Neurosurgery notes/Procedures/Cranial procedures/Approaches/Skull base approaches/Middle Fossa Approach

Middle Fossa Approach

View Details
Status
Done
logo
Parent item
Skull base approaches
Sub-item
Pterional Craniotomy
Subtemporal approach
Extended Middle Fossa (Anterior petrosectomy) approach

General

  • Goal of the middle fossa approach is to expose the IAC from above, from the roof of the petrous temporal bone

Indication

  • Approach adequately exposes the facial (CN VII) and superior vestibular nerves within the IAC. It permits exposure of the subarachnoid, intracanalicular, and labyrinthine segments, and the first portion of the horizontal (tympanic) segment of CN VII
  • Useful for removing small lesions (up to 2 cm in diameter), primarily intracanalicular, with minimal extension into the posterior fossa.
    • Depending on the size of a lesion, the possibility for preserving hearing with the middle fossa approach is good.
    • It is useful whenever the above segments of CN VII need to be decompressed.
    • Because the approach is from the superior surface of the temporal bone, the risk of inadvertently opening the posterior semicircular canal is less than that associated with the retrosigmoid approach, which involves drilling the posterior wall of the IAC.

Surgical technique

Positioning

  • The long axis of the head is positioned parallel to the floor with the ear facing the ceiling and the neck slightly extended.
  • Skin incision for the middle fossa approach (dashed line).
  • The patient’s head is rotated 20º to the contralateral side.
A white skull with holes in it AI-generated content may be incorrect.
•ostero-medial Angle of Attack
Postero-medial angle of attack

Incision

  • A vertical incision, 5 to 6 cm long, is made anterior to the tragus and perpendicular to the zygomatic arch
  • A question mark incision (dotted line) is preferred when anterior extension is desired (e.g., transKawase approach) and reaching the petrous apex is the goal.
  • Fascia and temporal muscle are exposed and split with the Bovie electrocautery device and then retracted with a self-retaining retractor or fishhooks.
A drawing of a person's head AI-generated content may be incorrect.

Craniotomy

(A) The skin incision and temporalis muscle have been retracted with fishhooks to expose the temporalis squama and the root of the zygomatic arch
(B) A square-shaped craniotomy 3.5-cm square.
  • Superior limit is the temporal squama
    • Squamous suture, which is a good landmark for identifying the superior limit of the craniotomy
  • Inferior limit is the zygomatic arch.
  • A third of the craniotomy is positioned posterior and two-thirds of it anterior to the external auditory canal
A diagram of the human skull AI-generated content may be incorrect.
 

Intradural

  • Sometimes part of the temporal rim must be removed to obtain a flat exposure and thus minimize retraction of the temporal lobe
  • Dura is elevated posteriorly to anteriorly to prevent injury to the GSPN and lesser petrosal nerve
    • Anterior portion
      • GSPN and Lesser petrosal nerve
        • Both nerves are just beneath the dura and become visible once it is elevated.
        • Both nerves are delicate and the operating microscope must be used to identify them.
        • Both nerves can be distinguished by electrical stimulation
      • MMA
        • When the dura is retracted medially, the middle meningeal artery, with its anterior and posterior branches, can be seen emerging from the foramen spinosum.
        • The middle meningeal artery is sectioned just after it exits the foramen spinosum, which is then exposed along with V3.
    • Posterior portion
      • Near the petrous ridge, the arcuate eminence becomes visible.
      • The angle between the arcuate eminence and the GSPN is approximately 120º.
A diagram of the ear AI-generated content may be incorrect.

IAC exposure

  • After the GSPN, arcuate eminence, and petrous ridge are identified, the IAC can be localized by any of the following three techniques.
      • (A) Fisch technique:
      • A line is drawn over the long axis of the arcuate eminence
      • Another line is drawn 60º to the first line away from the arcuate eminence.
      • IAC between the two lines
        • Typically is 3 to 4 mm below the petrous ridge
      (B) Garcia-Ibañez technique:
      • GSPN and the long axis of the arcuate eminence are identified by tracing two imaginary lines over these structures.
      • At their bisection, another line is drawn over the anterior lip of the IAC. Drilling proceeds along this line to expose the IAC.
      (C) House technique:
      • The GSPN is followed until the geniculate ganglion is exposed
        • Need to drill the floor of the middle fossa (2 to 3 mm), to identify the geniculate ganglion
      • Junction between the geniculate ganglion and the facial nerve is not on the same plane as the IAC but rather is slightly posterior. Therefore, most of the geniculate ganglion must be unroofed to expose the facial nerve medially.
      • The labyrinthine portion of the facial nerve is traced until it reaches the IAC.
      • It is unnecessary to expose the arcuate eminence, thereby reducing retraction of the temporal lobe
      A diagram of the structure of the body AI-generated content may be incorrect.
       

Exposure of IAC internal structures

  • After the roof of the IAC has been drilled (3 to 4 mm below the floor of the middle fossa), Bill’s bar is identified.
    • Anterior to Bill's Bar is the superior vestibular nerve.
  • Adjacent to the IAC is the meatal portion of the AICA
    • Forms an important loop, sometimes enters the IAC, and then passes through the nerves.
    • 54% of meatal segments protrude into the IAC.
    • Internal auditory artery sends branches to the bone and dura lining the IAC
      • Internal auditory artery emerged from the
        • Premeatal segment in 77% of their anatomical specimens,
        • Meatal segment in 21%
        • Post-meatal segment in 2%
        • This artery also can emerge outside or at the meatus.
      • Its patency is a prerequisite to the preservation of hearing.
  • Recurrent artery
    • Is also important because it supplies blood to the pons, middle cerebellar peduncle, and the entry zone of the trigeminal nerve (CN V).
    • It can be found between or anterior, anteroinferior, or superior to the facial and vestibular nerves.
    • Early identification and preservation of these arterial branches inside the IAC are fundamental to the preservation of hearing during the resection of acoustic neuromas.