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SUBAORTIC STENOSIS CAUSED BY ANOMALIES OF THE ATRIOVENTRICULAR VALVES
SUBAORTIC STENOSIS CAUSED B Y ANOMALIES OF THE ATRIOVENTRICULAR VALVES Robert D. Sellers, M.D.*
C. Walton Lïllehei, M.D., and
Jesse E. Edwards, M.D., Minneapolis
and St. Paul, Minn.
T
HERE are instances of subaortic stenosis in which the fundamental problem lies, not in primary disease of the left ventricular outflow tract but, rather, in anomalies of the atrioventricular valves. Emphasis on such entities is justified by the fact that, in these, surgical therapy for relief of subaortic obstruction must be directed toward correction of the primary valvular anomaly. Ferencz, 1 in 1957, and Björk, 2 in 1961, described cases in which malforma tions of the atrioventricular valves caused subaortic obstruction. The mechanisms by which abnormalities of the atrioventricular valves cause or contribute to subaortic stenosis are (1) obstruction by accessory tissue of the atrioventricular valves, (2) restriction of the movement of the anterior leaflet of the mitral valve either by accessory chordae running between the edges of a cleft in the valve and the ventricular septum or by fusion of valvular tissue to the septal wall of the ventricular outflow tract, and (3) attachment of the mitral valve to the ventricular septum when the latter is narrowed by a deficit in tissue in its basal portion. A study was made of the specimens in the combined pathologic collection of the Cardiovascular Registry of the Charles T. Miller Hospital and the Uni versity of Minnesota in a search for examples of subaortic stenosis resulting from abnormalities of the atrioventricular valves. Excluding cases of classical persistent common atrioventricular canal, there were 6 cases in which anomalies of the atrioventricular valves had caused, or appeared potentially capable of causing, subaortic stenosis. This report has two purposes. One is to describe anatomic details of the aforementioned 6 cases. The seeond is to emphasize the fact that in the complex malformation known variously as persistent common atrioventricular canal, A-V commune or endocardial cushion defect (herein called A-V commune malforma tion), an element of subaortic stenosis exists or may be caused by certain steps in correction of the malformation. Prom the Departments of Surgery and Pathology, University of Minnesota. Minneapolis. Minn., and the Department of Pathology, The Charles T. Miller Hospital, St. Paul, Minn. This study was supported by Research Grants HE-5694 and HB-830 and Research Training Grant 5-T1-HB 5570 of the National Heart Institute, U. S. Public Health Service, and by the Life Insurance Medical Research Fund. Received for publication Jan. 23, 1964. »United States Public Health Service Postdoctoral Research Fellow. 289
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Fig-. 1. Fig. 2. Fig. 1.—Diagram of left side of heart shows changes in position of mitral valve leaflets during ventricular diastole and systole. a, Ventricular (L.VJ diastole. The anterior mitral leaflet (A.) lies near the ventricular septum as the mitral valve is open. b, During ventricular systole the anterior leaflet of the mitral valve moves away from the ventricular septum to coapt with the posterior mitral leaflet (P.) and close the mitral valve. In this way it does not obstruct the left ventricular outflow tract. Fig. 2.—Diagram of left side of heart in the membranous type of subaortic stenosis. In this instance the membrane attaches to the anterior leaflet of the mitral valve at the junction of its lower two thirds and upper one third. In this position the membrane interferes with function of the mitral valve. More commonly, the membrane attaches to the anterior leaflet of the mitral valve at a more basal level and therefore interferes less with the function of the mitral valve than in the state illustrated here.
Before presenting our observations, certain anatomic and functional features of the mitral valve need to be recognized. The anterior leaflet of the mitral valve is an integral portion of the left ventricular outflow tract. This leaflet forms the posterolateral wall of the outflow tract and, as the leaflet moves with changing phases of the cardiac cycle, this aspect of the outflow tract changes. During ventricular diastole, the anterior leaflet bulges into the outflow tract, allowing free filling of the left ventricle (Pig. 1, a). During ventricular systole, the anterior leaflet moves away from the ventricular septum as it balloons freely toward the left atrium to coapt with the posterior leaflet. By moving away from the ventricular septum, the anterior leaflet leaves the outflow tract unobstructed (Fig. 1, b). By the same token, interference with movement of the mitral leaflet away from the ventricular septum causes the outflow tract to be obstructed. Such limitation of movement, which causes subaortic stenosis, may at the same time prevent the anterior leaflet from making full contact with the posterior leaflet and cause mitral insufficiency. The dynamics of the anterior mitral leaflet are minimally altered in the classical types of subaortic stenosis. When a membranous ring in the outflow
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Fig. 3.—Membranous subaortic stenosis. View of outflow tract of left ventricle (L.V.) and aorta (AJ. The subaortic membrane (arrows) in this instance is attached to the anterior leaflet of the mitral valve (M) near its lower extremity. Attachment of the membrane in this position tends to immobilize the anterior mitral leaflet and favors mitral incompetence in association with subaortic stenosis.
Fig. 4.—Diagrammatic portrayal of the left side of the heart during diastole and systole in muscular subaortic stenosis. In its purest form, as illustrated here, muscular subaortic stenosis does not affect the function of the mitral valve, a, Ventricular diastole, b, Ventricular systole.
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tract of the left ventricle is the cause of obstruction, the base of the anterior mitral leaflet is immobilized, but the remainder of the leaflet functions normally (Figs. 2 and 3). With the muscular type of subaortic stenosis, during ven tricular systole, the anterior mitral leaflet classically moves toward the posterior leaflet (Fig. 4). That the function of the mitral valve is not always normal in cases of muscular subaortic stenosis, however, is evidenced by the frequent coexistence of this lesion and mitral insufficiency. The observations of this study will be presented according to the type of malformation of the atrioventricular valves encountered as a cause of subaortic stenosis.
Fig. 5.—Diagrams of subaortic stenosis caused by accessory tissue of atrioventricular valves. a, Accessory tissue of the anterior leaflet of the mitral valve acting as a series of masses causing subaortic stenosis. b, An example of a ventricular septal defect with accessory tissue of the tricuspid valve. The latter tissue potentially may herniate through the ventricular septal defect to cause obstruction in the outflow tract of the left ventricle.
ACCESSORY ATRIOVENTRICULAR VALVULAR TISSUE
In 3 cases there was accessory tissue attached to one or the other atrio ventricular valve (mitral, 1; tricuspid, 2) (Fig. 5). The case of accessory tissue of the mitral valve (specimen submitted by Dr. C. P. Deal) involved an 11-year-old girl who was known to have subaortic stenosis. A pressure gradient of 70 mm. Hg between the left ventricle and the aorta had been demonstrated. In this patient, tissue of a membranous type was
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excised from the area of subaortic stenosis. Following operation, periods of tachypnea and cyanosis were observed and a supraventricular tachycardia de veloped. On the third postoperative day the patient died. A description follows of the left side of the heart in which were located the significant findings of the necropsy. The left atrium was hypertrophied and exhibited endocardial thickening. In the lowermost aspect of the left atrium, and just above the mitral valve, a ring of fibrous tissue protruded 1 mm. into the chamber. The mitral valve exhibited five spherical, balloon-like masses of accessory tissue, varying in size from 0.5 to 2.0 cm. in diameter (Fig. 6). Three of these were attached to the ventricular aspect of the anterior leaflet while two were attached to the chordae tendineae of the posterior leaflet. The former
Fig. 6.—Subaortic stenosis caused by accessory tissue of the mitral valve. a, Left ventricular outflow tract (L.V.), aortic valve (A.) and the mitral valve viewed from below. Attached to the posterior (P.M.) and to the anterior (A.M.) leaflets of the mitral valve is an aggregate of flve masses of balloon-like accessory tissue. Those presenting in the subaortic area were responsible for subaortic stenosis as illustrated in b. b, Outflow tract of the left ventricle as viewed from the apical region of the left ventricle. Immediately beneath the aortic valve (A.) the outflow tract of the left ventricle is obstructed by the series of balloon-like masses which represent accessory tissue of the mitral valve.
group protruded into the outflow tract of the left ventricle and, when expanded, caused subaortic stenosis. When empty, the balloon-like masses lay against the mitral valvular tissue from which they arose. At operation these collections were evidently collapsed and not identified. Three of the five masses of accessory valve tissue communicated with the left atrium through openings in the mitral valve, while the remaining two communicated only with the left ventricle. The left ventricular outflow tract also showed a circumferential fibrous ridge which
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was the remnant of the subaortic membrane, part of which had been removed surgically. The aortic valve was bicuspid. Both coronary arteries arose above the anterior cusp. In each of the 2 patients with accessory tissue of the tricuspid valve, there was a ventricular septal defect. In neither case was subaortic stenosis suspected clinically, yet the appearance of the specimens suggested that the accessory tissue might, under certain circumstances, have protruded through the ven tricular septal defect to obstruct the subaortic area (Fig. 7).
Fig. 7.—Subaortic stenosis caused by accessory tissue of tricuspid valve herniating through ventricular septal defect Diagram of this condition is illustrated in Fig. 5, 6. a, Interior of left ventricle (L.V.) and of aortic valve (A). Beneath the aortic valve is a pouch-like structure (T.j which protrudes into the outflow tract through a ventricular septal defect. The pouch-like structure represents accessory tissue of the tricuspid valve. b, Detailed view of aortic valve and subaortic area. Immediately beneath the aortic valve is a ventricular septal defect and herniating through the latter is a pouch-like structure (T.) which represents accessory tissue of the tricuspid valve. Theoretically, this accessory tissue could be responsible for subaortic stenosis. CLEFT MITRAL, VALVE AND ANOMALOUS CHORDAE
In considering the cleft mitral valve in relation to subaortic stenosis, it is well to recall that, normally, the chordae from the center of the anterior mitral leaflet insert into the papillary muscles. No chordae extend to the ventricular septum. Normally, therefore, during systole, the anterior mitral leaflet moves freely away from the vicinity of the ventricular septum. As stated, the anterior mitral leaflet forms part of the wall of the subaortic tract and normally does not restrict its caliber. In 3 cases of this study, an isolated cleft of the central part of the anterior leaflet of the mitral valve was present. The first of the 3 cases was a 9-year-old boy with a history of asthma-like attacks and upper respiratory infections be-
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ginning at 1 year of age. He first developed congestive heart failure at 3 years of age. His symptoms progressed and he died with cardiac failure. Necropsy revealed an isolated cleft mitral valve.* The second case was a 9-year-old men tally retarded boy. He had been known to have congenital heart disease since birth. His growth had been normal. At 8 years of age, and again 3 weeks before death, he developed febrile illnesses. Necropsy revealed an isolated cleft an terior mitral leaflet with fine granular vegetations. The third case will be dis cussed in more detail, subsequently.
Fig. 8.—Accessory chordae of the anterior leaflet of the mitral valve adhering to ventric ular septum and causing subaortic stenosis, as observed in cases of isolated cleft of the mitral valve. a. During ventricular diastole the anomalous chordae between the ventricular septum and the anterior mitral leaflet do not interfere with cardiac function. b, During ventricular systole, however, the anterior leaflet of the mitral valve is held in close apposition to the ventricular septum by the chordae and prevents development of a normal caliber to the outflow tract.
The cleft of the anterior mitral leaflet, in each case, had the characteristics of that in the fully developed A-V commune malformation, but in these par ticular cases the ventricular and atrial septa were intact. The cleft was inter preted as representing a minor form of A-V commune malformation. In each of the 3 cases, accessory chordae ran from the edges of the cleft in the mitral valve to that part of the ventricular septum which was the wall of the left ventricular outflow tract. It appeared that during ventricular systole the accessory chordae would hold the anterior mitral leaflet closer to the ventricular septum than would be the case in the normal, where no such chordae are present. Our interpretation •Case previously cited by Edwards and associates. 3
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was that fixation of the mitral leaflet in this position prevented the normal systolic excursion of the leaflet away from the septum and was the basis for subaortic obstruction (Fig. 8). In the third of the 3 cases with this type of abnormality, an element of muscular subaortic stenosis was also present. In this case a diagnosis of muscular subaortic stenosis had been made clinically and excision of the muscular sub aortic stenosis was done by a combined transaortic and transventricular ap proach. Death occurred 18 hours after operation. Pathologic examination then revealed the isolated cleft of the anterior mitral leaflet with anomalous chordae tendineae of the type described (Fig. 9).
Fig-. 9.—Isolated cleft of anterior leaflet of the mitral valve associated with accessory chordae inserting into ventricular septum. In this case the anomalous insertion of chordae contributed to subaortic stenosis as muscular subaortic stenosis was also present. a, Left atrium (L.A.), left ventricle (L.VJ, and mitral valve. At the center of the anterior leaflet of the mitral valve is a cleft (between arrows). b. Interior of left ventricle (L.V.), aortic valve (A.) and the mitral valve from below. From the edges of the cleft (arrows) in the anterior leaflet of the mitral valve, accessory chordae proceed to the ventricular septum. At the region of the chordal insertion, there is a raw area of the ventricular septum representing the site from which muscle of muscular subaortic stenosis had been resected surgically. Immobilization of the anterior mitral leaflet in the vicinity of the ventricular septum by the accessory chordae contributed to subaortic stenosis.
A similar cleft of the mitral valve was recently encountered at operation in a 27-year-old woman with mitral régurgitation but without subaortic stenosis. The cleft was repaired with fine silk buttressed with Teflon and Ivalon pledgets. Recovery was uneventful with disappearance of the pansystolic murmur. A careful inspection did not disclose any abnormal chordae and preoperative leftsided cardiac catheterization had established normal left ventricular pressures. CLASSIC A-V C O M M U N E MALFORMATION
Subaortic stenosis has been suspected in three instances in a series of 75 patients undergoing repair of various forms of A-V commune at the University
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of Minnesota Medical Center. Among the 25 pathologic specimens with the A-V commune malformation (exclusive of the 3 cases of cleft mitral valve and anomalous chordae) which were at our disposal, there are 4 in which subaortic stenosis was clearly present. In keeping with the observations of Wakai and Edwards, 4 · 5 each of the 25 specimens studied showed a broad, crescent-shaped deficiency in the pos terior aspect of the septal wall of the outflow tract of the left ventricle (Pig. 10). In the usual instance of the partial and transitional forms of the malforma tion, and in some examples of the complete form, the anterior mitral leaflet is adherent to the posterior edge of the deficient ventricular septum. This union prevents interventricular communication, but causes the left ventricular outflow tract to be narrow.
Pig. 10.—Diagrammatic portrayal of the left side of the heart during diastole (a) and systole ihj in persistent common atrioventricular canal. In this complex malformation there is a deficiency of the ventricular septum involving the wall of the outflow tract of the left ventricle. In some instances the mitral valve attaches intimately to the posterior edge of the deficient ventricular septum. In other instances, as illustrated in Fig. 11, the anterior leaflet of the mitral valve is attached indirectly by chordae to this portion of the ventricular septum. In the situation portrayed here, the direct attachment of the anterior leaflet of the mitral valve to the posterior edge of the deficient ventricular septum creates a complex situation in which subaortic stenosis is an integral part of the malformation. Because of the direct fusion of the anterior leaflet of the mitral valve to the deficient ventricular septum, there is little change in position of this part of the mitral valve during the cardiac cycle causing subaortic stenosis. The posterior leaflet of the mitral valve (P.M.) moves in a normal fashion.
In some cases of the complete tjTpe of the A-V commune malformation, the anterior leaflet of the common atrioventricular valve is not directly adherent to the posterior edge of the deficient ventricular septum but is joined to it by chordae. In such cases the anterior leaflet of the common A-V valve readily may move awav from the left ventricular outflow tract and no subaortic stenosis is
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present (Fig. 11). In such cases, however, when repair of the malformation includes suturing the central part of the anterior leaflet of the common atrioventricular valve to the ventricular septum, subaortic stenosis results. In such cases the surgeon accomplishes the subaortic stenosis, as nature does in those cases in which the anterior mitral leaflet is naturally adherent to the posterior aspect of the deficient ventricular septum. The following case is representative of the pathologic anatomy seen in the 4 cases of A-V commune malformation associated with subaortic stenosis fol lowing surgical repair.
Fig. 11.—Persistent common atrioventricular canal in which the anterior leaflet of the mitral valve is attached indirectly by chordae to the deficient posterior edge of the ventricular septum. a. During ventricular diastole the anterior leaflet of the mitral valve is pressed against the deficiency of the ventricular septum and allows normal opening of the mitral valve. b, During ventricular systole the anterior leaflet of the mitral valve balloons away from the ventricular septum and thereby prevents subaortic stenosis from being present. Direct suture of the anterior leaflet of the mitral valve to the deficient septum as part of a reparative process would create subaortic stenosis.
A 4-year-old girl presented with the classic picture of the A-V commune malformation. At operation, carried out with the aid of the pump oxygenator, a transitional type of the malformation was found. This was repaired by (1) suturing the clefts in the mitral and tricuspid valves, and (2) by inserting a prosthetic patch to close the atrial septal defect. The circulation was not satis factory after the surgical repair. The patient developed pulmonary edema and died in the operating room. Necropsy showed pulmonary edema and subaortic stenosis which had been accentuated by the repair. A description follows of the left side of the heart. The left atrium was dilated. There was a jet lesion on the anterior portion of the atrial septum. The anterior leaflet of the mitral valve was cleft and had
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been repaired by inserting a piece of Teflon cloth, 1 by 1.5 cm., between the two portions of the leaflet (Fig. 12, a ) . There were no anomalous chordae tendineae extending to the ventricular septum. The left ventricle was dilated and the outflow tract was narrow (Pig. 12, 6). This narrowing resulted from two factors: (1) the septal wall of the outflow tract was narrow, as an expression of the natural deficiency of ven tricular septal tissue seen in the A-V commune malformation, and (2) sutures placed in the ventricular septum through the anterior leaflet of the common A-V valve at a right angle to the axis of the left ventricular outflow tract caused tucks in that part of the left ventricular outflow tract which lay at the junc tion of the ventricular septum and the anterior mitral leaflet. These tucks ac centuated the intrinsically narrow state of the subaortic area.
Fig. 12.—Persistent common atrioventricular canal with relationship between ventricular septum and mitral valve as portrayed in Fig. 10. a, Left atrium (L.A.) and left ventricle (L.V.). In the lowermost portion of the atrial septum the defect of this malformation has been closed by the placement of a prosthetic patch. The cleft in the anterior leaflet of the mitral valve has been corrected by placement of a second prosthetic patch. ft. Outflow tract of the left ventricle (L.V.) and aortic valve (A.). The anterior leaflet of the mitral valve is attached (between arrows) to the posterior edge of the deficient ventricular septum. This basic arrangement is responsible for crowding of the outflow part of the left ventricle. COMMENT
Subaortic stenosis, in the majority of cases, results from either of the two classical types of lesions: (1) focal circumferential fibrous deposition, or (2) localized muscular overgrowth of the ventricular septum. In occasional cases, however, the obstructive mechanism will be related partially, or entirely, to abnormalities of the atrioventricular valves.
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Clinically, hints as to the presence of one of these A-V valvular malforma tions as the fundamental cause of subaortic obstruction may, at times, be en countered. With accessory valvular tissue, variations in the degree of subaortic obstruction hypothetically might occur from time to time and consequently be responsible for changing symptoms and physical signs. In patients with an isolated cleft anterior mitral valve leaflet, there is a significant cleft, so that signs of mitral insufficiency accompany those of subaortic stenosis. The possi bility of subaortic stenosis being present before operation or being induced by operation should be considered in every patient with an A-V commune defect. Left-sided cardiac catheterization and left ventriculography might be of considerable help in delineating these lesions and we now perform such studies before operation in each case of aortic or subaortic stenosis, and in all patients with an A-V commune malformation. Left ventriculography was not carried out in either our case with accessory tissue involving the mitral valve or in the case reported by MacLean and as sociates.6 We would expect, however, that left ventriculograms in this condition would reveal irregularity of the contour of the subaortic area. In addition, tracings on withdrawal of the catheter from the left ventricle would show the location of the obstruction to be subaortic. In the type of subaortic stenosis that is associated with a cleft mitral valve, left ventriculography should reveal the presence of mitral insufficiency. Subaortic stenosis caused by anomalies of the atrioventricular valves may coexist with one of the classical types of subaortic stenosis, as in 2 of the cases of our study. The relief of the obvious membranous or muscular subaortic stenosis without relieving the less apparent subaortic obstruction, resulting from the abnormality of the mitral valve, led to failure in both of these cases. Coexistent anomalies of the mitral valve and muscular subaortic stenosis may be more than a simple coincidence. Moberg, Fix, and Söderberg 7 considered the basic problem in muscular subaortic stenosis to lie in the mitral valve. They suggest that an unusually long anterior leaflet of the mitral valve protrudes into the left ventricular outflow tract to cause obstruction and that the muscular changes are secondary. In treating this condition, they suggest plastic repair of the anterior mitral valve leaflet without any direct attack on the hypertrophic muscle. Björk 2 also has emphasized the role of mitral valvular abnor malities in the etiology of muscular subaortic stenosis. He describes 2 cases in which the junction of the mitral valve and the aortic valve extended anteriorly as far as the right cusp of the aortic valve. I t was his opinion that this held the anterior leaflet of the mitral valve across the left ventricular outflow tract to produce subaortic stenosis. The muscular hypertrophy of the septum, he believed, was secondary. Accessory tissue of the mitral valve causing subaortic stenosis has an exact counterpart in subpulmonary stenosis caused by accessory tissue of the right atrioventricular valve in corrected transposition. The latter condition was re ported from our institutions by Levy and associates.8 The lesions which have been discussed in this report have been primarily involved as causes of subaortic stenosis and these lesions have either caused no
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dysfunction of the mitral valve or were associated with mitral insufficiency. There is yet another syndrome in which subaortic stenosis is associated with obstruction at the mitral valve. In the latter condition, reported by Shone and associates,9 the subaortic stenosis was of the classical muscular or membranous types. The obstruction at the mitral valve resulted from a supravalvular ring of the left atrium, a parachute deformity of the mitral valve, or a combination of both lesions. In this group of 8 cases there also was a tendency for the co existence of aortic coarctation. Thus, the occurrence of subaortic stenosis or congenital mitral stenosis should alert one to the possible presence of other lesions of the left side of the heart. Recently, one of us (C. ΛΥ. L.) and Levy10 described a trans-left atrial approach for surgical correction of subaortic stenosis. In this procedure the anterior leaflet of the mitral valve is detached from the annulus, allowing direct visualization of the subaortic area. By this approach the classical types of subaortic stenosis are easily dealt with. In addition, abnormalities of the mitral valve or of its chordae are directly visualized and can be corrected at the same time. Following the correction of the subaortic stenosis, the anterior mitral leaflet is re-attached to the annulus. SUMMARY
In addition to the classical muscular and membranous types of subaortic stenosis, there are instances of subaortic obstruction caused by abnormalities of the atrioventricular valves. These abnormalities take three forms. The first is obstruction of the left ventricular outflow tract by accessory tissue arising from the A-V valves. The second results from restriction of the movement of the anterior mitral valve leaflet by accessory chordae tendineae or by abnormal fusion of valvular tissue to the septal Avail of the outflow tract. The third form of subaortic stenosis occurs in the A-V commune malformation and is primarily related to deficiency of the ventricular septum. The importance of careful preoperative hemodynamic and angiographie studies is emphasized because certain of these abnormalities are not readily recognizable in the relaxed, nonfunctional state in which they are viewed during operation. REFERENCES 1. Ferencz, C. : Atrio-ventricular Defect of Membranous Septum. Left Ventricular-Bight Atrial Communication With Malformed Mitral Valve Simulating Aortic Stenosis : Report of a Case, Bull. Johns Hopkins Hosp. 100: 209, 1957. 2. Bjork, V. O., Hultquist, G., and Lodin, H. : Subaortic Stenosis Produced by an Abnor mally Placed Anterior Mitral Leaflet, J . THORACIC & CARDIOVAS. STJRG. 4 1 : 659, 1961.
3. Edwards, J . E., Dry, T. J., Parker, B, L., Burchell, H. B., Wood, E . H., and Bulbulian, A. H. : An Atlas of Congenital Anomalies of the Heart and Great Vessels, Spring field, 111., 1954, Charles C Thomas, Publisher, pp. 41-42. 4. Wakai, C. S., and Edwards, J . E . : Developmental and Pathologic Considerations in Per sistent Common Atrioventricular Canal, Proc. Mayo Clin. 3 1 : 487, 1956. 5. Wakai, C. S.. and Edwards, J . E . : Pathologic Study of Persistent Common Atrioven tricular Canal, Am. Heart J . 56: 779, 1958.
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6. MacLean, L. D., Culligan, J . A., and Kane, D. J . : Subaortic Stenosis Due to Accessory Tissue on the Mitral Valve, J . THORACIC & CARDIOVAS. SURG. 45: 382, 1963.
7. Moberg, A., Fix, P., and Söderberg, H . : On the Pathogenesis of Idiopathic Hypertrophie Subaortic Stenosis, J . CARDIOVAS. SURG. 4 : 602, 1963.
8. Levy, M. J., Lillehei, C. W., Elliott, L. P . , Carey, L. S., Adams, P . , J r . , and Edwards, J . E. : Accessory Valvular Tissue Causing Subpulmonary Stenosis in Corrected Trans position of Great Vessels, Circulation 27: 494, 1963. 9. Shone, J . D., Sellers, K. D., Anderson, R. C , Adams, P., J r . , Lillehei, C. W., and Edwards, J . E . : The Developmental Complex of "Parachute Mitral Valve," Supravalvular Ring of Left Atrium, Subaortic Stenosis, and Coarctation of Aorta, Am. J . Cardiol. 1 1 : 714, 1963. 10. Lillehei, C. W., and Levy, M. J . : Transatrial Exposure for Correction of Subaortic Stenosis, J . A. M. A. 186: 8, 1963.
C. Walton Lïllehei, M.D., and
Jesse E. Edwards, M.D., Minneapolis
and St. Paul, Minn.
T
HERE are instances of subaortic stenosis in which the fundamental problem lies, not in primary disease of the left ventricular outflow tract but, rather, in anomalies of the atrioventricular valves. Emphasis on such entities is justified by the fact that, in these, surgical therapy for relief of subaortic obstruction must be directed toward correction of the primary valvular anomaly. Ferencz, 1 in 1957, and Björk, 2 in 1961, described cases in which malforma tions of the atrioventricular valves caused subaortic obstruction. The mechanisms by which abnormalities of the atrioventricular valves cause or contribute to subaortic stenosis are (1) obstruction by accessory tissue of the atrioventricular valves, (2) restriction of the movement of the anterior leaflet of the mitral valve either by accessory chordae running between the edges of a cleft in the valve and the ventricular septum or by fusion of valvular tissue to the septal wall of the ventricular outflow tract, and (3) attachment of the mitral valve to the ventricular septum when the latter is narrowed by a deficit in tissue in its basal portion. A study was made of the specimens in the combined pathologic collection of the Cardiovascular Registry of the Charles T. Miller Hospital and the Uni versity of Minnesota in a search for examples of subaortic stenosis resulting from abnormalities of the atrioventricular valves. Excluding cases of classical persistent common atrioventricular canal, there were 6 cases in which anomalies of the atrioventricular valves had caused, or appeared potentially capable of causing, subaortic stenosis. This report has two purposes. One is to describe anatomic details of the aforementioned 6 cases. The seeond is to emphasize the fact that in the complex malformation known variously as persistent common atrioventricular canal, A-V commune or endocardial cushion defect (herein called A-V commune malforma tion), an element of subaortic stenosis exists or may be caused by certain steps in correction of the malformation. Prom the Departments of Surgery and Pathology, University of Minnesota. Minneapolis. Minn., and the Department of Pathology, The Charles T. Miller Hospital, St. Paul, Minn. This study was supported by Research Grants HE-5694 and HB-830 and Research Training Grant 5-T1-HB 5570 of the National Heart Institute, U. S. Public Health Service, and by the Life Insurance Medical Research Fund. Received for publication Jan. 23, 1964. »United States Public Health Service Postdoctoral Research Fellow. 289
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Fig-. 1. Fig. 2. Fig. 1.—Diagram of left side of heart shows changes in position of mitral valve leaflets during ventricular diastole and systole. a, Ventricular (L.VJ diastole. The anterior mitral leaflet (A.) lies near the ventricular septum as the mitral valve is open. b, During ventricular systole the anterior leaflet of the mitral valve moves away from the ventricular septum to coapt with the posterior mitral leaflet (P.) and close the mitral valve. In this way it does not obstruct the left ventricular outflow tract. Fig. 2.—Diagram of left side of heart in the membranous type of subaortic stenosis. In this instance the membrane attaches to the anterior leaflet of the mitral valve at the junction of its lower two thirds and upper one third. In this position the membrane interferes with function of the mitral valve. More commonly, the membrane attaches to the anterior leaflet of the mitral valve at a more basal level and therefore interferes less with the function of the mitral valve than in the state illustrated here.
Before presenting our observations, certain anatomic and functional features of the mitral valve need to be recognized. The anterior leaflet of the mitral valve is an integral portion of the left ventricular outflow tract. This leaflet forms the posterolateral wall of the outflow tract and, as the leaflet moves with changing phases of the cardiac cycle, this aspect of the outflow tract changes. During ventricular diastole, the anterior leaflet bulges into the outflow tract, allowing free filling of the left ventricle (Pig. 1, a). During ventricular systole, the anterior leaflet moves away from the ventricular septum as it balloons freely toward the left atrium to coapt with the posterior leaflet. By moving away from the ventricular septum, the anterior leaflet leaves the outflow tract unobstructed (Fig. 1, b). By the same token, interference with movement of the mitral leaflet away from the ventricular septum causes the outflow tract to be obstructed. Such limitation of movement, which causes subaortic stenosis, may at the same time prevent the anterior leaflet from making full contact with the posterior leaflet and cause mitral insufficiency. The dynamics of the anterior mitral leaflet are minimally altered in the classical types of subaortic stenosis. When a membranous ring in the outflow
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Fig. 3.—Membranous subaortic stenosis. View of outflow tract of left ventricle (L.V.) and aorta (AJ. The subaortic membrane (arrows) in this instance is attached to the anterior leaflet of the mitral valve (M) near its lower extremity. Attachment of the membrane in this position tends to immobilize the anterior mitral leaflet and favors mitral incompetence in association with subaortic stenosis.
Fig. 4.—Diagrammatic portrayal of the left side of the heart during diastole and systole in muscular subaortic stenosis. In its purest form, as illustrated here, muscular subaortic stenosis does not affect the function of the mitral valve, a, Ventricular diastole, b, Ventricular systole.
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tract of the left ventricle is the cause of obstruction, the base of the anterior mitral leaflet is immobilized, but the remainder of the leaflet functions normally (Figs. 2 and 3). With the muscular type of subaortic stenosis, during ven tricular systole, the anterior mitral leaflet classically moves toward the posterior leaflet (Fig. 4). That the function of the mitral valve is not always normal in cases of muscular subaortic stenosis, however, is evidenced by the frequent coexistence of this lesion and mitral insufficiency. The observations of this study will be presented according to the type of malformation of the atrioventricular valves encountered as a cause of subaortic stenosis.
Fig. 5.—Diagrams of subaortic stenosis caused by accessory tissue of atrioventricular valves. a, Accessory tissue of the anterior leaflet of the mitral valve acting as a series of masses causing subaortic stenosis. b, An example of a ventricular septal defect with accessory tissue of the tricuspid valve. The latter tissue potentially may herniate through the ventricular septal defect to cause obstruction in the outflow tract of the left ventricle.
ACCESSORY ATRIOVENTRICULAR VALVULAR TISSUE
In 3 cases there was accessory tissue attached to one or the other atrio ventricular valve (mitral, 1; tricuspid, 2) (Fig. 5). The case of accessory tissue of the mitral valve (specimen submitted by Dr. C. P. Deal) involved an 11-year-old girl who was known to have subaortic stenosis. A pressure gradient of 70 mm. Hg between the left ventricle and the aorta had been demonstrated. In this patient, tissue of a membranous type was
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excised from the area of subaortic stenosis. Following operation, periods of tachypnea and cyanosis were observed and a supraventricular tachycardia de veloped. On the third postoperative day the patient died. A description follows of the left side of the heart in which were located the significant findings of the necropsy. The left atrium was hypertrophied and exhibited endocardial thickening. In the lowermost aspect of the left atrium, and just above the mitral valve, a ring of fibrous tissue protruded 1 mm. into the chamber. The mitral valve exhibited five spherical, balloon-like masses of accessory tissue, varying in size from 0.5 to 2.0 cm. in diameter (Fig. 6). Three of these were attached to the ventricular aspect of the anterior leaflet while two were attached to the chordae tendineae of the posterior leaflet. The former
Fig. 6.—Subaortic stenosis caused by accessory tissue of the mitral valve. a, Left ventricular outflow tract (L.V.), aortic valve (A.) and the mitral valve viewed from below. Attached to the posterior (P.M.) and to the anterior (A.M.) leaflets of the mitral valve is an aggregate of flve masses of balloon-like accessory tissue. Those presenting in the subaortic area were responsible for subaortic stenosis as illustrated in b. b, Outflow tract of the left ventricle as viewed from the apical region of the left ventricle. Immediately beneath the aortic valve (A.) the outflow tract of the left ventricle is obstructed by the series of balloon-like masses which represent accessory tissue of the mitral valve.
group protruded into the outflow tract of the left ventricle and, when expanded, caused subaortic stenosis. When empty, the balloon-like masses lay against the mitral valvular tissue from which they arose. At operation these collections were evidently collapsed and not identified. Three of the five masses of accessory valve tissue communicated with the left atrium through openings in the mitral valve, while the remaining two communicated only with the left ventricle. The left ventricular outflow tract also showed a circumferential fibrous ridge which
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was the remnant of the subaortic membrane, part of which had been removed surgically. The aortic valve was bicuspid. Both coronary arteries arose above the anterior cusp. In each of the 2 patients with accessory tissue of the tricuspid valve, there was a ventricular septal defect. In neither case was subaortic stenosis suspected clinically, yet the appearance of the specimens suggested that the accessory tissue might, under certain circumstances, have protruded through the ven tricular septal defect to obstruct the subaortic area (Fig. 7).
Fig. 7.—Subaortic stenosis caused by accessory tissue of tricuspid valve herniating through ventricular septal defect Diagram of this condition is illustrated in Fig. 5, 6. a, Interior of left ventricle (L.V.) and of aortic valve (A). Beneath the aortic valve is a pouch-like structure (T.j which protrudes into the outflow tract through a ventricular septal defect. The pouch-like structure represents accessory tissue of the tricuspid valve. b, Detailed view of aortic valve and subaortic area. Immediately beneath the aortic valve is a ventricular septal defect and herniating through the latter is a pouch-like structure (T.) which represents accessory tissue of the tricuspid valve. Theoretically, this accessory tissue could be responsible for subaortic stenosis. CLEFT MITRAL, VALVE AND ANOMALOUS CHORDAE
In considering the cleft mitral valve in relation to subaortic stenosis, it is well to recall that, normally, the chordae from the center of the anterior mitral leaflet insert into the papillary muscles. No chordae extend to the ventricular septum. Normally, therefore, during systole, the anterior mitral leaflet moves freely away from the vicinity of the ventricular septum. As stated, the anterior mitral leaflet forms part of the wall of the subaortic tract and normally does not restrict its caliber. In 3 cases of this study, an isolated cleft of the central part of the anterior leaflet of the mitral valve was present. The first of the 3 cases was a 9-year-old boy with a history of asthma-like attacks and upper respiratory infections be-
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ginning at 1 year of age. He first developed congestive heart failure at 3 years of age. His symptoms progressed and he died with cardiac failure. Necropsy revealed an isolated cleft mitral valve.* The second case was a 9-year-old men tally retarded boy. He had been known to have congenital heart disease since birth. His growth had been normal. At 8 years of age, and again 3 weeks before death, he developed febrile illnesses. Necropsy revealed an isolated cleft an terior mitral leaflet with fine granular vegetations. The third case will be dis cussed in more detail, subsequently.
Fig. 8.—Accessory chordae of the anterior leaflet of the mitral valve adhering to ventric ular septum and causing subaortic stenosis, as observed in cases of isolated cleft of the mitral valve. a. During ventricular diastole the anomalous chordae between the ventricular septum and the anterior mitral leaflet do not interfere with cardiac function. b, During ventricular systole, however, the anterior leaflet of the mitral valve is held in close apposition to the ventricular septum by the chordae and prevents development of a normal caliber to the outflow tract.
The cleft of the anterior mitral leaflet, in each case, had the characteristics of that in the fully developed A-V commune malformation, but in these par ticular cases the ventricular and atrial septa were intact. The cleft was inter preted as representing a minor form of A-V commune malformation. In each of the 3 cases, accessory chordae ran from the edges of the cleft in the mitral valve to that part of the ventricular septum which was the wall of the left ventricular outflow tract. It appeared that during ventricular systole the accessory chordae would hold the anterior mitral leaflet closer to the ventricular septum than would be the case in the normal, where no such chordae are present. Our interpretation •Case previously cited by Edwards and associates. 3
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was that fixation of the mitral leaflet in this position prevented the normal systolic excursion of the leaflet away from the septum and was the basis for subaortic obstruction (Fig. 8). In the third of the 3 cases with this type of abnormality, an element of muscular subaortic stenosis was also present. In this case a diagnosis of muscular subaortic stenosis had been made clinically and excision of the muscular sub aortic stenosis was done by a combined transaortic and transventricular ap proach. Death occurred 18 hours after operation. Pathologic examination then revealed the isolated cleft of the anterior mitral leaflet with anomalous chordae tendineae of the type described (Fig. 9).
Fig-. 9.—Isolated cleft of anterior leaflet of the mitral valve associated with accessory chordae inserting into ventricular septum. In this case the anomalous insertion of chordae contributed to subaortic stenosis as muscular subaortic stenosis was also present. a, Left atrium (L.A.), left ventricle (L.VJ, and mitral valve. At the center of the anterior leaflet of the mitral valve is a cleft (between arrows). b. Interior of left ventricle (L.V.), aortic valve (A.) and the mitral valve from below. From the edges of the cleft (arrows) in the anterior leaflet of the mitral valve, accessory chordae proceed to the ventricular septum. At the region of the chordal insertion, there is a raw area of the ventricular septum representing the site from which muscle of muscular subaortic stenosis had been resected surgically. Immobilization of the anterior mitral leaflet in the vicinity of the ventricular septum by the accessory chordae contributed to subaortic stenosis.
A similar cleft of the mitral valve was recently encountered at operation in a 27-year-old woman with mitral régurgitation but without subaortic stenosis. The cleft was repaired with fine silk buttressed with Teflon and Ivalon pledgets. Recovery was uneventful with disappearance of the pansystolic murmur. A careful inspection did not disclose any abnormal chordae and preoperative leftsided cardiac catheterization had established normal left ventricular pressures. CLASSIC A-V C O M M U N E MALFORMATION
Subaortic stenosis has been suspected in three instances in a series of 75 patients undergoing repair of various forms of A-V commune at the University
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of Minnesota Medical Center. Among the 25 pathologic specimens with the A-V commune malformation (exclusive of the 3 cases of cleft mitral valve and anomalous chordae) which were at our disposal, there are 4 in which subaortic stenosis was clearly present. In keeping with the observations of Wakai and Edwards, 4 · 5 each of the 25 specimens studied showed a broad, crescent-shaped deficiency in the pos terior aspect of the septal wall of the outflow tract of the left ventricle (Pig. 10). In the usual instance of the partial and transitional forms of the malforma tion, and in some examples of the complete form, the anterior mitral leaflet is adherent to the posterior edge of the deficient ventricular septum. This union prevents interventricular communication, but causes the left ventricular outflow tract to be narrow.
Pig. 10.—Diagrammatic portrayal of the left side of the heart during diastole (a) and systole ihj in persistent common atrioventricular canal. In this complex malformation there is a deficiency of the ventricular septum involving the wall of the outflow tract of the left ventricle. In some instances the mitral valve attaches intimately to the posterior edge of the deficient ventricular septum. In other instances, as illustrated in Fig. 11, the anterior leaflet of the mitral valve is attached indirectly by chordae to this portion of the ventricular septum. In the situation portrayed here, the direct attachment of the anterior leaflet of the mitral valve to the posterior edge of the deficient ventricular septum creates a complex situation in which subaortic stenosis is an integral part of the malformation. Because of the direct fusion of the anterior leaflet of the mitral valve to the deficient ventricular septum, there is little change in position of this part of the mitral valve during the cardiac cycle causing subaortic stenosis. The posterior leaflet of the mitral valve (P.M.) moves in a normal fashion.
In some cases of the complete tjTpe of the A-V commune malformation, the anterior leaflet of the common atrioventricular valve is not directly adherent to the posterior edge of the deficient ventricular septum but is joined to it by chordae. In such cases the anterior leaflet of the common A-V valve readily may move awav from the left ventricular outflow tract and no subaortic stenosis is
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present (Fig. 11). In such cases, however, when repair of the malformation includes suturing the central part of the anterior leaflet of the common atrioventricular valve to the ventricular septum, subaortic stenosis results. In such cases the surgeon accomplishes the subaortic stenosis, as nature does in those cases in which the anterior mitral leaflet is naturally adherent to the posterior aspect of the deficient ventricular septum. The following case is representative of the pathologic anatomy seen in the 4 cases of A-V commune malformation associated with subaortic stenosis fol lowing surgical repair.
Fig. 11.—Persistent common atrioventricular canal in which the anterior leaflet of the mitral valve is attached indirectly by chordae to the deficient posterior edge of the ventricular septum. a. During ventricular diastole the anterior leaflet of the mitral valve is pressed against the deficiency of the ventricular septum and allows normal opening of the mitral valve. b, During ventricular systole the anterior leaflet of the mitral valve balloons away from the ventricular septum and thereby prevents subaortic stenosis from being present. Direct suture of the anterior leaflet of the mitral valve to the deficient septum as part of a reparative process would create subaortic stenosis.
A 4-year-old girl presented with the classic picture of the A-V commune malformation. At operation, carried out with the aid of the pump oxygenator, a transitional type of the malformation was found. This was repaired by (1) suturing the clefts in the mitral and tricuspid valves, and (2) by inserting a prosthetic patch to close the atrial septal defect. The circulation was not satis factory after the surgical repair. The patient developed pulmonary edema and died in the operating room. Necropsy showed pulmonary edema and subaortic stenosis which had been accentuated by the repair. A description follows of the left side of the heart. The left atrium was dilated. There was a jet lesion on the anterior portion of the atrial septum. The anterior leaflet of the mitral valve was cleft and had
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been repaired by inserting a piece of Teflon cloth, 1 by 1.5 cm., between the two portions of the leaflet (Fig. 12, a ) . There were no anomalous chordae tendineae extending to the ventricular septum. The left ventricle was dilated and the outflow tract was narrow (Pig. 12, 6). This narrowing resulted from two factors: (1) the septal wall of the outflow tract was narrow, as an expression of the natural deficiency of ven tricular septal tissue seen in the A-V commune malformation, and (2) sutures placed in the ventricular septum through the anterior leaflet of the common A-V valve at a right angle to the axis of the left ventricular outflow tract caused tucks in that part of the left ventricular outflow tract which lay at the junc tion of the ventricular septum and the anterior mitral leaflet. These tucks ac centuated the intrinsically narrow state of the subaortic area.
Fig. 12.—Persistent common atrioventricular canal with relationship between ventricular septum and mitral valve as portrayed in Fig. 10. a, Left atrium (L.A.) and left ventricle (L.V.). In the lowermost portion of the atrial septum the defect of this malformation has been closed by the placement of a prosthetic patch. The cleft in the anterior leaflet of the mitral valve has been corrected by placement of a second prosthetic patch. ft. Outflow tract of the left ventricle (L.V.) and aortic valve (A.). The anterior leaflet of the mitral valve is attached (between arrows) to the posterior edge of the deficient ventricular septum. This basic arrangement is responsible for crowding of the outflow part of the left ventricle. COMMENT
Subaortic stenosis, in the majority of cases, results from either of the two classical types of lesions: (1) focal circumferential fibrous deposition, or (2) localized muscular overgrowth of the ventricular septum. In occasional cases, however, the obstructive mechanism will be related partially, or entirely, to abnormalities of the atrioventricular valves.
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Clinically, hints as to the presence of one of these A-V valvular malforma tions as the fundamental cause of subaortic obstruction may, at times, be en countered. With accessory valvular tissue, variations in the degree of subaortic obstruction hypothetically might occur from time to time and consequently be responsible for changing symptoms and physical signs. In patients with an isolated cleft anterior mitral valve leaflet, there is a significant cleft, so that signs of mitral insufficiency accompany those of subaortic stenosis. The possi bility of subaortic stenosis being present before operation or being induced by operation should be considered in every patient with an A-V commune defect. Left-sided cardiac catheterization and left ventriculography might be of considerable help in delineating these lesions and we now perform such studies before operation in each case of aortic or subaortic stenosis, and in all patients with an A-V commune malformation. Left ventriculography was not carried out in either our case with accessory tissue involving the mitral valve or in the case reported by MacLean and as sociates.6 We would expect, however, that left ventriculograms in this condition would reveal irregularity of the contour of the subaortic area. In addition, tracings on withdrawal of the catheter from the left ventricle would show the location of the obstruction to be subaortic. In the type of subaortic stenosis that is associated with a cleft mitral valve, left ventriculography should reveal the presence of mitral insufficiency. Subaortic stenosis caused by anomalies of the atrioventricular valves may coexist with one of the classical types of subaortic stenosis, as in 2 of the cases of our study. The relief of the obvious membranous or muscular subaortic stenosis without relieving the less apparent subaortic obstruction, resulting from the abnormality of the mitral valve, led to failure in both of these cases. Coexistent anomalies of the mitral valve and muscular subaortic stenosis may be more than a simple coincidence. Moberg, Fix, and Söderberg 7 considered the basic problem in muscular subaortic stenosis to lie in the mitral valve. They suggest that an unusually long anterior leaflet of the mitral valve protrudes into the left ventricular outflow tract to cause obstruction and that the muscular changes are secondary. In treating this condition, they suggest plastic repair of the anterior mitral valve leaflet without any direct attack on the hypertrophic muscle. Björk 2 also has emphasized the role of mitral valvular abnor malities in the etiology of muscular subaortic stenosis. He describes 2 cases in which the junction of the mitral valve and the aortic valve extended anteriorly as far as the right cusp of the aortic valve. I t was his opinion that this held the anterior leaflet of the mitral valve across the left ventricular outflow tract to produce subaortic stenosis. The muscular hypertrophy of the septum, he believed, was secondary. Accessory tissue of the mitral valve causing subaortic stenosis has an exact counterpart in subpulmonary stenosis caused by accessory tissue of the right atrioventricular valve in corrected transposition. The latter condition was re ported from our institutions by Levy and associates.8 The lesions which have been discussed in this report have been primarily involved as causes of subaortic stenosis and these lesions have either caused no
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dysfunction of the mitral valve or were associated with mitral insufficiency. There is yet another syndrome in which subaortic stenosis is associated with obstruction at the mitral valve. In the latter condition, reported by Shone and associates,9 the subaortic stenosis was of the classical muscular or membranous types. The obstruction at the mitral valve resulted from a supravalvular ring of the left atrium, a parachute deformity of the mitral valve, or a combination of both lesions. In this group of 8 cases there also was a tendency for the co existence of aortic coarctation. Thus, the occurrence of subaortic stenosis or congenital mitral stenosis should alert one to the possible presence of other lesions of the left side of the heart. Recently, one of us (C. ΛΥ. L.) and Levy10 described a trans-left atrial approach for surgical correction of subaortic stenosis. In this procedure the anterior leaflet of the mitral valve is detached from the annulus, allowing direct visualization of the subaortic area. By this approach the classical types of subaortic stenosis are easily dealt with. In addition, abnormalities of the mitral valve or of its chordae are directly visualized and can be corrected at the same time. Following the correction of the subaortic stenosis, the anterior mitral leaflet is re-attached to the annulus. SUMMARY
In addition to the classical muscular and membranous types of subaortic stenosis, there are instances of subaortic obstruction caused by abnormalities of the atrioventricular valves. These abnormalities take three forms. The first is obstruction of the left ventricular outflow tract by accessory tissue arising from the A-V valves. The second results from restriction of the movement of the anterior mitral valve leaflet by accessory chordae tendineae or by abnormal fusion of valvular tissue to the septal Avail of the outflow tract. The third form of subaortic stenosis occurs in the A-V commune malformation and is primarily related to deficiency of the ventricular septum. The importance of careful preoperative hemodynamic and angiographie studies is emphasized because certain of these abnormalities are not readily recognizable in the relaxed, nonfunctional state in which they are viewed during operation. REFERENCES 1. Ferencz, C. : Atrio-ventricular Defect of Membranous Septum. Left Ventricular-Bight Atrial Communication With Malformed Mitral Valve Simulating Aortic Stenosis : Report of a Case, Bull. Johns Hopkins Hosp. 100: 209, 1957. 2. Bjork, V. O., Hultquist, G., and Lodin, H. : Subaortic Stenosis Produced by an Abnor mally Placed Anterior Mitral Leaflet, J . THORACIC & CARDIOVAS. STJRG. 4 1 : 659, 1961.
3. Edwards, J . E., Dry, T. J., Parker, B, L., Burchell, H. B., Wood, E . H., and Bulbulian, A. H. : An Atlas of Congenital Anomalies of the Heart and Great Vessels, Spring field, 111., 1954, Charles C Thomas, Publisher, pp. 41-42. 4. Wakai, C. S., and Edwards, J . E . : Developmental and Pathologic Considerations in Per sistent Common Atrioventricular Canal, Proc. Mayo Clin. 3 1 : 487, 1956. 5. Wakai, C. S.. and Edwards, J . E . : Pathologic Study of Persistent Common Atrioven tricular Canal, Am. Heart J . 56: 779, 1958.
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6. MacLean, L. D., Culligan, J . A., and Kane, D. J . : Subaortic Stenosis Due to Accessory Tissue on the Mitral Valve, J . THORACIC & CARDIOVAS. SURG. 45: 382, 1963.
7. Moberg, A., Fix, P., and Söderberg, H . : On the Pathogenesis of Idiopathic Hypertrophie Subaortic Stenosis, J . CARDIOVAS. SURG. 4 : 602, 1963.
8. Levy, M. J., Lillehei, C. W., Elliott, L. P . , Carey, L. S., Adams, P . , J r . , and Edwards, J . E. : Accessory Valvular Tissue Causing Subpulmonary Stenosis in Corrected Trans position of Great Vessels, Circulation 27: 494, 1963. 9. Shone, J . D., Sellers, K. D., Anderson, R. C , Adams, P., J r . , Lillehei, C. W., and Edwards, J . E . : The Developmental Complex of "Parachute Mitral Valve," Supravalvular Ring of Left Atrium, Subaortic Stenosis, and Coarctation of Aorta, Am. J . Cardiol. 1 1 : 714, 1963. 10. Lillehei, C. W., and Levy, M. J . : Transatrial Exposure for Correction of Subaortic Stenosis, J . A. M. A. 186: 8, 1963.