Page 422 - Textbook of Pathology, 6th Edition

406 weakening of the wall. Since atherosclerotic lesions are most
common and severe in the abdominal aorta, atherosclerotic
aneurysms occur most frequently here. In the thoracic aorta,
besides atherosclerotic lesions, medial degeneration is
another additional factor implicated in pathogenesis.

MORPHOLOGIC FEATURES. Atherosclerotic aneu-
rysms of the abdominal aorta are most frequently infra-
renal, above the bifurcation of the aorta but may extend
into common iliac arteries. They may be of variable size
but are often larger than 5-6 cm in diameter.
Atherosclerotic aneurysm is most frequently fusiform in
shape and the lumen of aneurysm often contains mural
thrombus.
Histologically, the wall of atherosclerotic aneurysm loses
its normal arterial structure. Instead, there is predo-
minance of fibrous tissue in the media and adventitia with
mild chronic inflammatory reaction. The intima and inner
part of the media show remnants of atheromatous plaques
and mural thrombus.
EFFECTS. The clinical effects of atherosclerotic aneurysms
are due to complications. These are as under:
Figure 15.16 Sites of major forms of aneurysms.
1. Rupture. Rupture of the atherosclerotic aneurysm is the
5. Racemose or circoid having mass of intercommunicating most serious and fatal complication. The risk of rupture
small arteries and veins. depends upon the size and duration of the aneurysm and
the blood pressure. Rupture of abdominal aneurysm may
C. Based on pathogenetic mechanisms: This classification occur either into the peritoneum or into the retroperitoneum
is followed most often (Fig. 15.16): resulting in sudden and massive bleeding. Occasionally,
SECTION III
1. Atherosclerotic (arteriosclerotic) aneurysms are the most there may be slow progressive leak from the aneurysm. A
common type. ruptured aneurysm is more likely to get infected.
2. Syphilitic (luetic) aneurysms found in the tertiary stage of 2. Compression. The atherosclerotic aneurysm may press
the syphilis. upon some adjacent structures such as compression of ureter
3. Dissecting aneurysms (Dissecting haematoma) in which the and erosion on the vertebral bodies.
blood enters the separated or dissected wall of the vessel.
4. Mycotic aneurysms which result from weakening of the 3. Arterial occlusion. Atherosclerotic aneurysms of the
abdominal aorta may occlude the inferior mesenteric artery,
arterial wall by microbial infection. or there may be development of occlusive thrombosis.
5. Berry aneurysms which are small dilatations especially However, collateral circulation develops slowly and is nearly
affecting the circle of Willis in the base of the brain (Chapter always sufficient so as not to produce effects of ischaemia.
30). Thromboembolism is rather common in abdominal
Systemic Pathology
The three common types of aortic aneurysms—athero- aneurysms.
sclerotic, syphilitic and dissecting, are described below:

Atherosclerotic Aneurysms Syphilitic (Luetic) Aneurysms
Atherosclerotic aneurysms are the most common form of Cardiovascular syphilis occurs in about 10% cases of syphilis.
aortic aneurysms. They are seen more commonly in males It causes arteritis—syphilitic aortitis and cerebral arteritis,
and the frequency increases after the age of 50 years when both of which are already described in this chapter. One of
the incidence of complicated lesions of advanced the major complications of syphilitic aortitis is syphilitic or
atherosclerosis is higher. They are most common in the luetic aneurysm that develops in the tertiary stage of syphilis.
abdominal aorta, so much so that all forms of aneurysms of It usually manifests after the age of 50 years and is more
abdominal aorta (fusiform, cylindrical and saccular) should common in men. The predominant site of involvement is the
be considered atherosclerotic until proved otherwise. Other thoracic aorta, especially in the ascending part and arch of
locations include thoracic aorta (essentially the ascending aorta. It may extend proximally into the aortic valve causing
part and arch of aorta), iliac arteries and other large systemic aortic incompetence and may lead to syphilitic heart disease.
arteries. Less often, it may extend distally to involve abdominal aorta.
PATHOGENESIS. Obviously, severe atherosclerotic lesions PATHOGENESIS. About 40% cases of syphilitic aortitis
are the basic problem which cause thinning and destruction develop syphilitic aneurysms. The process begins from
of the medial elastic tissue resulting in atrophy and inflammatory infiltrate around the vasa vasorum of the