Discussion

Aortic valve development begins at approximately days 31–35 from the endocardial cushions in the outflow tract and atrioventricular canal of the primitive heart tube. Cavitation of the cushions results in a central lumen of each cushion that separates the three-valve leaflets.^{Reference Spicer, Bridgeman, Brown, Mohun and Anderson5}

The bicuspid aortic valve develops when the excessive fusion of ridges results in conjoined leaflets with two commissures. When more leaflets have fused, this situation results in a unicuspid aortic valve. Two forms of unicuspid aortic valve have been described, the unicommissural variant which is characterised by one commissure, the presence of a lateral attachment to the aorta with an eccentric orifice and the acommissural form in case of the absence of such attachment and presence of a central orifice.^{Reference Essandoh, Castellon-Larios, Zuleta-Alarcon, Portillo and Crestanello6} In their recently published article, Slostad et al^{Reference Slostad, Witt and O’Leary7} reported that they detected unicommissural type unicuspid aortic valve in 100% of 75 patients after surgery and autopsy. In some patients, it is difficult to make a differential diagnosis on echocardiography from the bicuspid aortic valve. However, Ewen et al^{Reference Ewen, Karliova and Weber8} defined major and minor criteria for the echocardiographic diagnosis of the unicuspid aortic valve. Criteria are as follows: (I) single commissural attachment zone, (II) rounded, leaflet-free edge on the opposite side of the commissural attachment zone, (III) eccentric valvular orifice during systole, and (IV) patient age <20 years and mean transvalvular gradient >15 mmHg. The minor criteria were defined as an associated thoracic aortopathy and age <40 years. They claimed that three of the four main criteria or two of the four main criteria and one minor criterion distinguish unicuspid aortic valve patients from the bicuspid aortic valve or tricuspid aortic valve with high sensitivity and specifcity.^{Reference Ewen, Karliova and Weber8} The present case is of the unicommissural type unicuspid aortic valve with a single commissural attachment zone, a rounded, leaflet-free edge, and an eccentric orifice causing a slight narrowing of the aortic valve area. There may be a phenotypic continuum of similar disease between bicuspid and unicuspid aortic valves; however, in the unicuspid aortic valve, there is more smaller indexed aortic valve area that correlates with the lower number of cusps than bicuspid aortic valve.^{Reference Krepp, Roman and Devereux2} In addition, acommissural valves have a smaller effective orifice area than unicommissural valves and often presents at a younger age due to severe stenosis.^{Reference Armstrong, Crawford and Arnautovic9}

Trileaflet aortic valve is formed by the excavation of the mesenchymal ridge in three areas. Quadricuspid aortic valve may develop when one of these three mesenchymal ridges divides to form an additional leaflet.^{Reference Fernández, Durán, Martire, López and Sans-Coma10} Two classifications are usually used to describe quadricuspid aortic valves. In the first classification, Hurwitz and Roberts introduced the classification system in which seven subtypes, named A to G by the relative size of the four cusps. The most common types consist of four equal cusps (type A) and three equally sized cusps and one smaller cusp (type B) (both, 73% of all cases).^{Reference Hurwitz and Roberts11} In the second classification, Nakamura et al provided a simplified classification into four types based on the position of the supernumerary cusp.^{Reference Nakamura, Taniguchi, Saiki, Morimoto and Yamaga12} It is important to note that the position of the accessory cusp has no effect on the severity of the valvular regurgitation.^{Reference Tsang, Abudiab and Ammash13} The characteristic echocardiographic finding in the short-axis view is an X-shaped commissure pattern during diastole and a rectangular appearance during systole. The present case was showed a quadricuspid aortic valve classified as type A (four relatively equal cusps) in accordance with the Hurwitz and Roberts’ classification. No thickness and calcification were found.

Unicuspid and quadricuspid aortic valves have similar clinical presentations (aortic stenosis or regurgitation) with the bicuspid aortic valve.^{Reference Krepp, Roman and Devereux2} Because unicuspid aortic valve had a smaller indexed aortic valve area, most often present with aortic stenosis, with or without aortic valve regurgitation (93% mixed with aortic valve regurgitation, 7% isolated).^{Reference Slostad, Witt and O’Leary7} Aortic valve stenosis tends to start earlier and progress more rapidly in patients with the unicuspid aortic valve. Slostad et al^{Reference Slostad, Witt and O’Leary7} reported that 71 of the 75 patients with the unicuspid aortic valve needed aortic valve surgery during a median clinical follow-up of 13.2 years. Additionally, unicuspid aortic valve patients had greater mean and peak aortic valve gradients than the bicuspid aortic valve.^{Reference Krepp, Roman and Devereux2} In the present case, there is moderate aortic valve stenosis with mildly narrowed aortic valve area and mild aortic valve regurgitation, which requires close clinical follow-up. The quadricuspid aortic valve tends to present with aortic valve regurgitation due to incomplete coaptation of the valve leaflets. Aortic valve stenosis has been described more rarely (90% versus 8%, respectively).^{Reference Tsang, Abudiab and Ammash13} The present case of the quadricuspid aortic valve has mild aortic valve regurgitation due to incomplete coaptation of four cusps with thin and mobile leaflets.

Both unicuspid and quadricuspid aortic valves usually appear as an isolated congenital anomaly, but may also be associated with other abnormalities. Slostad et al^{Reference Slostad, Witt and O’Leary7} reported that, 27% of the patients with the unicuspid aortic valve, had additional structural cardiac comorbidities, including patent foramen ovale, coarctation of the aorta, hypertrophic cardiomyopathy, aortic dissection. Additionally, interatrial^{Reference Saito, Yoshimura, Miyatsu and Ikeda14,Reference Ermis, Bilgi, Altay and Erol15} and/or interventricular septal defect,^{Reference Demirkol, Balta, Arslan, Unlu, Kucuk and Iyisoy16,Reference Ishigami, Iwase and Hyoudo17} Fallot tetralogy,^{Reference Maheshwari, Tanwar and Mittal18} truncus arteriosus,^{Reference Franco, Gabriel and Ruehm19} coarctation of the aorta,^{Reference Agrawal, Kalavakunta and Gupta20} Turner syndrome,^{Reference Essandoh, Castellon-Larios, Zuleta-Alarcon, Portillo and Crestanello6} and Williams syndrome^{Reference Gouveia, Martins and Costa21} have been reported in the literature. However, unicuspid and quadricuspid aortic valves are most frequently associated with coronary abnormalities, such as a single coronary artery, origin and/or course abnormalities of coronary arteries.^{Reference Saito, Takaoka and Funabashi22} In addition, infective endocarditis has been reported in both unicuspid and quadricuspid aortic valves.^{Reference Yuzawa-Tsukada, Tanaka and Morimoto23,Reference Jackson, Sarwar, Hwang, Joglekar and Alsafwah24} While aortic valve disease was the only complication in the patient with quadricuspid aortic valve; aortic valve disease was accompanied by mitral valve prolapse and mild mitral valve regurgitation in the patient with the unicuspid aortic valve.

Aortic stenosis changes the jet of fluid emerging from the aortic valve leading to an increased risk for aortic dilatations, and aortic dissection. Because of the earlier presentation and increased likelihood of more severe stenosis, unicuspid aortic valves manifest with an increased incidence of ascending aorta dilatation,^{Reference Slostad, Witt and O’Leary7} and aortic complications than the bicuspid and quadricuspid aortic valves.^{Reference Krepp, Roman and Devereux2,Reference Naito, Sequeira-Gross, Petersen, Holst, Reichenspurner and Girdauskas25} In accordance with the literature, aortic dilatation was present in the patient with the unicuspid aortic valve, while aortic measurements were normal in the patient with the quadricuspid aortic valve.