Unlike the
mitral valve, the aortic cusps have no supporting apparatus; they
withstand the considerable diastolic pressure in the aorta by their
shape, and by virtue of the fact that they rest against each other
with some 30% of each cusp overlapping its neighbors. However, if
the region of the aorta where the cusps are attached is dilated
or distorted by disease, the cusps may fail adequately to withstand
aortic diastolic pressure and regurgitation will occur.
 |
| Fig. 52 |
Figure
52 shows a diagram of the aortic root. Between the aorta
proper and the lower annulus, which forms part of the fibrous
skeleton of the heart, is a tube comprised mainly of collagen,
which bulges outward to form the sinuses of valsalva and to which
the aortic cusps are attached. Dilation of the fibrous annulus
is rare.
 |
| Fig. 53 |
Dilation of the ascending aorta is commonly found in patients
with aortic stenosis, but does not normally affect the valve cusps
(otherwise almost all patients with aortic stenosis would have
regurgitation). Dilation of the supra-aortic ridge at the point
where the aorta meets the top of the collagen sleeve, however,
pulls the cusps apart and reduces their mutual support (Fig.
53). In this way, an enlargement in this area by as little
as 2mm above the normal range can be sufficient to cause regurgitation
severe enough to require valve replacement. Such dilation is readily
detected by echocardiography, since it is at this level that the
echoes from the cusps are routinely recorded.
 |
| Fig. 54 |
Aortic root disease can be divided into those characterized by
an inflammatory reaction, for example syphilis, Reiter's disease,
ankylosing spondylitis, and giant cell aortitis, and those in
which the elastic within the aorta is partially destroyed, as
in Marfan's disease. In nearly all cases, echocardiography shows
a dilated aortic root, though in some cases of syphilis, valve
incompetence results from distortion of the cusps rather than
dilatation. It may also be possible to infer the presence of inflammation
or calcium from dense aortic wall echoes. The cusps themselves
often generate multiple echoes in diastole, similar to those frequently
found with bicuspid valves
(Fig.
54). This is probably due to the thickening of the cusp edges
caused by their rubbing against each other as they partially prolapse
under the stress or aortic pressure. This "functional prolapse"
should be distinguished from that of the mitral valve, and occasionally
the aortic valve, in which the cusps themselves are abnormal.
 |
| Fig. 55 |
One of the features of Marfan's disease is the destruction of
the elastic in the aortic media, causing the root to dilate and
the valve to become incompetent
(Fig.
55). However, many patients with aortic regurgitation have
dilated aortic roots, biopsies from which are histologically identical
to those of Marfan's disease but have none of the other clinical
or echocardiographic stigmata of this condition. This "idiopathic
root disease" is in fact the most common cause of severe aortic
regurgitation in western countries where rheumatic fever is now
rare. Interestingly, it is frequently found in patients with aortic
regurgitation who also have bicuspid aortic valves.
2004
