Supravalvular Mitral Ring

Often considered a congenital anomaly of the mitral valve, the supravalvular mitral ring is a fibrous construction attached to the posterior anulus 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 anulus, it is to be differentiated from the cor triatriatum. Like the subaortic membrane in the left ventricular outflow tract, the supravalvular 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 very 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.^4,5 It is perhaps for these reasons that the supravalvular mitral ring is prone to reoccur after surgical resection, unless the underlying anatomic anomaly has been identified and corrected.

Figure 127–1 Two-dimensional echocardiogram of supravalvular mitral ring, apical view. The membrane attached at mid height of the anterior leaflet is characteristic. The posterior part of the membrane is only suggested by the hyperechogenicity of the posterior anulus.

Cleft Mitral Valve

The cleft mitral valve is very often isolated and can be easily differentiated from a left AV valve in a partial AV septal defect.^6,7 It is an actual cleft with no suspension apparatus on the edges of the defect.⁸ The cleft is centered on the aortic commissure between the noncoronary and left coronary cusps.⁶ Each half of the anterior leaflet at the midportion bears the attachment of the strut cordae. Rarely, the cleft mitral valve is associated with a leaflet tissue defect, which is an acquired defect resulting from the regurgitation through the cleft. The defect is never stenotic and may generate only little regurgitation for a long time.

Lesions Associated with Lack of Valvular Tissue

Three major anatomic types of lesions are almost always associated with a lack of valvular tissue to various degrees and are worth characterizing: parachute mitral valve, papillary-muscle-to-commissure fusion, and hammock or arcade valve. The functional lesion can be either predominantly regurgitant or predominantly stenotic, or it can be both stenotic and regurgitant. Rarely, 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,⁹ and it is often integrated in Shone syndrome.^10,11 It can also be seen in hypoplasia of the left ventricle, resulting in a 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.³ At the other end of the spectrum are cords that are relatively normal appearing, with good mobility of the leaflet (Fig. 127-2). The accessory papillary muscle is usually very 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 cord. The leaflet tissue can be intact or perforated.

Figure 127–2 Macroscopic view of a parachute mitral valve. A large predominant papillary muscle distributes anomalous cords to both commissures. A small diminutive papillary muscle is seen to the right of the valve.

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 cords.¹² 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.⁹

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,⁸ also called short chordae syndrome, is defined by the presence of very short cords 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 cords 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 valvular 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.

Figure 127–3 Macroscopic view of a mitral valve with papillary-muscle-to-commissure fusion. Extremely short cords are distributed to the anterior commissure (left). The posterior papillary muscle is almost fused to the posterior commissure (right).

Isolated Mitral Leaflet Hypoplasia

This very rare condition is almost restricted to the middle scallop of the posterior leaflet.^8,13

Isolated Dilation of the Mitral Valve Anulus and Isolated Elongation of the Cords 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 anulus and elongation of the suspension apparatus, but they are included in most studies of congenital anomalies of the mitral valve^8,14 and account for 15% to 40% of the patients in published studies of congenital mitral valve regurgitation.^12,14–16 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 anulus is not found at birth.

Both dilation of the anulus 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 anulus under the effect of the volume loading, followed by elongation of the marginal cords and prolapse of the free edge of the anterior leaflet. Rarely, minor anomalies of the valvular 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 an area of high prevalence.

Functional mitral regurgitations secondary to cardiomyopathies are not discussed here.

Accessory Mitral Valve Tissue

The intercordal spaces are filled with a dense network of valve tissue. When there is continuity between the anterior and the posterior leaflet, the accessory valvular tissue may generate a gradient directly related to the size of the perforations in the accessory tissue (Fig. 127-4).¹⁷ 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 valvular tissue on the anterior leaflet, which results in the valve’s opening at mid systole¹⁸; however, in that case, the left ventricular outflow tract obstruction is the predominant hemodynamic lesion and is usually responsible for the diagnosis.¹⁹ Often, the accessory mitral valve tissue does not generate significant gradient or insufficiency.²⁰

Figure 127–4 Accessory mitral valve tissue. A, Long-axis view. The accessory mitral valve tissue is attached to the anterior and posterior leaflets of the mitral valve and blown like a windsock. B, Macroscopic view of the same tissue after surgical resection is characteristic of accessory mitral valve tissue attached to the suspension apparatus.

Mitral Valve Disease with Excess of Leaflet Tissue, and Mitral Valve Prolapse

It is debatable whether the mitral valve prolapse syndrome—in its most common form, limited to the middle scallop of the posterior defect—is congenital. In a large population of neonates,²¹ and using strict criteria, the incidence of mild bulging of the anterior leaflet was negligible, and no prolapses were detected. This tends to prove that mitral valve prolapse is an acquired disease. In its common form, it is the exception when encountered in neonates and infants. In adults, the histologic anomalies are limited to the middle scallop of the posterior leaflet, with predominant elastic fiber alteration and myxomatous tissue proliferation.²²

The more extensive form of mitral valve prolapse is, however, seen in neonates and infants. In this form, an excess of tissue is distributed to both the anterior and posterior leaflets, and histologic examination reveals extensive infiltration of the spongiosa with myxomatous tissue. The histologic anomalies are identical to those found in patients with Marfan syndrome,^23,24 Ehlers-Danlos syndrome, and osteogenesis imperfecta. Marfan syndrome is an autosomal dominant disorder with varying penetrance. The mutation is found on the fibrillin gene. Ehlers-Danlos syndrome is represented by a constellation of mutations linked to different subtypes.²⁵ The extensive form of the mitral valve prolapse syndrome is encountered in sporadic cases, or in familial forms demonstrating autosomal dominant and X-linked inheritance. Three different loci, on chromosomes 16, 11, and 13, have been found to be linked to mitral valve prolapse, but no specific gene has been described.²⁶

Recurrent Left Atrioventricular Valve Regurgitation

After repair of complete or partial atrioventricular septal defect (AVSD), there are two main mechanisms for regurgitation in valves with a normally developed left lateral leaflet. The cleft may be open because the cleft closure performed in the initial surgery has ruptured or because it was never done. In this case, the regurgitation occurs through the cleft directly. On color Doppler examination, the regurgitation jet is oriented vertically through the cleft.

Alternatively, when the cleft was completely closed and has remained so, the predominant mechanism for the regurgitation is the absence of a coaptation surface in front of the tip of the left lateral leaflet. In the unrepaired AVSD, the very small surface of the zone of apposition is an inherent feature of these parts of the superior and inferior bridging leaflets. For this reason, we at the Royal Children’s Hospital in Melbourne believe that the latter mechanism is always involved, even when the cleft closure has ruptured. The direct closure of the cleft does not restore a coaptation surface; in fact, the little coaptation surface that exists can be reduced and distorted by the cleft closure. On color Doppler examination, the regurgitation jet is oriented posteriorly, hugging the posterior wall of the left atrium.

If the regurgitation is a long-standing condition, the secondary lesions or dysplastic lesions on the edges of the cleft are severe, with thickening and retraction of the leaflet tissue, and sometimes even calcification. On the other hand, the left lateral leaflet is usually thin and pliable, with no secondary or dysplastic lesion. There is no restriction of the left lateral leaflet motion and no prolapse.

In the presence of hypoplastic or absent left lateral leaflet, the cleft cannot be closed at the time of the primary repair without generating inflow restriction. The residual or recurrent regurgitation occurs through the cleft. On an echocardiographic study, the anatomy is suspected when a strong asymmetry of the papillary muscles can be seen on the short-axis view of the left ventricle. The predominant papillary muscle, usually the anterior one, is connected to both superior and inferior bridging leaflets. The presence of a double orifice is also a very strong indicator. It is usually directly suspended to the posterior papillary muscle and in the body of the inferior bridging leaflet. On Doppler examination, the regurgitation jet is oriented vertically through the cleft.