29 Anterior Petrosal Approach

Guilherme Henrique Weiler Ceccato, Gabriel H. Peres, Marcio S. Rassi, Jean G. de Oliveira, and Luis A. B. Borba

Anterior petrosectomy consists in an extension of middle fossa approach, combining drilling of petrous apex, especially the area called of “Kawase”, quadrilateral or posteromedial middle fossa triangle. This step allows the surgeon to have access to posterior fossa through middle fossa and even reach the lower basilar trunk.7 The middle fossa approach was first described by Parry in 1904, but was redefined and popularized using the operating microscope in 1961 by House, exposing internal auditory canal to treat acoustic neuromas with goal of hearing preservation.^{1 ,​ 2 ,​ 3} In 1965 Drake described a subtemporal transtentorial approach toward vertebrobasilar aneurysms, however how was necessary excessive brain retraction it carried risk of venous infarction by sacrificing vein of Labbé and exposure was not deep enough toward basilar artery.^{4 ,​ 5} Bochenek and Kukwa described in 1975 the extended middle fossa approach, an extradural subtemporal corridor to resect acoustic neuromas combining drilling of petrous temporal bone.^{1 ,​ 6} However, despite the approach of these authors allowed access deeper from level of superior petrosal sinus toward posterior fossa, bone drilling extended too much posteriorly until level of sigmoid plate jeopardizing semicircular canals.^{1 ,​ 6}

In 1985 Kawase et al described a more conservative variation of extended middle fossa approach for treatment of low-lying basilar aneurysms.^{1 ,​ 4} Drilling of petrous temporal bone was restricted to an area of petrous apex with no neurovascular structures, bordered medially by superior petrosal sinus, anteriorly by third branch of trigeminal nerve (V3), laterally by greater superficial petrosal nerve (GSPN) and posteriorly by arcuate eminence. Combined to this bone defect, tentorium is divided and increases posterior fossa exposure. In this way, it is possible to expose the upper petroclival area and ventrolateral brainstem (Fig. 29.1).

29.1.1 Anatomical Background

Middle fossa approach and its extension combining an anterior petrosectomy require deep anatomical knowledge of the anatomical structures of middle fossa floor. This approach uses mainly an extradural corridor, working underneath the brain and minimizing retraction of neurovascular structures.

Middle cranial base can be divided into a lateral part, containing the middle cranial fossa and upper surface of temporal bone, and a medial area, comprising the sellar and parasellar regions, where pituitary gland and cavernous sinus are located.⁸

Middle fossa is a complex skull base region limited anteriorly by sphenoid ridge, posteriorly by petrous ridge, laterally by squamous temporal bone and medially by sellar region.⁸ Medial aspect of middle fossa comprises body of sphenoid bone, tuberculum sellae, pituitary fossa, middle and posterior clinoid processes, dorsum sellae and carotid sulcus.⁸ Laterally middle fossa is composed by lesser and greater sphenoid wings, with superior orbital fissure between them, and the attachments of lesser sphenoid wing to body of sphenoid bone forming the optic canal. Greater sphenoid wing comprises the largest part of middle fossa, completing its surface with squamosal and petrous part of temporal bone.⁸ Superior orbital fissure transmits third, fourth, ophthalmic (V1) and sixth cranial nerves, besides a recurrent meningeal artery and ophthalmic veins. The maxillary (V2) and mandibular (V3) nerves run through foramen rotundum and ovale in the greater sphenoid wing.

While elevating dura mater and exposing middle fossa floor, the first structure to be identified is the middle meningeal artery (MMA) arising from the foramen spinosum, and this structures can be considered the most constant middle fossa landmarks.⁹ In its initial intracranial centimeter MMA gives branches to facial nerve near geniculate ganglion, and the artery should be sacrificed distal to this point.¹

Proceeding dural elevation, the next structures to be identified are V3 running toward foramen ovale and the greater superficial petrosal nerve (GSPN) passing underneath V3. The distance between foramen spinosum and intersection point of these two nerves is around 7.80 mm.² While working in the middle fossa floor, GSPN can be used as a landmark to estimate the location of petrous internal carotid artery, as this nerve runs superficial and medially to this vessel¹⁰ (Fig. 29.2).

Fig. 29.2(a) Cadaveric dissection exposing the Kawase’s triangle or posteromedial middle fossa triangle, bordered anteriorly by V3 and gasserian ganglion, laterally by GSPN, medially by the superior petrosal sinus and posteriorly by arcuate eminence. The middle meningeal artery is also demonstrated. The tegmen tympani is also exposed (roof of tympanic cavity), and care should be taken to not damage this region as it can leads to postoperative CSF leakage. (b) The Kawase’s triangle is a region of petrous apex, which one continues underlying V3 to join with clivus at the petroclival fissure. If necessary, careful elevation of V3 can expose more bone to be drilled. (c) and (d) Other cadaveric dissection demonstrating the anatomy surrounding the anterior petrosectomy. The trigeminal nerve can be exposed since the root in middle cerebellar peduncle until its three branches. If a tumor extends from posterior fossa toward cavernous sinus it is possible to use the same exposure provided by the extended middle fossa approach to treat this lesion. CN, cranial nerve; GG, gasserian ganglion; GSPN, greater superficial petrosal nerve; ICA, internal carotid artery; MMA, middle meningeal artery; PCA, posterior cerebral artery; SCA, superior cerebellar artery.

GSPN travels around 17 mm in length between geniculate ganglion and the level of V3.¹ GSPN arises from the geniculate ganglion of facial nerve and exit through the bone at hiatus fallopi, running in the sphenopetrosal groove to join with deep petrosal nerve from sympathetic carotid plexus, and form vidian nerve.⁹ Usually GSPN is covered by bone in its proximal segment before exiting through the hiatus for an average of 3.7 mm, but in 30% has no bony coverage.¹⁰ Lateral to GSPN lies the lesser superficial petrosal nerve, that runs to otic ganglion.⁹ The geniculate ganglion is located in 58% posterolateral to the posterior genu of carotid artery, in average separated by a distance of 6.5 mm. Geniculate ganglion is usually separated from the middle fossa floor by a layer of bone of 1.2 mm, with no bony coverage in 16% of cases.¹⁰

Foramen ovale is located anteromedial to spinosum and both of them are positioned anterior to petrous carotid artery, which runs in average 5.0 and 6.4 mm posterior to foramen spinosum and ovale, respecitvely.9 ^{,​ 10}

Cochlea is positioned in average 3.8 mm below the floor of middle fossa, superior, posterior and lateral to the posterior genu of carotid artery, separated by 2.2 mm of bone. It is located anteromedial to the fundus of internal auditory canal (IAC), encased by dense bone.¹⁰ Vestibule is a small cavity created by confluence of semicircular canals (SCCs), and is located posterior and lateral to the fundus of IAC. And SCCs are positioned posterior and superior to vestibule. The superior semicircular canal creates an elevation in the middle fossa floor, called of arcuate eminence, and laterally a thin layer of bone covers the tympanic cavity.

The internal carotid artery enters the carotid canal and runs 7.8 mm upwards as the posterior vertical segment until the posterior genu. Then it turns anteromedially and travels around 15.7 mm until the anterior genu, medial to tensor tympani muscle and Eustachian tube more laterally. Then the artery turns upwards and runs for 5.6 mm until the petrolingual ligament below gasserian ganglion, where it enters in cavernous sinus.¹⁰ Bone covering petrous carotid artery thins progressively toward V3, with no bony coverage proximal to the mandibular nerve in 80% of cases.¹⁰

The internal auditory canal makes an angle of 60 degrees approximately with the long axis of GSPN, at the mid portion of the angle between the arcuate eminence and GSPN.¹ During an anterior petrosectomy the posterior border of bone removal is the anterior wall of internal auditory canal. Also, it is important to mention that IAC is located about 5 mm deep from the middle fossa floor.¹¹

Superior petrosal sinus lies over the petrous ridge and corresponds to the lateral border of tentorium, connecting cavernous and sigmoid sinus, receiving blood also from the superior petrosal veins from the posterior fossa and closely related to the trigeminal nerve.

The bone of petrous apex medial to the carotid artery and anterior to the IAC can be safely drilled, as it contains no neurovascular structure. The region called Kawase’s triangle, bordered anteriorly by V3, laterally by GSPN, medially by superior petrosal sinus and posteriorly by the arcuate eminence gives surface landmarks to guide the surgeon while drilling petrous apex. It is interesting to comment that the Kawase’s triangle is a region of petrous apex, but the petrous apex continues underlying the trigeminal ganglion medially until join to clivus (Fig. 29.2). When drilling this region, care should be taken to not drill too much lateral or posterior to avoid injury to petrous carotid artery or cochlea, and also take care when drilling posteriorly to not damage the structures of IAC.

29.1.2 Indications

Anterior transpetrosal approach can be indicated in cases of upper clival, retrosellar, petroclival or Meckel’s cave tumors related to the upper two-thirds of clivus and medial to the internal auditory canal; tumors with attachment medial to trigeminal nerve; extension between middle and posterior fossae; basilar, AICA and vertebrobasilar aneurysms; intrapetrous lesions; ventrolateral midbrain or upper pontine intrinsic lesions.^{1 ,​ 12 ,​ 18}

Fig. 29.1Three-dimensional models simulating the anterior transpetrosal approach. (a) and (b) The neurovascular structures related with the approach are demonstrated through the bone window of a frontotemporal craniotomy. A zygomatic osteotomy is performed, and this is important in order to provide further temporalis muscle displacement and allow better exposure close to middle fossa floor. (c) The anatomical structures exposed during the extradural dissection in the middle fossa floor are exposed by the surgical point of view. (d) The exposure provided after the anterior petrosectomy is demonstrated, exposing the sixth nerve, AICA and even part of basilar trunk. (e) The seven/eight cranial nerves complex is also demonstrated in the posterior side of the exposure. (f) It is important to have a non-obstructed surgical field to work parallel to skull base at level of middle fossa floor, and that’s why the zygomatic osteotomy to further temporalis muscle displacement is important. Also, if necessary, drilling of temporal squamosal bone can be performed to reach the level of middle fossa floor and allow the surgeon to even see the upper clivus and retrosellar areas. ACP, anterior clinoid process; AICA, anterior inferior cerebellar artery; BA, basilar artery; CN, cranial nerve; GG, gasserian ganglion; GSPN, greater superficial petrosal nerve; ICA, internal carotid artery; MMA, middle meningeal artery; ON, optic nerve; PCA, posterior cerebral artery; SCA, superior cerebellar artery; SOF, superior orbital fissure; SPS, superior petrosal sinus; ** Kawase’s triangle.

Related posts:

The Microsurgical Transoral Approach Endoscopic Approach to the Craniocervical Junction Microsurgical Anatomy of the Foramen Magnum The Endoscopic Transpterygoid Approach to the Parasellar and Infratemporal Fossa Cranio-Orbit-Zygomatic (COZ) Approach Pretemporal Approach

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