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.