Aortic Stenosis Morphology

Natural History of Bicuspid Aortic Valves

Although a few patients with BAV may go undetected or without clinical consequences for a lifetime, most develop complications. The most important clinical consequences of BAV are valve stenosis, valve regurgitation, infective endocarditis, and aortic complications such as dilatation, dissection, and rupture ( Box 78.2 ). Estimates of the prevalence of these complications and outcomes have varied depending on the era of the study, the cohort selected, and the method used to diagnose BAV (clinical examination, cardiac catheterization, or echocardiography). Several large recent studies have helped better define the unoperated clinical course in the modern era.

Isolated AS is the most frequent complication of BAV, occurring in approximately 85% of all BAV cases. ^, BAV accounts for the majority of patients aged 15 to 65 years with significant AS. The progression of the congenitally deformed valve to AS presumably reflects its propensity for premature fibrosis, stiffening, and calcium deposition in these structurally abnormal valves.

Aortic regurgitation (AR), present in approximately 15% of patients with BAV, is usually caused by dilatation of the sinotubular junction of the aortic root, preventing cusp coaptation. It may also be caused by cusp prolapse, fibrotic retraction of the leaflet(s), or damage to the valve from infective endocarditis. Compared with AS, AR tends to occur in younger patients.

Why some patients with a BAV develop stenosis and others regurgitation is unclear. As mentioned, rarely, patients may not develop hemodynamics consequences. Roberts and colleagues reported three congenital BAVs in nonagenarians who underwent surgery for AS. Why some patients with a congenital BAV do not become symptomatic until they are in their 90s and why others become symptomatic early in life is also unclear.

Echocardiographic Features of Bicuspid Aortic Valves

The roles of echocardiography in the detection and evaluation are listed in Box 78.3 . The diagnosis of a BAV can usually be made by transthoracic echocardiography (TTE). When adequate images are obtained, sensitivities and specificities of up to 92% and 96%, respectively, have been reported for detecting BAV. The most reliable and useful views are the parasternal short-axis (PSAX) and long-axis (PLAX) views. The echocardiographic features and their respective views are summarized in Table 78.2 . The PAX view extremely useful to examine the number and position of the commissures, the opening pattern, the presence of a raphe, and the leaflet mobility. In contrast to the normal tricuspid aortic valve (TAV), which opens in a triangular fashion with straightening of the leaflets ( Figs. 78.1 and 78.2A ), the BAV opens in an elliptical (“fish-mouth” or “football”) shape with curvilinear leaflets ( Figs. 78.1, 78.3, and 78.4 ). There is typically a raphe, a fibrous ridge that represents the region where the cusps failed to separate. ^, The raphe is usually distinct and generally extends from the free margins to the base of the leaflet. Calcification commonly occurs first along this raphe, ultimately hindering the motion of the conjoined cusp. Rarely, the leaflets are symmetric, and there is no raphe—a “pure” or “true” bicuspid valve. Note that a false-negative diagnosis may occur when the raphe gives the appearance of a third coaptation line. In diastole, the normal trileaflet aortic valve appears like a Y (inverted “Mercedes-Benz” sign), with the commissures at 10, 2, and 6 o’clock positions (see Figs. 78.1 and 78.2B ). When the commissures are deviated from those clock-face position, one should suspect a BAV and evaluate carefully. An additional short-axis (SAX) feature is a variable degree of leaflet redundancy. In patients with very little redundancy of the leaflet margins, the development of stenosis is likely, whereas a significantly redundant leaflet with associated prolapse is more likely to lead to regurgitation.

The morphologic patterns of BAV vary according to which commissures have fused, and a number of classifications have been devised that pertain to the orientation of the leaflets ^{, , ,} ( Fig. 78.5 and Table 78.2 ). Fusion of the right and left cusps is the most common morphologic type. ^, In an echocardiographic study by Brandenburg and colleagues, the posterior commissure was located at the 4 or 5 o’clock position, and the anterior commissure was located at the 9 or 10 o’clock position when the valve is viewed in a PSAX view. The second most frequent type, fusion of the right and noncoronary cusps, has been linked to aortic arch involvement and may also be related to an increased risk of AS and regurgitation compared with the other anatomic types. The least common type is fusion of the left and noncoronary cusps. Michelena and colleagues similarly classified BAVs as typical (right–left coronary cusp fusion) if the commissures were at the 4 and 10 o’clock, 5 and 11 o’clock, or 3 to 9 o’clock (anterior–posterior cusps) positions and atypical (right-noncoronary cusp fusion) if the commissures were at the 1 and 7 o’clock or 12 and 6 o’clock positions.

The PLAX view typically shows systolic doming ( Figs. 78.4B; and 78.6 ) caused by the limited valve opening. In a normal TAV, the leaflets open parallel to the aortic walls. In diastole, one of the leaflets (the larger, conjoined cusp) may prolapse. The PLAX view with color Doppler is also useful to evaluate for AR (the diastolic aortic regurgitant jet is usually eccentric) and AS (turbulence in the aortic root and ascending aorta in systole). Last, the PLAX view is also important for sizing the sinus of Valsalva, sinotubular junction, and ascending aorta. With increasing age, as the leaflets become thickened, fibrotic, and calcified, systolic doming may no longer be evident, and the typical SAX appearance of the BAV may be difficult to distinguish from calcific AS of a TAV. In fact, there is an inverse association between the degree of valve stenosis and accuracy of echocardiographically determined valve structure and cause. The elliptical systolic opening in the SAX view is not easily appreciated in a severely stenotic valve. M-mode echocardiography of a BAV may demonstrate an eccentric closure line ( Fig. 78.7 ), but this sign is not reliable, and approximately 25% of patients with a BAV have a relatively central closure line. Moreover, occasionally TAVs can also appear to have an eccentric closure line depending on image quality and orientation of the echocardiography beam.

Common reasons that lead to misclassification or misdiagnosis of BAV include shadowing due to calcium, oblique axis imaging, and poor or suboptimal image quality. The diagnosis of BAV becomes more difficult in older individuals (older than 65 years) because of fibrosis or sclerosis. In such instances, transesophageal echocardiography (TEE) may improve visualization of the leaflets and may be helpful for accurate evaluation of the aortic valve anatomy and confirmation of a BAV. ^, However, studies have noted that even with TEE sensitivity and specificity for detecting BAV are reduced in the presence of more than moderate aortic valve calcification. In some instances, alternative cardiac imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), may help confirm BAV anatomy. More commonly, these imaging modalities are used to visualize the thoracic aorta.

The mechanism(s) of ascending aortic dilatation in BAV has long been debated, and this controversy remains. Two hypotheses include (1) abnormal hemodynamics causing increases wall stress related to eccentric turbulent blood flow and (2) genetic pathological (cellular) structural deficits in the aortic wall. In fact, evidence for each aspect of this “flow versus fate” controversy is not overwhelming.

The advent of time-resolved phase contrast four-dimensional magnetic resonance imaging (4D flow MRI) has shed some light on this controversy. 4D flow MRI has demonstrated abnormal three-dimensional flow patterns and hemodynamic forces that act on the aortic wall that may be implicated in aortic dilatation. ^, Flow through the aortic valve is normally laminar with little force directed toward the aortic wall. However, in the presence of a BAV (even with normal function), blood flow is abnormal with spiral or helical flow patterns impinging the outer curvature of the proximal aorta resulting in increased shear stress as shown in Fig. 78.8 . ^{, , ,} Furthermore, several groups using a similar imaging approach have demonstrated that the BAV cusp fusion pattern (i.e., right-left cusp fusion vs right noncusp fusion) impacts the type of flow eccentricity and helicity and the type and degree of wall shear stress. ^, If validated, the determination of the various transvalvular blood flow patterns by MRI may identify patients at risk for aortography progression and may enable the development of individualized approach to the management of BAV-related aortic disease beyond the traditional diameter-based guidelines.