Keywords aortic dilatation - bicuspid aortic valves
Introduction
Editor's Commentary
Dilatation of the ascending aorta in patients with congenitally bicuspid aortic valves
(CBAV) is a well-known phenomenon, which often requires a major surgical intervention.
Most members of our profession believe that this dilatation is caused by inherent
weakness of the aortic wall,[1 ] which should be treated with radical surgery, even if the dilatation is mild, or
even nonexistent.[2 ]
[3 ]
We believe that the condition (CBAV) is analogous with “poststenotic dilatation” (PD)—a
curious enlargement of a vascular segment distal to a stenotic area. This phenomenon
was already recognized by the great Italian anatomist Morgagni[4 ] and in modern times clinically studied by Halsted[5 ] and Holman.[6 ] They postulate that PD is acquired, caused by mechanical damage by the turbulent
blood flow streaming through the stenotic area ([Figs. 1 ]
[2 ]). This has been further studied by us in 1954.[7 ]
[8 ] Later we extended the theory of PD also to dilatation of the ascending aorta which
occurs in patients with CBAV.[9 ]
[10 ]
Fig. 1 In the in vitro model, the flow continues constricted past the stenosis (A ) and vortices continues to linger in the poststenotic segment (B ). (Adapted from Robicsek.[8 ])
Fig. 2 Vortex formation is extensive if the poststenotic segment is dilated. (Adapted from
Robicsek.[7 ])
The principal arguments that CBAV is associated with congenital weakness of the ascending
aortic wall, is that, disarray and fragmentation of the elastic fibers, as well as
disappearance of muscular elements and disturbed collagen structure, frequently occurs
in individuals with CBAVs, regardless of presence or absence of ascending aortic dilatation.[11 ] It should be noted that in most of these studies the aortic wall was sampled from
grown individuals. Also, similar, if not identical, changes occur in patients with
arteriosclerotic aortas or in those with Marfan syndrome,[1 ]
[11 ]
[12 ] especially if the biopsy was taken from a dilated segment.
Our previous observations[9 ]
[10 ] also challenge the concept that there are “non-stenotic” CBAVs. In a patient with
such condition, the bicuspid valve may not generate any transvalvular pressure gradient;
the patient may be asymptomatic, albeit every bicuspid aortic valve is physiologically stenotic . These studies were made in, the hearts of young individuals with CBAVs, ages 16
to 28 years, who had no cardiac symptoms and who died of non-cardiac causes. We found
all the bicuspid aortic valves to be 30 to 50% stenotic ([Fig. 3 ]). This is not surprising, if somebody considers the functional mechanism of the
bicuspid aortic valve, where dynamically thinking, some degree of stenosis is unavoidable
to assure adequate opening and closure. This “stenosis” may be only relative, considering that the annuli of bicuspid aortic
valves were found to be larger than those of normal tricuspid aortic valves .[13 ] Ergo propter it is logical to conclude that a bicuspid aortic valve, even if the patient is asymptomatic
and lacks significant aortic gradient, may generate enough turbulence to damage the
wall of the ascending aorta. The presence of significant turbulence in asymptomatic
patients is also supported by the presence of a murmur and often a palpable thrill,
over the aorta.[9 ]
[10 ]
Fig. 3 Opening cycle of a congenitally bicuspid valve obtained from a 18 years old patient,
who died of noncardiac causes and had neither symptoms nor measurable aortic valve
gradient. Note: At maximum opening, the aortic valve orifice is still 30” stenotic.
(Reproduced with permission from Francis Robicsek, Mano J Thubrikar, Joseph W Cook,
Brett Fowler. The congenitally bicuspid aortic valve: how does it function? Why does
it fail? Ann Thorac Surg 2004;77(1):177–185.)
Notably, abnormal flow patterns also occur in patients with stenosis of their initially
normal trileaflet aortic valve. In these patients, the ascending aortic dilatation/aneurysm
is usually symmetrical “pear” or “teardrop” shaped, while in patients with CBAV the
dilatation is convex toward the right anterolateral aspect where degenerative changes
of the media were also found to be the most severe. Studying the issue with computer
simulation, we found that the direction of the flow through the CBAV, due to asymmetry
of the valve leaflets, is not central but directed in the anterolateral direction.
Stress overload was also measured to be highest at the same location in patients with
“normally” functioning CBAVs.[14 ]
The fact that not every patient with bicuspid aortic valve develops ascending aortic
dilatation may be explained with individual variation in the strength of the elastic
elements in the aortic wall.
We realize that while these arguments are valid indeed, they still do not provide
an absolute proof of our postulate, that is, turbulence is the main reason for an ascending aortic
dilatation in patients with bicuspid aortic valves. The issue, therefore, was further
investigated.
Methods
We began our additional observations on the aorta of a 5 years old child ([Fig. 4 ]) who died of pneumonia and who had bicuspid aortic valve. The histological examination
of the ascending aorta showed not only the disruption of the elastic fibers but also
turbulence-induced plaque formation, which was readily visible even by the naked eye.
After that, we histologically examined the aortic roots obtained from infants with
CBAV who expired within a week of their birth. We were able to collect four such specimens.
Two infants with normal trileaflet aortic valves served as controls. Samples were
stained for hematoxylin–eosin and for elastic fibers. According to the findings, the
specimens were divided in three groups.
Fig. 4 Visual image (A ) and histologic picture (B ) (elastic staining) of the ascending aorta of a 5 years old boy with “normal” bicuspid
aortic valve who died of noncardiac causes. Note the plaque on the aortic wall caused
by “jet-damage” and the disintegration of the elastic fibers on histology. (Adapted
from Robicsek et al.[14 ]) (B ) 25x magnification. Movat pentachrome stain.
Results
Group 1 was composed of two specimens, where the full autopsy revealed CBAV as the sole cardiovascular
anomaly. In this group, the histological examination showed a completely normal ascending
aortic wall.
Group 2 consisted of two infants who—besides the bicuspid aortic valve—had other cardiovascular
anomalies as well. One infant had hypoplastic left ventricle, and the other had hypoplastic
left ventricle and coarctation. The histological examination of the ascending aorta
in both infants confirmed disruption of the elastic fibers.
Group 3. The two controls with normal aortic valves revealed no abnormalities in the wall
of the ascending aorta ([Fig. 5 ]).
Fig. 5 The histologic pictures of the ascending aortic wall (10X magnification, hematoxylin-eosin
and Moffat pentachrome stain) of three newborns with CBAV, who died in the first two
weeks of life: Picture (A ) is that of a newborn who had CBAV as the sole cardiovascular anomaly. Note that
the aortic wall appears to be normal. Picture (B ) is that of a newborn who, in addition of CBAV had additional cardiovascular anomalies
(hypoplastic left ventricle and coarctation of the aorta). Note that the aortic wall
shows disintegration of the normal structure and fragmentation of the elastic fibers.
Picture (C ) is that of a newborn (control) with normal tri-leaflet aortic valve. The aortic
wall, as pictured in A, is entirely normal. (Adapted from Robicsek et al[14 ]).
Conclusions
Because of the extreme difficulties in obtaining newborn cadaver hearts with bicuspid
aortic valves, our observations are limited to a very few samples. This is a shortcoming
of the study. Our findings, however, reinforce our previous postulates which are as
follows:
Patients with CBAVs may lack symptoms and transvalvular pressure gradients, but all
bicuspid aortic valves are inherently, anatomically stenotic to a various degree.
Patients with such “normally” functioning aortic valves may develop pathological changes
and poststenotic dilatation of the ascending aorta. In patients, where CBAV is the
sole anomaly, the aortic wall is normal at birth.
Apparently, there are several subgroups of bicuspid aortic valves. In patients with
additional cardiovascular anomalies, because of inborn weakness of the aortic wall,
the dilatation may occur earlier, may be more extensive, and may require more radical
replacement of the ascending aorta.
In most patients where the bicuspid valve is the only cardiovascular anomaly, dilatation
of the ascending aorta may be treated with a less aggressive surgical approach.[14 ]
[15 ]
