Mitral Valve Disease in Children
Precise knowledge of the normal anatomy of the mitral valve and the interpretation of echocardiographic studies is essential to understanding the mechanism of mitral valve regurgitation or stenosis (functional classification), elucidate the location of leaflet dysfunction (segmental valve analysis), and plan and achieve successful surgical repair. The anatomy of the normal mitral valve in the child does not differ from that in the adult.1
The embryology of the mitral valve is complex. Our understanding of the formation of the leaflets and suspension apparatus has evolved, so that the current approach is mainly based on immunohistochemistry, in vivo labeling of cushion tissue, and scanning electron micrography of human and avian embryos.2,3 In humans, the mitral valve develops between the 5th and 15th weeks of embryonic life. During the fifth week, the atrioventricular canal is defined and lined with two cushions mostly toward the left side of the canal. The anterior leaflet of the mitral valve derives from the junction of the superior and inferior cushions, whereas the posterior leaflet derives from an infolding of the atrioventricular–muscular wall and the development of a lateral cushion. The wedging of the aortic root into the superior bridging leaflet (mostly originating from the superior cushion) will separate the developing mitral valve from its septal attachments and create the aortomitral continuity.4 The process required for the transformation of the endocardial cushion into valvar tissue is poorly understood. The presence of calcineurin and periostin is required. As the cushion tissue elongates and grows toward the ventricular cavity, it is gradually delaminated from the underlying myocardium and the leaflet becomes progressively shaped as a funnel-like structure totally attached to the myocardium. Perforations then appear into the valve leaflet and grow to form the chordae tendineae. The atrial aspect of the cushion will generate the spongy atrial layer and the ventricular layer will generate the fibrous part of the mitral valve and chordal apparatus. The separation between atrial and ventricular myocardium is dependent on the sulcus tissue located on the epicardial side of the junction. The development of the papillary muscles takes place simultaneously, originating from the myocardium. A horseshoe-shaped ridge lies within the left ventricle. The anterior and posterior parts of this ridge lose contact with the ventricular wall, forming the papillary muscles and increasing in size, while maintaining structural continuity with the cushion tissue at the tip of the papillary muscle. The midportion of the muscular ridge will be incorporated into the apical trabeculations of the left ventricle.5
Congenital Anomalies of the Mitral Valve
Congenital stenosis and insufficiency of the mitral valve are presented together, as their pathology and associated lesions are similar. Moreover, they frequently coexist in the same patient.