[A case of the double mitral valve orifices]

In 1996 student course of gross anatomy dissection at Iwate Medical University School of Medicine, a case of the double mitral valve orifices was found in a heart of a 73-year-old male cadaver who died of acute pneumonia. No other congenital anomalies were detected in this heart. There was no prior history of heart disease. The left atrioventricular ostium was an oval with a long axis of 4 cm and a short axis of 2.5 cm. There were double mitral valve orifices, main and accessory (Fig. 1). The main orifice situated left ventrally with a long axis of 2.5 cm and a short axis of 1.7 cm, had a larger cleft. The accessory orifice situated right dorsally with a long axis of 2.5 cm and a short axis of 1.2 cm, had a smaller cleft. There was a bridge-like valvular leaflet between the two clefts (Fig. 1). Disregarding small incisions, both the main and the accessory mitral valves had two cusps, anterior and posterior. The anterior and posterior cusps of the main and accessory valves were continuous and considered to be homologous with the anterior and posterior cusps of the normal mitral valve. In other wards, this double mitral valve orifices was appeared to be formed by bridging the cleft between the anterior and posterior cusps of the normal mitral valve. The valvular cusps of the main or the accessory mitral valve had proper chordae tendineae and papillary muscles. The papillary muscles were classified into four groups, anterior (A), anterolateral (AL), posterior (P) and posterolateral (PL) according to Taniya (1974). The anterior papillary muscles group had a base at the anterior wall of the left ventricle, the anterolateral papillary muscles group had at the whole lateral wall, the posterior papillary muscles group had at the boundary between the interventricular septum and the posterior wall, and the posterolateral papillary muscles group had at the posterior part of the lateral wall (Fig. 3). The anterior and anterolateral papillary muscles groups, and the posterior and posterolateral papillary muscles groups of Taniya (1974) constituted the anterior and the posterior papillary muscles respectively in the general use of the anatomical term. The chordae tendineae attached to the cusps of the main orifice arose from the anterior, anterolateral and posterior papillary muscles groups (Fig. 2a). And those attached to the cusps of the accessory orifice arose from the posterolateral papillary muscles group (Fig. 2b). The chordae tendineae were also classified into three types after Taniya (1974). The A type chordae tendineae were thick and attached to the base of the cusps, the type B were moderately thick and attached to the midway between the base and the margin of the cusps, and the C type were thin and attached to the margin of the cusps. The main mitral valve had 6 type A, 8 type B and 40 type C chordae. The accessory mitral valve had 0, 6 and 30 chordae respectively (Fig. 2). This case of the double mitral valve orifices is considered to be included in the central type of Cascos et al. (1967) and the bridge type of Elfenbein et al. (1967). A morphogenetic mechanism of this type of the double mitral valve orifices is explained by an abnormal adhesion between the dorsal endocardial cushion and the left lateral endocardial cushion like mass in early developmental stage according to Wimsatt and Lewis (1948). But a possibility of an abnormal orifice in the posterior cusp which is produced in the process of formation of cusps, chordae tendineae and papillary muscles in later stage, still remains to be discussed.