Presentation on theme: "Facial Nerve Embryology, Anatomy, Evaluation Alice Lee October 28, 2004."— Presentation transcript:
Facial Nerve Embryology, Anatomy, Evaluation Alice Lee October 28, 2004
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: 4 th week By the end of the 4 th week, the facial and acoustic portions are more distinct The facial portion extends to placode The acoustic portion terminates on otocyst
Facial nerve embryology: 5 th week Early 5 th week, the geniculate ganglion forms Distal part of primordium separates into 2 branches: main trunk of facial nerve and chorda tympani
Facial nerve embryology: 5 th week Near the end of the 5 th 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
Facial nerve embryology: 7 th week Early 7 th 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?
Facial nerve embryonic development: Intratemporal course and branches
Facial nerve embryonic development: Extratemporal segment - branches Proximal branches form first 6 th week, posterior auricular branch>branch of digastric Early 8 th week,temporofacial and cervicofacial divisions Late 8 th week, 5 major peripheral subdivisions present
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
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
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
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 – 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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