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Middle Cranial Fossa Approach to the Internal Auditory Canal restricted

last modified on: Mon, 09/17/2018 - 10:52

return to: Otology - Neurotology 

Room Set-up

  1. Patient images;  Stenver view mastoid to assess bone thickness overlying superior canal; MRI coronal and axial (post-contrast) showing tumor and confirming operative side.

     
  2. Patient clinic notes.  Pre-operative note, audiogram, ABR test results and Internal medicine surgical co-management note placed on wall

White board:  intra-operative medications (1 gram Ancef Q8hrs, 10 mg Decadron q6 hrs, Mannitol at 0.4 mg/kg given after performing craniotomy).  Nursing instructions to call family every 3 hours

ABR settings (0.05 mA stimulation)




Patient positioning with 3 point straps to allow rolling of the bed to improve operative exposure





Electrode placement;  NIMS monitor electrodes in the orbicularis occuli and orbicularis oris with ground electrods on the chest;  ABR click generator in the operertive side ear canal;  3 ABR electrodes (one on each mastoid and one on vertex).  Also note the posteriorly based temporal skin flap design.



 
 

  1. OPERATIVE PROCEDURE
  2. Incision and Elevation of Skin Flaps
    1. Two possible incision designs:
      1. Anterior/inferiorly-based skin flap (MH)
        1. Incision starts anterior to tragus, extends posteriorly approximately 3-4 cm posterior to pinna, superiorly 5-6 cm, and anteriorly again to the temporal hairline
        2. Good for extended middle fossa approaches
        3. Temporalis muscle is then reflected inferiorly
      2. Posteriorly-based skin flap (BJG)
        1. Incision starts just behind temporal hairline and a rounded box shape approx 6 cm wide is carried back approximately 6-7 cm.
        2. Begin as low onto pinna as possible
        3. Temporalis muscle flap is then reflected anteriorly
      3. Keep temporoparietal fascia layer attached to scalp during skin flap elevation
    2. Harvest a large piece of temporalis fascia prior to elevation of the muscle flap
      1. leave cuff of fascia on either side of muscle flap
    3. Elevate muscle flap
      1. Anterior temporalis flap if posterior-based skin flap
      2. Inferiorly-based temporalis flap if anterior-based skin flap
    4. Should be able to see zygomatic root easily after elevation
  3. Craniotomy
    1. Have anesthesiologist administer 0.4 grams/kg of mannitol now
    2. Hyperventilate patient to end tidal CO2 of 30
    3. Center craniotomy on zygomatic root
    4. 4.5 x 4.5 cm wide bone flap
    5. Use a 4 mm (BJG) or 5 mm (MRH) cutting burr to remove the majority of the bone.
      1. Use diamond 4 mm burr, to remove the final layer of bone over the dura.
    6. Branches of the middle meningeal artery will be encountered, which are controlled with bone wax or bipolar cautery
    7. Mark bone flap to ensure easy and properly oriented replacement at end of case.
    8. Elevate bone flap off of dural with Joker elevator.
    9. Pass bone flap off to scrub nurse
    10. Keep bone flap moist
    11. Check exposure: if the bone window is not flush with the tegmen, remove excess bone with rongeur or drill
  4. Dural Elevation
    1. Circumferentially elevate the dura from the overlying cranium, using the bipolar cautery liberally to stop bleeding from the dura.
    2. Place Oxycel cigars under the bone flap anteriorly, posteriorly, and superiorly.
    3. Elevate the dura along the floor of the middle cranial fossa from posterior to anterior so as not to disrupt the greater superficial petrosal nerve (GSPN). (SEE FIGURE)
    4. Identify the arcuate eminence, GSPN, and petrous ridge.
    5. Use cottonoids anteriorly and posteriorly for dural retraction during elevation.
    6. Brisk bleeding from the middle meningeal artery at the foramen spinosum may be encountered and can be controlled with bone wax and Oxycel packing.
    7. Place the House-Urban retractor under the lip of the petrous ridge at the anticipated location of the IAC (based upon the arcuate eminence).
  5. Identification of the IAC
    1. Begin drilling using a 4-0 diamond burr over the arcuate eminence to identify the location of the superior semicircular canal.
      1. The SSCC will be perpendicular to the petrous ridge
      2. The Stenvers x-ray will demonstrate the amount of bone overlying the semicircular canal (SSCC).
    2. Blueline the superior canal.
    3. The IAC is located 60° anterior to the blue lined SSCC. (SEE FIGURE)
    4. Lower the meatal plane over the IAC until it is well-defined
  6. Tumor exposure
    1. Drill deep troughs 270 degrees around the IAC, down to the level of the posterior fossa dura
    2. Develop your exposure working medial to lateral
      1. Exercise caution when drilling laterally, due to the location of the cochlea and ampulla of the SSCC.
    3. Skeletonize the IAC up the level of Bill\’\’s Bar.
    4. Identify the labyrinthine segment of the facial nerve at the transverse crest and decompress the meatal foramen a few millimeters.
    5. The cochlea is deeper than the plane of the labyrinthine segment of the facial nerve.
      1. As long as one does not drill deep to the facial nerve anteriorly, the cochlea will not be violated.
    6. Remove the last flecks of bone from the IAC dura.
    7. Have audiologist start monitoring ABR
  7. Tumor Removal
    1. Open the dura of the IAC over the superior vestibular nerve with a 59-10 Beaver blade or 2.5 mm hook.
    2. Place a direct auditory nerve electrode between the dura of the IAC and the cochlear nerve for monitoring the cochlear action potentials in real time.
    3. Identify the separation between the facial nerve and superior vestibular nerve at the transverse crest.
      1. Separate the facial nerve from the superior and inferior vestibular nerves at this location.
      2. Avulse lateral ends of the vestibular nerves, and reflect medially.
    4. Use a 2.5 mm hook to separate the tumor from the facial and cochlear nerves.
      1. Always work from medial to lateral when possible.
      2. When tumor has been separated, cut the medial ends of the vestibular nerves along with the tumor, thus removing the tumor.
    5. Special considerations
      1. In larger tumors, it is often necessary to debulk the tumor as for TL excision, prior to establishing a plane between the facial nerve and the tumor.
      2. If the auditory response demonstrates prolongation of latency or a decrease in amplitude, switch strategies or pause tumor dissection for several minutes.
      3. A cochlear nerve that no longer responds intraoperatively may still have hearing postoperatively; unfortunately, the reverse is also true.
  8. Closure
    1. Ensure hemostasis in IAC/CPA
    2. Document stimulation of facial nerve with Prass Probe on 0.05 mAmp
    3. Document ABR/Direct Nerve Monitoring after tumor removal
    4. Repairing temporal bone defect
      1. Apply bone wax to all opened air cells.
      2. Place a large temporalis muscle plug (BJG) or abdominal fat (MRH) in the IAC defect.
      3. Place the previously harvested fascia over the entire temporal bone defect.
      4. Use the anterior corner of the bone flap, or a piece of the inner table, over the defect to prevent temporal lobe herniation into the middle ear, if it exposed.
      5. Bolster fascia into place with Oxycel ™ cigar
      6. Tisseal ™ is optional to further secure fascia
    5. Craniotomy closure
      1. Release the House-Urban retractor and allow the temporal lobe to re-expand.
        1. Relay to anesthesiologist that the EtCO2 can be allowed to rise
      2. Ensure an accurate neuro pattie count.
      3. Tack dura to pericranium with at least two 4-0 neurolon sutures to eliminate epidural potential space
      4. Double check epidural hemostasis
      5. Replace the bone flap.
        1. Simply replace and allow to float (BJG)
        2. Plate back with three plates (superior, anterior, posterior) reabsorbable system (MRH)
          9. Close the temporalis muscle with water-tight interrupted 3-0 vicryl sutures.
    6. Close the subcutaneous layer with water-tight interrupted 3-0 vicryl sutures.
    7. Close the skin with running-locking 3-0 nylon sutures.
    8. Place a large, tight mastoid dressing centered over surgical area.

MCF Anatomy

          

         

          

 
  1. For POSTOPERATIVE CARE , see Skullbase Post-Operative Care Map
  2. CPT CODING
    1. 61591 Middle cranial fossa approach
    2. 61616 Intradural removal of tumor
    3. 69990 Use of Operating Microscope
    4. 95867 Facial Nerve Monitoring
    5. 62140 ( <5 cm2) or 62141 ( > 5 cm2) cranioplasty
    6. 15770 Abdominal fat harvest
    7. *Audiologists bill and code the ABR monitoring separately*
  3. SUGGESTED READING
    1. Gantz BJ.  Middle Cranial Fossa Technique Lecture Slides
    2. Chen DA, Arriaga MA, Fukushima T. Technical refinements in retraction for middle fossa surgery. Am J Otol. 1998;19:208-211.
    3. Gantz BJ, Harker LA, Parnes LS, McCabe BF. Middle cranial fossa Acoustic neuroma excision: results and complications. Ann Otol Rhinol Laryngol. 1986;95:454-459.
    4. Nadol JB. Cerebellopontine angle tumors. In: Nadol JB, Schuknecht HF, eds. Surgery of the Ear and Temporal Bone. New York, NY. Raven Press, Ltd. 1993:391-413.
    5. Weber PC, Gantz BJ. Results and complications from acoustic neuroma excision via middle cranial fossa approach. Am J Otol. 1996;17:669-675.