Br Heart J: first published as 10.1136/hrt.47.5.419 on 1 May 1982. Downloaded from Br Heart J 1982; 47: 419-29 Anatomical-embryological correlates in atrioventricular septal defect SALLY P ALLWORK* From the Department ofSurgery, Division of Cardiovascular Disease, Royal Postgraduate Medical School, Hammersmith Hospital, London SUMMARY Recent embryological studies have supported the consideration that the ventricular sep- tum is multifocal in origin. These data have also provided excellent correlation of the morphology of malformed hearts with their embryology. In particular, atrioventricular septal defect correlates accurately with these observations on ventricular septation. Many of the names given to atrioven- tricular septal defect (for example ostium primum, persistent atrioventricular canal, endocardial cushion defect) indicate attempts at correlating the anatomy with embryology. None of these has been very convincing. In the light of this uncertainty, this review considers briefly the anatomy of the malformation and its ontogeny, and presents a hypothesis of the development of atrioventricular septal defect. Although there is almost always a communication above the atrioventricular valves, the malforma- tion lies in the ventricular, not the atrial septum. Hearts with inlet septal defect without interatrial communication represent one end of the spectrum of anomalies, and those with common atrioven- tricular orifice, in which Fallot's tetralogy or single outlet heart may be associated, mark the other end. The outflow tract malformations are not randomly associated, but are points in a huge range of cardiac malformations. http://heart.bmj.com/ Atrioventricular septal defect is a cardiac malforma- leaflets themselves are often poorly developed and tion characterised externally by an abnormally short may have a verrucous appearance. posterior (diaphragmatic) ventricular surface.' Inter- The short external diaphragmatic surface of the nally there is a gap between the concave inferior rim ventricular mass is reflected internally by a pro- of the atrial septum (which is usually well developed) nounced disproportion between the inlet and outlet on September 27, 2021 by guest. Protected copyright. and the atrioventricular valves. There may be a com- lengths of the left ventricle. In normal hearts these mon atrioventricular valve, usually with five or six two measurements are essentially the same.' In leaflets of which the two major ones, anterior and atrioventricular septal defect the outflow tract of the posterior, bridge the ventricular septum.2 Often there left ventricle is long and narrow compared with a are two valves, each with three leaflets3 (Fig. la, b). normal heart (Fig. 2a, b) and the left anterior leaflet The left component has an abnormally orientated inserts immediately behind and beneath the aortic anterior leaflet which is separated medially from the valve so that the latter is displaced. (Whether the septal or posterior leaflet by an unsupported division valve is really displaced has been debated at length46 which is usually called a cleft. Between the anterior but the appearance is that of displacement.) It has lost i and septal leaflets, at the obtuse margin, is the lateral its wedged position between the atrioventricular val- leaflet (Fig. lb). When there is a bridging leaflet bet- ves and the ventricular septum.78 Sometimes aortic ween either the anterior and lateral, or posterior and outflow tract obstruction occurs, and muscular obs- lateral leaflets, it produces an accessory orifice, and is truction may be further aggravated by hypoplasia of common in atrioventricular septal defect. 'I The the aortic valve.9 Pulmonary outflow tract obstruction is especially associated with a common valve with bridging *Supported by the British Heart Foundation. leaflets2 and may be the result of the infundibular Fallot's tetral- Accepted for publication 27 November 1981 septal derangement characteristic of 419 Br Heart J: first published as 10.1136/hrt.47.5.419 on 1 May 1982. Downloaded from 420 Allwork 3. Iff .:j-A >...i._ .A, t ..... .! ..Xo. os._- :::<:.. *.: i:' S.i _. ;. |__!gb$. :X.,_s .[.i. S o' W *:^ .... ,. S. .: :.,. *.\ ^ i so a.h qS ::. http://heart.bmj.com/ :. *: X [. A- *..|^ .... SWe a°' 0: AR..4 ... o.! .. A_ on September 27, 2021 by guest. Protected copyright. mk6 UI L- Fig. I (a) The atrioventncular valves in atrioventncular septal defect (AVSD) seenfrom the right atrium (RA). The atrial septum (AS) is intact and its concave nmforms the "roof' ofthe AVSD. The valve leaflets are attached at the same level to the summit ofthe muscular septum, but the anterior (AL) and posterior (PL), or septal (SL) bridging leaflets are separated by the "cleft" (atrowed). LLL, lateral left leaflet, ARL, anteriorright leaflet; CS, coronary sinus; FO, fossa ovalis; LFO, limbusfossa ovalis. (b) Same heart, left atrial (LA) view. The left valve does not resemble a normal mitral valve. It has three leaflets, anterior, septal or posterior, and lateral. The abnormally orientated anterior leaflet is separated by the cleft (arrowed) from the septal or posterior bridging leaflet. LAu, left auricle. Other abbreviations as in Fig. 1(a). Br Heart J: first published as 10.1136/hrt.47.5.419 on 1 May 1982. Downloaded from Embryology of atnioventricular septal defect 421 Ao r-^;~~~~~~~~~~~~~~~~~~1 N 'I~ ~ ~~~- L)S ~ ~ A http://heart.bmj.com/ J VOT on September 27, 2021 by guest. Protected copyright. Fig. 2 (a) The left ventricular outflow tract in the normal heart. The inlet septum (IS) and its atrioventricular component, indicated by the dotted line, give "depth" to the outflowv tract The atrioventricular septum extendsfrom the membranous part ofthe septum (MS) to the crux cordis permitting a posterior recess (arrowed). The anterior mitral leaflet (AML) is normnally orientated, and extends from the base ofthe left aortic leaflet (L) to the non-coronary leaflet (NC). Thus the aortic valve is "wedged" by the inlet septum and the mitral valve, so that it lies beside rather than anterior to the mitral valve. Ao, aorta; LVOT, left ventricular outflow tract; R, right aortic leaflet. (b) The left ventricular outflow tract in AVSD. The narrowness is partly the result ofthe deficiency ofthe inlet septum and absence ofits atrioventricular component (note the septal autachment ofthe anterior leaflet (AL) of the left valve) and partly ofattenuation ofthe outlet septum (OS). The loss ofthe normnal "'wedge" position ofthe aortic valve is shown by the position ofthe anterior leaflet which extends from the left aortic leaflet to the commissure between non-coronary and right (R) leaflets. The membranous part ofthe septum is small, but intact. APM, anterior papillary muscle group. Other abbreviations as in Fig. 1. Br Heart J: first published as 10.1136/hrt.47.5.419 on 1 May 1982. Downloaded from 422 Allwork ogy. Double outlet right ventricle, ventriculoarterial Table 2 Nomenclature and morphology in atrioventricular discordance, and single outlet heart are also some- septal defect in atrioventricular septal defect.10'12 times represented Peacock'3 1846 Imperfection of atrial and The atrial septum is usually well developed in ventricular septa atrioventricular septal defect; the anticipated normal Rokitansky'4 1875 Atrioventricular canal defect Partial -VSD anatomical elements are represented and atrial septa- Complete +VSD tion is often complete, especially in those with "par- Watkins and Gross'6 1955 Endocardial cushion defect Ostium primum ASD tial" or "transitional" defects (Fig. la and b). Patency Wakai and Edwards'7 1956 Persistent common AV canal of the foramen ovale, however, occurs with some fre- Partial separate atrioventricular valves quency, as in the normal heart, and secundum atrial Transitional almost separate septal defects are common. In view of the frequent atrioventricular valves a normal atrial septum it is, perhaps, sur- ±small VSD finding of Complete common atrioventricular prising that so many investigators have concluded that valve the development of this structure is at fault in Bedford et al. 18 1957 Atrioventricular defect Brandt et al. '9 1972 Endocardial cushion defect atrioventricular septal defect. The defect itself is the Partial -VSD gap between the inferior rim of the atrial septum and Complete +VSD Piccoli et al.' 6 1979 Partial separate atrioventricular the atrioventricular valves. The vertical dimension of valves this defect is approximately the same as the dispropor- Complete common atrioventricular tion between the inlet and outlet length of the ven- valve tricular septum measured in the left ventricle (Table ASD, atrial septal defect; VSD, ventricular septal defect. 1). These data strongly suggest that the anomaly con- cerns the inlet ventricular septum rather than the With respect to "partial" and "complete" defects, atrial septum. most authors have adhered to Rokitansky'sl4 division The foregoing paragraphs are not intended to be an between those with interatrial communication only exhaustive description of atrioventricular septal (partial) and those who had an interventricular defect defect, but to indicate the spectrum which can accu- as well. 15-19 More recently, however, the two terms rately be correlated with the ontogeny of the ventri- have been used to distinguish between separated cles and the ventricular septum. atrioventricular (partial), and common orifice (complete),5-6 but this distinction has not gained gen- eral currency, especially in surgical practice.2 3 Wakai Historical background and Edwards20