The bicuspid aortic valve complex: still a mystery

Abstract

The bicuspid aortic valve (BAV) still hides many challenging issues including definition and classification. Michelena et al. [1] present an interesting new, very comprehensive, partially complex BAV classification and beyond in written English, avoiding numbers and letters for a universal, audience. Included are 3 basic types: the fused BAV, the 2-sinuses BAV and the partial-fusion BAV (form fruste), and also aspects like fusion versus raphe, indeterminate fusion, fusion orientation, cusp symmetry and aortopathy. For everyday clinical purposes, this classification appears a little impractical compared to other classifications (see Table 1 in [1]) including that, published by our group, Sievers and Schmidtke [2]. From clinical experience, we defined a congenital aortic valve anomaly as a BAV when it presents with a fishmouth like, bifoliate, functionally biscupid systolic valve opening, validated at surgery in adults, independent of empryological considerations. Three basically different phenotypes were observed: the genuine BAV with 2 sinuses and 2 cusps but no raphe (BAV type 0, whereby a raphe was defined as a fibrous or calcified ridge extending from the wall to the free edge of the cusps), followed by phenotypes (similar to tricuspid valves), with 3 sinuses and 3 cusps, 2 of which are conjoined in 1 raphe (BAV type 1), and consequently a phenotype with also a bifoliate opening, 3 sinuses, 3 cusps but 2 raphes (BAV type 2). Although not yet observed in mice or hamsters it may surface as a member in the spectrum of endocardial cushion defects, typical for BAV’s, underlined by an earlier report, describing it as a unicuspid valve with 2 raphes ‘representing abortive commissures’, which is not the original unicuspid valve [3]. Lack of finding this entity in animals is not necessarily proof that it does not exist in human beings. As the main category, the number of raphes allowed for classifying the 3 groups with short, practical and memorable expressions (BAV type 0, BAV type 1 and BAV type 2), the essence of clinical classifications (e.g. Stanford A/B, de Bakey I, II, III). The subcategory includes letters for orientation e.g. L: left-, R: right-, N: non-coronary sinus. Thus, the numbers of the BAV types correspond to concrete BAV typical structures—the raphes—facilitating identification and clinical connotations. For example, BAV type 0: the Ross operation and even the implantation of conventional substitutes could be less straightforward, because of only 2 commissures. Repair is facilitated. The most frequent BAV type 1, L-R: how to manage the discrepancy between the more downstream displaced smaller left and right cusps and the more upstream located larger non-coronary cusp during the Ross operation, repair, remodelling, reimplantation or even replacement with conventional prostheses preventing leaflet prolapse or tilt? Since cusp symmetry seems favourable for repair, the question comes up how to achieve the ideal length of the cusps, 1.6 times the diameter is necessary (1.1 in tricuspid valves) to warrant a stenosis free valve opening without prolapse (commissural elongation, or windmill excessive cusps tissue?) BAV type 2: the nadirs of the 3 sinuses are at 1 plane facilitating valve replacement but rendering repair more complex. Furthermore, considering the complex BAV pathology and the excellent long-term results with surgery, warranting secure fixation of the prosthesis and concomitant aortic surgery, transcatheter aortic valve replacement may be applied carefully and benefit from BAV-specific management or even design innovations. Also, BAV development is complex, probably not single gene related, influenced by epigenetics such as embryonic blood flow and sometimes including also the myocardium, the ascending aorta and the mitral valve [4, 5]. Basically, endocardial cushions seem to be involved. As a novel mechanism of BAV development, displaced aortic intercalated valve swellings in human embryos were reported recently [6], possible correlating with our observation (see above) of displaced left and right coronary cusps in BAV type 1, L-R. BAV-associated aortopathy (AoP) also emerges with many questions, e.g. how is it best defined, and optimally treated? Fact is, AoP rarely presents with symptoms except when catastrophic aortic dissection occurs. This renders AoP surgery more prophylactic. Because it is predominantly performed at BAV surgery, indication and surgery are challenging since the dilatation of the aorta ascending often tapers into the arch [7]. The association of AoP and BAV seems overall weak [7]. Including haemodynamics in the analysis reveals ascending AoP more associated with stenotic BAVs, probably shear stress-related. Insufficient BAVs are associated with more extensive AoP [7], obviously more gene related inducing surgical implications for adequate reinforcement and replacement. Latest after Rylski et al. [8] reported a mean ascending aorta diameter of only 40 mm, measured within 2 years prior to dissection, other parameters in addition to size surface significant for decision-making, such as age, z-score, particularly history, growth rate, intraoperative tissue quality, haemodynamics, BAV type, and in the future potentially genes, blood pressure at rest and exercise, etc. First clinical results with this more individualized concept for treatment of the ascending aorta at BAV surgery are promising [9]. In this respect, calculation of the place value of each single variable and at the end the total personal risk, optimized by computational biology, may assist decision-making [10]. Also, surgical risk reduction to near zero; flexible, individually shaped prostheses; intraoperative histological analysis; automatical anastomoses, etc, are desirable goals. Proper communication is necessary, preferably accompanied either by novel classifications or evolving already used classifications with new knowledge or speciality requirements, e.g. adding the angle between the fused and non-fused cusps in degree indicating symmetry for repair purposes (BAV type 1, L-R, 150°), or the extend of fusion in %, a tissue quality score or a calcification score for transcatheter aortic valve replacement. However, in the clinical setting, more extended, complex classifications are less user friendly, potentially causing confusion. Often only short, concise, practical, partially idealized, but memorable expressions with speciality and clinical relevance and connotations are advantageous. Paying attention to these facts has probably made our classification [2], particulary the BAV type 0 and the BAV type 1, the so-called Sievers classification [1] popular and widely used. Anyway, progress in decrypting the BAV mystery, may profit from fruitful discussions including different definitions and classifications, their supposed limitations and benefits. (Babel also positive?) Only audience and time will tell.