Facial nerve anatomy/certified fixed orthodontic courses by Indian dental academy

Facial Nerve Embryology,
Anatomy, Evaluation
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Case presentation
 HPI: 20 yo M s/p fall from bike without helmet, +
LOC, +EtOH
 PMH/PSH/Med/All/Fam hx/Soc hx: neg
 PEX: AVSS, A&O x3, PERRLA
Ears: R hemotympanum,BC>AC
L TM WNL, AC>BC, Weber R
Nose/OC/OP/Neck: WNL
Face: Abrasions to R forehead, L lip
CN II-XII intact
 CT head: WNL
 Other injuries: R clavicle and scapula fx

Case presentation
 Returns to ER 5 days from trauma with
acute onset of R facial paralysis and with
R decreased hearing
 HB VI, R hemotympanum, R Weber, R
BC>AC
 CT temporal bone: Longitudinal R
temporal bone fracture, sparing otic
capsule
 2 week steroid taper, f/u clinic 5 days

Facial nerve embryonic
development
 Facial 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 or crest

Facial nerve embryology: 4th
week
 By the end of the 4th
week, the facial and
acoustic portions are
more distinct
 The facial portion
extends to placode
 The acoustic portion
terminates on otocyst

week
 Early 5th week, the
geniculate ganglion
forms
 Distal part of
primordium separates
into 2 branches: main
trunk of facial nerve
and chorda tympani

week
 Near the end of the
5th week, the facial
motor nucleus is
recognizable
 The motor nuclei of
CN VI and VII initially
lie in close proximity.
The internal genu
forms as
metencephalon
elongates and CN VI
nucleus ascends www.indiandentalacademy.com

week
 Early 7th week, geniculate ganglion is well-
defined and facial nerve roots are
recognizable
 The nervus intermedius arises from the
ganglion and passes to brainstem. Motor
root fibers pass mainly caudal to ganglion
 Can patients with congenital facial
paralysis have intact taste? Why?

week

development: Intratemporal course
and branches

development: Extratemporal
segment - branches
 Proximal branches form first
 6th week, posterior auricular
branch>branch of digastric
 Early 8th week,temporofacial and
cervicofacial divisions
 Late 8th week, 5 major peripheral
subdivisions present

segment – other nerves
 Facial nerve communicates with peripheral
branches of CN V, IX, X, cervical cutaneous
nerves
 greater auricular nerve and transverse cervical
branches of the cervical plexus (C2, C3)
 Trigeminal nerve: auriculotemporal, infraorbital,
buccal, mental branches
 All connections are complete by week 12 except
for 4 (connections to branches of CN V at orbit
periphery)-these are complete at 4.5 months

Peripheral communications of facial
nerve

segment – Parotid

Anatomic segments of facial
nerve
 Intracranial: brainstem to IAC
 Meatal: fundus of IAC to meatal foramen
(narrowest aperture of FN’s bony canaliculus
 Labyrinthine: meatal foramen to geniculate
ganglion (first genu)
 Tympanic/horizontal: ganglion  adj to oval
window  pyramidal eminence of stapedius
tendon
 Mastoid/vertical: second genu to SM foramen
 Extratemporal: SM foramen to facial muscles

3-D t bone

Facial nerve: types of fibers
 Special Visceral Efferent/Branchial Motor
 General Visceral Efferent/Parasympathetic
 General Sensory Afferent/Sensory
 Special Visceral Afferent/Taste

Special Visceral Efferent/Branchial
Motor
 Premotor cortex  motor cortex 
corticobulbar tract  bilateral facial motor
nuclei (pons)  facial muscles
 Stapedius, stylohyoid, posterior digastric,
buccinator

General Visceral
Efferent/Parasympathetic
 Superior salivatory nucleus (pons)  nervus intermedius
 greater/superficial petrosal nerve  facial
hiatus/middle cranial fossa  joins deep petrosal nerve
(symp fibers from cervical plexus)  thru pterygoid canal
(as vidian nerve)  pterygopalatine fossa 
spheno/pterygopalatine ganglion  postganglionic
parasympathetic fibers  joins zygomaticotemporal
nerve(V2)  lacrimal gland & seromucinous glands of
nasal and oral cavity
 Superior salivatory nucleus  nervus intermedius 
chorda  joins lingual nerve  submandibular ganglion
– postganglioic parasympathteic fibers  submandibular
and sublingual glands

General Sensory Afferent/Sensory
Sensation to auricular concha, EAC wall,
part of TM, postauricular skin
Cell bodies in geniculate ganglion

Special Visceral Afferent/Taste
 Postcentral gyrus  nucleus solitarius –>
tractus solitarius – nervus intermedius 
geniculate ganglion – chorda tympani 
joins lingual nerve  anterior 2/3 tongue,
soft and hard palate

__________

Facial nerve blood supply
 Intracranial/Meatal: labyrinthine branches
from ant inf cerebellar artery
 Perigeniculate: superficial petrosal branch
of middle meningeal artery
 Tympanic/Mastoid: stylomastoid branch of
posterior auricular artery

Nerve fiber components
 Epineurium – nerve sheath; vasa
nervorum
 Perineurium – surrounds endoneural
tubules; tensile strength, protects against
infection
 Endoneurium – surrounds axons, adherent
to Schwann layer, endoneural tubules
regeneration

Pathophysiology of nerve injury:
Sedon classification
 Neuropraxia – conduction blockade from
body to distal; distal nerve can still be
stimulated. External compress vs
intraneural edema
 Axonotmesis – wallerian degeneration
distal to lesion with preservation of
endoneural tubules
 Neurotmesis – wallerian degeneration and
loss of endoneural tubules/regen layer

Nerve injury

Causes of facial paralysisCauses of facial paralysis

 h/o recurrent
alternating facial
paralysis
 Recurrent orofacial
edema (lasts<48 hrs)
 chelitis
 Fissured tongue
 What do I have?

HB Facial Nerve Grading

Topognostic testing
 Mainly of historical interest; not prognostic
 Uses branching pattern of the facial nerve
to identify site of lesion, but is not reliable
 Tearing – Schirmer’s test
 Stapes reflex – Change in acoustic
impedence caused by superthreshold
stimulus; stapedial branch of FN is the first
efferent branch

Auditory testing
 To eval for concurrent SNHL or CHL
 CHL – middle ear tumors, cholesteatomas,
other processes involving tympanic
segment
 SNHL – acoustic neuromas, meningiomas,
congenital cholesteatoma, others involving
CPA or IAC

Electrophysiologic tests
 Measures nerve conduction; from proximal
to injury site to muscle/evoked electrical
signal.
 Cannot measure proximal to stylomastoid
foramen
 Require waiting until degeneration has
progressed enough to be detectable.

Nerve stimulation test
 NST -office-based, stim main branches with 1
millisec wave pulse, minimal thresholds for facial
muslce response are compared
 3.5 milliampere difference is pathologic; not
sens to lesser degrees of nerve transmission
that do not result in loss of visible face motion
 Why can’t this test be used during the first 72
hours after injury?

Maximal stimulation testing
 Variation of NST, but uses maximal stimulation
at a level sufficient to depolarize all motor axons
under the stimulator
 Stim 5 peripheral branches and main trunk
 Compares both sides; subj grading
 Bell’s – Equal B results up to 10 days, 92% with
full recovery. Response lost within 10 days,
100% had incomplete return (May, et al)

Electroneuonography ENog/
Evoked electromyography EEMG
 Similar to MST except the measured end
point is evoked muscle compound action
potential amplitudes and latencies (not
visible muscle movement); used after 2
weeks of injury
 Recording electrodes on nasal alae,
stimulator under zygomatic arch

EEMG
 The peak-to-peak
amplitude is
proportional to the
number of intact
motor axons
 Example: 10% of
normal amplitude =
90% degeneration

EEMG - tumor

EEMG – Bell’s
 Progressive
degeneration – 3,4,5
days post-onset
 MA = masseter
artifact, can be
confused with small
evoked potential, ID
by very short latency

Electromyography
 Measures activity of muscle (from
volitional contraction) instead of the nerve
 Measured at insertion, voluntary
contraction, at rest
 Helps to eliminate false positive
NET/MST/EEMG
 Diagnostic, not prognostic

EMG – insertional, at rest
 A – normal needle
insertional activity
(dec w/ muscular
fibrofatty changes)
 B – Positive sharp waves
(denervation)
 C – *Fibrillations
(denervation 10-20d)
 D – Bizarre formations
(myopathies,
neuropathies)

Motor unit action potential
 The motor unit tested by EMG is only a small portion
of the muscle fibers in an anatomic motor unit
 Motor unit action potential/MUAP is the sum of early
discharges of some muscle fibers of one motor unit
 Nl MUAP: bi/triphasic, amp 0.3-0.5mv, duration 3-
16ms

EMG
 A, inserting needle activity. For
suspected muscle atrophy-
reanimation usu doesn’t work 2
not enough muscle present.
 B. Fibrillation potentials can be
seen in conduction block and
complete disruption
 C. Contracting muscle/smile.
Polyphasic potentials indicative of
early nerve regenration;
polyphasic patterns can be seen
in myopathies
 D. Recruitment/interference
assessed my maximal contraction
of a muscle group

Limitations of electrophysiologic
testing
 72 hours delay for MST and EEMG
 EMG delay ~14 days until fibrillations seen
 Normal variations can be great. EEMG
response of 50% have been seen in
normal controls.
 Must correlate clinical findings with results
 Future? Magnetic nerve stimulation for
intracranial stim/stim prox to lesion

References
 May – The Facial Nerve
 Burgess – Reanimation of the Paralyzed Face
 Rubin – The Paralyzed face
 Netter – Collection of Medical Illustrations, Vol I:Nervous System
 May M, Blumenthal FS, Klein SR: Acute Bell’s palsy: prognostic value of evoked
electromyography, maximal stimulation, and other electrical tests. Am J Otol 5: 1,
1983.
 Darrouzet, et al. Management of facial paralysis resulting from temporal bone
fractures: Our experience ein 115 cases. Otol-Head Neck Surg 125:77-84, 2001.
 Jenny AB et al. Organization of the facial nucleus and corticofacial projection in the
monkey: a reconsideration of the upper motor neuron palsy. Neurology 37:930-939,
1987.

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Facial nerve anatomy/certified fixed orthodontic courses by Indian dental academy

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Facial nerve anatomy/certified fixed orthodontic courses by Indian dental academy