Very often isolated, the cleft mitral valve can be easily differentiated from a left atrioventricular valve in a partial AVSD. The cleft is centered on the aortic commissure between the noncoronary cusp and the left coronary cusp, and there is no suspension apparatus on the edges of the defect. The papillary muscles are normal. Lack of valvular tissue can be seen and is secondary to the regurgitation through the cleft. The defect is not stenotic and may generate only little regurgitation for a long time.

In these anomalies, often found in association with other valvar anomalies, the spaces between the chordae are filled with a network of myxoid, valve-like tissue. When there is continuity between the anterior and the posterior leaflet, the accessory tissue may generate a gradient directly related to the size of the perforations in the accessory tissue. When the accessory valve tissue is entrapped in the left ventricular outflow tract, the mitral valve may become regurgitant due to the traction exerted by the accessory valvular tissue on the anterior leaflet, opening the valve in mid systole; however, in such cases, the left ventricular outflow tract obstruction is the predominant hemodynamic lesion and is the most frequent mode of diagnosis. The accessory mitral valve tissue in isolation often does not generate significant gradient or insufficiency.

Three major anatomical types have been identified, although there is a continuum between them. Their recognition is useful for the planning of the repair. The functional lesion can be either predominantly regurgitant or predominantly stenotic, it can be both stenotic and regurgitant, or the valve can have a normal function.

There is no evidence of the congenital origin of these lesions. They are not found at birth unlike the previous anomalies described above. They are usually associated with significant volume loading of the left ventricle, that is large ventricular septal defect or large patent ductus arteriosus. Sometimes minor anomalies of the valvular tissue or the papillary muscles can give an indication toward a true congenital origin. The papillary muscle may have an ischemic aspect, and even rupture, this is mostly seen in neonates.

The mitral regurgitation in patients with anomalous coronary artery from the pulmonary artery is of ischemic origin. The anatomy is normal. The functional classification is of systolic restriction of one of the segments of the posterior leaflet (Carpentier type IIIb).

Acute rheumatic fever (ARF) is an autoimmune disorder. The immune response to group A streptococcal M protein generates T cells and antibodies that cross-react with cardiac antigens. In some patients, the acute damage to the valves will induce chronic and evolving lesions secondary to the scarring process and/or the hemodynamic modifications. This is known as rheumatic heart disease.

Bacterial endocarditis of the mitral valve is rare. It is always a regurgitant lesion. At the Royal Children’s Hospital, Melbourne, in the last decade most patients had normal native mitral valve. It is very important for the surgeon to be able to differentiate intact valvar tissue, supple thin and resistant from infected tissue, thickened edematous and friable.

The anatomy of the left atrioventricular valve after complete AVSD repair has standard features: The superior and inferior bridging leaflets are partitioned transversally at the level of the crest of the septum. Their mobility is very limited at that level; it increases further away from the partition point. The superior and inferior bridging leaflets face one another through the zone of apposition made of by rough surface. The quality of the suspension apparatus to the free edge varies greatly from patient to patient. The zone of apposition between these two leaflets has often been closed with continuous or interrupted sutures at the time of the primary repair. In the most common configuration, the left lateral leaflet (LLL) or posterior leaflet is normally developed and the superior and inferior bridging leaflets separate from one another and delineate a triangular orifice at the inferior aspect of the left AV valve annulus. This orifice is covered with the LLL. It is triangular; the base hinges at the posterior aspect of the annulus and the tip faces exactly where the superior and inferior leaflets separate. Between one-third and one-fifth of the circumference of the reconstructed left AV valve belongs to the LLL. Two papillary muscles make the suspension apparatus. The anterior papillary muscle underneath the zone of apposition between the superior bridging leaflet and the LLL and the posterior papillary muscle
underneath the zone of apposition between the inferior bridging leaflet and the LLL and their respective commissure. The leaflet has a broad base when it covers one-third of the annulus or a narrow base when it covers one-fifth. In that configuration, the LLL is tall and narrow and unstable.

In the least common configuration (6% to 10% of the complete AVSD), the LLL is diminutive or absent. Both superior and inferior bridging leaflets are supported by the anterior papillary muscle, while the posterior papillary muscle is commonly diminutive or more rarely absent. Both bridging leaflets reach the left lateral annulus where a true commissure can be seen. Most often, the diminutive posterior papillary muscle is supporting a second orifice in the inferior bridging leaflet.