Atrioventricular (AV) canal defect is also known as an AV septal defect, common AV canal, endocardial cushion defect, ostium primum atrial septal defect, or persistent ostium atrioventriculare commune. Complete AV canal defect consists of an unrestrictive atrial septal defect, an unrestrictive ventricular septal defect, and a single common AV valve. It occurs because of failure of development of the endocardial cushion during embryogenesis and persistence of the primitive single AV canal beyond 6 weeks of gestation. Although the AV canal defect may involve a wide spectrum of abnormalities of the atrial and ventricular septa and the AV valves, the complete form with associated AV valve regurgitation is the one that most commonly presents during the neonatal period (Chin et al., 1982).

AV canal defects can be partial or complete. With a partial AV canal defect, there are two separate AV orifices, with communication between two atria or between the left ventricle and the right atrium. While the right AV valve is usually normal, the left valve usually has three leaflets (Becker and Anderson, 1981). With a complete AV canal defect, there is a defect in the inferior portion of the atrial septum and the superior portion of the ventricular septum, together with a single AV valve orifice, which usually has five valve leaflets. The anterior and posterior valve leaflets are inserted on the anterior and posterior surfaces of the ventricular septum, respectively; the degree of bridging of the anterior leaflet across the ventricular septum determines the type of defect (Rastelli et al., 1966). For type A complete AV canal defects, the anterior leaflet does not bridge the ventricular septum and is attached on both sides of the septum by chordae tendineae (Silverman et al., 1986). For type B complete AV canal defects, the anterior leaflet somewhat bridges the ventricular septum and is attached to the right ventricle by an anomalous papillary muscle. For type C complete AV canal defects, the anterior leaflet is not attached to the septum, and completely bridges it, being attached at either side by papillary muscles. Type A is commonly seen in trisomy 21, and type C is commonly seen with atrial isomerism (Mann et al., 2004).

Complete AV canal defect accounts for 1% to 5% of all cases of congenital heart disease (Rowe et al., 1981). It is the most common cardiac defect detected prenatally. In one series of 357 cases of congenital heart disease 13% were AV canal defects, almost 70% of which were of the complete type (Fontana and Edwards, 1962). In a series of 49 AV canal defects from a single center, 88% were of the complete type and 12% were partial (Allan, 1999). Familial clustering of AV canal defects has also been described (Tennant et al., 1984; Disegni et al., 1985). Review of population-based series of congenital heart disease suggests that the incidence of AV canal defect is 0.3 per 1000 livebirths (Hoffman and Kaplan, 2002).

Prenatal diagnosis of a complete AV canal defect is relatively straightforward, because a large defect or hole is easily visualized at the crux of the heart during diastole on the four-chamber view (Figure 45-1). This hole represents the defects in the inferior portion of the atrial septum, the superior portion of the ventricular septum, and the single AV valve orifice. In the normal four-chamber view of the heart, the tricuspid valve inserts more apically than the mitral valve, leading to an offset appearance at the crux of the heart. With an AV canal defect, this offset appearance may be lost, with both valves forming a straight line across the crux of the heart when closed (Figure 45-2).

Because the defect can be large, the cardiac conduction apparatus is frequently abnormal, resulting in an increased frequency of bradyarrhythmias. In one series, 11 of 29 fetuses with AV canal defects also had complete heart block (Machado et al., 1988). In addition, all 11 of these fetuses also demonstrated features of the heterotaxy syndrome (see Chapter 56). Prenatal echocardiography should focus on the structure of the AV valve. The presence of a common leaflet at the level of the AV valve is suggestive of complete AV canal defect, while in its partial form the only abnormality may be a small defect in the inferior portion of the atrial septum (Romero et al., 1988).

Color and pulsed Doppler echocardiography may demonstrate turbulent systolic flow on the atrial side of the common AV valve because of significant valvular regurgitation (Barber and Chin, 1990). The presence of significant valvular regurgitation can be quantified, and when present throughout systole, it may be predictive of hydrops fetalis (Gembruch et al., 1991). In contrast, valvular regurgitation confined to early systole is not associated with hydrops and may be an indicator of improved prognosis.

Prenatal sonography should also focus on excluding other abnormalities, such as malformations associated with aneuploidies or additional cardiac malformations. Sonographic markers associated with aneuploidy should be specifically sought, as approximately 50% of cases of AV canal defects have a chromosomal abnormality, the majority of which are due to trisomy 21 or 8p deletion (Machado et al., 1988). Additional cardiac malformations may be present in more than 70% of cases, such as tetralogy of Fallot, coarctation, pulmonary stenosis, and double outlet right ventricle (Machado et al., 1988). In a series of 49 cases of AV canal defect from a single center, 31 (63%) had additional cardiac malformations, typically heterotaxia or isomerism, but also including hypoplastic left ventricle and tetralogy (Allan, 1999). In the Baltimore–Washington infant study, 336 children were identified with an endocardial cushion defect. Of these, 76% were identified as having a syndromic diagnosis (78% of these had trisomy 21) (Carmi et al., 1992). A more recent review of 301 cases of AV canal defect from a single center in London demonstrated that approximately 50% with known karyotype had aneuploidy, with the vast majority (80%) of these being trisomy 21, and others including trisomy 18, trisomy 13 (Huggon et al., 2000). In this series, 155 of 301 (52%) had normal atrial arrangement and no other evidence of additional cardiac abnormalities, while 40 of 301 (13%) had significant extracardiac abnormalities (Huggon et al., 2000). Chromosomal abnormality appears to be much more likely in the setting of AV canal defect with normal atrial connections.

The differential diagnosis for complete AV canal defects includes a large atrial or ventricular septal defect, or a single ventricle (Barber and Chin, 1990). While complete AV canal defects can usually be easily distinguished from these lesions by prenatal echocardiography, differentiation from partial AV canal defects may be more problematic. Genetic considerations in the differential diagnosis include trisomy 21 (see Chapter 131), heterotaxy (Ivemark syndrome; see Chapter 56), Ellis–van Creveld syndrome (see Chapter 94), Holt–Oram syndrome, and CHARGE association.