21 Temmuz 2014 Pazartesi

Facial Nerve (CN VII)

Facial Nerve (CN VII)


ANATOMY
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Corneal reflex: principal nucleus of V, motor nucleus of VII
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  • Bills's bar - separates facial and superior vestibular nerve
  • Transverse crest - separates facial and cochlear from superior and inferior vestibular nerves

PATHOLOGY
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Sarcoid



HISTORY & PHYSICAL EXAM
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  • Frontalis             --> wrinkle forehead, look up
  • Orbicularis oculi  --> close eye while examiner attempts to open
  • Buccinator          --> purse lips while examiner presses cheeks
  • Orbicularis oris    -> show teeth

House Brackmann Scale
1   Normal
2   Slight mouth asymmetry with effort, normal at rest
3   Obvious weakness / asymmetry
4   Incomplete eye closure
5   Asymmetric at rest, no forehead movement
6   Total paralysis

Vestibular Nerve

Vestibular Nerve
Cochlear / Auditory Nerve
(CN VIII)

  • Function: hearing and equilibrium

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PHYSICAL EXAM
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  • Brainstem auditory evoked potentials (BAEP)
    • (in blue on above diagram)
    • Electrical activity in first 10 msec after "click" stimulus
    • Uses: 
      • Hearing assessment 
      • CPA lesions - decrease amplitude, increase latency I/II, increase V latency
      • Cochlear lesions - no change in wave pattern or latency
      • Intra-op monitoring
      • Brainstem function in coma
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Bone better than air
Both bone and air poor
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  • Weber Test
    • Hold tuning fork against vertex
    • Ask pt where sound is loudest






  • Pure Tone Audiometry


  • Rhinne Test
    • Hold tuning fork against mastoid bone
    • Ask pt where sound is loudest

  • Speech Audiometry




  • Stapedial Reflex Decay









  • Head thrust test
    • Head rotates to right
  • Vestibuloocular reflex / Caloric testing 
    • Cold water in left ear
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Retrolabyrinthine Approach

Retrolabyrinthine Approach

    • Preserves hearing, but limited exposure compared to translabyrinthine approach
    • Exposes presigmoid posterior fossa dura, lateral and posterior semicircular canal, jugular bulb, superior petrosal sinus
    • Can be combined with subtemporal craniotomy, splitting of tentorium to expose vertebro-basilar junction to dorsum sellae

INDICATIONS
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    • Basilar trunk

POSITION
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  • Fukushima lateral 
    • Patient oblique
    • Back of shoulder at edge of table near surgeon
    • Upper arm / shoulder 45 degrees anterior caudal direction
    • Head in mayfield / gel ring / horseshoe, flexed, elevated up and off from lower shoulder, vertex down

SURFACE ANATOMY
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  • A: Fukushima Outer Mastoid Triangle
    • Posterior root of zygoma -> asterion -> mastoid tip
  • B: Fukushima Inner Triangle / Trautman's Triangle
    • Superior / anterior semicircular canal -> sigmoid sinus -> jugular bulb
  • C: Macewen's Triangle
    • Flat / depressive triangle behind EAC

INCISION
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  • A: Small C-shaped retromastoid (for small tumors)
  • B: Extended retroauricular incision (large tumors)
  • C: Curvilinear lazy-S incision (standard)








CREATE FLAP
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  • Incise scalp
  • Expose 2 layer musculofascial flap 
    • Superficial: pericranium
    • Deep: fascia of temporalis, SCM, periosteum
  • Elevate anteriorly with large blunt hooks

BONY ANATOMY
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  • Identify bony landmarks
    • Posterior root of zygoma
    • Spine of henle
    • Asterion
    • Supramastoid / temporal crest
    • Mastoid tip

REMOVE CORTEX
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  • Remove bony cortex of Fukushima's outer triangle
    • Anterior border (posterior root of zygoma to mastoid tip)
    • Superior border (posterior root of zygoma to asterion)
    • T intersection marks mastoid antrum and lateral / horizontal semicircular canal

ID SIGMOID SINUS & MASTOID ANTRUM
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  • When exposed to mastoid air cells, 
    • Change drill burr to extra coarse (to not injure sigmoid sinus)
    • Use wide shallow brush like strokes (for maximum exposure)
  • Remove bone
    • Start 1 cm behind sigmoid sinus, where bone is compact, maintaining uniform depth
    • Skeletonize sigmoid sinus
    • Remove mastoid air cells to expose middle fossa dura, compact bone of bony labyrinth / solid angle (anterior limit of bony removal, lateral / horizontal semicircular canal)

ID
JUGULAR BULB, DIGASTRIC RIDGE,
FACIAL NERVE
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  • Drill retrofacial air cells to expose jugular bulb
    • ID digastric ridge
      • Exit of CN VII in fallopian canal as it exits stylomastoid foramen
      • Stylomastoid foramen is medial to anterior limit of digastric ridge
  • Expose dura (middle fossa, pre-sigmoid / posterior fossa?, sinodural angle)
  • Define lateral / horizontal (LSC) and posterior (PSC) semicircular canals
    • LSC protrudes into antrum
    • Remove small air cells around labyrinth with medium diamond burr
  • Skeletonize fallopian canal and entire couse of facial nerve











OPEN IAC
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  • ??? IF NO TRANSLABYRINTHINE APPROACH, HOW TO GET A GOOD EXPOSURE OF IAC??

  • Remove bone around internal auditory canal (IAC) 
    • Begin at porous acusticus, thin compact bone with small smooth diamond burr until thin shell remains
      • Laterally, dura only covers 2/3 of IAC
      • Transverse crest separates superior and inferior vestibular nerves
    • Remove thin shell of bone with a fine dissector
      • Start medial to lateral
      • With care near superior lip of porous acusticus because of proximity to facial nerve

DURAL OPENING
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  • Open dura
    • Start medial to sigmoid sinus, 5-10 mm below superior petrosal sinus
    • Continue toward mid portion of IAC
    • At porous acusticus, extend dural incision superior and inferiorly
  • Open dura near IAC
    • Use #11 blade, expose superior and inferior vestibular nerves
    • Reflect laterally, exposing cochlear and facial nerves
      • Bills's bar - separates facial and superior vestibular nerve
      • Transverse crest - separates facial and cochlear from superior and inferior vestibular nerves

REMOVE TUMOR
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  • **GAP BETWEEN INSTRUCTION AND DISSECTION
  • 2MM space CSF aspiration
  • Clean, bloodless, sharp dissection
  • Maintain arachnoid plane
  • Sufficient internal tumor coring / debulking before capsule elevation
  • Preserve AICA, PICA, perforators, petrosal veins, VII nerve



CLOSURE
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  • Close dura near IAC with abdominal fat graft
  • Close musculofascial flap