Congenital Anomalies of the Mitral Valve

Supravalvar Mitral Ring

Often considered a congenital anomaly of the mitral valve, the supravalvar mitral ring is a fibrous construction attached to the posterior annulus of the mitral valve; it runs from both commissures to the mid height of the anterior leaflet. The lesion is stenotic, often to a greater extent than might be suggested by the extension of the ring. This is more a result of the limitation of the opening of the anterior leaflet than of the actual diaphragm effect of the ring ( Fig. 127-1 ). Strictly attached to the mitral valve annulus, it is to be differentiated from the cor triatriatum. Like the subaortic membrane in the left ventricular outflow tract, the supravalvar mitral ring is an acquired lesion resulting from turbulent flow through the mitral orifice. The primary lesion of the mitral valve responsible for the turbulent flow can be obvious, stenotic, and regurgitant, or it can be discrete or mild and difficult to identify. It can be related to a prominent coronary sinus, as found in the left superior vena cava draining into the coronary sinus. It is perhaps for these reasons that the supravalvar mitral ring is prone to reoccur after surgical resection, unless the underlying anatomic anomaly has been identified and corrected.

Cleft Mitral Valve

The cleft mitral valve is often isolated and can be easily differentiated from a left AV valve in a partial AV septal defect. It is an actual cleft with no suspension apparatus on the edges of the defect. The cleft is centered on the aortic commissure between the noncoronary and left coronary cusps. Each half of the anterior leaflet at the midportion bears the attachment of the strut chordae. With time, the cleft mitral valve regurgitation will generate secondary lesions at the edges of the cleft, such as thickening, rolling up, and retraction. The defect is never stenotic and may generate only little regurgitation for a long time.

Lesions Associated with Lack of Valvar Tissue

Three major anatomic types of lesions are almost always associated with a lack of valvar tissue to various degrees and are worth characterizing: parachute mitral valve, papillary muscle to commissure fusion, and hammock or arcade valve. There is, however, a continuum between these three types and with the normal anatomy. The functional lesion can be either predominantly regurgitant or predominantly stenotic, or it can be both stenotic and regurgitant. In rare cases, the valve functions normally.

Parachute Mitral Valve

The parachute mitral valve can be found in isolation. It is, however, almost always associated with another cardiac anomaly, such as atrial septal defect (ASD), ventricular septal defect (VSD), or coarctation, and it is often integrated in Shone syndrome. It can also be seen in hypoplasia of the left ventricle, resulting in the need for univentricular palliation. The gross pathology shows a predominant single papillary muscle, with the orifice of the mitral valve overriding the tip of the papillary muscle. The spectrum of lesions for the suspension apparatus starts with complete fusion of the tip of the papillary muscle to the free edge of the valve. At the other end of the spectrum are chords that are relatively normal appearing, with good mobility of the leaflet ( Fig. 127-2 ). The accessory papillary muscle is usually small and devoted to a short segment of the free edge, or even to the undersurface of the leaflet tissue, as would be seen if it were a larger-than-normal secondary chord. The leaflet tissue can be intact or perforated.

The functional class depends on the interaction between the amount of tissue and the mobility of the leaflet; presence and size of the fenestrations; and presence, length, and quality of the chords. The parachute mitral valve almost always has a stenotic component, because the gradient is fixed but the valve grows. These patients may not require a valve operation.

Double-orifice mitral valve, in which the lesser papillary muscle supports a complete orifice, is an exceedingly rare variation of the parachute mitral valve. It should be differentiated from the left AV valve, where an accessory orifice is often found when there is a diminutive or absent left lateral leaflet (mural leaflet).

Papillary Muscle to Commissure Fusion

Fusion of papillary muscle to the commissure, also called short chordae syndrome, is defined by the presence of short chords and a papillary muscle tip that is attached or fused to the commissural area of the free edge ( Fig. 127-3 ). In the most extreme form, the chords are completely absent. The papillary muscles can be of normal size and volume. The valve is then generally more regurgitant than restrictive, which is because of the lack of valvar tissue and the restriction of the leaflet motion. When the papillary muscles are hypertrophied, the bulk of their mass is generally responsible for a predominantly restrictive valve.

Isolated Mitral Leaflet Hypoplasia

This rare condition is almost restricted to the middle scallop of the posterior leaflet.

Isolated Dilation of the Mitral Valve Annulus and Isolated Elongation of the Chords and Papillary Muscle

When the anatomy of the mitral valve is otherwise normal, it is difficult to ascertain the congenital origin of dilation of the mitral valve annulus and elongation of the suspension apparatus, but they are included in most studies of congenital anomalies of the mitral valve and account for 15% to 40% of the patients in published studies of congenital mitral valve regurgitation. However, there is no evidence of their congenital origin. Elongation of papillary muscles can be found at birth in the mitral or the tricuspid apparatus, but the muscles usually have an ischemic, beige aspect. Sometimes, the ischemic origin is demonstrated by acute rupture at or shortly after birth. Isolated dilation of the annulus is not found at birth.

Both dilation of the annulus and elongation of the suspension apparatus are usually associated with significant volume loading of the left ventricle (e.g., with a large VSD or large patent ductus arteriosus). The pathophysiology is of initial dilation of the posterior annulus under the effect of the volume loading, followed by elongation of the marginal chords and prolapse of the free edge of the anterior leaflet. In rare cases, minor anomalies of the valvar tissue or the papillary muscles indicate a true congenital origin. When a patient older than 4 years has an isolated mitral regurgitation combining anterior leaflet prolapse and various degrees of posterior leaflet retraction, especially if the latter is thickened, a rheumatic origin should be ruled out when the patient originates from a geographical area of high prevalence.

Functional mitral regurgitations secondary to cardiomyopathies are not discussed here.

Accessory Mitral Valve Tissue

The interchordal spaces are filled with a dense network of valve tissue. When there is continuity between the anterior and the posterior leaflet, the accessory valvar tissue may generate a gradient directly related to the size of the perforations in the accessory tissue ( Fig. 127-4 ). When the accessory valve tissue is entrapped in the left ventricular outflow tract, the mitral valve may become regurgitant because of the traction exerted by the accessory valvar tissue on the anterior leaflet, which results in the valve's opening at mid systole ; however, in that case, the left ventricular outflow tract obstruction is the predominant hemodynamic lesion and is usually responsible for the diagnosis. Often, the accessory mitral valve tissue does not generate significant gradient or insufficiency.