Isolated, non-reoperative congenital mitral valve abnormalities in adults are rare.^1,2 Mitral pathology concurrent with other cardiac anomalies is less rare. Atrial septal defect is the third commonest congenital cardiac lesion presenting in adulthood, occurring in an estimated 1/5700 persons, nearly 10% are ostium primum defects, and almost all primum defects have an associated mitral valve cleft.³ Congenital mitral valve pathology such as arcade or parachute mitral valve very rarely present in adulthood, though the patients surviving unrepaired to adulthood may be milder forms, amenable to repair.

In contrast to the rarity of non-reoperative mitral pathology presenting in adulthood, patients needing reoperation for formerly corrected mitral pathology represent an expanding adult population. Survivors of corrective surgery in infancy and childhood add an estimated 8960 new adults cases annually to the adult congenital population in the US and there are over 1,000,000 adults living in the US today with congenital heart disease.⁴ The prevalence of atrioventricular septal defect (AVSD) is 5.3 per 10,000 live births.⁵ Mitral insufficiency is the principal reason for reoperation in the AVSD population. For patients undergoing an anatomic repair for complete or partial AVSD in infancy, freedom from reoperation for mitral regurgitation (MR) is 81 to 83% at 15 years, so the number of adults presenting with the need for reoperative mitral valve surgery is steadily increasing as successful infant repairs continue to accrue.⁶

Normal Mitral Valve

Consideration of the normal mitral apparatus is important to the success of any repair for congenital mitral valve abnormalities. Congenital mitral valve pathology is widely variable between patients, demanding an individualized approach to each. A systematic analysis of the annular, leaflet, and subvalvar support structures informs a surgical strategy that can be tailored to the individual.

Annulus

The normal mitral valve is attached to a dynamic, saddle-shaped fibrous annulus with the horn of the saddle at the anterior annulus, and the low points of the saddle at the commissures—a shape that minimizes leaflet stress through the cardiac cycle (Fig. 39-1).⁷ The annulus is dynamic through the cardiac cycle, with apical to basilar displacement of the entire annulus, annular “folding” that changes the planar versus saddle configuration of the annulus, and a 23 to 40% contraction in annular circumference between systole and diastole.⁸

Leaflets

The anterior, or aortic leaflet, occupies one-third of the annular circumference, is broader than the posterior leaflet, and is anchored in its continuity with the aortic annulus. The posterior, or mural leaflet occupies two-thirds of the annulus and is narrower than the anterior leaflet.⁸ The leaflets are divided into named regions A1, P1, A2, P2, A3, P3, descriptive designations that are useful to consider in a systematic examination of the valve at repair (Fig. 39-1).

Subvalvar apparatus

All chordal leaflet support radiates from the anterolateral and posteromedial papillary muscles that are anchored on the free wall of the left ventricle, and correspond to the anterolateral and posteromedial commissures of the valve. Chordal attachments are at the free edge (“primary” or “marginal” chordae), the mid leaflet, (“secondary” or “rough zone” chordae), or closer to the hingepoint/annulus (“tertiary” or “basal” chordae).⁸ Papillary muscle separation is important to normal valve excursion, and fusion or close spacing produces functional valve stenosis.

The physiologic patterns that result from congenital dysplasias of the mitral valve defy clean categorization, as several conditions produce stenosis and insufficiency together, in variable combination. We organize our description of anatomic mitral malformations into (1) lesions predominantly causing stenosis, (2) those commonly associated with combined stenosis and regurgitation, and (3) those predominantly associated with regurgitation.

Congenital Mitral Stenosis

Ruckman and Van Praagh categorized primary congenital mitral stenosis into four basic subtypes, based on autopsy data: (1) typical, (2) hypoplastic, (3) supravalvar ring, and (4) parachute mitral valve (Fig. 39-2).⁹ We will describe these categories, including a broader spectrum of supravalvar entities with supravalvar ring, and a variety of subvalvar pathologies in addition to parachute mitral valve.

Typical

In the “typical” variant of congenital valvar stenosis, the left ventricular size is normal, the mitral annulus is normal, and obstruction occurs within the valve elements themselves. The interchordal spaces are obstructed by fibrous tissue, leaflets may be thickened, chordae shortened, and papillary muscles, though distinct, are closely spaced.⁹ In some cases, the chordal apparatus is nearly absent, and papillary muscle tissue may insert directly and rigidly onto the leaflet itself, greatly reducing leaflet mobility and options for repair.

Hypoplastic

The hypoplastic variant is usually associated with other left-sided abnormalities such as ventricular hypoplasia, aortic stenosis, aortic arch hypoplasia, or coarctation of the aorta. The mitral annulus itself is small, and multiple valve elements are small. Hypoplastic mitral stenosis generally produces physiology that is either lethal or requires surgical palliation in infancy, and is therefore seldom found in the adult.⁹

Supramitral Ring and other Supravalvar Pathology

Supravalvar obstruction to mitral inflow presenting in adulthood can take a variety of forms, causing inflow obstruction from a level remote from the valve (such as Cor Triatriatum), adjacent to the valve (supravalvar ring), or intimately associated with the valve (intramitral ring) (Fig. 39-3).

Cor triatriatum is embrologically distinct from true mitral valve stenosis, buts acts similarly, is encountered in the adult, and is therefore included. Cor Triatriatum is a membranous subdivision of the left atrium into two chambers—an upper chamber that receives the pulmonary veins, and a lower chamber that excludes pulmonary veins but includes the left atrial appendage. An “os” in the membrane is the portal through which all pulmonary venous return passes to the lower chamber and in turn through the mitral valve. Cor triatriatum can present with mild-to-severe restriction to mitral inflow, depending on the size of the os and the alternate pathways of left ventricular filling that may exist (Fig. 39-3).^10,11

The true supramitral ring is a fibrous structure residing on the atrial side of the mitral annulus, with variable adherence to the mitral valve leaflet itself. It can produce restriction to inflow according to the size of the inflow orifice, and by restricting the movement of the leaflet itself when adherent (Fig. 39-3).

The intramitral ring is a thin fibrous membrane intimately associated with the leaflets and restricting leaflet excursion. The intramitral ring is always associated with subvalvar pathology (Fig. 39-3).¹²

A persistent left superior vena cava, present in 0.3 to 0.5% of population, courses across the floor of the left atrium in the supramitral area, and has been described to itself, uncommonly, produce a supravalvar obstruction.^13,14

Parachute Mitral Valve and Other Subvalvar Pathology

There are a great variety of types of subvalvar obstruction and an understanding of the differences is important to any operative strategy to repair them.

True “parachute mitral valve” presenting in adulthood is very rare, with only nine published cases of isolated parachute mitral valve in adults reported over 50 years.¹ An embryologic failure in the separation of papillary muscles into two distinct columns results in parachute mitral valve, with a single papillary muscle from which all chordae radiate.^1,15 Annular and subannular restriction to mitral inflow results from thickened, shortened chordae, tethered to a single papillary origin, with tight interchordal spaces and restriction of leaflet excursion.¹ Mitral insufficiency may accompany mitral stenosis, possibly due to ischemic dysfunction of the single papillary muscle.¹ Parachute mitral valve is commonly associated with additional left heart pathology. “Shone’s” complex is a constellation of left sided anomalies that includes a parachute mitral valve deformity, supramitral ring, subaortic stenosis, and coarctation of the aorta. The original description by John Shone in 1963 was from autopsy data from eight cases, including one adult, noting the tendency for additional left sided pathology to accompany parachute mitral valve, and the severity of disease that accounts for its rare presentation in adulthood.¹⁵ These concomitant pathologies commonly drive the need for palliative or corrective interventions in childhood, leaving rare instances of isolated parachute mitral valve to present in adulthood.¹⁷ “Parachute-like mitral asymmetry” is a similar but distinct entity where there are two distinct papillary muscles, but the anterolateral muscle is underdeveloped, short, and imparts restriction to the valve similar to that of true parachute valve.^12,17