**2. Anatomy and pathology**

The anatomic features of BAV are characterized by smooth cusps margins and the fusion of two cusps of unequal size. These cusps often include a central raphé or false commissure which are usually in the centre of the larger cusp [Figure 2]. The main difference between a commis‐ sure and a raphé is that the commissure does not completely span into the cusp [6]. The raphé is to be considered as a hypoplastic commissure between two partially fused cusps. The most commonly seen variant of BAV is the fusion of the left and right coronary cusps (L-R BAV)

© 2013 Tokmaji et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

also known as the latero-lateral cusps position which is seen in 70-86% of the BAV cases, whereas the fusion of the right and noncoronary cusps (R-N BAV; antero-posterior cusps position) are observed in 12-28% and the left and noncoronary cusps (L-N BAV) in 0.5-3% of the cases [7-11] [Figure 1]. R-N BAVs and asymmetrical sized cusps are relative risk factors that seem to accelerate the stenosis with 27 mm Hg per decade and are therefore more often associated with AS [12]. BAV with equal cusps and absence of raphé have also been reported [6, 7]. In some cases, the raphé can have a quite deep indentation, which could give a false echocardiographic image of a normal tricuspid aortic valve [13]. Calcium depositions are often confined to the raphé and the base of the cusps [7]. AS tend to develop in BAVs which contain no redundant cusp tissue whereas AR tend to develop in BAVs due to the different dimensions of the two cusps, valve prolapse or redundancy of one cusp [14-16]. Histological examination demonstrates that the raphé does not contain fibrous valve tissue but rather include elastin fibers [17]. BAV should also be distinguished from unicommisural valves that tend to calcify and degenerate even earlier in life. Unicommissural valves includes one commissure with normal height and two raphe's that are much lower height, while there is one large cusp, more or less moving like a bicuspid valve. Up to 90% of the individuals with normal tricuspid valve have right coronary artery dominance whereas 29% to 57% of the patients with BAV disease present with left coronary artery dominance. The average length of the left main stem for individuals with BAV and tricuspid valve is less than 5 mm (90% of the cases) and 10 mm in length, respectively. Recognition of these associations with BAV is mandatory due to the increased risk of perioperative myocardial infarction and a potential risk of insufficient myocardial preservation at the time of aortic valve replacement (AVR) [18, 19, 20]. L-R BAVs are often associated with right coronary artery taking its origin from the right sinus of Valsalva, while in the R-N BAVs, both coronary arteries derive from the anterior sinus [21]. The vast majority of the patients with COA present with a L-R BAV (66-90%) [9, 135, 136].

**Figure 2.** Macroscopic illustration of a bicuspid aortic valve with the fusion of the right and noncoronary cusps (left image) and a dilated ascending aorta (right image). A: Non coronary cusp, B: Right coronary cusp, C: Left coronary

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The embryogenesis of BAV is still not fully understood. It seems that both genetic predispo‐ sition and environmental factors, which could influence the valve morphogenesis, play an important role in the pathogenesis of BAV disease. Initially, the major factor in the formation of BAV is the fusion of the two cusps at the early foundation of the valvulogenesis [22]. The valve morphogenesis occurs in the early stage of foetal development. The heart is one of the first organs to develop through the specification and migration of the anterior lateral plate mesoderm cells which later forms the cardiac crescent [23]. At 3 weeks of gestation in humans, the cardiac progenitors migrate along the ventral midline where they fuse and form a linear heart tube. This beating heart tube is composed of an inner endocardial cell layer which is separated by the extracellular matrix (ECM). Cardiac looping occurs at 4 to 5 weeks of gestation which brings the atrial region of the linear tube into the posterior position of the common ventricles. This is followed by the increase of ECM production which causes the tissue to swell at several areas of the primitive heart, which leads to the formation of the endocardial cushions at the outflow tract (OFT) and atrioventricular (AV) canal. The inner endocardial cells transform into mesenchymal cells, also known as the epithelial-to-mesenchymal transforma‐ tion (EMT). EMT initiates the formation of the aortic valve in the OFT. Afterwards, the cushions undergo massive cell proliferation, as a result growing towards each other with cushion fusion

cusp, D: Raphé, E: Ascending aorta (dilated).

**3. Morphologenesis**

**Figure 1.** Schematic illustration of the anatomic variations of BAV. (A) normal tricuspid aortic valve. (B) Bicuspid aortic valve, fusion of the left and right coronary cusps. (C) Bicuspid aortic valve, fusion of the right and noncoronary cusps. (D) Bicuspid aortic valve, fusion of the left and noncoronary cusps.

**Figure 2.** Macroscopic illustration of a bicuspid aortic valve with the fusion of the right and noncoronary cusps (left image) and a dilated ascending aorta (right image). A: Non coronary cusp, B: Right coronary cusp, C: Left coronary cusp, D: Raphé, E: Ascending aorta (dilated).
