Facial Nerve Embryology, Anatomy, Evaluation

1. Facial Nerve Embryology, Anatomy, Evaluation Alice Lee October 28, 2004

2. 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, PERRLAEars: R hemotympanum,BC>AC L TM WNL, AC>BC, 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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?

9. Facial nerve embryology: 7th week

10. Facial nerve embryonic development: Intratemporal course and branches

11. Facial nerve embryonic 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

12. 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

13. Peripheral communications of facial nerve

14. Facial nerve embryonic development: Extratemporal segment – Parotid

15. 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

16. 3-D t bone

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

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

19. 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

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

21. 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

22. _____ _____

23. 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

25. 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

26. 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

28. Nerve injury

29. Causes of facial paralysis Causes of facial paralysis

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

32. HB Facial Nerve Grading

34. 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

35. 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

36. 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.

37. 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?

38. 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)

39. 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

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

41. EEMG - tumor

42. 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

43. 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

44. 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)

45. 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

46. 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

47. 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

49. 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.