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

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

After passing through the distal dural ring, the ICA runs posteriorly and then superiorly until it bifurcates into the anterior cerebral artery (ACA) and middle cerebral artery (MCA) at the circle of Willis. Before this bifurcation, the ICA gives off several critical branches: the superior hypophyseal artery, the ophthalmic artery, the posterior communicating artery, and the anterior choroidal 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

The medial aspect of the ophthalmic segment of the ICA may give off one or more superior hypophyseal arteries (SHAs) that supply the inferior and anterior aspects of the chiasm, stalk, and pituitary gland. A recent study, however, has shown that the origin of the SHA is often at the clinoidal ICA segment. ²⁵ The anatomy and potential displacement of the SHAs become particularly important during suprasellar surgery, as their displacement over the superolateral aspect of tumors such as meningiomas and craniopharyngiomas dispose them to a greater risk of injury from an “open,” lateral approach in comparison to an endonasal one. There are many variations in the branching pattern of the SHAs, but most commonly there are three branches: infundibular anastomotic, which supplies the stalk and universally anastomoses with its counterpart; optic or recurrent, which vascularizes the inferior and anterior aspects of the precanalicular optic nerve and chiasm; and the descending or diaphragmatic, which irrigates the dural diaphragm and/or the upper surface of the gland. The SHA branches supplying the chiasm and stalk should be preserved in every instance while SHA branches that supply the sellar diaphragm can be more readily sacrificed if necessary. It is important to note that craniopharyngiomas and other suprasellar lesions will often get vascular supply from the SHA branches, and selective cauterization is mandatory to preserve the critical main trunks.

The ICA next gives off the posterior communicating artery (PComA) either posteriorly or laterally. The PComA runs posteromedially to reach the posterior cerebral artery (PCA) above the oculomotor nerve, which is most commonly medial, although it can occasionally run lateral, to the nerve (Fig. 1.1c). Due to this proximity, PComA aneurysms can often present with third nerve palsies. ²⁶ Approximately 4 to 15 perforating vessels can be found throughout the length of the PComA, the largest of which is designated the anterior thalamoperforating artery. Perforator injury can be avoided closer to the PCA, where the frequency of PComA perforators decreases substantially. ^{27 ,​ 28} Not uncommonly the PComA terminates as the PCA, without a distinct PCA originating from the basilar artery (BA), termed fetal PCA. Thus, aneurysms of a fetal PCA should be dealt with high caution as occlusion of the vessel can lead to catastrophic PCA stroke. The PComA or proximal PCA is frequently adherent to the posterior or lateral aspect of suprasellar lesions, especially craniopharyngiomas, and should be carefully identified and preserved.

The anterior choroidal artery (AChA) arises from the inferolateral aspect of the ICA and runs inferior to the optic chiasm, crossing the optic tract first medially and then laterally. Branches of the AChA supply the uncus of the temporal lobe (unco-hippocampal artery), optic tract, and choroid plexus of the temporal horn. They also go through the anterior perforated substance to supply the posterior limb of the internal capsule. Hence, injury to this artery may result in hemianopia and contralateral hemiparesis. ²⁹ After entering the choroid plexus via the anterior choroidal point, the AChA may still give off branches to the pulvinar of thalamus and other relevant areas of the central core.

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. Importantly, AChA aneurysms commonly occur at the branch points between the parent AChA vessel and a perforator; these aneurysms are difficult to treat both endovascularly and microsurgically due to the proximity of perforating vessels and limited surgical corridor. ^{30 ,​ 31}

1.3.5 Anterior Cerebral Artery (A1 and A2 Segments)

The anterior branch of the ICA bifurcation is the ACA, which travels anterior, medial, and superior to the optic tract to reach the anterior communicating artery (AComA) (Fig. 1.1). The ACA is divided into five segments, the first two of which are important to understand their course and branches for skull base and cerebrovascular surgery. The most proximal segment, A1, is defined as the segment between the origin of the ACA from the ICA and the branching point of the AComA. The medial lenticulostriate arteries originate from the medial aspect of A1 to enter the anterior perforated substance. The AComA can also carry perforators to the superior aspect of the chiasm and hypothalamus. The subcallosal artery emerges from the posterior or posterosuperior surface of the AComA to supply the septal region and one or both fornices and is most commonly encountered inferior to the AComA through an endonasal approach. ³² Damage to this vessel can result in acute, severe memory loss. ³³

The proximal segment of A2 gives off one or more recurrent arteries of Heubner (RAH), which run parallel and lateral to A1 in opposite directions and end anterior to the carotid bifurcation where they enter the anterior perforated substance. In approximately half of the patients, the RAH can run anterior to the A1, which is particularly salient during the surgical treatment of AComA aneurysms. Damage to the RAH, which supplies portions of the basal ganglia, caudate, and internal capsule, can lead to hemiparesis and/or aphasia. ³⁴

The A2 segment also gives rise more distally to the fronto-orbital artery (FOA), which runs along the inferior surface of the frontal lobe and supplies the olfactory bulb and olfactory tract along with adjacent fronto-basal gyri. It courses anteriorly along the frontal lobe, crossing the olfactory tract, and enters the olfactory sulcus. As such, it is commonly involved in olfactory groove meningiomas (OGMs) and must be carefully dissected away from the resected tumor. ³⁴

The AComA itself, a small anastomosis between the A1 segments of the ACAs bilaterally, is the most common intracranial site of aneurysms. ³⁵ Although endovascular treatment of these aneurysms is possible, it can be difficult in some instances due to the tortuosity of A1 and wide involvement of the A1/A2 bifurcation. ³⁶ In such instances, surgical clipping may be indicated; medially projecting aneurysms may be rarely considered for endoscopic endonasal surgery, but these are generally best treated with microsurgical clipping. The A3 and A4 segments correspond to the callosomarginal artery at the superior surface of the corpus callosum and the pericallosal artery, respectively, followed by A5 terminal branches of the ACA.

1.3.6 Middle Cerebral Artery (M1 Segment)

After giving off the ACA, the ICA becomes the MCA, which supplies the majority of the lateral cerebral cortex as well as parts of the basal temporal and occipital lobes, and insula (Fig. 1.1c). The origin of MCA is commonly found medial to the sylvian fissure and lateral to the chiasm. ³⁷ The MCA is made up of four segments; the first segment, M1, is most relevant to skull base and cerebrovascular surgery. The M1 segment courses within the sylvian vallecula along the proximal or sphenoidal sylvian fissure, running posterior and parallel to the lesser sphenoid wing, superior and medial to the uncus, and inferior to the anterior perforated substance to which it delivers the lateral lenticulostriate arteries before becoming M2. However, in instances of a short M1 segment that ends before traversing the vallecula of the sylvian fissure, the perforating branches can originate from the proximal M2 as well. The lateral lenticulostriate arteries supply deep structures of the basal ganglia (the lentiform nucleus, portions of the internal capsule, and caudate) as well as the insular cortex, and their occlusion can lead to motor, cognitive, and speech impairments. Infarcts in lateral lenticulostriate arteries tend to present with more severe motor deficits than those in anterior lenticulostriate arteries. ^{38 ,​ 39}

The M1 segment of the MCA also gives off the anterior temporal artery (ATA), which supplies the temporal pole and is responsible for semantic and social processing. The M1 segment ends at the limen insulae, where the MCA typically bifurcates into a superior and inferior division.

It is important to note that M1 perforators, predominantly branching off the dorsolateral surface of the M1 segment, are present throughout the length of the segment as it crosses through the sylvian fissure. Therefore, temporary proximal clipping of M1 risks occlusion of these critical perforators; if necessary, clipping should be done as distally and as close to the lesion as possible to minimize unnecessary vessel occlusion. ⁴⁰ The M2 starts after the bifurcation to enter the distal or opercular sylvian fissure and circular sulcus of the insula, followed by the opercular (M3) and cortical (M4) segments of the MCA. ⁴¹ The cortical segments of the MCA are the common recipients of anastomosis in bypass surgery, given their ease of access, size match with donor and arterial interposition grafts, and avoidance of even transient M1 occlusion.