair must be “tension free”, postural maneuvers such as neck turning to allow primary repair are discouraged

• When tension free primary nerve repair is not possible, an interposition/interfascicular nerve graft recommended

Methods of repair

Fascicular repair

Conduit repair

• Only done in frog/toad models

• 1st attempted in 1979• Brogens demonstrated

that negative voltage applied to injured nerve on a toad resulted in both stumps regenerating towards each other

Recovery post surgery

• House brackmann scale guide for nerve compressions

• RFNRS used to grade nerve recovery post resection and grafting

Grade description

1 Normal function

2 Mild dysfunction, complete eye closure normal symmetry at rest

3 Moderate dysfunction, complete eye closure. Obvious assymetry at rest

4 Mod – severe dysfunction, incomplete eye closure, noticeable assymetry @ rest

5 Severe dysfunction, incomplete eye closure, only twitch of gross motor mvt

6 Total paralysis

Gardener-Robertson hearing classification for VIII nerve function

Repaired Facial Nerve Recovery Scale (RFNRS)

Score Function

A Normal nerve function

B Independent movement of eyelids and mouth, slight mass motion, slight movement of forehead

C Strong closure of eyelids and oral sphincter, some mass movement, no forehead movement

D Incomplete closure of eyelids, significant mass motion, good tone

E Minimal movement of any branch, poor tone

F No movement

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