Tribute: The creation of this page is a direct result of the catastrophe wrecked on the United States Northeast by Hurricaine Sandy, which extensively damaged both NYU Langone Medical Center and Bellevue leading to their prolonged shutdown and our secondary over-indulgencein academic productivity. This page is dedicated, in small measure, to all who suffered loss as a result of this unprecedented disaster.

Introduction

The aim of this page is to review the anatomy of the internal carotid artery proper, from the cervical segment to its intracranial bifurcation, particularly asregardsits geometry (with secondary endovascular interventional implications) andlocation of its various, and often complex aneurysms. Patients seeking information on treatment ofcerebral aneurysms mayvisit the page titled Patient Information: Cerebral Aneurysm. The author of the website,Maksim Shapiro, MDis a practicing neurointerventional radiologist in at theNYU Langone Medical Centerin New York City, and can be reached with questions, comments, appointment requests, etc. via theContact Ussection.

The branches ofintracranial ICA are described in exhaustive detail on their respective pages. It would take take hundreds of pages, with associated surgical dissection images and videos, to describe surgical anatomy of the carotid siphon,and so we will touch upon this vast topic somewhat, mainlyin connection withstrategiesin aneurysm treatment. Our primary focus is endovascular, andwe will present information in angiogrpahic format, without too many diagrams, which often appear to the trainee sofrustratingly comprehensiblewhen comparedwithreality.

A brief overview of ICA anatomy. The ICA in the neck (cervical ICA) extends fromcarotid bifurcation to skull base. It then goes through the petrous bone of the skull base (petrous segment), and turns up within the foramen lacerum, existing the bone. It passes under a key landmark structure called petrolingual ligament, and enters the cavernous sinus, where it usually has an s-shaped look, though much variability exists. In the cavenous sinus, the artery is surrounded by venous plexus, such that carotid rupture there leads to a carotid-cavernous fistula. After an anterior turn (genu), the ICA leaves the cavernous sinus, passing through the dura cover of the sinus that is called the proximal dural ring. The ICA then goes through a small but important region where, though already out of the cavernous sinus, it is not yet subarachnoid, or intradural. This transitional or clinoid area has been subject of much surgical attention. After this short segment, the ICA goes through another dural ring, called the distal dural ring, and then becomes intradural, or subarachnoid.This transition is critical, since aneurysms past the distal dural ring are located in the subarachnoid space, and their rupture leads to subarachnoid hemorrhage. The ophthalmic artery is usually (90% of time) located just distal to the distal dural ring (i.e. intradural, i.e. subarachnoid), and this region is home to many kinds of complex aneurysms. Other times, the ophthalmic arises more proximally, from the transitional (extradural) or the cavernous segment, or from the external carotid all very important variants.Past the ophthalmic segment,artery continues into the hypophyseal region (with inconstantly observed superior hypophyseal arteries), where otherkinds aneurysms can form. The next major branch of the ICA is the posterior communicating artery, home to particularly notorious PCOM aneurysms, which seem to rupture with increased frequency for given size, when compared to other aneurysms of the ICA (ISUIA data). Next comes the anterior choroidal artery and its aneurysms, which can be mistaken for the PCOM type when the latter is hypoplastic. Finally, after a short terminus segment, home to some perforating branches, the ICA bifurcates into the MCA and ACA. This fairy tale has many variations and inconsistencies, but is useful as a general guide. Now that we have the general layout, before getting into pathology, we must review some segmental classifications of the ICA.

Segmental Classifications of the ICA

The ICAhas been repeatedlysubdivided into discrete parts, or segments, to aid description of its pathology. We, at NYU, are also to blame for one such scheme. A brief review of the more popular classificatons is necessary and useful for the trainee and lay professional.

The first classification was devised by Fischer in 1938, designating intracranial ICA from C1-C5, against direction of blood flow. Its aim was to help localize skull base lesions via their mass effect on different ICA segments, before the era of cross-sectional imaging. It was not designed to describe ICA aneurysms.

The Fischer classification endured until development of reliable microsurgical and catheter angiographic technique, which paved the way for development of predominantly non-lethal aneurysm neurosurgery.

In 1981, Gibo, Lenkey, and Rhoton, based on incredible supracliniod ICA dissections which became a landmark in vascular neurosurgery, classified their findings according the the Gibo system, which numbered 4 segments cervical, petrous, cavernous, and supraclinoid, with an alphanumeric designation of C1 thru C4, in direction of blood flow. The C4 segment is subdivided into ophthalmic, communicating, and choroidal (see below). The C3 segment began wherever the ICA emerged from the dural covers as a subarachnoid vessel. This simple and elegant classification, predating the era of dural rings and clinoid discussions, continues to be in use.

The landmark present-day classification, however, belongs to Bouthillier and collegues, whoproposed a Modified Fischer Classification in 1996, with alphanumeric designation of ICA segments in direction of blood flow. This system was also based on careful microsurgical dissections and optimized for present-day aneurysm clipping. Importantly, it formally recognized theclinoid segment as a transitional area between the cavernous and intradural ICA, as will be discussed below.

The Bouthillier classification was widely adopted, despite some criticism. For example, Ziyal and collegues questioned the need for a discrete Lacerum Segment, and dispensed with it based on their careful dissections. They also got rid of the ophthalmic and communicating segments, in favor of simple cisternal segment. This classification, shown below, did not achieve widespread use.

An entirely different approach was taken by the great Lasjaunias who, together with Santoyo-Vazquez, subdivided the ICA based on embryologic considerations rather than anatomical landmarks adjacent to the ICA. The article is available free of charge: http://link.springer.com/article/10.1007%2FBF01773165?LI=true#page-1Segment boundarieswere defined by intracranial ICA branches, such as mandibulovidian artery, MHT, ILT, ophthalmic. This systemmakes the most physiologic sense, andconceptualizes many variants of the ICA and its branches, but it was not designed to facilitate surgical dissections or emerging endovascular methods.

Much surgical work was done to address the complex anatomy of the ICA surrounding the region of the anterior clinoid process, including the transitional segment between the cavernous and intradual ICA, and the various ophthalmic segment aneurysms. Surgery which required removal of the clinoid process was rather complex, and aneurysms within the cavernous segment were regarded by many as either unclippable or clippable given superb skill and acknowledgement of higher stakes. Work on aneurysms near the ophthalmic artery (and optic nerve) was associated with a well-known risk of visual loss. At the same time, more reliable endovascular techniques were emerging with introduction of the GDC. This modified the conceptual framework, emphasizing aneurysm dome morphology and neck anatomy, with less critical attention to surgical landmarks. Finally, emergence of primary endoluminal (flow diversion) methods allowed for treatment of the un
derlying ICA dysplasia, which often transcends artificial segmental boundaries. Our own NYU classification of ICA segments, developed as aresult ofangiograhic and cross-sectionalreview anddiscussion, is based predominantly on endovascular considerations.

Finally, it is not the purpose of this page to advance a particular classification: the object is to illustrate the anatomy and pathology of the ICA; the NYU classification is used because we find it most useful at the moment as long as there is understanding of whatever anatomy the classification describes, any scheme is fine. For classical descriptive purposes, one can use the following system:

From an endovascular standpoint, however, we find that aneurysms which lie on the Transitional-Ophthalmic-Hypophyseal continuum have particular common endovascular (rather than surgical) features, which unite rather than divide them, as has been previously the case. We therefore hold, somewhat boldly, that all of these can be considered as paraophthalmic. This is not the system in current use, though we hope it gains following, which would look something like this:

This is nicely illustrated in the artwork below:

In the following section, each segmentis discussed in more detail, and relevant aneurysms are shown.

A note regarding aneurysms: The following section will repeatedly emphasize a key point: aneurysms in general, andthose of ICA in particular, are not perfect spheres with slender, elegant necks these can be encountered in diagrams, pamphlets, and other works of art and safely cured with a permanent marker, at zero risk. Real aneurysms tend to be irregular growths which arise on basis of underlying ICA dysplasia, and frequentlytranscending arbitrary and even embryologically-dictated boundaries. Images in this section will direct one towards recognition of this state and its therapeutic implications.

Cervical Internal Carotid Artery

The carotid artery usually bifurcatesbetween C3-5, except when it does not. High bifurcations are disadvantageous for vascular surgeons but not for carotid stents per se.

Atherosclerotic disease of the carotid bifurcation and its treatment is a separate topic. It is potentially important in terms of accessing the internal carotid artery with large-diameter catheters,increasingly utilized in modernendoscular procedures. In such cases, we try to keep the exchange wire in the ECA, and bring the guide into the ECA as well and flush it there, or keep it in the CCA, and go through the stenosis with a smaller profile and more compliant distal support catheter (these catheters are getting better and more numerous, which is excellent news). If additional support becomes necessary later on, the guide can then be more safely advanced into the internal carotid artery over the larger diameter distal access catheter, ratherthan primarily over a smaller cross-section guidewire, thereby minimizing the step-off

The cervical internal carotid artery is supposed to have no branches, except when it does. Persistent hypoglossal artery is one such branch (See neurovascular evolution). The ascending pharyngeal artery occasionally takes off from the proximal ICA also, as does the occipital. Aside from embryologic implications, it underscores the potential costs of catheterizing the ICA without a roadmap, which is also useful to visualize the not so rare Cervical Internal Carotid Artery Loops. They are felt to represent a kind of embryologic redundance, which can also be observed intracranially involving the PCOM and A1 segments for example (Lasjaunias and Berenstein)as opposed to the tortuous vessels seen in the vasculopaths.

Loops are, of course, significant from an endovascular access standpoint, presenting challenges for distal catheterization and delivery of larger caliber devices. This is fortunately becoming less problematic, as distal support catheter technology rapidly improves.

Retropharyngeal ICA: Distinct from loops are carotids with unusual courses, particularly those that swing anteromedially towards the back of the oropharyngeal wall, best appreciated on cross-sectional imaging. They are surgically important, particularly when it comes to procedures related to the posterior nasopharyngeal and oropharyngeal walls. Apulsatile mass in the back of the throat is probablythe ICA, and should be treated with appropriate respect.

Webs the cervical ICA, in particular its proximal aspect, are sometimes seen to harbor a particular narrowing which is caused by shelf-like proliferation of connective tissue, probably similar to that of fibromuscular dysplasia (FMD). The angiographic appearanceis very different, however, with a single shelf of stenosis. This can be occasionally a cause of embolic stroke due to blood stasis over the shelf, more likely than hemodynamic narrowing. This young man presented with a transient language dysfunction:

MRA and angio of the same patient, left ICA. A second patient, with a similar angiographic appearance of carotid web, noted incidentally. For more info, see dedicated Carotid Web page

Aberrant Carotid Artery fully treated in the Ascending Pharyngeal Artery Section, as this vessel is, in fact, not the ICA, but rather ascending pharyngeal reconstitution of the true ICA in the petrous segment, due to cervical ICA agenesis. The aberrant carotid is made up of the ascending pharyngeal artery, its inferior tympanic branch, and the caroticotympanic branch of the ICA. The vessel has a characteristic lateral swing within the petrous bone (red arrows), bringing it into the middle ear cavity, which can be appreciated on MR, CT, and angio. This variant comes up with unfortunate regularity as a middle ear pulsatile mass, subjected to an unwitting biopsy. Case courtesy of Dr. Howard Antony Riina, NYULMC

The same appearance angiographically, with a somewhat posterior course in the lateral projection (yellow arrow)

Finally, a VERY cool stereo 3D-DSA, visualizing the aberrant ICA within the ear canal.

Carotid Occlusion Vasa Vasorum Reconstitution

An occasionally seen, quite fascinating consequence of carotid occlusion. Since all tissues require blood supply, it stand to reason that walls of larger blood vessels, such as the aorta and carotid arteries, contain smaller arteries for the nourishment of the various connective, muscular, and other tissues which make up the wall. These are called Vasa Vasorum. On some occasions, occlusion of the primary carotid artery lumen is followed by hypertrophy and, possibly, hyperplasia of these vasa vasorum to reconstitute the carotid artery distal to the site of occlusion. When encountered, these vasa vasorum networks tend to be long, extending from the carotid bulb (usual site of atheromatous occlusion) to the pertous segment, where the native carotid artery is again opacified. This pathway is likely to be present only when the more typical collateral pathways (circle of Willis, ophthalmic artery) remain insufficient (see Collateral Circulation page for extensive discussion on the topic). Below is a typical example of carotid vasa vasorum (red) a tortuous channel or multiple channels, having no resemblance to the native lumen. A normal petrous carotid is artery is labeled (yellow).

Another example, with a duplicated channel (pink)

Aneurysms

In cervical ICA, these aretypically of dissecting type, and therefore pseudoaneurysms (white arrow), such as this one.

Dissecting aneurysms are a heterogeneous bunch. Many seem to be either asymptomatic or clinically benign, generating much harm in terms of patient anxiety but little beyond that. They can, when particularly large, act as embolic sources.

Carotid Dissection

This is an important topic, addressed in a dedicated Patient Information Carotid/ Vertebral Dissection page. While most carotid dissections do not lead neurologic dysfunction, a fact which patients should remember, the range of occasional issues is large, including em
bolic stroke from dissection-related thrombus formation and distal embolization, flow-limiting stroke due to insufficiency of distal collateral pathways, and occasional rare issues such as pulsatile tinnitus or lower cranial nerve dysfunction due to dissection-related mass effect.

Petrous Segment

Petrous segment This is the ICA segment inside the petrous bone and partially within foramen lacerum. The artery enters the skull at right angle and has an initially ascending course (vertical petrous subsegment), turning anteromedially (horizontal petrous subsegment) and exiting the petrous bone at foramen lacerum, where it turns up and travels a short distance before issuing from the foramen above the horizonal plane of the petrous bone (lacerum subsegment). At this point, it passes beneath the Petrolingual Ligament (PLL), an important landmark which defines entry of the ICA into the cavernous sinus (cavernous segment). The PLL cannot be angiographically seen, and the plane of the temporal petrous bone can be taken as its landmark (distal yellow line). As everyone knows, the ICA does not go thru and thru the foramen Lacerum, but runs into it, as T-intersection.Bouthillier defined a discrete Lacerum segment where the ICA ascends inforamen lacerum.The existence of this discrete segment was questioned by Ziyal. Weagree that a separate lacerum segmentdoes not seem to be necessary, both anatomically and pathophysiologically, as aneurysms of the petrous segment almost always extend into the Lacerum portion of the ICA, butvery rarely beyond the PLL. There is much variability in the lengths and angles of the petrous segments. At the genu between the vertical and horizontal segments, the ICA gives off the caroticotympanic branch, which courses posteriely towards the middle ear (this is the route aberrant carotid takes to hook up with the petrous carotid). This branch is rarely seen, as it is encased inthick temporal bone. At the distal horizontal petrous segment, before the artery heads superiorely into the lacerum (or transitional) segment, it gives off the mandibulovidian artery, which courses anteriorly through the vidian canal. A small waist (pink arrow) is sometimes seen where the artery enters the petrous bone.

Stereo 3D-DSA of petrous ICA.Short horizontal segment (white), vertical segment (red), lacerum subsegment (purple) and mandibulovidian artery (lower purple arrow) bifurcating into mandibular and vidian branches.

Purple arrow demonstrating a small mandibulovidian artery. Notice how the petrous carotid often appears somewhat attenuated and mottled due to overlap of the petrous bone, compared with the uniform dark color of the cervical and cavernous segments not to be confused with thrombus.

Petrous Segment Aneurysms

Aneurysms of the petrous segment seem to come in two types post-traumatic and other. Post-traumatic (not aneurysms but pseudoaneurysms) are usually created by skull base fractures involving the temporal bone, with secondary petrous segment tear/dissection/pseudoaneurysm formation. The other category usually looks like fusiform large/giant petrous bone blowout, often partially thrombosed, and often extending into the cervical ICA but rarely past the PLL into thesegment. When the bone is often extensively remodelled, attesting to long-standing anerysm presence, and the abnormality is easily appreciated on a non-contrast head CT. In my experience, most patients or parents cannot recall any impressive head trauma. Patients with such long, irregular, and partially thrombosed aneurysms can present withan embolic stroke. Historically, treatment was based on a deconstructive approach of carotid sacrifice, withor without bypass, depending on results of test occlusion. Now, many such cases are being treated with Pipeline or similar endoluminal devices.

Illustrations:

Frontal (top) and lateral (bottom) projection digital subtraction angiographic (DSA) and native images demonstrate a fusiform aneurysm involving distal cervical and petrous segments. The long-standing aneurysm, partially thrombosed, produced extensive remodeling (yellow arrows) and erosion (red arrow) of the petrous bone, with bowing of the posterolateral right sphenoid sinus wall and dehiscence of lateral petrous apex (red arrow); status post pipeline embolization. A coil mass in the posterior fossa (black arrows) also seen on CT scan, belongs within a dissecting aneurysm of the mid-basilar artery (purple arrow), also treated with Pipeline (rightmost three images); coils were placed into the aneurysm after documenting its rapid short-term expansion in a patient presenting with new headaches.The case highlights an innate predispostion for aneurysm formation in this patient population.

This incidentally discovered petrous segment aneurysm, with secondary osseous remodeling (yellow arrows), is associated with dorsal ophthalmic artery variant (red arrows), which I believe also supports thenotion of a congential predispostion; there is no history of trauma.

Three petrous segment aneuyrsms, all extending into the lacerum subsegment, but not distal to the petrolingual ligament, as landmarked by the horizontal plane of the temporal bone (white arrows). This is the pathophysiologic side of argument againsta separate Lacerum segment.

3D-DSA of petrous segment aneurysm, confined below the PLL.

A mirror image of giant holo-Fischer aneurysm which involves all post-petrous carotid segments, and also does not violate the PLL. Poster case for trans-segmentaldysplasia.A small mandibulovidian artery (white arrow) is seen.

CAVERNOUS SEGMENT

Defined as that portion of the ICA located within the cavernous sinus see dedicated Cavernous Sinus page for more venous details. In practice, the anatomy of Cavernous Segmentis dependent onsize and morphology of the cavernous sinus, which has a variable and complex anatomy, both in terms of size and compartmentalization. Injection of the ICA or, more appropriatelyCCA, does not necessarily visualize the entire ipsilateral cavernous sinus, particularly when its main cerebral tributary the superficial Sylvian venous system is underdeveloped. From an microsurgical standpoint, the cavernous sinus begins (lets assume, please) at the petrolingual ligamen, and ends at the proximal dural ring. Neither structure isvisible angiographically or bycross-sectional imaging. One can only guess, on angio, where cavernous sinus begins and ends. When the cavernous sinus is well-formed, and when itscompartmentsreceive amplevenous drainage from the ipsilateral common carotid territory, you can guesstimateits boundaries by superimposing arterial and venous phases on each other, as shown below. In practice, this is of little value, since ballpark estimates can be made anyway, and precise localization (say when a transitional aneurysm is present) leaves room for doubt anyway. In the image below, the posterior cavernous (dark blue)sinus is well-developed, receiving a large superficial sylvian / sphenoparietal sinus tributary (orange), allowing one to visualize the proximal boundary of the cavernous sinus (yellow arrow) as a line, against the background of the arterial phase. The inferior petrosal sinus is marked by light blue arrow. The same information can be gathered from a CT angiogram, whenever it is contaminated with venous state.

The more typical uncertainty of cavernous sinus borders, particularly at the distal aspect, has important clinical significance in terms of deciding whether a given aneurysm is purely intracavernous (and therefore extradural), or distal to the cavernous sinus (clinoid, paraophthalmic, or whatever your boss calls it, and therefore potentially intradural). Consequently, aneurysms in uncertain locations (probably distal to the cavernous segment, andprobably not yet intradural) are sometimes called transitional, underscoring the uncertainty.

The cavernous segment can be subdivided into various segments, as seen below. There exists simply endlessvariability in cavernous sinus and ICA cavernou
s segment geometry, withall manner of straight andcurved segments and subsegments to the great delight of computational fluid dynamics enthusiasts and classification junkies (like us). There are also immediately practical endovascular implications in terms of navigability, catheter support, and implant (stent) behaviour around the various curves.

The important Meningohypophyseal Trunk arises from the genu (bend) of this segment. Its prominence is variable, of course, as its territory is in balance with those of the ILT, clival branches of the Ascending Pharyngeal Artery, and with the MMA. It most typically will be seen as supplying the hypophysis, with a characteristic early blush and early venous phase (not to be mistaken for a dural fistula). The famed artery of Bernasconi-Cassinari comes from there also. In the image below, the lateral tentorial arcade arising from the proximal genu supplies a small sigmoid sinus fistula (orange).

The second important branch of the cavernous segment is the Inferolateral Trunk (ILT), which supplies the floor of the middle cranial fossa, cranial nerves of the cavernous sinus, and is in balance with the Middle and Accessory Meningeal Arteries. Therefore, it is a potential conduit to the ophthalmic artery, expressed in its full prominence as the dorsal ophthalmic (red arrow).

On occasion, one can appreciate slight enlargement in ICA caliber within the cavernous segment. Whether this is physiologic, within a particular cavernous compartment (akin to constriction of the vertebral or radiculomedullary artery when piercing the dura), or a marker for future Cavernous Segment aneurysmdevelopment is unclear. The distal constriction (distal yellow arrow) is as reliable an angiographic marker of the proximal dural ring as any other. This is a lateral left ICA injection in a young epileptic patient status post craniotomy (blue arrow) and subdural strip placement (purple arrow) for invasive EEG monitoring (study done as part of Wada evaluation). Notice enlarged ICA caliber of the cavernous and transitional segments, between two yellow arrows. The distal arrowpoints to vessel constriction which probably marks the location of the dural ring, and its corresponding intradural transition. The ophthalmic artery ostium may be extradural. Notice hypertrophied anterior meningeal artery, post craniotomy-related MMA sacrifice.

On the other hand, it is also important to recognize the physiologic variability in vessel size based on local and systemic factors spasm and vasodilatation. When catheter-related, these are usually straightforward, but it is not always so. As an example, see pre- and post- AVM resection angiograms of this patient, where the cavernous segment is perfectly delineated as a region or relative vascular constriction (left image, yellow arrow), whereas the subsequent study the same area (red arrow) actually marks a subtle change towards relative dilatation. This is not related to any catheter manipulation. The MHT is labeled with blue, and ILT with purple arrows.

ANEURYSMS

What is certain is that nontraumatic cavernous aneurysms are usually fusiform, and have a strong female predominance. The former observation seems to run somewhat at odds with the theory of preferred aneurysm origin at vessel ostia, as championed by the superb works of Rhoton. For example, the superior hypophysealaneurysms are felt to arise at the ostia of superior hypophyseal arteries, not to mention the ophthalmic, PCOM, choroidal, etc. Curiously,saccular aneurysms rarely form in association withthe more consistently visualized MHT and ILT, whichare first in line to receive the brunt of supra-petrous ICA inflow. The hypertension theory is also suspect, as there are many patients with such aneurysms having nohypertension, and incidence in men is rare. It seemsmuch more likelythat the underlying cause has a primary genetic basis.

The majority of cavernous aneurysms preferentially expand laterally, into the cavernous sinus.It is a fact of singular consistency that the proximal vertical subsegment (from the PLL to the posterior genu, yellow arrows) is very rarely involved, even when the remaining cavernous ICA is transformed into a monstrous deformity (see image below). The explanation for this observation seems to be missing in the literature (please correct me if you come across any!)

Even when involvement of the vertical segment is suggested by some images, angiograhic techniques such as earlier phase or 3D-DSA imaging can help clarify the situation (below). Note actual transition into the aneurysmal segment (red arrows). A small waist marks the petro-cavernous transition in the upper case.

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