Extended Middle Fossa (Anterior petrosectomy) approach

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Middle Fossa Approach

Indications

• Petrous Apex Lesions
• Cholesterol granulomas, chordomas, chondrosarcomas, meningiomas

• Brainstem Lesions
• Anterolateral pontine cavernomas

• Upper to Mid-Basilar Aneurysms

• Can be combined with posterior petrosectomy (Combined Petrosal Approach), or FTOZ

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.

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.

Craniotomy/Bony Work:

• Includes sphenotemporal junction.

• Expose petrous apex extradurally until petrous ridge is identified.

• Elevate dura and coagulate the middle meningeal artery at foramen spinosum.

• Identify V3 (mandibular nerve), greater superficial petrosal nerve (GSPN), superior petrosal sinus, and petrous ridge.
• Peel middle fossa dura from back to front to protect the GSPN otherwise you can pull the GSPN and damage the gaserian ganglion

• Drill the Kawase triangle
Kawase triangle
• Location: between the greater petrosal nerve and the lateral edge of the trigeminal nerve (behind the point where the greater petrosal nerve passes below the lateral edge of the trigeminal nerve).
• Borders
• Anterior: Mandibular nerve (V3)
• Lateral: The major petrosal groove
• Is one of the anatomical landmarks of Kawase triangle.
• Lateral border of the Kawase triangle
• GSPN courses in this groove
• The GSPN (Greater Superficial Petrosal Nerve) is the most reliable superficial landmark on the middle cranial fossa for drilling of the petrous apex (Kawase’s triangle) in the extradural anterior transpetrosal approach (ATPA)
• The GSPN should be clearly recognized before drilling at Kawase’s triangle to avoid the risk of injury to the facial nerve and ICA.
• Especially, the GSPN can be considered as the superficial lateral border of anterior petrosectomy on the middle fossa to avoid risk of the ICA injury, because of the GSPN path running parallel in the sagittal plane to the ICA.
• GSPN confirmation for a safe extradural anterior petrosectomy and GSPN preservation to prevent symptomatic dry eye are not always easy to achieve.
• Both facial nerve-evoked EMG by antidromic GSPN stimulation and free-running facial muscle EMG monitoring are useful for the confirmation of the GSPN as a landmark for safe extradural anterior petrosectomy and for the preservation of the GSPN itself
• Formed between the
• Sphenopetrosal fissure
• Hiatus facialis
• Posterior: Internal auditory canal (IAC)
• The arcuate eminence (a marker of the Superior semicircular canal) and lies just posterior and lateral to the IAC.
• Medial: Petrous ridge/insertion of the tentorium
• Defines the location of the superior petrosal sinus.
• Contents
• Petrous apex
• IAC
• Cochlear
• Located in the angle formed by the GSPN and the facial nerve (CN VII, labyrinthine portion).
• Surgical Exposure:
• Drilling the bony floor of the triangle, specifically in the area behind the internal carotid artery and medial to the cochlea, exposes the lateral edge of the clivus.



• The posterolateral triangle (Glasscock’s triangle) is limited by V3 anteriorly, the GSPN medially, and an imaginary line from the origin of the MMA (foramen spinosum) to the origin of the GSPN from the geniculate ganglion.
• This triangle is an important landmark for exposing the petrous portion of the internal carotid artery. V1, V2=branches of CN V

• Operative exposure achieved with the Kawase approach.
• The dura is opened and the superior petrosal sinus is ligated anterior to the point where the petrosal vein reaches the superior petrosal sinus.
• The trigeminal nerve is exposed from its origin at the pons to its entrance into Meckel’s cave.
• The subarachnoid and labyrinthine portions of the facial nerve (CN VII) are exposed.
• Posterior to CN VII in the internal auditory canal (IAC), the superior vestibular nerve and, inferior to it, the inferior vestibular nerve, are visible.
• The cochlear nerve is anterior and inferior to CN VII.
• Three tributaries (i.e., the transverse pontine vein, pontotrigeminal vein, and vein of the cerebellopontine fissure) of the superior petrosal vein are exposed.
• The premeatal and meatal segments of the branch inside the IAC and the anterior inferior cerebellar artery (AICA) are visible.



• Right cerebral hemisphere: Removed to expose the middle fossa.
• Dura of the middle fossa: Peeled away.
• Middle meningeal artery (MMA): Exposed at the foramen spinosum.
• Greater superior petrosal nerve (GSPN): Visible from the geniculate ganglion to below the mandibular branch of CN V.
• GSPN travels in the floor of the middle fossa under the dura in the sphenopetrosal groove and can be either partially covered or not covered at all by bone. It is medial to the lesser petrosal nerve and could easily be mistaken for the latter.19 Hence, nerve stimulation can help identify the GSPN.
• Geniculate ganglion: Shown.
• Facial nerve portions: Labyrinthine, tympanic, and subarachnoid portions are displayed.

• Technical notes of drilling Kawase
• Begin drilling in the center of the defined Kawase triangle—the area “devoid of important structures.”
• The GSPN forms the lateral boundary and is protected.
• The GSPN must be preserved and kept moist; accidental transection or retraction can cause facial nerve dysfunction or dry eye.
• Stay anterior to the IAC and lateral to the petrous ridge, avoiding injury to the cochlea, ICA, and facial/vestibulocochlear nerves.
• Drill down through the petrous apex bone systematically and with copious irrigation to avoid thermal injury.
• Continue until the dura of the posterior fossa is exposed, which is visible as a blue membrane deep to the apex.
• Do not extend drilling too far posteriorly or inferiorly, or you risk injuring the semicircular canals, facial nerve (in the IAC), or ICA.
• Adjust the angle of the microscope for visibility—this can improve working freedom despite the limited corridor.

• Ligate/section superior petrosal sinus as needed for exposure.

• Drill bone “devoid of important structures” for safe entry to posterior fossa.

Dural Opening:

• Middle fossa dura elevated

• Superior petrosal sinus may be ligated before dural opening.
• To identify the sinus:
• The sinus runs along the petrous ridge, just posterior to the porus trigeminus (trigeminal nerve entry to Meckel's cave).
• Trace the sinus to the area just before it receives the superior petrosal vein. It's crucial to ligate the sinus anterior to the vein's entry point, preserving normal venous drainage into the transverse sinus.
• Incision of dura
• Make small incisions above and below the petrous ridge, reflecting the dura to expose the superior petrosal sinus fully.
• Carefully protect the trochlear nerve (CN IV), which courses beneath the tentorium and can be accidentally cut at this point
• Use micro clips or cautery to close off the sinus segment.
• Ligation is performed by gently dissecting and placing ties or clips around the sinus, ensuring minimal disturbance of adjacent vein influx.
• Hemostasis is confirmed before proceeding.

• Posterior fossa opening
• Localization: The posterior fossa (lateral pontine zone, petroclival region) dura is visible as a blue sheath deep to the petrous apex.
• Incision:
• Make a precise, linear or curvilinear incision in the posterior fossa dura with a microsurgical blade or scissors.
• The incision is typically made parallel to the petrous ridge and anterior to the sigmoid sinus.
• Reflection:
• Reflect the dural leaflets gently to reveal the CPA cisterns, exposed cranial nerves, and vascular structures (e.g., basilar trunk, AICA, trigeminal root, VII/VIII complex).
• Safety Checks:
• Avoid inadvertent injury to the underlying brainstem, nerves, or vessels.
• Maintain CSF egress for brain relaxation if needed; use small cottonoids or patties to protect underlying anatomy during entry.
• Carefully handle the dural edges to prevent vein or nerve entrapment during closure.
• Exposure the basilar trunk, the emergence of the AICA, and abducens nerve are visible medially
• The nerves inside the IAC are visible posteriorly.
• The brain stem is exposed from the pontomedullary sulcus, and the anterolateral portion of the pons between CN V and CN VII is also visible.

Important neurovascular structures

• Middle meningeal artery (coagulated/sectioned early)

• Mandibular nerve (V3)

• Greater superficial petrosal nerve (GSPN)

• Superior petrosal sinus (ligated)

• Facial nerve (VII) and cochlear nerve (VIII) in IAC
Facial Nerve Visualisation
• The GSPN is a critical landmark for safe drilling.
• Facial nerve’s labyrinthine and tympanic segments visualized, with intraoperative EMG recommended for safety.
• Drilling within Kawase triangle is safe for the facial nerve if boundaries respected.
• Facial nerve inside IAC posterior to Kawase triangle is identified; injuring beyond this zone risks nerve damage.
Hearing Preservation
• Safe extradural drilling within Kawase triangle typically preserves cochlea and semicircular canals, maintaining hearing.
• Hearing loss risk is very low if drilling is not extended too far posteriorly or inferiorly.
• Landmark-based technique enables consistent preservation of critical structures affecting hearing, as shown in clinical outcomes.

• Trigeminal nerve (CN V, including root entry at pons and Meckel’s cave)

• Internal carotid artery (petrous segment)

• Cochlea and semicircular canals (preservation crucial for hearing)

• Abducens nerve (VI) and AICA, as the approach exposes the basilar trunk and adjacent cranial nerves

Pros

• Direct anterolateral access to petroclival region and ventral brainstem with less cerebellar or brainstem retraction.

• Good exposure of the basilar trunk, upper brainstem, and major vascular structures.

• Uses reproducible anatomical landmarks for safe drilling (Kawase triangle reduces risk of vascular/neural injury).

• Preserves hearing and facial nerve function if drilling remains within safe limits.

• Can be combined with other approaches for expanded exposure.

Cons

• Technically demanding with narrow operative corridor.

• Risk of injury to GSPN, facial nerve, cochlear nerve, superior petrosal sinus, and ICA if anatomical landmarks not properly identified.

• Requires careful dural and vein handling (risk to superficial middle cerebral vein or petrosal vein).

• Anatomy can be variable, requiring adaptation during surgery.

• Posterior/inferior extension beyond IAC or into the labyrinth increases risks to hearing and facial nerve.

Reference

• Maximizing the Corridor for the Anterior Petrosal Kawase's Approach

• Middle cranial fossa triangle

 

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