AJR 2003; 181:303-307
© American Roentgen Ray Society
Pathogenesis in Acute Aortic Syndromes: Aortic Aneurysm Leak and Rupture and Traumatic Aortic Transection
Katarzyna J. Macura1,
Frank M. Corl,
Elliot K. Fishman and
David A. Bluemke
1 All authors: The Russell H. Morgan Department of Radiology and Radiological
Science, Johns Hopkins Medical Institutions, 600 N. Wolfe St., Baltimore, MD
21287-0750.
Received July 26, 2002;
accepted after revision December 17, 2002.
Address correspondence to K. J. Macura.
Introduction
This pictorial essay focuses on the pathophysiology of enlargement and
rupture of the atherosclerotic aortic aneurysm and on mechanisms involved in
traumatic aortic transection related to deceleration injury.
Aortic Aneurysm Leak and Rupture
The wall stress related to blood pressure in the nonaneurysmal aorta is
relatively low and uniformly distributed, whereas within the aortic aneurysm,
regions of high- and low-stress distribution are present
[1]. Increased tension stress
results in progressive vessel dilatation and weakening of the aortic media.
According to Laplace's law, wall tension is proportional to the vessel radius
for a given blood pressure. When an artery wall develops a weak spot and
expands as a result, it might seem that the expansion would provide some
relief, but in fact the opposite is true. The expansion subjects the weakened
wall to even more tension. The weakened vessel continues to expand
(Fig. 1). A localized weak spot
in an artery might gain temporary tension relief by expanding toward a
spherical shape because a spherical membrane has half the wall tension for a
given radius. Unfortunately, in an expanding aneurysm, forming a
near-spherical shape cannot give sufficient tension relief. Aortic aneurysm
rupture is believed to occur when the mechanical stress on the wall exceeds
the strength of the wall tissue (Fig.
2A,
2B). Infected aortic aneurysm
is a rare lesion, which may progress rapidly to aortic rupture or uncontrolled
sepsis with high mortality rate.
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Fig. 1. —Drawing shows enlarging aortic aneurysm. First, wall tension is
proportional to vessel radius, according to Laplace's law: T =
P x r, where T is circumferential wall
tension, P is transmural pressure, and r is mean vessel
radius. Second, increased tension stress from blood pressure results in
progressive vessel dilatation and weakening of aortic media, which lead to
enlargement of aortic aneurysm. Third, when mechanical stress on wall exceeds
strength of wall tissue, aortic aneurysm ruptures.
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Fig. 2A. —79-year-old man with acute rupture of abdominal aortic aneurysm that
occurred during CT. Contrast-enhanced CT scan obtained at level of
extravasation (open arrow) shows large left retroperitoneal hematoma
and enhanced blood in left paracolic gutter (solid straight arrow).
Note eccentric intramural hematoma (curved arrow) within aortic lumen
on side of extravasation.
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Fig. 2B. —79-year-old man with acute rupture of abdominal aortic aneurysm that
occurred during CT. Coronal multiplanar reformatted CT image shows aortic
rupture with active extravasation from irregular ulcerlike lesion in distal
abdominal aorta (open arrow). Note enhanced blood (solid
arrow) around spleen (S) and that spleen is displaced medially.
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Abdominal aortic aneurysms expand at a rate of 2–4 mm per year for
aneurysms smaller than 4 cm, 2–5 mm for aneurysms 4–5 cm, and
3–7 mm for those larger than 5 cm. The rupture risk at 4 years is 2%,
10%, and 22%, respectively [2].
Abdominal aortic aneurysm is seen concomitantly in 42.1% of patients with
penetrating aortic ulcers and 29.4% of patients with intramural hematomas
[3]. Aneurysms in patients with
aortic ulcer and intramural hematoma tend to be larger than those associated
with classic aortic dissection (6.2 and 5.5 cm vs 5.2 cm, respectively)
[3]. In the presence of
penetrating aortic ulcer, gradual enlargement of the aorta and extension of
intramedial hematoma cause stretching of the weakened aortic wall and may lead
to a saccular aortic aneurysm or pseudoaneurysm (Figs.
3A,
3B and
4A,
4B). The weakened aortic wall
may eventually rupture. Most spontaneous aortic ruptures are believed to be
associated with perforation through the atheromatous plaque. The proposed
mechanism for rupture is pressure atrophy of the media due to overlying
intimal atherosclerotic plaque with localized distention of the aortic wall
resulting from intramural hematoma before perforation
[4].
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Fig. 3A. —76-year-old woman with disseminated tuberculosis and saccular
pseudoaneurysm arising from distal descending thoracic aorta. Volume-rendered
CT scan of axial view of descending thoracic aorta shows large pseudoaneurysm
(straight arrow) that arises posteriorly and causes flattening of
aortic lumen (curved arrow).
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Fig. 3B. —76-year-old woman with disseminated tuberculosis and saccular
pseudoaneurysm arising from distal descending thoracic aorta. Volume-rendered
CT scan of left anterior oblique view of aorta shows calcified atherosclerotic
plaque (arrowheads), limiting superior and inferior extent of
pseudoaneurysm.
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Fig. 4A. —71-year-old woman with thoracic aortic pseudoaneurysm due to
penetrating atherosclerotic ulcer. Volume-rendered CT scan of axial view shows
contrast material leaking from aortic lumen (solid arrow) into
posterior pseudoaneurysm (open arrow) of proximal descending thoracic
aorta.
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Fig. 4B. —71-year-old woman with thoracic aortic pseudoaneurysm due to
penetrating atherosclerotic ulcer. Volume-rendered CT scan of oblique view of
pseudoaneurysm shows site of contrast leak (arrow).
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Traumatic Aortic Transection
One of the accepted mechanisms for traumatic aortic rupture from rapid
deceleration involves a combination of traction, torsion, and hydrostatic
forces created by differential deceleration of thoracic structures. Unequal
horizontal shear forces that are applied during high-speed deceleration cause
the mobile ascending and descending aorta to lag behind the transverse aortic
arch, which is relatively fixed by the brachiocephalic vessels
[5]. Deceleration forces place
the maximal stress on those segments of the aorta and great vessels at the
points of attachments, the aortic isthmus, and the aortic root. Another
hypothesis involves the osseous pinch theory
[6], which proposes that aortic
rupture occurs when the aorta is pinched between the spine and the anterior
bony thorax (the manubrium, clavicle, and first ribs) during chest compression
caused by abrupt deceleration. The most commonly injured site is just distal
to the left subclavian artery. The ligamentum arteriosum and the intercostal
vessels fix the distal arch and descending thoracic aorta in apposition to the
vertebral bodies. The superior portion of the arch is held in place by the
great vessels extending from the thoracic inlet into the neck. Therefore, the
relatively fixed proximal descending aorta cannot move away from the bony
structures as they pinch and transect it.
In more than 80% of the cases, rupture is complete—through all three
layers of the aorta—and results in exsanguination and death at the site
where the individual sustains the trauma. The spectrum of findings in
incomplete rupture in traumatic aortic transection includes intramural
hematoma without intimal tear, intimal tear, abrupt change in the contour of
the aorta, diminished caliber of the descending aorta (pseudocoarctation),
pseudoaneurysm (Figs. 5,
6,
7A,
7B,
7C,
7D,
8A,
8B), and extravasation of
contrast material from the aorta.
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Fig. 6. —33-year-old man involved in deceleration injury that caused
traumatic transection of aorta. Left anterior oblique thoracic aortogram shows
pseudoaneurysm (arrow) in classic location, approximately 2 cm distal
to origin of left subclavian artery.
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Fig. 7A. —13-year-old boy who was struck by car while riding his bicycle.
Patient sustained subarachnoid hemorrhage; traumatic aortic transection;
contusions of liver, spleen, and kidneys; and pelvic and lower extremity
fractures. Contrast-enhanced CT scan shows periaortic hematoma. Note
difference in attenuation between thymus (curved arrow) and hematoma
(straight arrow). Nasogastric tube is not deviated.
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Fig. 7B. —13-year-old boy who was struck by car while riding his bicycle.
Patient sustained subarachnoid hemorrhage; traumatic aortic transection;
contusions of liver, spleen, and kidneys; and pelvic and lower extremity
fractures. Axial CT scan shows intimal disruption with pseudoaneurysm
(arrow) filled with contrast material.
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Fig. 7C. —13-year-old boy who was struck by car while riding his bicycle.
Patient sustained subarachnoid hemorrhage; traumatic aortic transection;
contusions of liver, spleen, and kidneys; and pelvic and lower extremity
fractures. Volume-rendered CT scan shows small pseudoaneurysm (arrow)
in classic location distal to left subclavian artery.
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Fig. 7D. —13-year-old boy who was struck by car while riding his bicycle.
Patient sustained subarachnoid hemorrhage; traumatic aortic transection;
contusions of liver, spleen, and kidneys; and pelvic and lower extremity
fractures. Left anterior oblique and anteroposterior angiograms of thoracic
aorta show contained transection of proximal descending aorta
(arrows), just below level of ligamentum arteriosum.
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Fig. 8A. —69-year-old man involved in severe motor vehicle collision 34 years
earlier. Volume-rendered CT scan obtained in left anterior oblique projection
shows classic location for traumatic aortic transection (arrowheads),
just beyond origin of left subclavian artery. Pseudoaneurysm is partially
thrombosed with peripheral calcifications that indicate its chronic
nature.
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Fig. 8B. —69-year-old man involved in severe motor vehicle collision 34 years
earlier. Volume-rendered CT scan of anteroposterior view of thoracic aorta
shows extent of pseudoaneurysm (arrowheads).
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Aortic ductus diverticulum is an anatomic variant that should not be
mistaken for a pseudoaneurysm. This variant typically occurs at the aortic
isthmus and presents as a smooth focal bulge that forms gentle, obtuse angles
with the aortic wall. It is usually located at the anteromedial aspect of the
aorta. Aortic ductus diverticulum is best visualized on angiography or
reformatted CT. On axial CT scans, diverticulum has smooth transition between
the CT slices and is difficult to diagnose. In contrast, a posttraumatic
pseudoaneurysm is variable in shape and usually has sharp margins. On axial
images, the transition from a normal aorta to the site of transection is
abrupt, with a change in contour, presence of intimal flap, or contrast
extravasation.
The most common sites of arterial injuries are aortic rupture alone (81%),
aortic arch branches alone (16%), and both aorta and aortic branches (3%).
Among the aortic injuries, 96% occur at the aortic isthmus distal to the left
subclavian artery, 1% at the aortic isthmus and proximal ascending aorta, 1%
at the proximal ascending aorta only, 1% at the distal ascending aorta only,
and less than 1% at the descending aorta
[7].
Summary
Diagnostic algorithms in evaluation of aortic emergencies are changing.
Catheter angiography has traditionally been the gold standard for evaluating
patients with aortic disease. However, currently helical CT plays the dominant
and critical role in the evaluation of patients presenting with aortic
emergencies. In the evaluation of patients with acute thoracic injuries,
helical CT had a sensitivity and negative predictive value equivalent to those
of aortography [8]. CT
angiography was 100% sensitive and performed better than conventional
angiography [9] in the
evaluation of aortic aneurysm extent. The advancement in helical CT brings
this modality to the forefront of acute aortic imaging as a reliable and
noninvasive technique for definitive evaluation of patients with aortic
aneurysm and traumatic aortic transection and not just as a screening modality
before ordering an aortogram.
References
- Raghavan ML, Vorp DA, Federle MP, Makaroun MS, Webster MW. Wall
stress distribution on three-dimensionally reconstructed models of human
abdominal aortic aneurysm. J Vasc Surg2000; 31:760
–769[Medline]
- Hallin A, Bergqvist D, Holmberg L. Literature review of surgical
management of abdominal aortic aneurysm. Eur J Vasc Endovasc
Surg 2001;22:197
–204[Medline]
- Coady MA, Rizzo JA, Elefteriades JA. Pathologic variants of
thoracic aortic dissections: penetrating atherosclerotic ulcers and intramural
hematomas. Cardiol Clin North Am1999; 17:637
–657
- Hayashi H, Matsuoka Y, Sakamoto I, et al. Penetrating
atherosclerotic ulcer of the aorta: imaging features and disease concept.
RadioGraphics2000; 20:995
–1005[Abstract/Free Full Text]
- Parmley LF, Mattingly TW, Manion WC, Jahnke EJ. Non-penetrating
traumatic injury of the aorta. Circulation1958; 17:1086
–1101[Medline]
- Crass JR, Cohen AM, Motta AO, Tomashefski JF, Wiesen EJ. A proposed
new mechanism of traumatic aortic rupture: the osseous pinch.
Radiology1990; 176:645
–649[Abstract/Free Full Text]
- Ahrar K, Smith DC, Bansal RC, Razzouk A, Catalano RD. Angiography
in blunt thoracic aortic trauma. J Trauma1997; 42:665
–669[Medline]
- Parker MS, Matheson TL, Rao AV, et al. Making the transition: the
role of helical CT in the evaluation of potentially acute thoracic aortic
injuries. AJR2001; 176:1267
–1272[Abstract/Free Full Text]
- Errington ML, Ferguson JM, Gillespie IN, Connell HM, Ruckley CV,
Wright AR. Complete preoperative imaging assessment of abdominal aortic
aneurysm with spiral CT angiography. Clin Radiol1997; 52:369
–377[Medline]
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Introduction
Aortic Aneurysm Leak and...
