Valve Surgery for Congenital Anomalies and Degenerative Diseases
There are essentially two classification systems for congenital mitral lesions that are described by Carpentier and Metruka/Lamberti. These classifications characterize the different lesion sets by leaflet motion, deformed annulus, cleft leaflets, agenesis, commissural fusion, chordal shortening/elongation, and subvalvar apparatus anatomy. The following is a description of some of the more common lesions with traditional methods for treatment.
15.3 Stenotic Lesions
15.3.1 Supravalvar and Intravalvar Mitral Rings
Supravalvar mitral stenosis is a restrictive fibrous membrane that is attached and continuous with the mitral valve annulus. This thick fibrous plate of tissue can resemble a membrane in its central portion. It is usually attached at the level of the annulus or just above it. The membrane generally has a single central opening that may be eccentric in its position. The size of the opening defines the grade of obstruction and correlates well with the severity of symptoms. The underlying mitral valve may be functionally normal, but more often it is small and may be anatomically abnormal in some patients. Supravalvar mitral stenosis should be distinguished from and not confused with cor triatriatum. The fibrous membrane in cor triatriatum divides the atrium into two chambers. The atrial appendage originates downstream from the membrane in cor triatriatum, whereas the left atrial appendage is upstream from a supravalvar ring.
Figure 15.3 shows a lateral coronal view of supravalvar mitral stenosis. The plane between the supravalvar ring and the mitral orifice has been exaggerated to highlight the anatomic relationship. Generally, these patients have longstanding mitral stenosis and a large left atrium; this makes the approach through the interatrial groove preferable. The key to proper exposure is to employ a large left atrial incision with traction annular sutures to bring the annulus into the operative field for the membrane excision (Fig. 15.4). A Beaver blade (Fig. 15.5) is employed to find the dissection plane between the fibrous ring and the annulus using endarterectomy techniques. The dissection plane can be developed much like removal (i.e., enucleation) of a subvalvar aortic ridge. Careful and deliberate dissection effectively removes the fibrous ring without injury to the underlying mitral annulus. With complete excision of the ring, the restricted motion of the leaflets is alleviated, resulting in a much larger mitral orifice (Fig. 15.6). The mitral valve can then be tested for competence with a cold saline injection into the left ventricle. The atrium is closed with proper deairing of the left side of the heart, and the cross clamp is removed. After separation from cardiopulmonary bypass, transesophageal echocardiography is used for valve analysis. Occasionally, the mitral valve ring is situated within the body of the mitral valve, as shown in Fig. 15.7. Under these circumstances, the valve is approached and exposed in the same manner as for supravalvar stenosis. The dissection plane is developed using endarterectomy techniques, and the fibrous ring is carefully dissected from the surrounding valvar tissue with special care taken not to injure the underlying valve. In the event that unwanted mitral leaflet perforation complicates the excision, a piece of bovine pericardium can be used to repair the leaflet perforation. Ordinarily, the orifice of the mitral valve can be enlarged significantly. When the subvalvar papillary muscle structure is also stenotic, carefully calculated papillary muscle incisions between the chordae can be accomplished to allow further mobility of the leaflets and to maximize inflow to the body of the left ventricle (Fig. 15.7).
15.3.2 Double Orifice Mitral Valve
Double orifice mitral valve is an anatomic abnormality in which there are two reasonably formed inlets into the left ventricle. Figure 15.8 shows a left atrial superior view of the double orifice mitral valve. Figure 15.9 shows the same atrioventricular valve in a coronal view. Typically, the smaller of the two orifices is rightward, while the larger orifice is leftward as seen by the surgeon across from the left atrium. The degree of stenosis is variable, and effective orifice enlarging operations have not been reported. The most important tenet when approaching this valve is not to cut the isthmus between the two orifices. This results in severe regurgitation that cannot be repaired because of a lack of subtended supporting chordae tendineae in the middle of the valve. The best approach to a stenotic double orifice valve is to look for and treat any secondary causes of stenosis such as papillary muscle obstructive lesions. Sometimes a cleft is present in the anterior leaflet of the larger orifice. Experience and judgment determine whether the cleft should be closed, depending on the amount of resultant stenosis if the cleft is closed or the amount of regurgitation if the cleft is left unrepaired.
15.3.3 Mitral Valve Hypoplasia and Arcade (Hammock) and Parachute Mitral Valve
When mitral valve stenosis and annular hypoplasia are associated with an inadequately sized left ventricle, the patient is generally treated along the single ventricle pathway en route to a Fontan circulation. A hypoplastic valve and annulus can occasionally be associated with a functionally adequately sized left ventricle, especially if a ventricular septal defect is present. The hypoplastic valve may appear to be quite similar to a normal mitral valve in so far as there are two leaflets with corresponding chordae tendineae that appear appropriate. The leaflets are generally mobile with nonfused commissures. Alternatively, the leaflets are thickened, and the commissures are fused. These valves can be associated with thickened, short, and inadequate chordae tendineae with poorly developed interchordal spaces. If the chordae are shortened, a funnel-like appearance may be seen. Most often these characteristics match up very well with what most anatomists call an arcade mitral valve. The anterior and posterior groups of papillary muscles join together and fuse with the entire leading edge of the leaflet without the presence of well-formed chordae. Only one papillary muscle may participate in the arcade in some cases. The arcade mitral configuration can produce mitral stenosis or regurgitation or a mixed lesion. It may also be associated with a parachute valve. In general, therapy is aimed at incising the elongated and narrowed papillary muscle that is responsible for the central obstructing orifice and making fenestrations in the existing valve leaflets. Figure 15.10 shows a coronal atrioventricular view, and Figs. 15.11 and 15.12 show atrial views of a stenotic parachute valve that has similar anatomic characteristics to an Arcade/Hammock mitral valve and is discussed below in the section on Parachute Mitral Valve. Once the appropriate incisions and fenestrations are performed, the valve orifice can be measured with dilators. The predictive adequacy of the valvuloplasty can be evaluated by comparing it with standard measurements of normal mitral valves for somatic size. The mitral valve competence can be assessed with a saline-filled bulb syringe. The most important advice and technical pearls one surgeon can give to another is to “establish excellent exposure with retraction sutures in the annulus, measure ten times, and cut once.” Incision into the papillary muscle must be made accurately, deliberately, and centrally to avoid papillary muscle avulsion and to maintain chordal attachments.
Parachute mitral valve is characterized by total chordal attachment to a single papillary muscle that often causes significant mitral stenosis. In general, the entire leaflet is connected to the posterior papillary muscle, with absence of the anterior papillary muscle. In rare instances, the entire leaflet is connected to an anterior papillary muscle. Figure 15.10 shows the parachute mitral valve attaching to the single papillary muscle arising from the posterior left ventricular wall. Figure 15.11 shows the dotted lines that illustrate the areas of proposed leaflet fenestrations and papillary muscle incision (Fig. 15.12) to open the ventricular inlet. The valve is approached in a manner similar to that outlined in the previous sections; the same precautions apply, and the same methods of intraoperative evaluation are used.
The above valve repair techniques are applied in childhood when surgery is most often performed for mitral stenosis. It is rare that these procedures are successfully performed in a redo setting when they have been performed earlier in childhood. Leaflets are often thickened, restricted, and calcified in adulthood, and valve excision with prosthetic replacement is the more common course of action.
15.4 Regurgitant Lesions
Congenital MR can be associated with many of the previously noted abnormalities of mitral valve stenosis. It can also be primarily structural in etiology caused by annular dilation, degenerative disease that results in chordal elongation, and leaflet anomalies. Mitral regurgitation may also present as a secondary manifestation caused by ventricular dilation, cardiomyopathy, and aortic regurgitation. In particular, significant functional MR can also occur in association with an anomalous left coronary artery arising from the pulmonary artery as a result of decreased myocardial oxygen delivery and poor ventricular function. While it is not a mitral valve, the common atrioventricular valve found in AVSD is often associated with regurgitation of the left atrioventricular orifice. A cleft of the anterior leaflet of the left atrioventricular valve may produce important regurgitation that is more common in the adult with undiagnosed partial AVSD. Young patients with Marfan syndrome (or other connective tissue disorders) who present early with MR have abnormal mitral valves with billowing leaflets, elongated chordae, and dilated annuli. The double-orifice mitral valve discussed in the previous section (Figs. 15.8 and 15.9) can also be associated with AVSD with a cleft in the larger of the two orifices. This configuration may or may not result in regurgitation. Mitral valve prolapse caused by degenerative disease is characterized by excess valvar tissue involving the posterior or anterior leaflet, elongated or ruptured chordae, and annular dilation. The Barlow valve is severe redundancy and thickening of both anterior and posterior leaflets as shown in Fig. 15.13.