C H A P T E R 12

Aortic Dissection
David Hirschl and Vineet R. Jain

Definition and may compress or occlude the true lumen. Over time,
Aortic dissection is the most common acute aortic disor- the false lumen often becomes aneurysmal.
der. It has a high morbidity and mortality and an incidence Most dissections travel within the media distal to
of 0.2–0.8%. It is caused by an intimal tear within an abnor- the initial intimal tear, but some also travel retrograde to
mal, weakened aortic wall. The intimal tear allows blood involve the aorta proximal to the initial tear. Re-entry tears
to enter the wall, with subsequent extension proximally may occur within the aorta, creating more than one inti-
and distally, resulting in inward displacement of the vessel mal flap. In some aortic dissections, the initiating event was
intima. There may be one or more tears that allow blood to not the intimal tear but hemorrhage in the vasa vasorum,
communicate between two lumens. which are small vessels that arborize in the media of the
aortic wall.
Conditions that predispose a person to aortic dissec-
Clinical Features tion include, most commonly, hypertension. Other etiolo-
The classic clinical presentation is that of sharp chest pain gies include connective disuse disorders such as Marfan’s
that radiates to the back. Other common symptoms include and Ehlers-Danlos syndromes, congenital bicuspid aortic
syncope and shortness of breath. Risk factors for aortic dis- valve, iatrogenic aortic injury, and vasculitis.
section include hypertension, connective tissue disorders, The classification of an aortic dissection is based on
trauma, iatrogenic causes such as cardiac surgery, arte- the proximal-most location of the intimal tear. If the tear
ritis, and congenital lesions such as bicuspid aortic valve involves the ascending aorta, proximal to the origins of
and aortic coarctation. The imaging workup of acute aortic the arch vessels, it is known as a Stanford type A dissec-
dissection often begins with a chest x-ray. Further evalu- tion (60–70% of cases). Type A dissections may involve and
ation can subsequently be performed with multidetector extend into the great vessels leading to impaired cerebral
computerized tomographic angiography (MDCTA), MRI, blood flow. Stanford type A dissection may also extend into
or transesophageal echocardiography (TEE). MDCTA is the descending aorta. A Stanford type B dissection occurs
the modality most frequently used in the setting of sus- when the dissection flap involves the descending thoracic
pected aortic dissection because of its widespread availabil- aorta distal to the origin of the left subclavian artery and
ity, rapid imaging time, and high sensitivity. MDCTA has extends caudally (30–40% of cases). As the dissection
largely replaced invasive aortography. The benefit of using extends distally, it may involve the renal, celiac, or mesen-
MRI or TEE in the workup of aortic dissection includes teric arteries. Significant narrowing or occlusion of any of
lack of contrast injection and no exposure to ionizing radi- the aortic branch vessels may result in end-organ ischemia
ation; however, both of these modalities are more time con- or infarction.
suming and may not be appropriate for an unstable patient.

How to Approach the Image

Anatomy and Physiology The chest radiograph may be normal in up to 40% of
The aortic wall is composed of three layers: the intima, patients with dissection. The most common finding on
media, and adventitia. The intima, the inner layer, is in plain film in a patient with dissection is a widened medias-
direct contact with flowing blood. When aortic dissection tinum (61.1%). Acute enlargement of the cardiac silhouette
occurs, blood breaks through the intima and enters the may represent hemopericardium, a complication of type
media. Typically, the media is separated into two layers by A dissection. Another potential complication, hemothorax,
the high-pressure pulsatile flowing blood, which creates an can also be appreciated on chest radiography as a pleural
abnormal double-lumen aorta with a true lumen and false effusion.
lumen. The true lumen is the normal pathway of blood Multidetector CT imaging of acute aortic syndromes
in the aorta and the false lumen is the abnormal pathway is often performed with and without intravenous contrast.
within the media. As inflow occurs, the false lumen expands On non-contrast images, high attenuation within the aortic

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(a) (b)

Figure 12.1. Stanford type A aortic dissection. (a) Non-contrast axial CT shows a thick-walled ascending aorta with increased density
(arrow) representing intramural hemorrhage (hematoma). The true lumen is also seen (*). (b) Contrast-enhanced axial CT of the
same patient demonstrates an intimal flap (arrowhead), as well as the true (*) and false (curved arrow) lumens.

wall represents intramural hematoma, which may clinically CT angiographyin order to minimize motion artifact, par-
mimic or coexist with acute aortic dissection (Fig. 12.1a). ticularly in the ascending aorta, and more clearly delineate
An intramural hematoma is blood within the aortic wall the intimal flap.
media. Another feature of acute dissection detectable on Using non-contrast MR sequences, aortic dissection
non-contrast CT is medial displacement of atheroscle- can be diagnosed by observing an intimal flap. The intimal
rotic intimal calcifications. These features are particularly flap is best appreciated on spin-echo black-blood sequences.
important to detect in patients with renal impairment, con- The true lumen will demonstrate a flow void, while the
trast allergy, or other contraindication to receiving intrave- false lumen will show higher signal related to slow flow or
nous contrast. thrombus. A bright-blood cine sequence can be used to dif-
Contrast-enhanced CT in the arterial phase will dem- ferentiate slow flow from thrombus within the false lumen
onstrate an intimal flap separating the false and true lumens (Figs. 12.2, 12.3). Contrast-enhanced MR angiography can
(Fig. 12.1b). The false lumen is typically larger than the true also be performed, which will demonstrate similar features
lumen and may contain thrombus. In type A dissections, to those seen on CT. The evaluation of the dissection flap in
the true lumen is most often along the right anterolateral multiple planes will help in determining its extent.
wall of the ascending aorta and extends distally in a spiral
fashion along the left posterolateral wall of the descending
aorta. Linear low-attenuation areas (aortic cobwebs) rep- What Not To Miss
resenting incompletely dissected vessel media seen only in When evaluating a patient with suspected aortic dissection,
the false lumen may aid in the differentiation of false from an intimal flap is diagnostic. If no intimal flap is seen, intra-
true lumen. If differentiation remains difficult, the lumen mural hematoma may still be present. This is best observed
that extends most caudal is the true lumen. Multiplanar on non-contrast images as increased density within the
analysis including 3D reconstruction can aid in determin- aortic wall. Classification of intramural hematoma is simi-
ing the extent of the dissection, as well as help the referring lar to the Stanford classification system of aortic dissection.
clinician better understand the extent of the dissection. When an intramural hematoma involves the ascending
Special attention, including evaluation of the intimal aorta, urgent surgical management is warranted. If dis-
flap in multiple planes, should be given to patients with a section or intramural hematoma is not seen, other causes
tortuous aorta. A redundant aorta may mimic a dissection of chest pain, such as pulmonary embolism, myocardial
flap on axial slices; however, when viewed in orthogonal infarction, vasculitis, or pulmonary parenchymal processes,
planes, the course of a tortuous vessel will be clearly delin- should be considered and correlation made with imaging
eated. Some radiologists advocate performing ECG-gated features as well as relevant clinical and laboratory data.
 Aortic Dissection 65

Figure 12.2. Stanford type A aortic dissection. Coronal

balanced turbo field echo non-contrast MRI show an intimal
flap (white arrows) within the ascending aorta. Figure 12.4. Aortic dissection with complications. Axial
post-contrast CT shows a type B aortic dissection with an intimal
flap (black arrow). There is an associated hemopericardium
(thin white arrow) and hemorrhagic left-sided pleural effusion
Clinical Issues (thick white arrow).
The prompt diagnosis and classification of aortic dissection
is necessary for proper patient management. Type A dis-
sections require urgent surgical intervention because of the or medium-sized arteries, complications related to slow
possibility of life-threatening complications such as trans- or absent blood flow may result in myocardial infarction,
mural aortic rupture, aortic valve rupture with acute aortic cerebral infarction, renal insufficiency, and mesenteric
insufficiency, hemopericardium, dissection involving the infarction. It is therefore necessary to describe the extent
coronary arteries, mediasinal hematoma, and hemotho- of the dissection as well as all involved vessels, including
rax (Fig. 12.4). When the false lumen gives rise to large coronary arteries, arch vessels, renal arteries, and mesen-
teric vessels.
Type B dissections are often medically managed with
blood pressure control. Indications for surgical or endovas-
cular graft placement in type B dissections include rupture,
hemodynamic instability, and descending thoracic aorta
diameter greater than 6 cm.

Differential Diagnosis
When an intimal flap is present, the diagnosis of aortic
dissection can most often be made with confidence. One
must also consider the following when evaluating for aortic
pathology:

■ Intramural hematoma
■ Penetrating aortic ulcer
■ Thrombosed aortic aneurysm

■ Trauma with aortic laceration

■ Vasculitis

Intramural hematoma and penetrating aortic ulcer often

Figure 12.3. Stanford type B aortic dissection. Axial T1, coexist with an aortic dissection. A penetrating aortic ulcer
non-contrast MRI shows an intimal flap (arrowhead). occurs when an atherosclerotic plaque ulcerates, penetrates
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the intima, and extends into the media as a focal process.

It often leads to intramural hematoma, dissection, and/ without a dissection flap present. Management is sim-
or pseudoaneurysm. Acute traumatic aortic injury may ilar to dissection.
appear similar to an aortic dissection but is different mech- ■ The most common cause of aortic dissection is hyper-
anistically, usually because of severe deceleration injury. tension. Other etiologies include connective tissue
A large-vessel vasculitis such as an aortitis will cause abnor- disorders such as Marfan’s or Ehlers-Danlos syn-
mal thickening of the vessel wall often with concomitant dromes, congenital anomalies (coarctation, bicuspid
aneurysms and/or stenoses. aortic valve), iatrogenic disorders, or vasculitis.
■ Complications include aortic rupture, hemopericar-
dium, cardiac tamponade, and end-organ ischemia
Key Points (myocardial, cerebral, renal, or mesenteric infarcts).
■ Aortic dissection is present when an intimal flap

divides the aorta into a false lumen and a true lumen.

■ Stanford classification:
Suggested Reading
Hagan PG, Nienaber CA, Isselbacher EM, et al. The International
■ Type A (60%) involves the ascending aorta and
Registry of Acute Aortic Dissection (IRAD): new insights into
may extend into the descending aorta. It requires an old disease. JAMA. 2000;283(7):897–903.
surgical management. Lansman SL, Saunders PC, Malekan R, Spielvogel D. Acute aortic
■ Type B (40%) involves the descending aorta, dis- syndrome. J Thorac Cardiovasc Surg. 2010;140(6 Suppl):S92–
tal to the origins of the great vessels. It requires S97; discussion S142–S146.
medical management unless the patient is unsta- Litmanovich D, Bankier AA, Cantin L, Raptopoulos V, Boiselle
ble, there is vascular compromise to an organ, PM. CT and MRI in diseases of the aorta. AJR Am J Roentgenol.
or there is a concomitant large aortic aneurysm 2009;193(4):928–940.
present. McMahon MA, Squirrell CA. Multidetector CT of aortic dissec-
■ Intramural hematoma is present when increased
tion: a pictorial review. Radiographics. 2010;30(2):445–460.
density is seen within a thickened aortic wall on CT, Prêtre R, Von Segesser LK. Aortic dissection. Lancet.
1997;349(9063):1461–1464.