Diagnosis and Treatment of Diseases of the Aorta
Structurally a simple conduit, the aorta can manifest disease in only a limited
number of ways. When weakened by disease, its wall may dilate,
producing an aneurysm, or it may split in its long axis, producing
dissection. In either case, fatal rupture may result. Moreover,
like all pipes, it may become obstructed. More often than narrowing
of the main trunk, however, obstruction at the origin of a main
branch is encountered. In contrast to these relatively few clinical
manifestations, an array of disease processes can involve the
aorta. Not surprisingly, there is considerable overlap in the
clinical presentation of these disorders.
This chapter will first discuss the various diseases that involve
the aorta together with their pathogenetic mechanisms, characteristic
pathologic features, and typical clinical findings. A description
of the common clinical manifestations of aortic disease for
which there may be several etiologies or for which the etiology
is unknown will follow.
Etiologic And Pathogenetic Considerations In Aortic Disease
Medial Changes of Aging
Circumferential plates or lamellae of elastin
fibers constitute the most conspicuous feature of the aortic
media when it is examined histologically. Dispersed between
the circular elastic fibers are longitudinally oriented smooth
muscle cells, collagen fibers, microfibrils, and ground substance.
Clinicians have long recognized dilatation and elongation of
the aorta in the elderly. Characteristic alterations in the
structure of the aortic wall accompany these changes. Schlatmann
and Beckeridentified these as fragmentation of elastic fibers
and loss of smooth muscle cell nuclei, so-called medionecrosis.
Moreover, collagenous tissue and basophilic ground substance
deposits are a feature of the aging aorta.
By middle life, aortic
atherosclerosis is nearly universal in the Western world. ts
severity varies from individual to individual.
Diabetes, hypercholesterolemia, smoking, and
hypertension are among the factors promoting it. The pathology
of atherosclerosis is discussed in elsewhere.
Advanced atherosclerotic changes display a
characteristic distribution, and involvement is most severe
below the renal arteries in the abdominal aorta, is common but
less severe in the descending thoracic segment, and is least
severe in the ascending segment. With diabetes mellitus, however,
disease is frequently severe throughout. Individuals with familial
hypercholesterolemia are a second exception to the rule that
the ascending aorta is spared. Also, the aortic root and aortic
valve may be severely involved in familial cholesterolemia.
Both supravalvular and valvular aortic stenosis may develop.
Finally, syphilitic ascending aortitis promotes severe atherosclerosis.
Aortic atherosclerosis is manifest as aneurysm, obstruction
of the infrarenal aorta, embolization from atheromatous plaques
to distal arterial beds, and medial dissection initiated by
penetration of a plaque into the media.
Aneurysm of the abdominal aorta has long been
presumed to result from penetration into and weakening of the
media by atherosclerosis. Thus abdominal aortic aneurysms characteristically
appear in individuals with the most severe aortic atherosclerosis
in nonaneurysmal segments.
Recent recognition of familial clustering of patients with abdominal
aortic aneurysm, the identification of genetic defects in collagen
in a family with multiple aneurysms' and the detection of abnormal
collagenase and elastase in tissue from aortic aneurysms resected
at operation have led some to the assumption that atherosclerosis
is invariably the underlying pathophysiologic mechanism. These
findings suggest that atherosclerosis represents a secondary
response to dilatation of the aorta resulting from medial weakness.Aneurysm
of the descending thoracic aorta also traditionally has been
attributed to atherosclerosis, since such lesions are commonly
accompanied by an infrarenal aneurysm.
Obstruction of the Terminal Aorta
Obstruction of the main aortic channel most
often develops in the infrarenal aorta and may extend into the
proximal iliac arteries. Obstruction of branch arteries is more
common than aortic obstruction.
The luminal surface of a severely diseased
aortic segment is often rough and covered with thrombus. Embolization
of plaque material and thrombus from these surfaces now appears
to he far more common than was once appreciated. Emboli to the
brain, the lower extremities, and the coronary, renal, or visceral
circulations have been reported.
Transesophageal echocardiography now provides rather startling
views of pedunculated thrombus or other atherosclerotic material
waving in the aortic blood flow ( Fig.
105d ). Aortas with ulcerated plaques, pedunculated or mobile
thrombi, or spontaneous echo contrast are more apt to embolize
than are those with flat, layered atherosclerosis. Anticoagulant
therapy may be protective against future events.
A variation on the theme, the clinical syndrome labeled cholesterol
embolization, with small atherosclerotic particles obstructing
small arteries, is a rare complication of severe aortic atherosclerosis.
Clinical signs include mottled skin and "purple toes"
in the lower extremities together with renal insufficiency and
visceral ischemia in more severe cases.this rarely recognized
condition may he spontaneous but is more commonly encountered
as a complication of intraaortic catheter manipulation. Because
eosinophilia is frequent in the initial phases of this event,
an immune reaction to the free particles has been suggested.
Penetrating Atherosclerotic Ulcers
Atherosclerotic plaque penetration into the
media predisposes to formation of an intramural hematoma. Extension
circumferentially and in the long axis of the media may produce
a limited medial dissection ( Fig.
105e ). Radial extension results in pseudoaneurysm or rupture.
Penetrating ulcers are most commonly recognized in the descending
The clinical picture resembles that of aortic dissection or
of expansion/rupture of a preexisting aneurysm. Sudden onset
of severe back pain in a hypertensive patient or one known to
have atherosclerosis is typical. Many are identified in the
course of imaging for suspected aortic dissection. Since they
are more limited in axial length than typical dissection, and
since they arc located in the descending thoracic aorta, aortic
regurgitation and altered pulses are not characteristic features.
Surgical treatment is often indicated, although some patients
survive without operation.