Anatomy of Atrial and Ventricular Septal Defects

Transcription

1 Anatomy of Atrial and Ventricular Septal Defects SIEW YEN HO, PH.D., F.R.C.PATH, KAREN P. MCCARTHY, B.Sc., and MICHAEL RIGBY, M.D. From the Paediatrics, National Heart & Lung Institute, Imperial College of Science Technology and Medicine; and Paediatrics, Royal Brompton and Harefield NHS Trust, London, United Kingdom The anatomy of atrial and ventricular septal defects is reviewed with reference to the normal cardiac septum. Owing to the limited extent of the atrial septum, true septal defects are confined to the area of the oval fossa. The sinus venosus and coronary sinus defects are interatrial communications being outside the conjhes of the atrial septum and close to important atrial structures. The description of ventricular septal defects as perimembranous, muscular and doubly committed, and juxtaarterial defects highlights the anatomy of the margins, the location, and proximity to the conduction system and valvar structures. Malalignment of septal structures can complicate the anatomy of ventricular septal defects producing inlet or outlet obstruction. (J Interven Cardiol2000; 13: ) Introduction Congenital deficiencies of the atrial and ventricular septums are among the most common cardiac lesions in infants. Ventricular septal defects occur in 24%-35% of live newborns while atrial septal defects occur in 4%-11%. These defects can occur in isolation, in combination, or in association with many other defects. A ventricular septal defect is an integral part of tetralogy of Fallot, double outlet ventricles, and most cases of common arterial trunk. It is also frequently encountered in association with complete and congenitally corrected transposition, pulmonary atresia, univentricular atrioventricular connections and coarctation, or interruption of the aortic arch. When occurring in isolation, diagnosis may be delayed, sometimes well into adult life. This article focuses on these septal defects as isolated lesions and some of the complicating anomalies are discussed. Atrial Septal Defects Although generally categorized as atrial septal defects, some of these defects are interatrial communica- Address for reprints: S.Y. Ho, Ph.D., Paediatrics, National Heart & Lung Institute, Imperial College, Dovehouse St., London SW3 6LY. Fax: ; yen.ho@ic.ac.uk tions rather than deficiencies of the atrial septum. This is because the extent of the atrial septum is limited. A cursory look from the right atrium gives the impression of an extensive septal structure, whereas sectional cuts show the septum limited to the floor or valve, of the oval fossa, and the muscular rim immediately around it (Fig. 1). The peripheral structures revealed by the cuts are the infolded right atrial wall superiorly, posteriorly and inferiorly, and the fibrofatty sandwich of the atrial and ventricular musculature anteriorly.2 The latter was described previously as the muscular atrioventricular septum. Closer examination shows this part of the heart to be no more septal than the superior, posterior, and inferior infolding of the atrial wall since it too encloses epicardial tissues. The superior and posterior parts of the rim, often called the septum secundum, is mainly the infolded right atrial wall between the base of the superior caval vein and the insertion of the right pulmonary veins to the left atrium. This infolding from the epicardial aspect is known to surgeons as Waterston s groove through which the left atrium can be accessed without entering the right atrium. Posteroinferiorly, the rim is continuous with the wall of the inferior caval vein. Anterosuperiorly to the oval fossa, the seemingly vast expanse of the atrial septum is the right atrial wall overlying the aortic root. Therefore, the septum that separates the two atrial chambers is the valve that is circumscribed by the rim Vol. 13, No. 6,2000 Journal of Interventional Cardiology 475

2 HO, ET AL. Figure 1. (a) The right atrium opened to show the apparently vast expanse of the septal area. The true atrial septum, however, is limited to the oval fossa and its muscular rim (stippled area). (b) This four chamber section shows the thin valve of the fossa. Superiorly, the muscular rim is an infolding of the right atrial wall. Inferiorly to the oval fossa is a sandwich-like arrangement of atrial wall and ventricular septum with fibrofatty tissues in between. of the oval fossa. Defects within this area, usually termed securzdum defects are true atrial septal defects. In contrast, sinus venosus defects, coronary sinus defects and ostium primum defects are outside the confines of the atrial septum although they permit unequivocal interatrial shunting3 and are indeed interatrial communications (Fig. 2). Defects Within the Oval Fossa. These secundum defects are located at the site of the embryonic ostium secundum rather than a deficiency of the septum secundum since the septum secundum is largely the infolded right atrial wall. In the normal heart, the valve of the oval fossa is large enough to overlap and fuse with the left atrial aspect of the rim of the fossa. Even so, the anterosuperior border is not fused in one fourth of individuals and the oval fossa is probe ~atent. ~ Since left atrial pressure is normally higher than right atrial pressure under ordinary circumstances, interatrial shunting does not occur. Deficiencies, perforations, or complete absence of the valve that is formed by the embryonic septum primum are the most common types of interatrial communications with a spectrum of sizes. Defects that are the result of previous atrial septectomy may be difficult to distinguish from congenital defects. Perhaps the simplest congenital defect is that due to the valve being too small to overlap the muscular rim. These are most amenable to transcatheter repair providing they have adequate muscular borders without impinging upon the orifices of the pulmonary veins, the atrioventricular valves, or the caval veins.536 The valve of the fossa may be perforated with single or multiple fenestrations (Fig. 3). Sometimes it has a fishnet appearance or is represented by a filigreed remnant. When the valve is completely absent, the defect is that hole surrounded by the rim of the fossa. Occasionally, the defect may extend toward the inferior caval vein. In the most extreme form, the defect is so extensive as to efface nearly the entire muscular rim. Although defects in the oval fossa do not alter the basic disposition of the sinus and atrioventricular nodes, these very large defects will reduce the distances between the margin of the defects and the atrioventricular node. Sinus Venous Defects. These defects are usually located in the mouth of the superior caval vein and de- 476 Journal of Interventional Cardiology Vol. 13, No. 6,2000

3 ANATOMY OF ATRIAL AND VENTRICULAR SEPTAL DEFECTS Oval fossa de Superior sinus venosus defect inferioisinus I venosus defect Coronary I sinus defect Figure 2. Variants of interatrial communications scribed as superior sinus venous defects. The inferior sinus venosus defects are related to the inferior caval vein and are far less common. The key feature of sinus venosus defects is that they exist outside the confines of the true atrial septum (see above). This is not to say that they cannot become confluent or coexist with deficiency of the oval fossa. In the case of a superior sinus venosus defect, the mouth of the superior caval vein typically ovemdes the atrial septum above the superior rim of the oval fossa. Anomalous insertion of the right pulmonary veins into the wall of the superior caval vein is usual in this situation. The defect, therefore, has a well-defined inferior border, the superior rim of the oval fossa, which encloses epicardial fat (Fig. 4). Roofing the defect is the overriding caval vein. Repair of this defect should take account of potential obstruction to the superior caval pathway following restoration of pulmonary venous return to the left atrium. Also at risk is the sinus node and its arterial supply should there be the need to widen the cavoatrial junction (Fig. 4).* Sinus venosus defects related to the mouth of the inferior caval vein have comparable features to superior sinus venosus defects. In the inferior position, the de- Figure 3. (a) The oval fossa defect is small and crescent-shaped. (b) Fenestrations in the valve producing multiple defects. (c) Right and (d) left atrial views of a large defect in the oval fossa. The left atrial view shows the proximity of the defect to the right pulmonary vein. fect s roof is delineated by the posteroinferior rim of the oval fossa. The orifice of the inferior caval vein opens to the left and right atriums. The lower right pulmonary vein can attach anomalously to the wall of the inferior caval vein. This type of defect is remote from the anticipated locations of the sinus and atrioventricular nodes. In closing the inferior sinus venosus defect, care should be taken not to mistake the defect for an oval fossa defect, or the Eustachian valve for the valve that guards the oval fossa, since suturing the Eustachian valve to the muscular rim of the defect will surely occlude the inferior caval vein! Coronary Sinus Defects. Defects termed coronary sinus defects range from a defect at the site of the orifice of the coronary sinus and absence of the coronary sinus itself, to a single or multiple fenestrations along the course of the coronary sinus allowing it to commu- Vol. 13, No. 6, 2000 Journal of Interventional Cardiology 41 1

4 HO, ET AL. a b Figure 4. (a) The superior sinus venoms is located superior to the rim of the oval fossa (dots). The caval vein has biatrial connections. Note the connection of the right pulmonary vein to the superior caval vein. (b) This subcostal view shows the superior caval vein overriding the atrial septum. FO = oval fossa: LA = left atrium: RA = right atrium: RUPV = right upper pulmonary vein: SCV = superior caval vein. nicate with the left atrium (Fig. 5). The latter is also described as unroofing of the coronary sinus. The defect usually leaves the persistent left superior caval vein connecting directly to the left atrium. Repair of a large defect at the site of the orifice of the coronary sinus may jeopardize the atrioventricular node since the triangle of Koch becomes foreshortened. Ostium Primum Defect. This type of defect, although producing an interatrial shunt, is not a true atrial septal defect. In many cases, the oval fossa is well formed and the defect exists between the free margin of the atrial septum and the atrial aspect of the conjoined leaflets of the atrioventricular valves (Fig. 6A). Hearts with this type of defect do not have discrete right and left atrioventricular junctions. It is more logical to consider these hearts as having atrioventric- ular septal defects since they have comparable morphologies unified by a common atrioventricular junction. The main variants in valvar morphology are those with common valvar orifices ( canal defect Fig. 6B) and those with separate valvar orifices ( ostium primum defect ) which are also known as partial atrioventricular septal defects (Fig. 6A). Further variations in tethering or attachments of the leaflets determine the level of shunting between left and right hearts, namely interatrial, interventricular, or both (Fig. 6). Implications for Interventional Cardiology. Interatrial communications suitable for transcatheter device closure must be sufficiently remote from the atrioventricular valves, coronary sinus, and pulmonary and caval veins. Therefore, only defects within the 478 Journal of Interventional Cardiology Vol. 13, No. 6, 2000

5 ANATOMY OF ATRIAL AND VENTRICULAR SEPTAL DEFECTS Figure 5. (a) Diagram showing (I) persistent left superior caval vein (LSCV) draining into the coronary sinus (CS) with intact walls between the sinus and left atrium, (2) the LSCV drains directly into the left atrium and there is an interatrial communication at the orifice of the coronary sinus where the party wall between the vein and the atrium is absent, and (3) the party wall is fenestrated, representing partial unroofing. (b) This specimen viewed from behind shows partial unroofing. The coronary sinus has been dissected along its length and displayed to show the small communications (arrows) with the left atrium. (c) Right and (d) left atrial views of a heart showing an interatrial communication at the site of the coronary sinus. Figure 6. Two specimens with atrioventricular septal defect and well-formed atrial septum. (a) Fusion between the superior and inferior bridging leaflets along the crest of the ventricular septum results in shunting only at the atrial level. (b) The valvar leaflets are not adherent to the septal crest in this case with a common valvar orifice. Shunting is at the atrial and ventricular levels. Vol. 13, No. 6, 2000 Journal of Interventional Cardiology 479

6 HO, ET AL. oval fossa can be considered for device closure. Even then, some of these defects because of their very large size, or because of their close relationship to these vital structures within the atria, cannot be considered for this approach. In general, there must be a rim of approximately 6-7 mm between the oval fossa defect and the atrioventricular valves or the caval and pulmonary veins. The initial selection of patients who might be suitable for transcatheter closure is made on transthoracic echocardiography, but the final arbiter is almost always transesophageal echocardiography. About 70% of all oval fossa defects, measuring 2 10 mm in diameter, will be suitable for device closure. Ventricular Septa1 Defect. The most common of congenital heart malformations, ventricular septal defects existing in isolation, require little if any attention. A major determinant of outcome is size. The majority become proportionally smaller with time. Spontaneous closure of the defect occurs in up to half of recognized cases in childhood".'* and also has been suggested in adult life. Generally, those who are asymptomatic are likely to have small defects. This review of the morphology is restricted to isolated ventricular septal defects. For reasons already discussed, the ventricular component of atrioventricular septal defects will not be included. Also excluded are septal defects following myocardial infarction. For a logical approach to ventricular septal defects, we must consider not only its size but its location relative to the landmarks of the ventricular septum. The location determines the proximity of the defect to the atrioventricular conduction tissues and to the leaflets of the atrioventricular and arterial valves. All these considerations are important irrespective of whether the defect is to be closed transcatheter or at surgery. The anatomy of the defect is assessed from the right ventricular aspect since this is the most common surgical approach (Fig. 7). The defect's location is then described in terms of the inlet, trabecular, or outlet components of the right ventricle. Furthermore, any malalignment of septal structures is noted. This classification does not rely on developmental concepts and is applicable to all hearts however comple~.'~.'~ ~n y defect can be placed in one of the following three anatomic groups: perimembranous, muscular, and doubly committed and juxtaarterial defects. Perimembranous Defects. Most ventricular septal defects are perimembranous. This is to say that in the otherwise normal heart there is fibrous continuity between the leaflets of the aortic and tricuspid valves in the posteroinferior margin that incorporates the remnant of the membranous septum. The defect may be roofed by override of the aortic valve. The perimembranous defect thus excavates the muscular ventricular septum in the environs of the membranous septum and Aortic and pulmonary - Perimembranous VSD -1 :-*: Muscular VSD Doubly committed and juxtaarterial VSD Figure 7. Ventricular septal defects are described according to their right ventricular margins. 480 Journal of Interventional Cardiology Vol. 13, No. 6, 2000

7 ANATOMY OF ATRIAL AND VENTRICULAR SEPTAL DEFECTS Figure 8. The posteroinferior margin of the perimembranous ventricular septal defect (VSD) is related to the course of the atrioventricular conduction bundle (dots). (a) This perimembranous defect excavates toward the outlet portion of the right ventricle. The medial papillary muscle is near the posteroinferior margin. A triangular-shaped remnant of the membranous septum marks the posteroinferior margin. (b) This perimembranous defect points toward the apical and inlet portion. Note the location of the medial papillary muscle. (c) This four chamber section through a specimen with a penmembranous inlet ventricular septal defect (VSD) shows the loss of the off-set arrangement between the tricuspid and mitral valves. The valves are at the same level and are not separated by a ridge of muscle. (d) In contrast, this echocardiogram through a heart with a muscular inlet defect (arrow) shows the off-set between the atrioventricular valves and a ridge of muscle between them. LA = left atrium; LV = left ventricle; RA = right atrium; RV = right ventricle. can be further described as opening within the inlet, apical, or outlet components of the right ventricle (Fig. 8). Large defects opening to more than one of the three components are then called confluent defects. Irrespec- tive of the extension of the perimembranous defect into the muscular septum, the anatomic hallmark is the fibrous posteroinferior margin through which the atrioventricular bundle passes. When a flap-like remnant of the interventricular membranous septum is also present, it lies immediately on top of the bundle.15 If substantial, the surgeon may be able to use the flap for securing sutures to patch the defect, always being mindful of the close proximity to the conduction bundle. Defects confined to the membranous septum or a true deficiency of its atrioventricular component are extremely rare and are small. More usually, defects that are described as giving rise to left ventricular to right atrial shunting (Gerbode defects) are perimembranous defects associated with a breach in the leaflet of the tricuspid valve. Muscular Defects. In contrast to perimembranous defects, muscular defects have complete muscular borders. Muscular defects can be described as being located in the inlet, outlet, or apical trabecular portions of the right ventricle. Muscular defects, especially those in the trabecular portion, may be multiple giving the septum a swiss cheese appearance. The thick right ventricular trabeculations overlying the septum may obscure the surgeon s view of some of these defects (Fig. 9). Owing to the different relationship of the conduction bundles to the defects, a muscular defect between the ventricular inlets (muscular inlet defect) must be distinguished from a perimembranous defect with inlet extension (Figs. 8 and 9C). Compared to perimembranous defects opening between inlets, the offsetting between tricuspid and mitral valves is preserved and there is musculature between the two valves in the superior margin of the defect.i4 Small to moderately sized defects in the apical trabecular septum often close by hypertrophy of the musculature. Muscular defects in the outlet portion are relatively rare. These may be closed by muscular hypertrophy or prolapse of leaflets of the aortic valve. Doubly Committed and Juxtaarterial Defects. The hallmark of these defects is fibrous continuity between the leaflets of the aortic and pulmonary valves in the superior border. The outlet septum and the septal component of the subpulmonary infundibulum are absent (Fig. 10). Often the valve of one (or both) arterial trunks overrides the muscular septum. In some cases, there may be a firm fibrous raphe between the leaflets so that aortic and pulmonary valves are attached at the same level. Alternatively, off-setting may Vol. 13, No. 6, 2000 Journal of Interventional Cardiology 48 1

8 HO, ET AL. Figure 9. (a) and (b) show a specimen with a muscular defect in the trabecular portion. The anticipated locations of the atrioventricular conduction tissues are shown as dots (atrioventricular bundle) and stipples (left bundle branch). (c) There are two ventricular septal defects (VSDs) in the inlet portion of this specimen. The atrioventricular conduction bundle (dots) is anterosuperior to the muscular inlet defect but posteroinferior to the perimembranous inlet defect. be present when part of the aortic sinus interposes between the arterial valves. In all cases, it is the nature of the posteroinferior border that determines the proximity of the atrioventricular conduction bundle (Fig. 10). When there is a muscular border, as is often the case in doubly committed and muscular defect, the conduction bundle is further away from the edge of the defect.15 However, when the posteroinferior border is fibrous with continuity between the leaflets of the tricuspid and aortic valves (doubly committed and perimembranous defect), the conduction bundle is then much closer to the fibrous rim (Fig. 10). Figure 10. (a) This simulated subcostal cut through the normal right ventricle shows the muscular flange (small arrows) comprising the ventricular infundibular fold and the subpulmonary infundibulum that separates the aortic root from the right ventricle. The septal component of the infundibulum off-sets the pulmonary and aortic valves. (b) and (c) show two specimens with doubly committed and juxtaarterial defects (ventricular septal defect [VSD]) characterized by a lack of muscular separation (arrow) between the arterial valves. The VSD is perimembranous in (c) and has a muscular posteroinferior rim in (b). Aortic valvar prolapse leading to partial, or even complete, occlusion is not uncommon in doubly committed defects. This may occur with or without progressive aortic regurgitation. Malalignment of Septa1 Structures. Descriptions of ventricular septal defects are not complete without considering if the septal components are aligned. Malalignment between atrial and ventricular septums or between components of the muscular ventricular septum have important consequences on the structures in the vicinity of the septal defect. Perhaps the best known situation of septal malalignment is anterocephalad deviation of the outlet septum in hearts with tetralogy of Fallot. In normal 482 Journal of Interventional Cardiology Vol. 13, No. 6, 2000

9 ANATOMY OF ATRIAL AND VENTRICULAR SEPTAL DEFECTS Figure 11. (a) Anterocephalad deviation of the outlet septum narrows the subpulmonary outlet in tetralogy of Fallot. The ventricular septal defect (VSD) is perimembranous in this heart. (b) Right and (c) left ventricular views of a specimen with interruption of the aortic arch. The VSD is perimembranous and malalignment of the outlet septum obstructs the subaortic outlet. hearts, the outlet septum is minuscule since the major component of the right ventricular outflow tract is the free-standing subpulmonary infundibulum (Fig. 10). Much of the septal aspect is the ventriculo-infundibular fold that is continuous with the infundibulum. In the heart malformed by a defect that opens between the outlets, however, the outlet septum is identifiable as a discrete structure. It is a right ventricular structure in hearts with tetralogy of Fallot. Its malalignment produces overriding of the aortic valve and subpulmonary stenosis (Fig. 11). In contrast, malalignment of the outlet septum into the left ventricular outflow is associated with obstructive lesions of the aortic arch (Fig. 11). Malalignment between atrial ventricular septums is exemplified by cases with straddling and overriding of the tricuspid valve (Fig. 12). Whether existing with isolated ventricular septal defects or with other intracardiac defects, the cardinal feature is that the muscular septum does not extend to the crux of the heart but inserts to the right of the crux. The malalignment results in an abnormally located atrioventricular conduction axis. The atrioventricular node is then located in the posterolateral margin of the tricuspid orifice and the bundle penetrates at the point where the ventricular septum meets the right atrioventricular junction. Implications for Interventional Cardiology Perimembranous ventricular septal defects are bordered by the tricuspid and aortic valves, which are in fibrous continuity. In the current era, the majority of these defects are not suitable for transcatheter device closure because of the risk of interference with aortic and tricuspid valve function. There is also a significant risk that having closed the perimembranous ventricular septal defect with a device, there will be a residual shunt. Closure of ventricular septal defects, therefore, in the cardiac catheterization laboratory should be undertaken under exceptional circumstances and only by an experienced operator. A patient is considered a candidate for ventricular septal defect device closure if they have a muscular ventricular septal defect, a residual defect at the margins of a patch following surgery proving the defect is not close to the aortic valve, and a ventricular septal defect following a myocardial infarction, It is important that these defects are not very close to the aortic, pulmonary, or tricuspid valve. Indeed the majority of muscular defects that are large and require closure are in the anterior part of the apical muscular ventricular septum. Only a small proportion of individuals with a ventricular septal defect are candidates for device closure. The procedure can be technically difficult, so that an Vol. 13, No. 6,2000 Journal of Interventional Cardiology 483

10 HO, ET AL. Figure 12 (a) and (b). There is straddling and overriding of the tricuspid valve (TV) across a perimembranous inlet ventricular septal defect owing to malalignment between atrial and ventricular septums. The atrioventricular (AV) conduction bundle connects to an anomalously located atrioventricular node (*) instead of to the regular atrioventricular node (3. experienced team of cardiologists, anaesthetists, and surgeons needs to be available to ensure that device closure can be performed with adequate safety. Conclusions Description of septal defects need not rely on developmental concepts. Instead, descriptions based on morphology enhance understanding and draw attention to key features that are relevant and applicable to all hearts, however complex. With ventricular septal defects, whether treatment is to close or to enlarge, it is the right ventricular margin that requires close scrutiny. Finally, appreciation of the limited extent of the atrial septum helps in distinguishing true atrial septal defects from all other forms of interatrial communications. References I. Hoffman JIE. Incidence, mortality and natural history. In: Anderson RH, Macartney FJ, Shineboume EA, et al., eds. Paedi atric Cardiology. Edinburgh: Churchill Livingstone, 1987, pp Ho SY, Sanchez-Quintana D, Cabrera JA, et al. Anatomy of the left atrium: Implications for radiofrequency ablation of atrial fibrillation. J Cardiovasc Electrophysiol 1999;lO: Ho SY, Baker EJ, Rigby ML, et al. Interatrial communications and other atrial malformations. In: Colour Atlas of Congenital Heart Disease. London: Mosby-Wolfe, 1995, pp Edwards JE. Congenital malformations of the heart and great vessels. A malformation of the atrial septal complex. In: Gould SE, ed. Pathology of the Heart. 2"* ed. Springfield: Charles C. Thomas, 1960, pp Ferreira SMAG, Ho SY, Anderson RH. Morphologic study of defects of the atrial septum within the oval fossa: Implications for transcatheter closure of left-to-right shunt. Br Heart ;67: Rigby ML. The era of transcatheter closure of atrial septal defects. Heart 1999;81: a1 Zaghal AM, Li J, Anderson RH. Anatomical criteria for the diagnosis of sinus venosus defects. Heart 1997;78: Anderson KR, Ho SY, Anderson RH. The location and vascu- Iar supply of the sinus node in the human heart. Br Heart J 1979;4 1 : Raghib G, Ruttenberg HD, Anderson RC, et al. Termination of left superior vena cava in left atrium, atrial septal defect, and absence of coronary sinus. A developmental complex. Circulation 1965;3 1 : Journal of Interventional Cardiology Vol. 13, No. 6, 2000

11 ANATOMY OF ATRIAL AND VENTRICULAR SEPTAL DEFECTS Becker AE, Anderson RH. Atrioventricular septal defects. What s in aname. J Thorac Cardiovasc Surg 1982;83: Alpert BS, Mellitis ED, Rowe RD. Spontaneous closure of small ventricular septal defects: Probability rates in the first five years of life. Am J Dis Child 1973;125: Corone P, Doyen F, Gaudeau S, et al. Natural history of ventricular septal defect. A study involving 790 cases. Circulation 1977;55: Soto B, Becker AE, Moulaert AJ, et al. Classification of ventricular septal defects. Br Heart J 1980;43: Gatzoulis MA, Li J, Ho SY. The echocardiographic anatomy of ventricular septal defects. Cardiol Young 1997;7: Milo S, Ho SY. Wilkinson JL, et al. The surgical anatomy and atrioventricular conduction tissues of hearts with isolated ventricular septal defects. J Thorac Cardiovasc Surg 1980;79: Vol. 13, No. 6, 2000 Journal of Interventional Cardiology 485