Vasculopathy

Main Text

Preamble

The generic term “vasculopathy” literally means blood vessel pathology—of any kind, in any vessel (artery, capillary, or vein). Evaluating the craniocervical vessels for vasculopathy is one of the major indications for neuroimaging. Large vessel atherosclerotic vascular disease is the single most prevalent vasculopathy in the head and neck, whereas carotid stenosis or embolization from atherosclerotic vascular disease plaques are the most common causes of ischemic strokes.

In this chapter, we discuss diseases of the craniocervical arteries, first laying a foundation with normal anatomy. We then review atherosclerosis, starting with a general discussion of atherogenesis. Extracranial atherosclerotic vascular disease (ASVD) and carotid stenosis are followed by a brief overview of intracranial large and medium artery ASVD. We conclude the discussion with arteriolosclerosis, the most common brain “microvascular disease.”

The broad spectrum of nonatheromatous vasculopathy is then addressed. Finally, we devote the last section of this chapter to non-ASVD diseases of the cerebral macro- and microvasculature. While arteriolosclerosis is by far the most common cause of small vessel vascular disease, nonatherogenic microvasculopathies, such as amyloid angiopathy, can have devastating clinical consequences.

Normal Anatomy of Extracranial Vessels

Aortic Arch and Great Vessels

The aorta has four major segments: The ascending aorta, transverse aorta [primarily consisting of the aortic arch (AA)], aortic isthmus, and descending aorta.

Aortic Arch

The AA lies in the superior mediastinum. It begins at the level of the second right sternocostal articulation. It then curves backward and to the left over the pulmonary hilum. The AA has two curves, one convex upward anterior to posterior and another that is right to left in the superior mediastinum.

The AA is anatomically related to a number of important structures. The cervical sympathetic plexus branches and left vagus nerve, CNX, lie in front of the AA. The trachea, left recurrent laryngeal nerve, esophagus, thoracic duct, and vertebral column lie behind the arch. The great vessels lie above the AA, as does the left brachiocephalic vein. The pulmonary bifurcation, ligamentum arteriosum, and left recurrent laryngeal nerve lie below the arch.

Great Vessels

Three major vessels arise from the AA. From right to left, there are the brachiocephalic trunk (BCT), the l eft common carotid artery (CCA), and the left subclavian artery (SCA) (10-1). The BCT, a.k.a. the innominate artery, is the first and largest branch of the AA. It bifurcates into the right SCA and the right CCA.

The major branches of the right SCA are the right internal mammary (thoracic) artery, right vertebral artery (VA), right thyrocervical trunk, and right costocervical trunk. The right CCA bifurcates into its two terminal branches, the right internal carotid artery (ICA) and the right external carotid artery (ECA).

The left CCA arises from the AA just distal to the BCT origin. The left CCA bifurcates in the left ICA and left ECA near the upper border of the thyroid cartilage. The left CCA lies anterior and medial to the internal jugular vein.

The left SCA arises from the AA distal to the left CCA origin. It ascends into the neck, lateral to the medial border of the anterior scalene muscle. The major branches are the left internal mammary (thoracic) artery, the left VA, left thyrocervical trunk and the left costocervical trunk.

Normal Variants

The “classic” AA has the three“great vessel” branching pattern, which is present in 80% of patients (10-1). Normal variants are seen in ~ 20% of patients. In 10-25% of cases, the left CCA shares a common V-shaped origin with the BCT. This is sometimes called a bovine configuration, which is a misnomer. The left CCA arises from the proximal BCT in 5-7% of cases. The left CCA and left SCA share a common origin, a left BCT, in 1-2% of cases. Finally, the left VA arises directly from AA in 0.5-1% of cases (10-2).

Cervical Carotid Arteries

The CCAs and their branches provide the major blood supply of much of the neck, all of the face, and the entire brain. The right CCA arises at the sternoclavicular level. The left CCA originates from the AA and ascends in front of and then lateral to the trachea. The CCAs course superiorly in the carotid space, anteromedial to the internal jugular vein. The CCA bifurcations are typically at C3-C4 or C4-C5.

Internal Carotid Artery

The cervical ICA is entirely extracranial and is designated as the C1 segment. In 90% of cases, the cervical ICA arises from the CCA posterolateral to the ECA.

The C1 has two parts, the carotid bulb and the ascending segment. The carotid bulb is the most proximal aspect of the cervical ICA and is seen as a focal dilatation with a cross-sectional area nearly twice as large as that of the distal ICA. Flow reversal occurs in carotid bulb normally. The ascending ICA segment courses cephalad in the carotid space, a fascially defined tubular sheath that contains all three layers of deep cervical fascia. The cervical ICA has no normal branches in the neck. It enters the carotid canal at the skull base in the petrous temporal bone.

External Carotid Artery

Each ECA has eight major branches (10-3)and is smaller and lies medial to the ICA. The first ECA branch is usually the superior thyroid artery, which may also arise from the CCA bifurcation. The superior thyroid artery arises anteriorly from the ECA and courses inferiorly to supply the superior thyroid and larynx. The ascending pharyngeal artery arises posteriorly from the ECA (or CCA bifurcation) and courses superiorly between the ECA and ICA to supply the nasopharynx and oropharynx, middle ear, eustachian tube dura, and CNIX-XI.

The lingual artery is the third ECA branch. It loops anterior and inferiorly, then courses superiorly to supply the tongue, oral cavity, and submandibular gland. The facial artery arises just above the lingual artery, curving around the mandible before it passes anterior and superiorly to supply the face, palate, lips, and cheek.

The occipital artery originates from the posterior aspect of the ECA and courses posterosuperiorly between the occiput and C1. It supplies the scalp, upper cervical musculature, and posterior fossa meninges. The occipital artery has extensive anastomoses with muscular VA branches.

The posterior auricular artery arises from the posterior ECA above the occipital artery. It courses superiorly to supply the pinna, scalp, external auditory canal, and chorda tympani. The superficial temporal artery (STA)is the smaller of the two terminal ECA branches. It runs superiorly behind the mandibular condyle, across the zygoma. It supplies the scalp and gives off the transverse facial artery.

The maxillary artery is the larger of the two terminal ECA branches. The maxillary artery arises within the parotid gland, behind the mandibular neck. It gives off the middle meningeal artery, which supplies the cranial meninges and runs anteromedially in the masticator space. Within the pterygopalatine fossa, it sends off terminal branches to the deep face and nose.

Numerous anastomotic channels exist between nearly all of the extracranial branches of the ECA (except the superior thyroid and lingual arteries) and the intracranial branches of the ICAs or musculospinal branches of the VAs (10-4). These anastomoses both provide an important pathway for collateral blood flow and pose a potential risk for intracranial embolization during neurointerventional procedures.

The major anastomoses include the ascending pharyngeal artery with the middle/accessory meningeal artery and caroticotympanic artery. The facial artery has anastomoses with the ophthalmic artery (ICA branch) and other ECA branches. The occipital artery has extensive anastomoses with muscular VA branches. The maxillary artery has anastomoses with the inferolateral trunk of the cavernous ICA, the ophthalmic artery, vidian artery, and the recurrent meningeal arteries.

Atherosclerosis

Preamble

Atherosclerotic vascular disease (ASVD) is by far the most common cause of mortality and severe long-term disability in industrialized countries, so it is difficult to overemphasize its importance. It affects all arteries, of all sizes, in all parts of the body.

The principal cause of cerebral infarction is atherosclerosis and its sequelae. Over 90% of large cerebral infarcts are caused by thromboemboli secondary to ASVD. We begin with extracranial ASVD before concluding with a brief discussion of the clinical and imaging manifestations of intracranial ASVD, including its microvascular manifestations.

Atherogenesis and Atherosclerosis

Terminology

The term “atherosclerosis” was originally coined to describe progressive “hardening” or “sclerosis” of blood vessels. The term “atheroma” (Greek for “porridge”) designates the material deposited on or within vessel walls. “Plaque” is used to describe a focal atheroma together with its epiphenomena, such as ulceration, platelet aggregation, and hemorrhage.

Atherogenesis is the degenerative process that results in atherosclerosis. Atherosclerosis is the most common pathologic process affecting large elastic arteries (e.g., the aorta) and medium-sized muscular arteries (e.g., the carotid arteries and VAs). Arteriolosclerosis describes the effects of atherogenesis on smaller arteries (and is treated separately at the end of this section). ASVD is the generic term describing atherosclerosis in any artery, of any size, in any area of the body.

Etiology

General Concepts

Plasma lipids, connective tissue fibers, and inflammatory cells accumulate at susceptible sites in arterial walls, forming focal atherosclerotic plaques. Angiogenic factors cause vasa vasorum proliferation, formation of immature vessels, and loss of capillary basement membranes. Neoangiogenesis is closely associated with plaque progression and is likely the primary source of intraplaque hemorrhage.

Pathology

Location

ASVD occurs preferentially at highly predictable locations. In the extracranial vasculature, the most common sites are the proximal ICAs and CCA bifurcations, followed by the AA and great vessel origins.

Size and Number

ASVD plaques vary in size from small, almost microscopic lipid deposits to large, raised, fungating, ulcerating lesions that can extend over several centimeters and dramatically narrow the parent vessel lumen. Multiple lesions in multiple locations are common.

Gross Pathology

ASVD plaques develop in stages (10-5). The first detectable lesion is lipid deposition in the intima, seen as yellowish “fatty streaks.” Other than “fatty streaks” and slightly eccentric but smooth intimal thickening, visible changes at this early stage are minimal.

Microscopic Features

ASVD plaques are classified histopathologically as “stable,” “vulnerable,” or “ulcerated.”

Stable Plaques

Uncomplicated stable plaques—the basic lesions of atherosclerosis—consist of cellular material, lipid, and an overlying fibrous cap. The intima covering a stable plaque is thickened, but its exterior surface remains intact. No ulceration or intraplaque hemorrhage is present (10-6).

Vulnerable Plaques

As a necrotic core of lipid-laden foam cells, cellular debris, and cholesterol gradually accumulates under the elevated fibrous cap, the cap thins and becomes prone to rupture (“vulnerable” plaque) (10-6).

Proliferating small blood vessels also develop around the periphery of the necrotic core. Neovascularization can lead to subintimal hemorrhage,  which  enlarges the necrotic core, further weakening the overlying fibrous cap.

Ulcerated Plaques

Plaque ulceration occurs when the fibrous cap weakens and ruptures through the intima, releasing necrotic debris (10-7)(10-8). Plates and fibrin aggregate within the ulcerated denuded endothelium. These aggregates can be pulled into the rapidly flowing main artery slipstream, causing arterioarterial embolization to distal intracranial vessels.

Clinical Issues

Epidemiology and Demographics

Most patients with symptomatic lesions are middle-aged or older adults. However, atherosclerosis is increasingly common in younger patients, contributing to the rising prevalence of strokes in patients < 45 years old.

Presentation

Many ASVD lesions remain asymptomatic until they cause hemodynamically significant stenosis or thromboembolic disease. Transient ischemic attacks (TIAs) and “silent strokes” are common precursors of large territorial infarcts.

Natural History

The natural history of ASVD is also highly variable. ICA occlusion poses an especially high risk for eventual stroke with > 70% of these patients eventually experiencing ischemic cerebral infarction.

Treatment Options

Treatment options include prevention, medical therapy (lipid-lowering regimens), and surgery or endovascular therapy.

Extracranial Atherosclerosis

Extracranial ASVD is the single largest risk factor for stroke. That risk starts with the AA, an underrecognized source of intracranial ischemic strokes. Complete imaging evaluation of patients with thromboembolic infarcts in the brain should include investigation of the AA.

Aortic Arch/Great Vessels

Aortic ASVD is more common in the descending thoracic aorta than in the ascending aorta or arch (10-8). However, late diastolic retrograde flow from complex plaques in the proximal descending aorta distal to the left SCA origin can reach all supraaortic arteries. Retrograde flow extends to the left SCA orifice in nearly 60% of cases, the left CCA in 25%, and the BCT in 10-15%.

Aortic emboli involve the left brain in 80% of cases and show a distinct predilection for the vertebrobasilar circulation. This striking geographic distribution is consistent with thromboemboli arising from ulcerated plaques in the descending aorta that are then swept by retrograde flow into left-sided arch vessels.

Carotid Bifurcation/Cervical Internal Carotid Arteries

Between 20-30% of all ischemic infarcts are caused by carotid artery stenosis. Therefore, determining the degree of carotid stenosis on imaging studies is now both routine and required.

Using data from the North American Symptomatic Carotid Endarterectomy Trial (NASCET), carotid stenosis is classified as moderate (50-69%), severe (70-93%), and “preocclusive” or critical (94-99%) (10-9). Patients with critical stenosis are at high risk for embolic stroke as long as the ICA lumen is patent.

In addition to stenosis degree, several recent studies have demonstrated the importance of also assessing the morphologic features of ASVD plaques. Rupture of an “at-risk” plaque with a large, necrotic core under a thin, fibrous cap is responsible for the majority of acute thrombi. As distal embolization from proximal ASVD-related clots is a common cause of cerebral ischemia/infarction, identifying rupture-prone “vulnerable” plaques is at least as important as determining stenosis!

CT Findings

The most common imaging findings in extracranial ASVD are mural calcifications, luminal irregularities, varying degrees of vessel stenosis, occlusion, and thrombosis (10-10). Elongation, ectasia, and vessel tortuosity can occur with or without other changes of ASVD.

NECT scans easily show vessel wall calcifications. Large atherosclerotic plaques may demonstrate one or more subintimal low-density foci. These represent the lipid-rich core of a “soft” plaque. High-density subintimal foci indicate intraplaque hemorrhage. Both findings carry increased risk of plaque rupture and concomitant distal embolization.

CTA source images display the carotid lumen in cross section (10-11). CTA is as accurate as DSA for ICA stenosis measurements. Smooth luminal narrowing is the most common finding in ASVD. The percent of stenosis is typically measured with NASCET criteria. However, there are also single-diameter thresholds for CTA: 2.2 mm (50% stenosis) and 1.3 mm (70% stenosis). Ulcerations—seen as irregularly shaped contrast-filled outpouchings from the lumen—are detected with 95% sensitivity and 99% specificity. Intraluminal thrombi are also readily demonstrated (doughnut sign) (10-12). Carotid occlusion is seen as contrast ending blindly in a blunted, rounded, or pointed pouch in the proximal ICA (10-13).

In addition to calculating percentage of stenosis (10-5), plaque morphologic characteristics should be described in detail, as stenosis alone does not define complete stroke risk in symptomatic patients. Intraplaque hemorrhage has been identified as an independent risk factor for ischemic stroke at all degrees of stenosis, including symptomatic patients with low-grade lesions (< 50%). CTA rim sign of adventitial calcification with internal soft plaque is highly predictive of carotid intraplaque hemorrhage. Therefore, accurate characterization of plaque morphology is important for patient management.

MR Findings

High-resolution MR imaging can be used to characterize carotid plaques, allowing identification of individual plaque components, including lipids, hemorrhage, fibrous tissue, and calcification.

High signal intensity on T1-weighted fat-suppressed scans, MRA source images, or MP-RAGE sequences represents hemorrhage into complicated “vulnerable” atherosclerotic plaques, not lipid accumulation (10-14). Unlike intraparenchymal brain bleeds, plaque hemorrhages may remain hyperintense for up to 18 months. Vulnerable plaques are usually hyperintense on T2WI, whereas stable plaques are isointense on both T1- and T2WI.

T1 C+ FS scans may show enhancement around plaque margins, consistent with neovascularity in a vulnerable “at-risk” plaque.

Contrast-enhanced or unenhanced 2D TOF MRA is 80-85% sensitive and 95% specific for the detection of ICA > 70% ICA stenosis. Signal loss with a “flow gap” occurs if the stenosis is > 95%. Compared with CTA and DSA, MRA tends to overestimate the degree of stenosis.

Vertebral Arteries

ASVD in the extracranial VAs accounts for up to 20% of all posterior circulation ischemic strokes. Although mid- and distal cervical segment lesions occur, extracranial ASVD is most common at or near the VA origin (10-15).

A special type of VA pathology is called subclavian steal. Here, the SCA or BCT is severely stenotic or occluded proximal to the VA origin. Flow reversal in the affected VA occurs as blood is recruited (i.e., “stolen”) from the opposite VA, crosses the basilar artery (BA) junction, and flows in retrograde fashion down the VA into the SCA to supply the shoulder and arm distal to the stenosis/occlusion (10-16).

Noninvasive imaging of subclavian steal can be problematic. Because superior saturation bands are applied in 2D TOF MRA, reversed flow direction in a VA can mimic occlusion. Standard TOF MRA alone may not be adequate to differentiate reversed flow from absent flow, so confirmation and quantification with additional imaging is usually required.

Differential Diagnosis

The major differential diagnoses of extracranial ASVD include dissection, dissecting aneurysm, vasospasm, and fibromuscular dysplasia (FMD). All usually spare the carotid bulb.

Dissection (either traumatic or spontaneous) is more common in young/middle-aged patients and occurs in the middle of extracranial vessels. Extracranial dissections typically terminate at the exocranial opening of the carotid canal. Most are smooth or display minimal irregularities, whereas calcifications and ulcerations—common in carotid plaques—are absent.

Midsegment vessel narrowing with a focal mass-like outpouching of the lumen is typical of dissecting aneurysm. Vasospasm is more common in the intracranial vessels. When it involves the cervical carotid arteries or VAs, it also typically spares the proximal segments.

FMD spares the carotid bulb and usually affects the middle or distal aspects of the extracranial carotid arteries and VAs. A string of beads appearance is typical. Long-segment tubular narrowing is less common and may reflect coexisting dissection.

Intracranial Atherosclerosis

One of the most serious and disabling manifestations of ASVD is stroke. Most acute ischemic strokes are thromboembolic, most often secondary to cardiac sources or plaques in the cervical ICA.

Many clinicians focus on extracranial carotid artery disease, considering intracranial ASVD a relatively infrequent cause of stroke. However, intracranial ASVD accounts for 5-10% of all ischemic strokes. Nearly 1/2 of all patients with fatal cerebral infarction have at least one intracranial plaque-associated luminal stenosis at autopsy (10-17).

Ectasia

Generalized nonfocal vessel elongation is called “ectasia,” “dolichoectasia,” “arteriectasis,” or “dilative arteriopathy.” Ectasias can involve any part of the intracranial circulation but are most common in the vertebrobasilar arteries (“vertebrobasilar dolichoectasia”) and supraclinoid ICA.

Atherosclerotic Fusiform Aneurysm

Atherosclerotic fusiform aneurysms (FAs) are focal arterial enlargements that are usually superimposed on an ectatic artery. ASVD FAs were discussed in detail in Chapter 6. ASVD FAs are most common in the vertebrobasilar circulation. When they occur in the anterior circulation, they can produce a rare but dramatic manifestation called a giant “serpentine” aneurysm.

Intracranial Stenoocclusive Disease

Atherosclerosis that causes large artery intracranial occlusive disease (LAICOD) is now a well-defined yet relatively neglected and poorly understood stroke subtype. Recent studies have shown that the overall prevalence of intracranial ASVD in patients with concurrent extracranial disease varies between 20-50%, and 12% of patients have diffuse (multifocal) intracranial ASVD (10-20).

Overall, symptomatic patients with moderate to severe stenosis (i.e., 70-99%) in the intracranial circulation have a 25% two-year risk for recurrent stroke.

The availability of endovascular techniques, such as intracranial angioplasty, has opened new treatment avenues for LAICOD. A variety of balloon-expandable, drug-eluting, and self-expanding stents are also now available as options.

Imaging

Mural calcifications are common on NECT with patterns varying from scattered stippled foci to thick continuous linear (“railroad track”) deposits. CTA or DSA may show solitary or multifocal stenoses alternating with areas of poststenotic dilatation (10-19). When ASVD affects distal branches of the major intracranial vessels, the appearance can mimic that of vasculitis (see below).

High-resolution “black blood” vessel wall imaging on MR directly depicts intracranial ASVD and is a reliable tool for identifying intracranial ASVD and measuring plaque burden (10-18). Intramural hemorrhage and irregular, noncircumferential short-segment enhancing foci are common findings.

Differential Diagnosis

The major differential diagnoses of intracranial ASVD are vasculitis, vasospasm, and dissection.

Vasculitis occurs at all ages but is more common in middle-aged patients. Vasculitis and ASVD appear virtually identical on angiography (MRA, CTA, or DSA). Remember: The most common cause of a vasculitis-like pattern in an older patient is not vasculitis; it is ASVD!

Vasospasm spares the cavernous ICA and is usually more diffuse than ASVD. A history of trauma, subarachnoid hemorrhage (SAH), or drug abuse (typically with sympathomimetics) is common. Intracranial dissection—especially in the anterior circulation—is rare and usually occurs in young patients.

Arteriolosclerosis

Arteriolosclerosis, also known less specifically as cerebral “microvascular disease,” is a microangiopathy that typically affects small arteries (i.e., arterioles), especially in the subcortical and deep cerebral white matter (WM). Aging, chronic hypertension, hypercholesterolemia, and diabetes mellitus are the most common factors that predispose to cerebral microvascular disease.

Pathology

Generalized volume loss, multiple lacunar infarcts, and deep WM spongiosis are typical. Stenosis or occlusion of small vessels by arteriolosclerosis and lipohyalinosis probably results in WM microinfarctions.

Imaging

CT scans show patchy &/or confluent subcortical and deep WM hypodensities (10-21). Periventricular lesions have a broad or confluent base with the ventricular surface and are especially prominent around the atria of the lateral ventricles.

MR shows patchy or confluent periventricular and subcortical WM hypointensities on T1WI. The lesions are hyperintense on T2WI and are especially prominent on FLAIR (10-22). T2* (GRE, SWI) sequences often demonstrate multifocal “blooming” hypointensities, especially in the presence of chronic hypertension.

Differential Diagnosis

The major differential diagnosis is age-related hyperintensities in the periventricular WM. Scattered T2/FLAIR WM hyperintensities are almost universal after age 65. Enlarged perivascular (Virchow-Robin) spaces (PVSs) can be seen in patients of all ages and in virtually all locations, although they do increase with age. Unlike arteriolosclerosis, PVSs suppress on FLAIR.

Demyelinating disease typically causes ovoid or triangular periventricular lesions that abut the callososeptal interfaces, which are rarely involved by arteriolosclerosis.

Nonatheromatous Vascular Diseases

Preamble

Although atherosclerotic vascular disease (ASVD) is by far the most common disease to affect the craniocervical vasculature, a number of other nonatheromatous disorders can affect the brain, causing stroke or stroke-like symptoms. In this section, we briefly discuss some of the most important entities, including fibromuscular dysplasia (FMD), vasculitis, and non-ASVD noninflammatory vasculopathies, such as cerebral amyloid disease.

Fibromuscular Dysplasia

Terminology

FMD is an uncommon segmental nonatherosclerotic, noninflammatory disease of unknown etiology. FMD is a polyvascular disease that affects medium and large arteries in many areas of the body.

Pathology

Location

Although virtually any artery in any location can be affected, FMD affects some arteries far more than others. The renal arteries are affected in 75% of cases; ~ 35% of these are bilateral. Patients with known renal artery FMD often have cerebrovascular disease (and vice versa).

The cervicocephalic vessels are involved in up to 75% of cases. The ICA is the most common site; FMD typically involves the middle of the ICA and spares the bifurcation. VA FMD is seen in 20% of cases. Approximately 1/2 of all cervicocephalic FMD cases involve more than one artery (usually either both ICAs or one ICA and one VA). Intracranial FMD is very rare.

Multiple arterial systems are involved in 25-30% of cases. When multisystem disease is present, the renal arteries are almost always involved.

FMD carries an increased risk of developing intracranial saccular aneurysms. Intracranial saccular aneurysms are present in ~ 7-10% of patients with cervical FMD.

Staging, Grading, and Classification

FMD is classified histologically into three categories according to which arterial wall layer is affected (media, intima, or adventitia) (10-23).

By far, the most common type (type 1)is medial fibroplasia, accounting for ~ 60-85% of all FMD cases. The media has alternating thin and very thick areas formed by concentric rings of extensive fibrous proliferations and smooth muscle hyperplasia.

Intimal fibroplasia (type 2)accounts for < 10% of FMD cases. The intima is markedly thickened, causing smooth, long-segment narrowing.

Adventitial (periarterial) fibroplasia (type 3)is the least common type of FMD, accounting for < 5% of cases. Dense collagen replaces the delicate fibrous tissue of the adventitia and may infiltrate the adjacent periarterial tissues.

Clinical Issues

Epidemiology

Once thought to be a relatively rare vasculopathy, overall prevalence of FMD is estimated between 4-6% in the renal arteries and 0.3-3.0% in the cervicocephalic arteries. FMD is identified in 0.5% of all patients screened with CTA for ischemic neurologic symptoms.

FMD primarily affects individuals between 20-60 years of age. Sex disparity in FMD is striking with a 9:1 female predominance.

Presentation

Sudden onset of high blood pressure in a young woman is a classic presentation of renal FMD. FMD is found in ~ 1% of hypertensive patients and is the second leading cause of renovascular hypertension (after ASVD).

Cervical FMD can present with headache, pulsatile tinnitus/bruit, dizziness, neck pain, TIA, stroke, or dissection (often with Horner syndrome, i.e., ptosis, pupil constriction, and facial anhidrosis). Between 5-6% of patients are asymptomatic at the time of diagnosis.

Natural History

The natural history of FMD is unclear, as many cases are now discovered incidentally on imaging studies. Overall, 1 in 5 patients with FMD will have dissections and 1 in 5 patients will have aneurysms.

Imaging

Technical Considerations

CTA is noninvasive and accurately depicts FMD in the cervicocephalic arteries (10-25). Visualization of the intracranial vessels should be included to detect the presence of associated aneurysms. TOF MRA can be problematic, as artifacts caused by patient motion or in-plane flow and susceptibility gradients can mimic the appearance of FMD.

Because multisystem disease is common, patients with newly diagnosed carotid &/or vertebral FMD should also have their renal arteries examined. The prevalence of renal FMD is 40-45% in patients with cephalocervical disease.

Imaging Findings

Type 1 FMD (medial fibroplasia) is seen as an irregular corrugated or string of beads appearance with alternating areas of constriction and dilatation (10-24) (10-26). In type 2 (intimal fibroplasia), a smooth, long-segment tubular narrowing is present. In type 3 (adventitial) FMD, asymmetric diverticulum-like outpouchings from one side of the artery are present (10-27).

All three cervical FMD subtypes spare the carotid bifurcations and great vessel origins, involve the middle segments, and are most common at the C1-C2 level. Complications of cervicocephalic FMD include dissection, intracranial aneurysm with or without SAH (10-27), and arteriovenous fistulas. Other less common manifestations of FMD include vascular loops and fusiform vascular ectasias.

Differential Diagnosis

The major differential diagnosis of FMD is atherosclerosis. FMD is most common in young women, a group that is generally at low risk for ASVD. FMD involves the middle to distal portions of the affected arteries, sparing the carotid bifurcation. Atherosclerosis is typically short-segment stenosis at or above the carotid bifurcation with mural calcifications.

Arterial standing waves are a form of transient vasospasm that can be misinterpreted as type 1 FMD on catheter angiograms. The regular corrugated appearance contrasts with the irregular “string of beads” of FMD (10-33). Standing waves resolve spontaneously on repeat angiography (including CTA or MRA) or after the administration of vasodilators.

The smooth, tapered “tubular” narrowing of type 2 (intimal FMD) can be difficult to distinguish from spontaneous dissection, which also occurs as a complication of FMD. The asymmetric, diverticulum-like outpouchings sometimes seen in FMD can mimic traumatic cervical pseudoaneurysms.

Other  nonatherosclerotic vasculopathies, such as Takayasu arteritis and giant cell arteritis, can mimic tubular (i.e., intimal) FMD.

Dissection

Craniocervical arterial dissection (CAD) is strongly associated with ischemic events, primarily artery-to-artery embolism, so early diagnosis and appropriate treatment are essential.

Terminology

A dissection is a tear in at least one layer of the vessel wall that permits blood to penetrate into and delaminate (split apart or “dissect”) wall layers (10-28).

A dissecting aneurysm is a dissection characterized by an outpouching that extends beyond the vessel wall. Most occur with subadventitial dissections and are more accurately designated as pseudoaneurysms (i.e., they lack all normal vessel wall components).

Etiology

Extracranial CAD is more common than intracranial dissection. Almost 60% of extracranial dissections are “spontaneous,” i.e., nontraumatic. Most have an underlying vasculopathy, such as FMD, Marfan syndrome, or other connective tissue disorder (e.g., Ehlers-Danlos type 4) (10-34). Less common predisposing conditions include hypertension, migraine headaches, vigorous physical activity, hyperhomocysteinemia, and recent pharyngeal infection.

Traumatic extracranial dissections occur with blunt or penetrating injury, but sports (e.g., wrestling) or chiropractic manipulations have also been implicated (10-31)(10-35).

Intracranial dissections can be either traumatic or spontaneous. Iatrogenic dissections (typically secondary to endovascular procedures) are becoming increasingly common.

Pathology

Location

Extracranial dissections typically occur in the most mobile segment of a vessel, often starting or ending where the vessel transitions from a relatively free position to a position fixed by an encasing bony canal. The extracranial ICA is the most common overall site in the head and neck. Extracranial ICA dissections spare the carotid bulb and often extend up to—but only occasionally into—the skull base (10-33). Vertebral dissections are most common between the skull base and C1 and between C1 and C2(10-36).

The most frequently involved intracranial site is the VA. Dissections in the anterior circulation are even less common. They almost always involve the supraclinoid ICA with or without extension into the proximal middle cerebral artery (MCA).

Size and Number

Dissections can be limited to a focal intimal tear and small subintimal hematoma. Most are solitary, long-segment lesions that extend for several centimeters. Approximately 20% involve two or more vessels. Multiple dissections are more common if an underlying vasculopathy, such as Marfan, Ehlers-Danlos type 4, or FMD, is present (10-34).

Gross Pathology

An intimal tear permits dissection of blood into the vessel wall, resulting in a medial or subendothelial hematoma that may narrow or occlude the vessel lumen (10-30). Occasionally, dissections—especially in the VA—extend through the adventitia and present with SAH.

Clinical Issues

Epidemiology and Demographics

CAD is the most common cause of ischemic stroke in young and middle-aged adults (10-35). Peak age is 40 years.

Presentation

Neck pain and headache are the most common symptoms. One or more lower cranial nerve palsies, including postganglionic Horner syndrome, may occur.

Natural History

The natural history of most extracranial CADs is benign. Approximately 90% of stenoses resolve and 60% of all occlusions recanalize. Recurrent dissection is rare. Intracranial CAD is much more problematic. Stroke is more common, and spontaneous recanalization is less frequent.

Imaging

General Features

Dissections can present as stenosis, occlusion, or aneurysmal dilatation.

CT Findings

NECT may show crescent-shaped thickening caused by the wall hematoma. Posterior fossa SAH is seen in 20% of intracranial VA dissections.

MR Findings

Fat-saturated T1WI is the best sequence for demonstrating CAD. A hyperintense crescent of subacute blood adjacent to a narrowed “flow void” in the patent lumen is typical (10-32). T2WI may show laminated thrombus that “blooms” on T2*.

At least 1/2 of all patients with cervicocephalic dissections have cerebral or cerebellar infarcts, best depicted on DWI. Multiple ipsilateral foci of diffusion restriction are typical findings.

Angiography

Extracranial ICA dissections typically spare the carotid bulb, beginning 2-3 cm distal to the bifurcation and terminating at the exocranial opening of the carotid canal (10-33)(10-35). Vertebral dissections are most common around the skull base and upper cervical spine.

CTA and MRA demonstrate an eccentrically narrowed lumen surrounded by a crescent-shaped mural thickening (10-32). A dissection flap can sometimes be identified (10-31). Pseudoaneurysms are common. An opacified double lumen (“true” + “false” lumen) occurs in < 10% of cases.

The most common finding on DSA is a smooth or slightly irregular, tapered midcervical narrowing (10-33)(10-35). Craniocervical dissections with occlusion shows a tapered “rat-tail” termination (10-36). Occasionally, a subtle intimal tear or flap, a double lumen, narrowed or occluded true lumen, or pseudoaneurysm can be identified. If the dissection is subadventitial and does not narrow the vessel lumen, DSA can appear entirely normal; the paravascular hematoma must be detected on cross-sectional imaging.

Intracranial dissections are more difficult to diagnose than their extracranial counterparts. They are significantly smaller and findings are often subtle.

DISSECTION

Terminology

• Vessel wall tear → blood penetrates into, splits layers apart

Intramural hematoma formed

± pseudoaneurysm

Pathoetiology

• “Spontaneous” (nontraumatic): 60%; traumatic: 40%

 Underlying vasculopathy (FMD, Marfan, etc.): 40%

• Location

 Extracranial ICA (spares bulb, usually terminates at skull base)

 VA (skull base-C1, C1-C2 most common)

 Intracranial = extracranial (vertebral > > carotid)

 Multiple arteries: 20% (look for underlying vasculopathy)

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