Diagnostic ultrasound ( PDFDrive )

434 PART II Abdominal and Pelvic Sonography
A
B
FIG. 12.2 Isolated Aneurysms of the Suprarenal Abdominal Aorta Are Rare. Most suprarenal abdominal aorta aneurysms (AAAs) also involve
the thoracic aorta above the diaphragm. Isolated suprarenal AAAs are included in the Crawford classiication of thoracoabdominal aneurysms.
(A) Longitudinal ultrasound image shows isolated suprarenal AAA measuring 5.5 cm. (B) Sagittal reformatted CT demonstrates the isolated
suprarenal AAA.
aneurysms 10 (Fig. 12.2). In the literature, unless otherwise speciied,
the term “abdominal aortic aneurysm” usually is used to
mean “infrarenal abdominal aortic aneurysm.”
Pathophysiology
Arteries have three layers: intima, media, and adventitia. he
intima, the inner layer, is composed of the endothelium and
the internal elastic lamina, along with scant intervening tissue.
he intima is the main layer involved in the formation of atherosclerotic
plaques. he adventitia, the outer layer, consists of
connective tissue and carries nerves and the vasa vasorum. 1
he media supplies much of the strength of the aorta and
consists of elastin, collagen, smooth muscle cells, and extracellular
matrix proteins. AAA formation is a disease largely involving
the media and also the adventitia. 11,12 he aorta is an elastic
artery and the aortic media has high elastin content. 1 With AAA,
there is a signiicant decrease in elastin and collagen content in
the media and adventitia. 11 Destruction of elastin is mediated
by enzymes such as matrix metalloproteinases (MMPs). Medications
may be able to reduce the rate of elastin destruction, either
by suppression of the MMPs or inhibiting other biochemical
pathways. 13 Statins and doxycycline are known inhibitors of MMP
enzymes. Good evidence, however, is lacking regarding what
may be the best medical therapy to help reduce the rate of AAA
growth. 14
Natural History and Medical Therapy
AAA is a disease of older persons. AAAs rarely occur before age
50 and afect men four times more oten than women. he risk
of developing an AAA also increases with smoking and with a
family history of AAA in a irst-degree relative. Other factors
that increase risk are a history of peripheral vascular disease,
cardiovascular disease, and hypertension. 15 AAAs typically
increase in diameter at a rate of 1.7 to 2.6 mm per year. his
rate of growth increases as the AAA becomes larger. 15 he rate
of growth is faster in women. 16,17
As the size of the AAA increases, the risk of rupture increases.
Rupture is rare when the AAA is less than 4.0 cm in diameter,
with near 0% risk per year. he rate of rupture increases to 1%
when the AAA is 4 to 4.9 cm, 1.0% to 11 % for AAAs 5.0 to
5.9 cm in diameter, 10% to 22% for AAAs 6.0 to 6.9 cm, and
30% to 33% for aneurysms larger than 7 cm. 18 Although data
for larger aneurysms are conlicting, when the diameter is greater
than 6.0 cm, clearly the risk of rupture is signiicantly higher
than for smaller aneurysms. 19,20 Other risk factors for rupture
include current smoking and chronic obstructive pulmonary
disease. In addition, the rupture rate for women is four times
that of men. 21 Rupture in women occurs in smaller aneurysms,
although the exact amount of increased risk is not well quantiied.
Because smaller AAAs in women have a risk similar to larger
ones in men, elective treatment in women should occur at a
smaller aneurysm size than in men. 17,18 Recommendations about
how much smaller vary from 3 mm to greater than 5 mm. 22
Current medical therapy is limited in its ability to prevent
the growth of an AAA. Rate of aneurysm growth is increased
in smokers and may be decreased in diabetics. Medications
including statins and doxycycline have been suggested as possibly
reducing the rate of aneurysm growth. 23-25 To date, none of the
current strategies for reducing rate of growth through medications
has been shown to be very efective. 14,26
Screening
Recent Studies
he malady of AAA meets the 10 World Health Organization
criteria for the institution of screening programs. 27 Research has
been extensive, particularly in Europe, on the efectiveness and
cost-efectiveness of ultrasound screening for AAA.
A meta-analysis commissioned by the US Preventive Services
Task Force (USPSTF) combined analyses from four studies,
including European studies. he analysts concluded that “for
men age 65 to 75, an invitation to attend AAA screening reduces