The Management of the Cleft Mitral

Transcription

1 The Management of the Cleft Mitral Valve in Endocardia1 Cushion Defects L. H. S. Van Mierop, M.D., and Ralph D. Alley, M.D. C ustomarily, in the surgical treatment of endocardial cushion defects the cleft in the anterior or aortic cusp of the mitral valve is repaired by suture in an effort to correct the mitral incompetence usually present, to a varying degree, in this type of defect. Indeed, the cleft has been regarded as an anatomical abnormality demanding closure even in the absence of mitral incompetence. Though superficially a logical step in the operative repair, the functional result has often been disappointing. Frequently, mitral incompetence is increased by closure of the cleft, incompetence may actually be created by closure of the cleft, and commonly the valve appears somewhat stenotic in consequence of a decreased mobility of the reconstructed anterior cusp. After careful study of the pathological anatomy of endocardial cushion defects and comparison of the mitral valve in such cases with that of a normal heart, we came to the conclusion not only that it was unnecessary to suture the cusp in cases where there was little or no insufficiency, but that it may actually be contraindicated. Accordingly, for the past few years (though it was with some trepidation initially) we have left the cleft alone in such cases. Thus far we have had no reason to regret this decision. Frater, in a recent excellent paper [4], expressed in no uncertain terms the same opinion and gave a number of rules for the management of endocardial cushion defects; we are in full agreement with his rules. THE NORMAL MITRAL VALVE EMBRYOLOGY AND ANATOMY The normal embryology [5, 81 of the atrioventricular valve region and the anatomy [3,6-81 of the normal mitral valve have been described previously. Normally, the endocardial cushions after fusion assume the From the Departments of Pediatrics (Cardiology) and Anatomy, and the Subdepartment of Thoracic Surgery, Albany Medical Center, Albany, N.Y. Supported by U.S. Public Health Service Grant No. HE Accepted for publication Mar. 8, THE ANNALS 0.F THORACIC SURGERY

2 Cleft Mitral Valve shape of an arch with the convexity of the arch directed toward the atria. Septum primum meets the fused endocardial cushions about midway between their right and left extremities at the summit of the arch and fuses with them, thus closing ostium primum. The part of the fused endocardial cushions to the left of this union becomes the anterior cusp of the mitral valve and does not participate in the septation of the heart. The right half of the fused endocardial cushions forms the muscular portion of the atrioventricular septum, both the atrioventricular and interventricular parts of the membranous septum, and contributes to the formation of the medial (septal) cusp of the tricuspid valve. In so doing, it forms a considerable portion of the wall of the left ventricular outflow or aortic vestibule. Formation of the arch also makes it possible for the aortic valve to eventually seat itself between the septum and the anterior cusp of the mitral valve. A further consequence is that only a small inferior portion of the anterior mitral cusp has direct relations with the ventricular septum; most of the cusp has a common origin with the adjacent halves of the left coronary and noncoronary cusps of the aortic valve. Strictly speaking, the normal mitral valve consists of four cusps. Usually, however, two of these are very small (commissural cusps), and two are large, so that the normal mitral valve is essentially a bicuspid valve. The anterior cusp, as all other atrioventricular valve cusps, is roughly trapezoid in shape. Its origin is formed by the longer of the two parallel sides, and is, in this case, actually not a straight line, but convex. The other parallel side is very short and free. The two converging sides are connected by chordae tendineae to the papillary muscles. Two groups of chordae can be distinguished: Chordae of the first order are fine and insert into the very edge of the cusp; chordae of the second order are less numerous, but larger, and they insert into the ventricular surface of the cusp some distance away from the edge (about 3 to 6 mm. in the adult) and blend into the body of the cusp. The cusps are somewhat irregularly thickened in the area of insertion of the second order chordae, and discrete nodules (of Albinus) are present here, particularly in very young individuals. In adults these nodules tend to disappear. The 3 to 6 mm. wide border of the valve cusps, defined by the insertion of first- and second-order chordae, appose when the ventricle contracts, thus insuring a secure and watertight closure. FUNCTIONAL CONSIDERATIONS A bicuspid valve in a circular orifice is mechanically not exactly ideal, particularly if the borders of the orifice (and therefore the origin of the valve cusps) are in a single, flat plane, and if the cusps are of equal size. A tricuspid valve would be much better, since it can open more VOL. 2, NO. 3, MAY,

3 VAN MIEROP AND ALLEY widely with less impediment to flow. (Compare, for instance, the function of a bicuspid aortic valve with that of a normal, tricuspid one.) Fortunately, a normal mitral valve shows two features which help to overcome the disadvantages inherent in a bicuspid valve (Figs. 1, 2). First, the cusps are very unequal in size, the aortic or anterior leaflet being much the larger; and second, the origin of the larger cusp is not straight but convex, toward the atria (upstream) side (see Fig. 1A). This allows the anterior cusp to be taut in the closed position, but when it opens it becomes relatively flaccid, falls together in folds, and rides with the bloodstream. That inequality of cusp size, given a bicuspid valve in a circular orifice, is more favorable and results in more adequate opening of the valve than does a bicuspid valve with equal cusps has been recognized by Edwards et al. [2] for the bicuspid aortic valve. And, of course, the origins of arterial valve cusps (and for that matter venous valve cusps, too) are always convex toward the upstream side. FIG. 1. Left side af cardiac septum. (A) htormal heart. (B) Endocardia2 cushion defect, partial type, with interatrial communication. 418 THE ANNALS OF THORACIC SURGERY

4 Cleft Mitral Valve LEFT ATRIUM FIG. 2. SECOND ORDER CHORDAE FIRST ORDER CHORDAE Diagrammatic cross section of normal mitral valve. THE NATURE OF THE ANOMALY IN ENDOCARDIAL CUSHION DEFECTS The basic developmental abnormality in endocardial cushion defects appears to be a failure of fusion, to a varying degree, of the superior and inferior atrioventricular endocardial cushions, and, therefore, also a failure of the fused endocardial cushions to arch toward the atria [ 1,4,5, 8-10]. Since the anterior mitral cusp is formed by the left halves of the endocardial cushions, failure of fusion results in a cleft in this cusp. Such a cleft may be partial and may only involve the marginal (peripheral) part of the cusp, but usually it is complete, i.e., it most often extends all the way up to the origin of the cusp. In the complete type of endocardial cushion defect, in which there has been no fusion at all of the endocardial cushions, the entire central part of the cardiac septum is missing and there is only one common atrioventricular orifice. In such cases each of the endocardial cushions forms a cusp which crosses the defect. These common cusps may or may not be attached to the top of the ventricular septum by chordae tendineae. It is particularly common for the superior common cusp not to be attached, while the inferior cusp almost always is. More important, from a practical and therapeutic point of view, are the partial forms of endocardial cushion defect; of these the so-called ostium primum type of defect (see Fig. 1B) is by far the most common. For the purpose of this discussion, we will use this type as a model. It shows a number of characteristic features, some of which, as will become apparent, have very important implications: 1. There is a cleft in the anterior cusp of the mitral valve. The other cusps are normal. If there is mitral incompetence, the edges of the cleft are generally abnormal and thickened. It is, however, not always easy to say whether these changes are primary or secondary. In cases where there is no insufficiency, the edges of the cleft may look surpris- VOL. 2, NO. 3, MAY,

5 VAN MIEROP AND ALLEY ingly normal, and one is impressed again, as elsewhere in embryology, by the ability of the embryo to compensate for deficiencies and abnormalities. It tries to make a competent valve with whatever material is available and often succeeds amazingly well. New chordae are created where there are none in a normal heart, and the edges of the cleft look like normal valve edges elsewhere, with a closing border, nodules of Albinus, etc. In fact, instead of one anterior cusp there are now two, each of the two being formed like any other cusp and each attached by two sets of chordae to the ventricular wall-one set to one of the normally present papillary muscles, and the other directly to the top of the ventricular septum. The latter may at times be quite normally formed and delicate, particularly in the complete forms of endocardial cushion defects and particularly if in these cases the common valve is competent. Also, the younger the patient, the more normal in architecture the abnormal cusps and chordae appear. Excellent pictures of such wellformed cusps have been published by Edwards [l] (his Figs. 1, 2) and others [2]. 2. The top of the ventricular septum is scooped out. Not only do the endocardial cushions fail to fuse and do not form the typical arch seen in normal cardiac development, but in addition, the endocardial cushions fail to contribute to the formation of the posterior part of the ventricular septum. The result is that the distance between the apex and the atrioventricular valve is considerably less than normal, i.e., the muscular septum is scooped out and the origin of the medial atrioventricular valve cusps i s much lower than normal. The origin of the cleft anterior mitral cusp is not convex toward the atrial side but concave. Such a cusp, unless it is extremely redundant, can only open adequately and freely without causing turbulence if it is cleft (Fig. 3). In other words, mechanically the cleft in the cusp in endocardial cushion defects is a favorable feature. Surgical closure of the cleft may result in an inability of the cusp to open completely and freely, inviting turbulence, and hence, future damage to the valve. A comparable sequence of events is illustrated by the natural history of a bicuspid aortic valve. It is not uncommon here to find the cusps to be structurally normal, thin, and delicate in newborns and very young infants, but we have yet to see a bicuspid aortic valve in an adult that is not thickened, with or without calcification. 3. There is a large communication between the two atria. This communication is the result of the fact that the endocardial cushions have failed to fuse and have failed to form a typical, normal arch, making it impossible for the atrial septum, which in many cases is formed normally, to reach and fuse with it. The location of the interatrial communication corresponds to the position taken up in the normal heart by the atrioventricular septum and most posterior portion of the mus- 420 THE ANNALS OF THORACIC SURGERY

6 Cleft Mitral Value PZG. 3. Two pieces of cloth, cut in identical fashion, have been taped to a board to simulate the anterior cusp of the normal mitral valve, A, and the same cusp in endocardial cushion defects, B. (Left) Zn the open position, A falls down in folds, while in B the cleft opens up. (Right) If the cleft, B, is closed with Scotch tape, the cusp will not fall open easily. cular ventricular septum. The border of the interatrial communication is formed by the free edge of the atrial septum and corresponds to the line of origin of the normal anterior mitral cusp. There is, therefore, neither an atrioventricular septum nor a membranous interventricular septum in endocardial cushion defects. 4. The superoinferior diameter of the heart is increased at the level of the atrioventricular va2ve. This can be explained on a hydraulic basis. The heart is shaped like a cone, the base of which, however, is not circular but more or less elliptical, i.e., the cone is flattened. Increase of pressure within the cone will tend to transform the ellipse into a circle: The short diameter of the ellipse will become larger, unless it is kept constant by a strut. In a normal heart the strut is formed by the fused endocardial cushions; but in endocardial cushion defects the strut is broken, and with increased pressure these broken ends become separated. This effect can be expected to be more pronounced in the complete type of endocardial cushion defect than in the partial or ostium primum type, but even in the latter it will manifest itself to some extent. It may be responsible, at least in part, for the valve incompetence so common in these defects. It may even be the initial factor in creating the vicious circle of incompetence leading to dilatation leading to more incompetence, etc. It cannot always be held responsible, since in some cases there is unquestionably a deficiency of one or more valve cusps. How much of a contributory factor the presence of a large left-to-right shunt is, is difficult to say. 5. Most of the cleft anterior cusp (all of the inferior half and the VOL. 2, NO. 3, MAY,

7 VAN MIEROP AND ALLEY posterior part of the superior half) originates from the ventricular septum. The result is that: 6. The left ventricular outflow is long and narrow, and the aortic valve is located in an abnormally high position. MANAGEMENT OF THE CLEFT MITRAL VALVE On the basis of the foregoing observations, we have decided to manage patients with endocardial cushion defects as follows: 1. Cardiac catheterization studies are carried out generally at a younger age, even in asymptomatic children, than in asymptomatic children with other types of intracardiac shunts. Although we cannot prove this, we have the impression that mitral insufficiency is more common in older patients. We are also inclined to operate upon such patients at a younger age, particularly if there is no mitral insufficiency. 2. If some mitral insufficiency has been demonstrated angiocardiographically, the source of it is located at the time of surgery. We agree with Frater [3, 41 that commonly the jet is located right next to the septum, in which case only at that point a few stitches should be placed. If there is some insufficiency at the other end of the cleft, at the meeting point of the cusps, this should be left alone, and the cleft should remain unsutured. A competent valve is left alone. 3. The interatrial communication should be closed very accurately with a patch without causing distortion, i.e., the patch should be tailored in such fashion as to be of exactly the same size as the defect. An exception to this which may have some merit is to make the longer diameter of the patch a bit too short in order to somewhat reduce the corresponding superoinferior measurement of the defect. Every effort should be made not to distort the mitral valve. SUMMARY The pathological anatomical features of endocardial cushion defects are described and their functional significance is discussed. In particular, the anatomy of the normal mitral valve is compared with that of the cleft mitral valve in endocardial cushion defects. The conclusion is reached that, mechanically, the cleft actually is a favorable feature without which the valve cannot open adequately. Its presence per se should not, therefore, be an indication for suture, and certainly in cases in which there is little or no insufficiency, it should be left alone. REFERENCES 1. Edwards, J. E. The problem of mitral insufficiency caused by accessory chordae tendineae in persistent atrioventricular canal. Proc. Mayo Clin , THE ANNALS OF THORACIC SURGERY

8 Cleft Mitral Valve 2. Edwards, J. E., Carey, L. S., Neufeld, H. H., and Lester, R. G. Congenital Heart bisease. Philadelphia: Saunders, Vol. 2, p Frater, R. W. M. Mitral valve anatomy and prosthetic valve design. Proc. Mayo Clin. 36:582, Frater, R. W. M. Persistent common atrioventricular canal: Anatomy and function in relation to surgical repair. Circulation 32: 120, Patten, B. M. Persistent interatrial foramen primum. Amer. J. Anat. 107: 271, Tandler, J. Anatomie des Herzens. In K. Von Bardeleben (Ed.), Handbuch der Anatomie des Menschen. Jena, Germany: G. Fisher, Vol. 111, Part Van Mierop, L. H. S. Anatomy of the heart. Ciba Clin. Sympos. 17:67, Van Mierop, L. H. S., Alley, R. D., Kausel, H. W., and Stranahan, A. The anatomy and embryology of endocardia1 cushion defects. J. Thorac. Cardiov. Surg. 4371, Wakai, C. S., and Edwards, J. E. Developmental and pathologic considerations in persistent common atrioventricular canal. Proc. Mayo Clin. 31 :487, Wakai, C. S., and Edwards, J. E. Pathologic study of persistent common atrioventricular canal. Amer. Heart J. 56:799, 1958.