DISCUSSION
In this study of 48,503 patients aged ³ 18 years undergoing
echocardiography,, we found the reported incidence of BAV to be 0.51%.
In the literature, reported incidence of BAV incidence of BAV ranges
between 0.4% to 2.25%. (11) Given the variability in BAV detection
using current imaging techniques, autopsy studies can provide a reliable
estimate of disease burden. In an autopsy study of 9966 cases from
London Hospital published in 1938, incidence of BAV was 0.52%.(12) In a
retrospective cohort study published in 2011 comprising of 41,687
cohorts undergoing echocardiography, Michelena et al., reported presence
of BAV in 1% of the studied population.(13) In the same paper,
published in JAMA, 20% of the diagnosed patients with BAV were less
than 18 years of age. This was not a true population-based study as
cohorts were chosen for echocardiography based on any cardiac disease
history, or presence of positive auscultatory findings. In an
echocardiographic survey of 817 apparently healthy primary school
children, incidence of bicuspid aortic valve was 0.5%.(14) BAV runs a
benign course during early life and most of the patients remain
asymptomatic until their middle age, when abnormal shear stress due to
altered valve geometry and inherent aortic wall defect likely starts to
take its toll. We divided our study population in three groups namely,
young age group (>18 and <40 years old), middle
age group (>40 and <60 years old), and old age
group (>60 years old). We used this stratification system
based on the literature which suggests that most of the major events
related to BAV occur between 40 and 60 years of age. In a study by
Elefteriades group, in which they reported incidence of BAV in known
patients of ascending aortic aneurysm being followed clinically.(15)
Mean age at presentation in their study was 49 years. In an autopsy
study of deceased with bicuspid aortic valve, mean age at the time of
death was 46 years.(16) In a community based study, Michelena et al
showed that 40% of cardiac events in a patient with BAV occur at a mean
age of 52 years. (17) In our study, 58% of patients with BAV were first
identified in the middle age group (40-65 years). We also found out that
increased BSA was an independent risk factor for aneurysm formation. We
know that increased BSA and BMI will demand increased flow across the
BAV possibly leading to more shear stress on the valve leaflet and
inherently deficient aortic wall. Theoretically this can confront us on
two fronts. First, increased turbulence in the aorta and shear stress on
the aortic wall could potentially lead to increased rate of aortic
dilatation. Secondly, it can hasten the process of valvulopathy, and
especially in the case of aortic stenosis, can give rise to higher
gradient across the valve and hence potential early intervention.
In our study, 65-72% of patients with BAV, identified across all age
groups, were of male sex. A cohort study from Toronto using
echocardiography to identify patients with BAV reported that 64% to
80% of patients with BAV were of male sex.(18) Michelena et al., in
their population-based study from Olmsted County, also reported that
69% of patients identified to have BAV were of male sex.(13)
Osler in 1886, first reported susceptibility of BAV for IE in his
autopsy study of over 800 patients.(19) Incidence of endocarditis was
3.2% in our study group, with a significantly higher incidence in the
young age group compared to the older age group (9.5% versus 0%),
which could be related to IVDA in this age group. We also identified the
same trend for severe AI, with significantly higher trend in young age
group when compared with old age group (11.9% versus 0%).The risk of
native valve endocarditis in patients with BAV is much higher than
patients with tricuspid aortic valve,(20) and it tends to affect younger
population, mostly undiagnosed BAV patients, and also runs a more
aggressive course, with significantly more peri-annular abscess
formation(21, 22).
17% of patients in our study had moderate or severe AI, with a
significantly higher prevalence of severe AI in the young age group
compared to the old age group. This is interesting because we know that
one important contributing factor for AI in BAV is annular dilatation
and effacement of sino-tubular junction. Since BAV is a progressive
disease, one would anticipate that there would be higher incidence of
severe AI in the old age group. This signifies the relevance of other
factors like leaflet asymmetry and prolapse also impacting the
progression of valvulopathy. In a database review of 158 young male
(mean age of 18 years) diagnosed with BAV, 12.6% were found to have
moderate to severe AI.(23) Same study also showed that those young males
with AI had significantly enlarged aortic root and ascending aorta. In
contrast to that, in our study we found that aortic root and ascending
aortic diameter were significantly higher in the Middle and Old age
group when compared with the Young age group. This is important because
larger aorta at the baseline is a risk factor for progressive dilatation
of the aorta.(24) In a retrospective study of 50 patients with BAV,
ascending aortic dimeter greater than 34 mm at baseline was a risk
factor for significantly higher rate of aneurysm formation.(25) In
Olmsted County study, during a follow up of 416 patients with BAV, risk
of development of aortic aneurysm was 26% at 25 years, with baseline
aortic diameter greater than 40 mm being a risk factor for aneurysm
formation.(13) Same study also reported increased incidence of aortic
dissection (0.5% per year) in patient with aortic aneurysm and BAV. In
our study, 40% of patients diagnosed with BAV had concomitant ascending
aortic aneurysm. However, distribution of patients with aortic root or
ascending aortic aneurysm were not significantly different across the
three defined age groups.
We also found that ascending aorta/height index and aortic root/height
index increased with age especially in Middle age group, but this growth
was not seen in the Old age group other than small number of outlier
subjects. An echocardiography study of 280 BAV patients also reported
that prevalence of ascending aorta dilatation across various age groups
increased consistently till the age of 60 years followed by a
decline.(26) This argues against aortic intervention for elderly
patients with BAV and dilated aortas less than 5.5 cm as the rate of
growth probably will slow down.