Skull base approaches can be used to augment all standard cranial approaches to aneurysms. Anterior petrosectomy and posterior petrosectomy with mastoidectomy, condylar fossa, sphenoid, or temporal bone drilling all help reduce or avoid brain retraction when accessing deep-seated aneurysms. Even removal or disconnection of the zygomatic arch, when added to frontotemporal craniotomy, enables the visualization and treatment of high-riding lesions. In this chapter, the application of skull base approaches to intracranial aneurysm surgery will be described.
Key wordsFrontotemporal craniotomy – orbitozygomatic approach – transzygomatic approach – anterior clinoid process – cavernous sinus – petrous bone – suboccipital muscles – condylar fossa
7 Skull Base Approaches for Aneurysm
7.1 Key Learning Points
It can be used for distal basilar aneurysms and all anterior circulation aneurysms except distal anterior cerebral artery aneurysm.
Frontal and temporal lobes should be exposed evenly to place the sylvian fissure in the middle of the craniotomy window.
Resecting the lateral sphenoid ridge to flatten the base of the craniotomy provides maximal exposure of the supraclinoid carotid artery.
Drilling and skeletonizing the posterolateral wall of orbit extend the basal operative space.
It can be used for high-positioned aneurysms like basilar tip aneurysms, carotid bifurcation aneurysms, and superiorly projecting M1 aneurysms.
Further inferior retraction of the temporalis muscle by removing the zygomatic arch extends the basal operative field at the pterion compared to standard frontotemporal craniotomy.
Lateral cavernous wall dissection:
Increases the anterior temporal space without sacrificing tributaries of superficial sylvian veins crossing the sylvian fissure by retracting the temporal lobe epidurally.
Reduces mechanical injury to the oculomotor nerve by opening the oculomotor foramen and releasing the cavernous segment of the nerve by incising the connective tissue between it and the cavernous sinus.
The operative field obtained via anterior petrosectomy is the space between the trigeminal foramen and internal auditory canal (IAC).
Midbasilar aneurysms between the IAC and trigeminal foramen can be exposed through this approach.
Important landmarks, like superior orbital fissure, foramen rotundum, spinosum, ovale, posterior border of V3, greater superficial petrosal nerve (GSPN), C6 segment of internal carotid artery (ICA), petrosal edge, superior petrosal sinus (SPS), arcuate eminence, geniculate ganglion, inner petrosal dura, IAC, and cochlea must all be identified during middle fossa dissection.
It is key to identify Glasscock’s and Kawase’s triangles and the rhomboid of middle fossa.
It can be used to access distal vertebral or lower basilar aneurysms located between the jugular foramen and IAC.
The space between superior semicircular canal and sinodural angle is opened via a mastoidectomy with removal of the petrous edge.
Mastoidectomy is necessary to perform a posterior petrosectomy.
Important anatomical landmarks for mastoidectomy are the root of zygoma, supramastoid crest, parietomastoid suture, lambdoid suture, occipitomastoid suture, asterion, spine of Henle, mastoid tip, outer mastoid triangle, sinodural angle, SPS, sigmoid sinus, transverse sinus, digastric ridge, jugular bulb, mastoid antrum, incus, lateral semicircular canal, Fallopian canal, posterior semicircular canal, superior semicircular canal, endolymphatic sac, presigmoid dura, stylomastoid foramen, chorda tympani, facial recess, and the outer rim of the tympanic membrane.
Suboccipital muscular layer-by-layer dissection:
This technique provides a shallow operative field without a bulky suboccipital muscle mass and enables use of shorter instruments (e.g., during occipital artery–posterior inferior cerebellar artery [OA-PICA] bypass).
The layers of muscle are divided with suboccipital muscles, occipital muscle, sternocleidomastoid, and trapezius in the first layer; splenius capitis, longissimus capitis, semispinalis capitis, and digastric muscle in the second layer; and superior oblique, rectus capitis major and minor, and inferior oblique muscles in the third layer.
All muscles except the sternocleidomastoid, trapezius, and occipital muscles are innervated by a posterior branch of the C2 spinal nerve. The sternocleidomastoid and trapezius muscles are innervated by the accessory nerve, and the occipital muscle is innervated by the facial nerve.
Lateral suboccipital craniotomy:
The intracranial vertebral artery can be exposed by this approach if the aneurysm is located lower than the eighth nerve.
If the vertebral aneurysm is shifted ipsilaterally this approach alone is adequate.
Transcondylar fossa approach:
Aneurysms involving the entire intracranial vertebral artery to the lower basilar artery (BA) can be exposed using this approach.
This provides the extended view necessary for distal vertebral or lower BA shifted contralaterally.
Important landmarks are the hip of sigmoid sinus where the sigmoid sinus curves anteromedially toward the jugular bulb, occipital condyle, C1 condylar facet where it faces the occipital condylar facet, posterior condylar emissary vein and canal, marginal sinus, vertebral venous plexus covering the V3 segment of vertebral artery, and the hypoglossal canal.
The concept of a skull base approach is the removal of bone to reduce brain retraction and neurovascular manipulation when approaching various deep-seated lesions.
An accurate knowledge of osseous, vascular, and neural anatomy is critical. Their relationships are relatively simple because every important neural and vascular structure is surrounded by compact bone which works as a protector and potential conduit for these vital structures.
7.3 Transzygomatic Approach
Removal of the zygomatic arch provides wide subtemporal exposure with added inferior retraction of the temporalis muscle. As a result, the retracted temporalis muscle bulk does not encroach on the middle fossa floor and pterion enabling a bright and wide operative field. Although the orbitozygomatic approach may be necessary if the main operative corridor is a subfrontal or transcavernous route, a transzygomatic approach is sufficient when a distal transsylvian with anterior temporal approach is the main corridor to the target lesion. For example, basilar tip superior cerebellar artery aneurysms are a good indication for a transzygomatic approach.
The patient should be supine with the head rotated 30 degrees toward the contralateral side and the head of bed elevated 15 to 20 degrees (Low-Fowler’s position).
The skin incision is designed as a curved line which starts in front of the tragus just posterior to the trunk of the superficial temporal artery, in order to avoid its injury, then curving up toward the midline hairline (Fig. 7.1).
The skin flap is elevated just above the temporalis fascia in a two-layer/differential fashion until exposing the orbitozygomatic process with frontozygomatic suture which is covered by the periosteum and connective tissue connecting to the deep temporal fascia. The fat pad of the superficial temporal fascia is elevated with a semicircular arc incision in order to expose the zygomatic arch between the root of zygoma and temporal zygomatic process (Fig. 7.2).
The exposed zygomatic arch is cut obliquely at just anterior to the root of the zygoma and just anterior to the temporal zygomatic process with a bone saw or reciprocating saw as a T-bone shape. The masseter muscle, which attaches to the inferior border of zygomatic arch, and temporalis muscle, which connects to the inner surface of the zygomatic arch, must be detached to remove the zygomatic arch (Fig. 7.3).
The temporalis muscle can be incised in line with the posterior vertical part of skin incision down to the root of the zygoma, then detached from the superior temporal line and dissected and retracted inferiorly (Fig. 7.4).
Next, a frontotemporal craniotomy can be performed. After the bone flap is removed, the remaining lateral sphenoid ridge is drilled away until the meningo-orbital band is exposed (Fig. 7.5). Drilling the remaining bone lateral to the meningo-orbital band extends the basal operative field and ensures no disturbance of microscope illumination regardless of zygoma retraction
7.4 Transpetrosal Approach (Includes Posterior Petrosectomy and Anterior Petrosectomy)
7.4.1 Posterior Petrosectomy
A posterior petrosectomy can be done via mastoidectomy which involves bone removal posterolateral to the superior semicircular canal, whereby the semicircular canals and Fallopian canal are skeletonized, exposing the sigmoid sinus, SPS, endolymphatic sac, presigmoid dura, and digastric ridge. This is used to access distal vertebral or lower basilar aneurysms located between the jugular foramen and IAC.
Mastoidectomy 1 with preservation of the labyrinth is not easy; it requires not only careful training with a high-speed drill, but also a thorough comprehension of the anatomy. The landmarks to begin a mastoidectomy are the root of the zygoma, external ear canal, spine of Henle, supramastoid crest, mastoid tip, digastric groove, and asterion. The outer mastoid triangle can be defined by the root of zygoma, spine of Henle, mastoid tip, and asterion (Fig. 7.6).
The rough drilling of superficial compact bone along the outer mastoid triangle to expose the cancellous bone and mastoid air cells is the first step of a mastoidectomy. The superoposterior wall of the external ear canal must be preserved as a thin wall of 0.5 to 1.0 mm thickness 1 while preserving the spine of Henle which attaches to the posterior external ear canal in order to preserve the very thin skin layer of this canal (Fig. 7.7).
The sigmoid sinus can be skeletonized as a blueish bulging along the posterior aspect of the outer mastoid triangle. The wall of the sigmoid sinus should be protected by retaining an “eggshell” or “paper thin” compact bone on it to not disrupt the venous sinus wall. The presigmoid dura can be exposed gradually after skeletonization of the sigmoid sinus.
The temporal dura can be exposed by drilling the bone along the superior aspect of the outer mastoid triangle and the SPS can be exposed by the skeletonization of the temporal dura and sigmoid sinus at the sinodural angle.
The mastoid antrum will be opened in the depth posterior to the root of the zygoma and a yellow compact bone can be found beside the mastoid antrum which is the lateral semicircular canal. The lateral semicircular canal is generally 15 mm from the surface of the mastoid bone. 1 It is important to find the mastoid antrum first, not to try to find the lateral semicircular canal directly. As the lateral semicircular canal is in the posterior part of the antrum, it can then be exposed just after opening the mastoid antrum (Fig. 7.8).
The tympanic segment of the facial nerve lies just anterolateral to the lateral semicircular canal and the corner between the tympanic segment and the Fallopian segment is called the genu of the facial nerve.
As the semicircular canals are surrounded by cancellous bone in the mastoid air cells, their yellow, hard, compact bone can be recognized easily under microscope. Each plane of the semicircular canals crosses at right angles: the posterior semicircular canal is posterior to the lateral semicircular, and the superior semicircular canal is superior to the lateral semicircular canal (Fig. 7.9).
The jugular bulb can be skeletonized by taking cancellous bone inferior to the posterior semicircular canal and posterior to the Fallopian canal, which overhangs the anterior half of the jugular bulb. In order to remove the bone over the jugular bulb, accurate skeletonization of the Fallopian canal and eggshell exposure of the facial nerve for its protection is necessary. The digastric ridge is found by the drilling cancellous bone lateral to the digastric groove where the posterior belly of the digastric muscle attaches. The Fallopian canal connects to the stylomastoid foramen at the anterior part of digastric ridge (Fig. 7.10).
The purpose of this approach is to obtain a sufficient operative field between the labyrinth and cerebellum to approach the ventral brainstem and the distal vertebral and lower BA. Complete drilling of the petrous edge posterior to the superior semicircular canal maximizes the operative field because the posterior edge of the petrous bone remaining after mastoidectomy blocks the corridor to the ventral pons.
7.4.2 Anterior Petrosectomy
Midbasilar aneurysms between the IAC and trigeminal foramen can be exposed through the anterior transpetrosal approach. The boundary between the posterior and anterior petrosectomy is the superior semicircular canal which is localized by the arcuate eminence. The temporal rhomboid is defined by the posterior border of the mandibular branch of the trigeminal nerve (V3), the petrous edge, arcuate eminence, and GSPN and can be exposed by the elevation of the middle fossa dura after ligating the middle meningeal artery at foramen spinosum and incision of the dura propria to expose V3 at foramen ovale. Care is taken to preserve GSPN to avoid damage to the facial nerve via retraction injury to the geniculate ganglion. 1 GSPN must be identified in the bony hiatus between the geniculate ganglion and V3. GSPN continues into the vidian canal under the mandibular nerve (V3) as the vidian nerve which connects to the pterygopalatine ganglion which innervates secreting glands in the head and neck including the lacrimal glands.
Glasscock’s triangle (G) and Kawase’s triangle (K) are important triangles to locate the petrous (C6) portion of ICA just beneath GSPN (Fig. 7.11).
Drilling the temporal rhomboid begins at the anteromedial corner of the rhomboid and proceeds toward the posteromedial corner at the crossing point of the petrous edge and arcuate eminence. The bone along the petrous edge is a safe area to drill and the superior wall of the IAC is covered by thick bone (Fig. 7.12).
On the other hand, drilling near GSPN must be done carefully because the C6 segment of ICA is just below GSPN and the cochlea is just medial to the corner of C6–C7 segment of ICA (Fig. 7.13).
The petrous apex can be drilled by elevating the trigeminal root after releasing the fibrous ring of the trigeminal foramen. Exposure and drilling of the petrous apex is only necessary to approach midbasilar to lower basilar lesions. Upper basilar lesions can be exposed in the space anterior to the trigeminal nerve root by opening the porus trigeminus.
The IAC is skeletonized by drilling the posterior part of the temporal rhomboid. The axis of the IAC divides the angle between GSPN and the superior semicircular canal at 60-degree angles between the cochlea and superior semicircular canal. 1