CLASSIFICATION OF THE PATHOLOGIC PROCESSES THAT CAUSE MITRAL REGURGITATION
The pathologic processes that lead to mitral regurgitation include degenerative valve processes (fibroelastic deficiency, rheumatic valvulitis, myxomatous [Barlow's] disease and Marfan's syndrome) (1), congenital malformation, destruction by endocarditis, and ischemic dysfunction. Carpentier noted that though the etiology of regurgitation may vary, the approach to repair is based on the valvular deformity caused by the deformative process. Treating regurgitation based on the deformity is called the “functional approach” (2). The goal of the functional approach is to restore normal function rather than anatomy. This points to a final common pathway for mitral regurgitation—the initial abnormality leads to regurgitation, which promotes ventricular and annular dilatation, worsening the regurgitation.
The three types of abnormalities are based on the positions of the leaflets during valve closure (Fig. 3.1). In Type I, the leaflets are in normal position relative to the annulus. This is usually associated with normal or abnormal leaflets, and the regurgitation is a result of a hole in the leaflet, central regurgitation secondary to annular dilation, leaflet abnormalities preventing proper leaflet coaptation, or a combination of these abnormalities. In a Type II abnormality, one or both of the leaflets prolapses relative to the valvular plane. This is due to papillary muscle or chordal rupture; or elongation or redundancy of the body of the leaflet. Type III abnormalities are related to restricted leaflet motion. The restriction prevents leaflet coaptation. This can be due to leaflet abnormalities, such as thickening or commissural fusion; chordal abnormalities, including fusion and thickening; papillary muscle contraction; or ventricular abnormalities, such as scarring or dilatation. In Type IIIa the motion of one or both leaflets is restricted throughout the cardiac cycle secondary to rheumatic changes. In Type IIIb the motion of one or both leaflets is restricted during systole secondary to papillary muscle displacement (1).
Fibroelastic deficiency occurs mostly in the elderly with a short history of valvular dysfunction. The leaflets are transparent, and except for the prolapsing segment there is no excess tissue. The chordae are thin, fragile, and elongated. The annulus is dilated and often infiltrated with calcium (1).
Figure 3.1 Carpentier's functional classification is based on the opening and closing motions of the mitral leaflets. Type I has normal motion of the leaflets and mitral regurgitation is on the basis of the leaflet perforation or annular dilatation. In Type II dysfunction (increased leaflet motion) the free edge of the leaflet travels above the plane of the mitral annulus during systole due to chordal elongation or rupture. Type IIIa dysfunction implies restricted opening leaflet motion during diastole and systole due to rheumatic changes. Type Illb dysfunction correlates to restricted leaflet motion during systole secondary to papillary muscle displacement. (From Adams DH, Fisoufi F. Another chapter in an enlarging book: repair degenerative mitral valves. J Thorac Cardiovasc Surg. 2003; 125:1197-1199.)
MYXOMATOUS MITRAL VALVE DISEASE (BARLOW'S DISEASE)
Myxomatous disease of the mitral valve generally appears early in life. Patients present with a prolonged history of a murmur, thickened leaflets, substantial excess tissue, and a dilated annulus, which may be calcified (1). Chordae may be elongated and thinned. Often isolated ruptures are present, contributing to the focal prolapse. Classically, the most common abnormality is focal enlargement of the posterior central scallop (P2) with an associated ruptured chorda. Mills et al. comparing unileaflet versus bileaflet prolapse, found that patients with unileaflet prolapse were younger and had a higher incidence of flail leaflets (3). Patients with bileaflet prolapse were less likely to be hypertensive and had mechanically stronger chordae though leaflet strength was similar to patients with unileaflet prolapse.
Marfan's syndrome of the mitral valve is characterized by excess tissue, thickened leaflets, and a dilated annulus (1).
RHEUMATIC MITRAL VALVE DISEASE
Rheumatic disease of the mitral valve more commonly results in stenosis than regurgitation. However, regurgitation can occur with or without associated stenosis. Leaflets become thickened and stiff, and can be calcified. Annular calcification is also common. In contrast to other disease processes chordae become thickened and foreshortened and fuse together.
Chordal rupture is uncommon. The foreshortened chordae pull the leaflets towards the papillary muscle tips, restricting ventricular enlargement during ventricular relaxation.
ISCHEMIC MITRAL VALVE DISEASE
Ischemia-related mitral regurgitation represents a spectrum of disease processes often lumped together into a single group for analysis. Acute ischemia can result in mitral regurgitation, which often resolves over time. However, mitral regurgitation accompanying acute myocardial infarction worsens long-term prognosis, increasing mortality (4). Implications, safety of surgery, and likelihood of repair differ greatly based on the time and type of presentation. In most cases, the anatomic substrate of the valve itself is normal; the leaflets bear no structural defects, the chords are not elongated or ruptured, and the annulus is not dilated. Acutely the defect in valvular competence is related to the changed relationships of the structures supporting the valve, specifically the infarcted myocardial wall causes apical displacement of the leaflets and abnormal tethering, causing leaflet deformation and preventing proper coaptation (5,6). In contrast to regurgitation secondary to dilated cardiomyopathy, which is associated with symmetrical mitral valve deformation (Fig. 3.2), mitral valve deformation is asymmetric with an ischemic cardiomyopathy (7). Timek et al. demonstrated, in a sheep model of ischemic mitral regurgitation, that proximal left circumflex coronary artery occlusion causes central and holosystolic regurgitation, by delaying valve closure, increasing mitral annular area and displacing both papillary muscle tips away from the septal annulus at end systole (8).
The concept of papillary muscle dysfunction as a cause of mitral regurgitation is simplified and incorrect. In dogs, isolated papillary muscle infarction does not cause regurgitation, instead the infarction of the supporting wall leads to malposition and abnormal motion of the papillary muscle (9). Tibayan et al. observed in a sheep model that ischemic changes included septal-lateral dilatation and lateral displacement of the posterior papillary muscle (10), and Lai et al. observed malcoaptation of the posterior leaflet scallops, suggesting all of these changes be addressed during repair (11). One approach is to undersize the valve to improve long-term results (12).
As described above, ischemic mitral regurgitation results from tethering, however ischemic mitral regurgitation can also be secondary to abnormal scarring and healing, specifically the papillary muscle can elongate or rupture, causing prolapse instead of regurgitation (Fig. 3.3).
Figure 3.2 Mechanical and geometric changes that lead to ischema-induced mitral regurgitation. (From Kumanohoso T, Otsuji Y, Yoshifuku S, et al. Mechanism of higher incidence of ischemic mitral regurgitation in patients with inferior myocardial infarction: quantitative analysis of left ventricular and mitral valve geometry in 103 patients with prior myocardial infarction. J Thorac Cardiovasc Surg. 2003; 125:135-143.)
Figure 3.3 Ischemic cardiomyopathy: Angiogram demonstrating a low ejection fraction and severe mitral regurgitation. (Clip 1, Case 2)
Acute vs. Chronic Regurgitation
The scenario of presentation has significant implication in the approach to ischemia-related mitral regurgitation. In the acute situation after myocardial infarction, the principle cause of regurgitation is tethering. There is no associated annular dilation. Therefore, repair in the acute situation is difficult and replacement is often necessary. Often regurgitation in the peri-infarct period will improve spontaneously or with revascularization. If possible a delay in operation may be advisable to avoid valve replacement.
Papillary Muscle Rupture
A small subset of patients develops mitral regurgitation secondary to partial or complete rupture of the infarcted papillary muscle. As the papillary muscle has multiple heads, the rupture can involve a single head, multiple heads, or the whole muscle. The level of symptoms will be related to the degree of regurgitation predating the rupture, the change in the degree of regurgitation with the rupture, and the extent of the associated infarction. Complete rupture is generally very poorly tolerated and must be addressed emergently. Complete rupture with extensive necrosis may best be treated with chordal-sparing valve replacement (13).
Rupture of the posterior papillary muscle is five times more common than rupture of the anterior muscle (14). This is based on the blood supply to the papillary muscle. The supply to the anterior papillary muscle comes from dual sources. The supply to the posterior papillary muscle comes from a single source.