facial nerve anatomy for medical students and ent postgraduates
TRANSCRIPT
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