Facial Nerve Embryology, Anatomy, Evaluation

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Facial Nerve Embryology, Anatomy, Evaluation

• Alice Lee
• October 28, 2004

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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, AO x3, PERRLAEars R
  hemotympanum,BCgtAC L TM WNL, ACgtBC,
  Weber RNose/OC/OP/Neck WNLFace Abrasions to R
  forehead, L lipCN II-XII intact
• CT head WNL
• Other injuries R clavicle and scapula fx

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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 BCgtAC
• CT temporal bone Longitudinal R temporal bone
  fracture, sparing otic capsule
• 2 week steroid taper, f/u clinic 5 days

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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

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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

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Facial nerve embryology 5th week

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

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Facial nerve embryology 5th 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

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Facial nerve embryology 7th 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?

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Facial nerve embryology 7th week
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Facial nerve embryonic development Intratemporal
course and branches
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Facial nerve embryonic development Extratemporal
segment - branches

• Proximal branches form first
• 6th week, posterior auricular branchgtbranch of
  digastric
• Early 8th week,temporofacial and cervicofacial
  divisions
• Late 8th week, 5 major peripheral subdivisions
  present

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Facial nerve embryonic development Extratemporal
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

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Peripheral communications of facial nerve
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Facial nerve embryonic development Extratemporal
segment Parotid
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Anatomic segments of facial nerve

• Intracranial brainstem to IAC
• Meatal fundus of IAC to meatal foramen
  (narrowest aperture of FNs 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

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3-D t bone
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Facial nerve types of fibers

• Special Visceral Efferent/Branchial Motor
• General Visceral Efferent/Parasympathetic
• General Sensory Afferent/Sensory
• Special Visceral Afferent/Taste

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Special Visceral Efferent/Branchial Motor

• Premotor cortex ? motor cortex ? corticobulbar
  tract ? bilateral facial motor nuclei (pons) ?
  facial muscles
• Stapedius, stylohyoid, posterior digastric,
  buccinator

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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

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General Sensory Afferent/Sensory

• Sensation to auricular concha, EAC wall, part of
  TM, postauricular skin
• Cell bodies in geniculate ganglion

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Special Visceral Afferent/Taste

• Postcentral gyrus ? nucleus solitarius gt tractus
  solitarius nervus intermedius ? geniculate
  ganglion chorda tympani ? joins lingual nerve ?
  anterior 2/3 tongue, soft and hard palate

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_____
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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

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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

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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

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Nerve injury
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Causes of facial paralysis
Causes of facial paralysis
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• h/o recurrent alternating facial paralysis
• Recurrent orofacial edema (lastslt48 hrs)
• chelitis
• Fissured tongue
• What do I have?

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HB Facial Nerve Grading
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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 Schirmers test
• Stapes reflex Change in acoustic impedence
  caused by superthreshold stimulus stapedial
  branch of FN is the first efferent branch

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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

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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.

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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 cant this test be used during the first 72
  hours after injury?

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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
• Bells Equal B results up to 10 days, 92 with
  full recovery. Response lost within 10 days, 100
  had incomplete return (May, et al)

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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

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EEMG

• The peak-to-peak amplitude is proportional to the
  number of intact motor axons
• Example 10 of normal amplitude 90
  degeneration

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EEMG - tumor
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EEMG Bells

• Progressive degeneration 3,4,5 days post-onset
• MA masseter artifact, can be confused with
  small evoked potential, ID by very short latency

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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

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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)

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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

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EMG

• A, inserting needle activity. For suspected
  muscle atrophy-reanimation usu doesnt 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

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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

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References

• May The Facial Nerve
• Burgess Reanimation of the Paralyzed Face
• Rubin The Paralyzed face
• Netter Collection of Medical Illustrations, Vol
  INervous System
• May M, Blumenthal FS, Klein SR Acute Bells
  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
  12577-84, 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 37930-939, 1987.