Skull Base Approaches for Aneurysm

Rokuya Tanikawa and Kosumo Noda

Summary

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.

Keywords: Frontotemporal craniotomy, orbitozygomatic approach, transzygomatic approach, anterior clinoid process, cavernous sinus, petrous bone, suboccipital muscles, condylar fossa

7.1 Key Learning Points

●Frontotemporal craniotomy:

–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.

●Transzygomatic approach:

–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.

●Anterior petrosectomy:

–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.

●Posterior petrosectomy:

–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:

–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.

7.2 Introduction

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).

Fig. 7.1 L- or reverse L-shaped skin flap for frontotemporal craniotomy is designed with preservation of the superficial temporal artery. The skin incision is started just in front of the tragus and extends superiorly before turning toward the midline hairline in order to not cut the trunk of the superficial temporal artery.

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).

Fig. 7.2 The temporal fascia can be incised and elevated along with the fat pad deep to it as a semicircular curve between orbital zygomatic process and root of zygoma in order to expose the zygomatic arch.

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).

Fig. 7.3 L-shaped zygoma removal can be performed using a sagittal saw for two osteotomies. The frontal zygomatic process is cut vertically, and the temporal zygomatic process is cut obliquely just anterior to the root of the zygoma.

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).

Fig. 7.4Coronal, squamous, sphenoparietal, sphenofrontal, sphenozygomatic, and sphenosquamous sutures are exposed by elevation of temporalis muscle after it is incised posteriorly in line with the vertical skin incision.

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

Fig. 7.5 Meningo-orbital band can be exposed by the skeletonization of orbit to reduce brain 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).

Fig. 7.6 The outer mastoid triangle (left side), which is defined by the root of the zygoma, the spine of Henle, mastoid tip, and asterion, is exposed by elevating the sternocleidomastoid muscle anteriorly, detaching the splenius capitis from the posterior border of the mastoid body, and elevating the occipital muscle.

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).

Fig. 7.7 Drilling of surface cortical bone of the left mastoid process should be done in a flat drilling manner, not going deeper locally to maintain the thin compact bone on important dural, vascular, or neural structures.

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).

Fig. 7.8 The first landmark to find the labyrinth is the antrum of the middle ear. The incus, one of the ear ossicles, can be found in the antrum and the yellow compact bone which is the lateral semicircular canal will be recognized just posterior to the incus.

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).

Fig. 7.9 The genu of the facial nerve can be found at the same depth as the lateral semicircular canal. The lateral, posterior, and superior semicircular canals cross at 90 degrees to each other.

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).

Fig. 7.10 The posterior petrous bone surrounding the labyrinth can be drilled away to maximize the presigmoid operative space. The superior petrosal sinus between sinodural angle and superior semicircular canal can be exposed by removing the petrous ridge.

Only gold members can continue reading. Log In or Register to continue