Congenital malformations of the aortic valve are characterized by their considerable heterogeneity. It is not surprising that numerous classifications have been proposed in the past 15 years. For bicuspid aortic valves alone, up to 12 different classifications have been published. The most recent international nomenclature document for bicuspid aortic valves marked a significant step toward unifying criteria. Today’s article aims to take it further by attempting to encompass all phenotypic expressions of the aortic root using a very similar classification. With minor nuances, the classification system can be considered nearly identical. It is based on simplifying the classification and distinguishing different types of aortic valves by the number of sinuses, the number of leaflets, commissural orientation, and the morphology of the subcommissural triangles.

COMMENTARY:

The aortic root is a complex three-dimensional structure separating the left ventricle from the tubular portion of the aorta. It comprises three semilunar leaflets, each supported by its corresponding sinus. These sinuses are separated by triangular spaces known as subcommissural triangles. The leaflets are anchored distally at the sinotubular junction, while proximally, their nadirs may join, forming a virtual ring. This ring should not be confused with the ventriculo-arterial junction, which is a muscular-arterial interface located within the sinuses from which the coronary arteries originate. The noncoronary leaflet, in contrast, is supported by the central fibrous body, composed of the membranous septum and the fibrous trigones.

Globally, the framework of the aortic root can be divided into three zones: the sinotubular junction, the surgical annulus, and the virtual ring. The latter is the “aortic annulus” typically measured by echocardiography. Regarding the semilunar valves, their middle portion, between commissures, features the nodule of Arantius. The apposition surfaces on both sides of this nodule are called lunules. The non-coapting bellies of the semilunar leaflets extend to the arterial wall, forming the hemodynamic interface of the ventriculo-arterial junction during diastole.

The nomenclature proposed by Tretter et al. simplifies prior frameworks by first assessing the number of sinuses. Most often, malformed roots present three sinuses, occasionally two, and rarely four. This is the first distinction from the international nomenclature proposed by Michelena et al., which focuses on describing bicuspid aortic valves.

Aortic valves with three sinuses but bicuspid opening are classified as functionally bicuspid. In the international nomenclature, they are categorized as “fused bicuspid aortic valve.” These congenital malformations account for more than 90% of cases, reflecting the fusion of two leaflets during development, resulting in a raphe. The resulting subcommissural triangle is hypoplastic and therefore does not extend distally to the sinotubular junction. The raphe may vary in extent or even be absent, necessitating the identification of subcommissural triangle hypoplasia when in doubt. Tretter et al. classify incomplete raphes as “forme fruste” bicuspid aortic valves, aligning with the international nomenclature’s separate category for this variant.

The next step after identifying this type of aortic valve with fused leaflets is to determine the degree of asymmetry to evaluate repairability and durability. In symmetrical forms, the two commissures are positioned at 160–180°. When commissures are positioned between 120–139°, the valve is highly asymmetrical, resembling a tricuspid valve configuration but with two fused leaflets. Depending on the repair strategy used, commissural orientation and repositioning have been shown to improve functionality and durability.

Ten percent of malformed roots present two sinuses. This architecture is more frequently associated with syndromes such as Turner syndrome. Roots with two sinuses and two leaflets will have only two subcommissural triangles. Each triangle will be of normal height, resulting in two normal commissures. This configuration produces an orifice parallel to the normal plane of the sinotubular junction and may explain why these roots dilate less than the fused forms. This group can be further subclassified based on leaflet orientation: anteroposterior or laterolateral.

Unicuspid valves often have three sinuses and are exceedingly rare in the adult population. They are the most common variant in critical neonatal aortic stenosis, where the apposition zones of the noncoronary and left coronary leaflets often fuse. This typically results in two zones of fusion, each with its corresponding raphe and hypoplastic subcommissural triangle. In infants and neonates, the leaflets are frequently thickened, further limiting valve opening.

The quadricuspid valve is the unicorn of variants, with an incidence of less than 0.005% among adults undergoing cardiac imaging. It features four sinuses with four leaflets; fusion of one or more leaflets can occur, complicating diagnosis.

The classification proposed by Tretter et al. closely resembles that of Michelena et al. However, it introduces subtle differences, such as avoiding the term cusp, including forme fruste bicuspid valves under fused forms, and referring to fused forms as functionally bicuspid. More importantly, it seeks to encompass all possible variants of aortic roots. This nomenclature represents a significant advance over prior alphanumeric systems, enhancing comprehension and clinical applicability. This classification could also be applied to describe truncal valves in repair surgery for truncus arteriosus.

The article highlights the limitations of the continuity equation in 2D echocardiography for oval annuli and emphasizes the use of magnetic resonance imaging or computed tomography for adequate structural assessment. It underscores the lack of evidence regarding different measurement methods, both geometric and across the cardiac cycle, leaving substantial room for improvement in the near future.

In conclusion, the articles by Tretter et al. and Michelena et al. are essential reading for anyone managing aortic valves—not only for their proposed classification but also for their educational value and exquisite iconography.

REFERENCE:

Tretter JT, Spicer DE, Franklin RCG, Béland MJ, Aiello VD, Cook AC, et al. Expert Consensus Statement: Anatomy, Imaging, and Nomenclature of Congenital Aortic Root Malformations. Cardiol Young. 2023 Jul;33(7):1060-1068. doi: 10.1017/S1047951123001233.

Michelena HI, Della Corte A, Evangelista A, Maleszewski JJ, Edwards WD, Roman MJ, et al. International consensus statement on nomenclature and classification of the congenital bicuspid aortic valve and its aortopathy, for clinical, surgical, interventional and research purposes. Eur J Cardiothorac Surg. 2021 Sep 11;60(3):448-476. doi: 10.1093/ejcts/ezab038.