Atrioventricular septal defects comprise a disease spectrum characterized by deficient atrioventricular septation, with several common features seen in all affected hearts and variability in atrioventricular valve morphology and interatrial and interventricular communications. Atrioventricular septal defects are among the more common defects encountered by pediatric cardiologists and echocardiographers. Despite advances in understanding, standard two-dimensional echocardiography may not be the optimal method for the morphologic and functional evaluation of this lesion, particularly malformations of the atrioventricular valve(s). In this review, the authors summarize the role of three-dimensional echocardiography in the diagnostic evaluation of atrioventricular septal defects.
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Embryologic Perspective and Nomenclature
The nomenclature of the AV valves can be confusing in AVSDs, and an understanding of the embryologic development is the Rosetta stone of this lesion. It may well be that an initial failure of fusion of the endocardial cushions is the first step toward the development of an AVSD. This can explain the “cleft” (zone of apposition) between the bridging leaflets in this lesion. In the normal heart, there is complete fusion, resulting in the anterior or what is more recently referred to by embryologists as the aortic leaflet of the normal mitral valve. The other leaflet of the normal mitral valve is the posterior leaflet. The mural leaflet in an AVSD is in effect equivalent to the posterior leaflet of the normal mitral valve, except that it is smaller and supported by papillary muscles that are closer together. The smaller mural leaflet in an AVSD is the result of lack of expansion of the inferior quadrants of the left AV junction, involving the parietal wall of the left ventricle, and the lateral cushion. In the normal heart, expansion and growth of the lateral cushion result in the posterior mitral leaflet occupying two thirds of the circumference of the normal mitral valvar orifice.
Abnormalities of the cushions are not the whole explanation for the development of AVSDs, because they do not provide the total substrate for the development of the AV valve leaflets or the other components that constitute these lesions. As a result of observations in hearts with AVSDs, it was decided to name the two components that make up the cleft the superior and inferior bridging leaflets, because of their positions within the AV junction. The superior and inferior bridging leaflets have no counterparts in the normal heart. Thus, the common AV valve is basically composed of the superior and inferior bridging leaflets, the left mural (confined to the left ventricle) and right mural and right anteroseptal leaflets (confined to the right ventricle). The cleft in an AVSD points toward the interventricular septum, whereas an isolated cleft of the normal mitral valve points toward the left ventricular outflow tract (LVOT). The reason for this is that fusion of the cushions is necessary for the LVOT to be incorporated into the normally developing left ventricle. In an AVSD, the aortic outflow tract is sprung and not wedged, this being one of the hallmarks of this lesion.
Embryologic Perspective and Nomenclature
The nomenclature of the AV valves can be confusing in AVSDs, and an understanding of the embryologic development is the Rosetta stone of this lesion. It may well be that an initial failure of fusion of the endocardial cushions is the first step toward the development of an AVSD. This can explain the “cleft” (zone of apposition) between the bridging leaflets in this lesion. In the normal heart, there is complete fusion, resulting in the anterior or what is more recently referred to by embryologists as the aortic leaflet of the normal mitral valve. The other leaflet of the normal mitral valve is the posterior leaflet. The mural leaflet in an AVSD is in effect equivalent to the posterior leaflet of the normal mitral valve, except that it is smaller and supported by papillary muscles that are closer together. The smaller mural leaflet in an AVSD is the result of lack of expansion of the inferior quadrants of the left AV junction, involving the parietal wall of the left ventricle, and the lateral cushion. In the normal heart, expansion and growth of the lateral cushion result in the posterior mitral leaflet occupying two thirds of the circumference of the normal mitral valvar orifice.
Abnormalities of the cushions are not the whole explanation for the development of AVSDs, because they do not provide the total substrate for the development of the AV valve leaflets or the other components that constitute these lesions. As a result of observations in hearts with AVSDs, it was decided to name the two components that make up the cleft the superior and inferior bridging leaflets, because of their positions within the AV junction. The superior and inferior bridging leaflets have no counterparts in the normal heart. Thus, the common AV valve is basically composed of the superior and inferior bridging leaflets, the left mural (confined to the left ventricle) and right mural and right anteroseptal leaflets (confined to the right ventricle). The cleft in an AVSD points toward the interventricular septum, whereas an isolated cleft of the normal mitral valve points toward the left ventricular outflow tract (LVOT). The reason for this is that fusion of the cushions is necessary for the LVOT to be incorporated into the normally developing left ventricle. In an AVSD, the aortic outflow tract is sprung and not wedged, this being one of the hallmarks of this lesion.
Morphologic Features of AVSD
The AVSD spectrum ranges from hearts with primum atrial defects with separate left and right AV valve orifices, a cleft left AV valve (LAVV), and no interventricular communication to those with a common AV valve orifice and defects at both the atrial and ventricular levels. There are common features seen in all hearts with AVSDs, irrespective of whether they have atrial or ventricular communication. Because of the complete absence of the muscular AV septum in an AVSD, both the right AV valve and LAVV components insert at the same level. There is “unwedging” and anterior displacement of the aortic valve that contribute to elongation of the LVOT, resulting in inlet/outlet disproportion. The left AV junction in an AVSD consists of the mural leaflet, which extends from the anterolateral to the posteromedial papillary muscle, and the two bridging leaflets ( Figure 1 ). The superior and inferior bridging leaflets arise from their appropriate papillary muscle and bridge the interventricular septum, resulting in the cleft, which is common to virtually all hearts with this defect. The commissures represent the zones of coaptation between the superior and inferior bridging leaflets and their mural counterpart ( Figure 2 ).
What differentiates one heart from the other is the status of the AV valves as they bridge the interventricular septum. In some, the superior and inferior bridging leaflets are fused with the interventricular septum, resulting in no communication at the ventricular level, whereas in others, there are varying degrees of bridging, with a defect at the ventricular level beneath the superior leaflet, with or without one beneath its inferior counterpart. In general, there is little correlation between the degree of bridging and leaflet attachment to the interventricular septum between the superior and inferior bridging leaflets. As the leaflets bridge the interventricular septum they may be attached to the crest of the interventricular septum, its right ventricular side, or a papillary muscle within the right ventricular cavity. The key to determining whether there are two orifices or a common one is the presence or absence of a connecting tongue of tissue between the superior and inferior bridging leaflets. This is an important concept, because it is possible to have a common orifice with no ventricular communication beneath the bridging leaflets but one between them when the connecting tongue is absent. As well, it is possible to have an AVSD with all the common features described above but without a shunt at atrial or ventricular level. Thus, the relationship of the bridging leaflets to each other determines the presence of common or separate valvar orifices, and their relationship to the septal structures, which in turn determines the shunting across the defect.
Advantages of 3DE Over 2DE in the Assessment of AVSD
Two-dimensional echocardiography is the primary imaging modality used in the diagnostic evaluation of AVSDs. Although many of the features of AVSD can be appreciated by 2DE, there are several limitations to the 2D technique, as well as the potential for improved diagnostic detail when working in a 3D medium. Three-dimensional echocardiography allows imaging of the heart the way it really looks, as opposed to a 2D slice, which allows no depth perception. Surgeons are increasingly relying on 3DE for the assessment of AV valve morphology and function before valve repair. Surgeons often will immediately understand a cropped or reconstructed 3D data set image displayed in a surgical view, and with a minimal amount of training, they are able to fully appreciate details of valve anatomy.
The real strength of 3DE is not its ability to make the diagnosis of an AVSD, which 2DE does very well, but its ability to identify complex AV valve and LVOT abnormalities associated with the defect. This is important, because intraoperative assessment of AV valve regurgitation on the basis of saline testing performed in a flaccid heart is nonphysiologic and prone to errors. It has been shown by our group that 3DE provides additional information regarding the mechanisms and sites of LAVV regurgitation in AVSDs compared with transesophageal 2DE. Importantly, there was low agreement between 2DE and 3DE regarding commissural formation and poor coaptation ( Table 1 ). The sites of poor coaptation and specific commissural detail as mechanisms of LAVV regurgitation were much more readily appreciated by 3DE. Moreover, 3DE permits evaluation of the complete line of apposition between the superior and inferior bridging leaflets (cleft), which often is a difficult diagnosis to make in patients who have previously undergone valve repair. Two-dimensional echocardiography provided an overestimation of the numbers of jets in comparison with 3DE. As well, we have examined the utility of 3DE in determining mechanisms and sites of AV valve regurgitation using surgical findings as the reference standard for structural abnormalities ( Table 2 ) and 3DE as the reference standard for the site of regurgitation ( Table 3 ). Compared with 2DE, transthoracic 3DE provided superior detail of the leaflet and anterior commissural abnormalities, thereby improving the detection rate for anatomic and functional LAVV problems.
| 3D findings | 2D findings | 2D-3D agreement | |
|---|---|---|---|
| SBL | Small (1) | Normal (1) | 6 (55%) |
| Small and abnormal chordae (1) | Normal size structure (1) | ||
| Deformity (2) | Normal (2) | ||
| Normal (1) | Small (1) | ||
| IBL | Abnormal (3) (deformity 1, stenotic 1, double orifice 1) | Normal (3) | 7 (64%) |
| Small (1) | Normal (1) | ||
| ML | Small (2) | Normal (2) | 5 (45%) |
| Small and deformed (1) | Normal (1) | ||
| Normal (1) | Small and double orifice (1) | ||
| Commissure | Abnormal between IBL and ML (3) | Normal (3) | 5 (45%) |
| Normal (3) | Abnormal commissure (3) | ||
| Cleft | Partially open (3) | Closed (3) | 8 (73%) |
| Prolapse | No prolapse (2) | Prolapse (2) | 8 (73%) |
| Prolapse (1) | No prolapse (1) | ||
| Mechanisms of regurgitation | |||
| Poor coaptation | Between SBL and ML (1), between IBL and ML (2) | Normal coaptation (3) | 8 (73%) |
| Cleft | Partially opened (3) | Closed (3) | 7 (64%) |
| Closed (1) | Partially opened (1) | ||
| Commissure | Deformities between SBL and ML (2); between IBL and ML (2); among SBL, IBL, and ML (1) | No abnormality (5) | 5 (45%) |
| No abnormality (1) | Abnormality between SBL and ML (1) | ||
| LAVV | Sensitivity (%) (95% CI) | Specificity (%) (95% CI) | ||
|---|---|---|---|---|
| 2DE | Real-time 3DE | 2DE | Real-time 3DE | |
| Leaflet abnormality | ||||
| Aortic leaflet | 30.0 (2–58) | 40 (10–70) | 69 (52–86) | 93 (84–100) |
| Mural leaflet | 37.5 (4–71) | 100 (100–100) | 29 (5–52) | 62 (39–86) |
| Commissural abnormality | ||||
| SBL-ML or A1-P1 | 30 (2–58) | 50 (19–81) | 20 (0–45) | 100 (100–100) |
| IBL-ML or A3-P3 | 90 (71–100) | 90 (71–100) | 30 (2–58) | 80 (55–100) |
| Prolapse | 54 (35–72) | 39 (21–57) | 91 (74–100) | 100.0 (100–100) |
| Poor coaptation | 67 (29–100) | 17 (0–47) | 60 (35–85) | 80.0 (60–100) |
| Cleft AVSD | 94 (83–100) | 94 (83–100) | — | — |
| Site of regurgitant jet | Sensitivity (%) (95% CI) | Specificity (%) (95% CI) | ||
|---|---|---|---|---|
| 2DE | Surgical findings | 2DE | Surgical findings | |
| Between SBL-ML or A1-P1 | 57.1 (20.5–93.8) | 28.6 (0.0–62.0) | 71.4 (47.8–95.1) | 100.0 (100.0–100.0) |
| Between IBL-ML or A3-P3 | 36.4 (7.9–64.8) | 18.2 (0.0–41.0) | 50.0 (19.0–81.0) | 90.0 (71.4–100.0) |
| Central portion | 83.3 (66.1–100.0) | 100.0 (100.0–100.0) | 100.0 (100.0–100.0) | 66.7 (13.3–100.0) |
| Cleft (AVSD only) | 100.0 (100.0–100.0) | 58.3 (30.4–86.2) | 50.0 (1.0–99.0) | 25.0 (0.0–67.4) |
The sensitivity of 3DE for leaflet abnormalities of the mural leaflet was higher than that of 2DE. For both the mural and anterior leaflets, 3DE overall provided greater degrees of accuracy and specificity for detecting abnormalities. This is consistent with findings in adults with mitral valve disease. For commissural abnormalities between the superior bridging and mural leaflets, 3DE appeared to be more sensitive and very specific compared with 2DE. Although sensitivity was similar for the inferior bridging leaflet and the mural leaflet, 3DE was more specific and accurate. For other mechanisms of LAVV regurgitation (prolapse and cleft), accuracy was similar. These mechanisms and other AV valve abnormalities with important surgical implications, including parachute and double-orifice AV valves, are discussed below under “Presurgical and Postsurgical AV Valve Abnormalities.”
Specific Considerations and Optimization of 3DE for AVSD
Inlet/Outlet Disproportion and Sprung AV Junction
In an AVSD, there is variable scooping of the inlet septum, and the inlet is shorter than the outlet. The inlet/outlet ratio in an AVSD is <1 (the ratio in normal hearts is approximately 1). This feature can be appreciated from the parasternal long-axis or subxiphoid long-axis imaging window on 2DE. Three-dimensional echocardiographic full-volume data sets acquired from the parasternal long-axis window allow precise determination of the extent of scooping ( Figure 3 ). The 3D echocardiographic data set is cropped from the anterior, such that the interventricular septum is removed, and then the image is rotated so that the heart is imaged from above, or anterior. In this view, the two papillary muscles supporting the superior and inferior bridging leaflets can be seen, as well as the cleft between them. The extent of the scooping of the interventricular septum can be appreciated in this image after minor adjustments to remove the two leaflets. A similar view can be obtained from the subxiphoid position, albeit at reduced image quality. The unwedged aorta in an AVSD is best appreciated on 3DE from an apical four-chamber view, with the atria removed and cropped from above ( Figure 4 ). The aorta is positioned in front of the common AV valve annulus, the appearance being the same whether or not there is a common orifice or there is partitioning by a connecting tongue.
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