1 Vascular Anatomy of the Head and Neck/Circle of Willis*

Aneek Patel, Hussam Abou-Al-Shaar, Maximiliano A. Nuñez, Georgios A. Zenonos, Paul A. Gardner, and Juan C. Fernandez-Miranda

Summary

This chapter reviews the pertinent anatomy of head and neck vasculature as it relates to skull base and cerebrovascular surgery. Understanding this anatomy is a foundational step to selecting surgical approaches and treatment modalities, knowing the clinical consequences of intraoperative decisions, and avoiding complications. In this chapter, we will break down the head and neck circulatory system into anterior and posterior circulation and review the major branches, common variants, and their clinical significance.

Keywords: Internal carotid artery, vertebral artery, anterior cerebral artery, middle cerebral artery, basilar artery, posterior cerebral artery, posterior communicating artery

1.1 Key Learning Points

●The vasculature of the head and neck can have a considerable level of anatomic variability, including variations in origin points, origin vessels, collateralization, and trajectories in relation to other anatomical landmarks.

●An understanding of the origins, courses, and variants of head and neck vessels is essential for the successful planning of skull base and cerebrovascular surgery, including the selection of the optimal approach, visualization of vital structures, proximal and distal vascular control, and limitations.

●The cavernous internal carotid artery (ICA) has the following components: short vertical or ascending segment, posterior genu, horizontal segment, and anterior genu.

●The communicating segment of the ICA carries the largest numbers of perforators to the anterior perforated substance and optic tracts. Injury to these small vessels that lie posterior to the carotid bifurcation will cause a dense contralateral motor deficit.

●The second division of the anterior cerebral artery (A2) gives off the recurrent artery of Heubner after the anterior communicating artery, which is the most common site of intracranial aneurysms. It is critical to preserve this branch whose occlusion typically results in a caudate infarct.

●The M1 segment of the middle cerebral artery delivers the lateral lenticulostriate arteries as well as the anterior temporal artery. Temporary clipping of M1 should be done as distally as possible to avoid occluding these critical M1 perforators.

●The ophthalmic artery and other branches of the ICA commonly anastomose with extracranial vessels, including the internal maxillary artery and ethmoidal arteries.

●Access to the basilar apex often requires a posterior clinoidectomy and/or transcavernous corridor; for low-lying apical aneurysms, an endoscopic endonasal approach may be an option.

1.2 Introduction

The vasculature of the head and neck consists of an anterior and a posterior circulations that give off branches as they travel up the neck and partially anastomose at the circle of Willis to provide blood supply throughout the brain. The circle of Willis sits in the center of the cranial base and can be accessed through a variety of skull base and cerebrovascular surgical techniques, each one with its advantages and limitations. The circle of Willis plays a vital role in providing adequate collateral supply to both hemispheres through communicating arterial trees both anteriorly and posteriorly. A comprehensive understanding of head and neck vasculature is fundamental for the treatment of vascular and skull base lesions as well as for preventing complications and identifying surrounding critical structures. However, this anatomic understanding must also remain fluid, as head and neck vasculature can often include natural variations or distortions because of pathology which should be taken into account preoperatively and adapted to intraoperatively. In this chapter, we will review the anatomy of the head and neck vasculature, and discuss the major anatomical variants and clinical significance of the vasculature in the head and neck as they are pertinent to skull base and cerebrovascular surgery.

1.3 Anterior Circulation

1.3.1 Cervical Carotid Artery

The internal carotid artery (ICA) has seven segments: cervical (C1), petrous (C2), lacerum (C3), cavernous (C4), clinoid (C5), ophthalmic (C6), and communicating (C7) segments (Fig. 1.1).

The common carotid artery (CCA) branches directly off the aortic arch on the left and the brachiocephalic artery on the right, which then becomes the right subclavian artery. This brachiocephalic bifurcation most commonly occurs posterior to the sternoclavicular joint.¹ The most common anatomical variation is known as a “bovine aortic arch,” in which both the left and right CCAs originate from the brachiocephalic artery.² Most often incidentally found, this variation has a prevalence of 11 to 27%.³ Bilaterally, the CCAs travel up the neck within the fibrous carotid sheath, which is made up of deep fascial layers and also contains the internal jugular vein (IJ) and vagus nerve (Fig. 1.2). Within the carotid sheath, the CCA runs medial to the IJ and anterior to the vagus nerve in most individuals.⁴ The CCA then bifurcates into the ICA and external carotid artery (ECA). The level of this bifurcation varies and is most commonly at the level of C3, approximately 1 to 2 cm above the superior border of the thyroid lamina, although it can also bifurcate as low as the level of the cricoid cartilage or as high as the hyoid cartilage.^{5 ,​ 6 ,​ 7} High-bifurcating CCAs become clinically important because they serve as cautionary surgical landmarks for a nearby hypoglossal nerve and marginal mandibular nerve.^{8 ,​ 9} For this reason, carotid stenting may be preferable over carotid endarterectomy in cases of high-bifurcating CCAs.¹⁰

After the carotid bifurcation, the ECA exits the carotid sheath and the ICA continues within the sheath. The origin of the superior thyroid artery (STA), the first branch of the ECA, varies widely between the CCA, the carotid bifurcation, and the ECA, and studies largely disagree on which variant is most prevalent.7 ^{,​ 8 ,​ 11} After potentially giving off the STA, the ECA then gives off the ascending pharyngeal artery, which supplies the larynx, after which it gives off the lingual artery.⁹ The other branches of the ECA in order include the facial, occipital, and posterior auricular arteries (Fig. 1.2). The ECA ends as the internal maxillary and superficial temporal artery, both of which are readily utilized during bypass surgery. After the bifurcation, the ICA continues within the carotid sheath toward the skull base, where it enters the carotid canal of the temporal bone.

It should be noted that the common classification system used for the segments of the ICA was made to describe the course of the ICA based on pertinent anatomical landmarks as they are encountered from an open, microsurgical perspective. However, with the increasing applications of endoscopic endonasal approaches to skull base lesions, the standard classification scheme for the ICA will also be compared to a classification scheme that is more suitable for endonasal surgery (Table 1.1). As such, what has been described microscopically as the cervical segment of the ICA, C1, can also be classified as the parapharyngeal segment of the ICA; through an endoscopic corridor, this segment is defined as the portion of ICA found behind the lateral cartilaginous eustachian tube spanning to the external opening of the carotid canal.¹²

Table 1.1 Correlating traditional ICA segments to their nearest anatomic counterparts from an endoscopic endonasal ICA classification scheme

1.3.2 Petrous and Lacerum Carotid Artery

The petrous segment of the ICA, C2, describes the portion of the ICA that courses first vertically and then horizontally through the carotid canal of the temporal bone, entirely encased in bone; however, the superior aspect of the canal may be dehiscent, placing the ICA at risk of inadvertent injury during middle fossa approaches. Within the carotid canal, the ICA is surrounded by periosteum and gives off no branches.¹³ While coursing anteromedially, C2 runs deep and medial to the greater and lesser superficial petrosal nerves and to the tensor tympani and eustachian tube.¹⁴

Upon exiting the petrous carotid canal, the ICA courses along the superior aspect of the foramen lacerum. This lacerum or C3 segment describes the stretch of ICA that bends and courses medial to the petrolingual ligament and lingual process to enter the cavernous sinus (Fig. 1.1 and Fig. 1.3). Throughout this segment, the ICA continues to be surrounded by periosteum and has a constant anatomic relationship with the pterygosphenoidal fissure and vidian nerve.^{14 ,​ 15} In fact, the pterygosphenoidal fissure represents a highly reliable landmark to identify and expose the lacerum ICA during endonasal endoscopic approaches.

1.3.3 Paraclival and Cavernous Carotid Artery

After exiting the carotid canal of the petrous temporal bone and passing through foramen lacerum, the ICA courses parallel to the clivus before entering the cavernous sinus (Fig. 1.1 and Fig. 1.4). At this level, the ICA runs within the carotid sulcus or groove, which is located in the lateral aspect of the body of the sphenoid. In cases of well-pneumatized sphenoid sinuses, the carotid grooves are readily identified at the lateral aspect of the clival recess; that is why this ICA segment has been classically named “paraclival.” This ICA segment also courses medial to V2 and the inferior aspect of gasserian ganglion for which it has also been called “paratrigeminal.”¹⁶

The upper petroclival fissure runs just behind the ICA at the carotid groove, with the petrous apex laterally and the petrosal process of the sphenoid bone medially; the top of this process can be used as a reliable landmark to identify the floor of the cavernous sinus where the abducens nerve enters from Dorello’s canal.17 ^{,​ 18}

The cavernous ICA has the following components: short vertical or ascending segment, posterior genu, horizontal segment, and anterior genu (Fig. 1.4). The posterior genu commonly serves as the origin of the meningohypophyseal trunk (or the inferior hypophyseal, tentorial, and dorsal meningeal arteries separately), which supplies the posterior pituitary gland, dorsum sella, clival dura, and tentorium, while the lateral aspect of the proximal horizontal segment is typically the origin of the inferolateral trunk that gives off branches to the lateral wall of the cavernous sinus and related cranial nerves (Fig. 1.4).¹⁹ The horizontal segment of the cavernous ICA delimits the venous compartments of the cavernous sinus into: superior, inferior, posterior, and lateral;²⁰ each compartment has distinct boundaries and dural and neurovascular relationships: the superior compartment relates to the interclinoidal ligament and oculomotor nerve, the posterior compartment bears the gulfar segment of the abducens nerve and inferior hypophyseal artery, the inferior compartment contains the sympathetic nerve and distal cavernous abducens nerve, and the lateral compartment includes all cavernous cranial nerves and the inferolateral arterial trunk.

The ICA then ascends lateral to the medial wall of the cavernous sinus and medial to V1, trochlear, and oculomotor nerves as it continues superiorly until it reaches the proximal dural ring, which is formed ventrally by the carotido-clinoidal ligament and dorsally by the carotid-oculomotor membrane.²¹ Thus, cavernous ICA aneurysms are extradural and their rupture does not lead to subarachnoid hemorrhage but may lead to the formation of spontaneous carotid-cavernous fistulae.

The segment between the proximal and distal dural rings is known as the clinoid segment (Fig. 1.4). It is not uncommon to have bony dehiscence over the ventral aspect of the clinoid segment of the carotid artery, which is vital to identify during endoscopic endonasal surgery to avoid ICA injury. Identifying the clinoid segment is particularly important in the surgical management of paraclinoidal aneurysms because this segment is the site of proximal control. Microsurgically, it can be accessed by performing an anterior clinoidectomy and distal annulectomy. Endoscopically, this segment is entered by transecting the carotido-clinoidal ligament, which forms the ventral aspect of the proximal dural ring.¹⁷ This ligament can also be calcified, connecting the middle clinoid to the anterior clinoid, making its removal significantly more difficult. The aforementioned bony dehiscence and calcified rings make studying the preoperative imaging and computed tomography scans as well as meticulous dissection intraoperatively of paramount importance. After passing through the distal dural ring, the ICA enters the intradural space.

1.3.4 Supraclinoid Internal Carotid Artery

The first major branch of the ICA is the ophthalmic artery (OphA), which arises from the medial surface of the ICA (Fig. 1.5). The OphA is responsible for supplying the muscles of the orbit as well as several facial muscles. It runs inferior to the optic nerve to enter the optic canal in the lesser wing of the sphenoid bone. The OphA branches into the critical central retinal artery (usually medial to the ciliary ganglion), which supplies the retina.²² The OphA typically branches off the inferior surface of the ICA, near the distal dural ring, and therefore risk of OphA occlusion needs to be taken into account when deciding between flow diversion and clipping for proximal ICA aneurysms.²³ However, it should be noted that the distal ophthalmic artery has significant collateral from the ethmoidal arteries, making occlusion of the proximal ophthalmic artery often asymptomatic.²⁴

Related posts:

Endovascular Options to Treat Iatrogenic Vascular Injury and Tumor Involvement of the Skull Base Dealing with Vascular Injury During Middle Fossa Surgery Simulation and Training—Preparing for Vascular Injury Dealing with Major Intraoperative Vascular Injury Extracranial Anterior Cranial Base Surgery for Vascular Tumors Extracranial Lateral Cranial Base Vascular Tumor Surgery

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