INTRODUCTION — Atrioventricular (AV) canal defects are a group of congenital cardiac defects involving the AV septum and AV valves (ie, mitral and tricuspid valves) (figure 1). They are also referred to as AV septal defects, endocardial cushion defects, or persistent AV ostium. Combinations of these anatomic abnormalities result in complete (both atrial and ventricular septal defects [ASD/VSD]) and partial (only ASD) forms that are manifested in varying clinical presentations.
The anatomy, clinical features, and diagnosis of AV canal defects will be reviewed here. The management and outcome of patients with AV canal defects are discussed separately. (See "Management and outcome of atrioventricular (AV) canal defects".)
EPIDEMIOLOGY — AV canal defects account for approximately 4 to 5 percent of congenital heart defects, with a reported prevalence of 0.3 to 0.4 per 1000 live births [1,2]. AV canal defects in fetuses accounted for a larger proportion of detected congenital heart disease, approaching 18 percent of congenital heart disease cases based on data from large fetal echocardiographic series [3,4]. The male-to-female distribution of AV canal defect is approximately equal [5], and complete AV canal (CAVC) defects with both atrial and ventricular septal defects (ASD/VSD) occur in one-half of the cases [1,2].
There is a strong association between AV canal defects and trisomy 21 (also referred to as Down syndrome), with a 40 to 50 percent risk of Down syndrome in fetuses in whom an AV canal defect is detected [6]. Nonsyndromic AV canal defects appear to be associated with maternal diabetes and obesity [7]. (See 'Association with Down syndrome' below.)
CLASSIFICATION — AV canal defects encompass a broad range of AV septal defects and abnormalities of the AV valves. An initial classification separates the different types of AV canal defects based on anatomical features (eg, complete versus partial form) and their impact on physiology. In addition, variations of complete AV canal (CAVC) defects are categorized by AV valve morphology (Rastelli classification) and relative ventricular size. These classifications are used to guide surgical management.
●Complete, partial, transitional, and intermediate forms – AV canal defects are initially divided between complete (which contain both atrial and ventricular septal defects [ASD/VSD]) and partial defects (which have an ASD without a VSD). The two other forms are transitional (physiologically similar to the partial subtype) and intermediate (physiologically similar to the complete subtype) (figure 1).
•CAVC is a result of complete failure of fusion between the superior and inferior endocardial cushions. It is characterized by a primum ASD that is contiguous with a posterior (or inlet) VSD and a common AV valve.
•Partial AV canal defect is due to incomplete fusion of the superior and inferior endocardial cushions and consists of a primum ASD and a single AV valve annulus with two separate valve orifices. Due to abnormal fusion of the left tubercle of the superior and inferior cushions, the anterior leaflet of the mitral valve typically is cleft.
•Transitional AV canal defect consists of a large primum defect, cleft mitral valve, and inlet VSD. However, dense chordal attachments to the ventricular septum lead to small insignificant ventricular shunting and delineation of distinct left and right AV valve orifices, resulting in a defect that is similar to the physiology of a partial AV canal defect.
•Intermediate AV canal defect is a rare subtype of CAVC, in which a bridging tongue of tissue divides the common AV valve into two distinct orifices. This defect usually has both a large primum ASD and inlet VSD. Due to the natural division of the common AV valve into left and right AV valve components by the tongue of tissue, surgical division is not required.
There is variability within the congenital heart community regarding the nomenclature used to describe AV canal defects. For example, some experts refer to AV canal as AV septal defect and describe the cleft in the left AV valve as the zone of apposition between the superior and inferior bridging leaflets.
●Balanced versus unbalanced CAVC – CAVC defects can be further classified based upon the relative sizes of the ventricles. In a balanced CAVC, the ventricles are relatively equal in size and are amenable to biventricular repair. In the unbalanced form, there is hypoplasia of either the right ventricle (RV) or left ventricle (LV) and the common AV valve sits predominantly above the dominant ventricle. In most unbalanced cases, the RV is the dominant ventricle. If there is significant hypoplasia of either ventricle, complete two-ventricle surgical repair may not be possible, in which case, surgical intervention consists of univentricular palliation. Echocardiographic features used to define balanced versus unbalanced CAVC and the surgical management of patients with unbalanced CAVC are discussed separately. (See "Management and outcome of atrioventricular (AV) canal defects", section on 'Single-ventricle palliation'.)
●Rastelli classification – The classification system initially developed and revised by Rastelli and others is used to describe the anatomy of the superior bridging leaflet of the common AV valve in patients with a CAVC [8,9]. The common AV valve is composed of five leaflets: left superior and inferior bridging leaflets, left lateral leaflet, right anterior leaflet, and right lateral leaflet. The superior bridging leaflet is more variable in its size and attachments than the other four leaflets. Initially, the Rastelli classification was important in describing morphologic details of the AV valve that was used for surgical repair. However, over time, the clinical and surgical significance of the Rastelli classification has become less important (figure 2).
•Rastelli type A defects are the most common form and are frequently found in patients who have Down syndrome. In this form, the superior bridging leaflet is committed to the LV with chordal attachments to the crest of the ventricular septum. It shares a commissure with the anterior tricuspid leaflet at the ventricular septum. Interventricular shunting may occur between the anterior and posterior bridging leaflets and underneath the anterior leaflet in the interchordal spaces.
•Rastelli type B is the least common form, in which the superior bridging leaflet extends into the RV with a corresponding decrease in size of the anterior tricuspid leaflet and with chordal attachments into the body of the RV. Because of the lack of chordal insertions to the septum, interventricular shunting occurs under the superior bridging leaflet.
•Rastelli type C is frequently found in combination with other cardiac defects, such as tetralogy of Fallot, transposition of the great arteries, and heterotaxy syndromes [10,11]. In this form, the superior bridging leaflet is even larger and has no chordal attachment to the interventricular septum, referred to as "free floating" [10-12].
ANATOMY AND PATHOGENESIS
Genetics — AV canal defects are thought to be caused by a disruption of the normal development of the endocardial cushions due to an underlying genetic defect or predisposition.
There is a strong association between Down syndrome and AV canal defect, as discussed below (see 'Association with Down syndrome' below). However, there is genetic heterogeneity and other chromosomal abnormalities and single-gene defects have been linked to AV canal defect [13,14]:
●Approximately 40 to 50 percent of cases occur in patients with Down syndrome
●Approximately 30 to 40 percent of cases are nonsyndromic, and the AV canal defect occurs as an isolated finding
●Approximately 20 percent of cases are associated with other chromosomal abnormalities (eg, deletion of 8p23 or 3p25) or genetic syndromes, including Noonan, Ellis-van Crevels, VACTERL, Smith-Lemli-Opitz, DiGeorge, Bardet-Biedl, and CHARGE syndromes
Data from animal studies suggest that the abnormal transformation of epithelial cells to mesenchymal cells during the development of the endocardial cushions leads to AV canal defects [15-18]. In a mouse model of Down syndrome (trisomy 16 mouse), there is a 100 percent incidence of AV canal defects. In these affected mice, the mesenchymal cell density is lower and their migration is slower, which results in an elongated endocardial cushion region compared with normal development in controls [16]. Transforming growth factor beta appears to be a mediator of epithelial-mesenchymal cell transformation during endocardial cushion development [17]. Bone morphogenetic proteins also appear to be essential to normal epithelial-mesenchymal cell transformation and normal AV canal development [18-21].
Association with Down syndrome — There is a strong association between AV canal defects and Down syndrome (trisomy 21). When an AV canal defect is identified on prenatal screening, the likelihood of the fetus having Down syndrome is approximately 40 to 50 percent. Conversely, approximately 40 percent of fetuses with trisomy 21 have an AV canal defect, usually the complete form. In particular, the combination of AV canal defect and tetralogy of Fallot is highly suggestive of Down syndrome because 75 percent of patients with both defects have Down syndrome [12].
Based on molecular mapping of small regions of duplication on chromosome 21, the region at 21q22.1-qter has been designated an AV canal critical region (also referred to as the Down syndrome critical region) [6]. Molecular studies have identified a candidate gene for Down syndrome-associated congenital heart disease called DSCAM [22]. DSCAM encodes for a cell adhesion protein that is expressed in the heart during cardiac development. Other genes beyond chromosome 21 may interact with the gene(s) responsible for Down syndrome-associated congenital heart disease and impact the phenotypic expression. Variants in six genes in the vascular endothelial growth factor A (VEGF-A) pathway have been found in up to 20 percent of patients with Down syndrome and AV canal defects, compared with only 3 percent of patients with Down syndrome without congenital heart disease [23].
Screening for Down syndrome is recommended when AV canal defects are diagnosed by fetal echocardiography [24]. (See "Down syndrome: Overview of prenatal screening" and "Congenital heart disease: Prenatal screening, diagnosis, and management", section on 'Postdiagnostic evaluation'.)
Anatomy
Normal embryology — In normal development, the primitive AV canal connects the atria to the ventricles. At four to five weeks of gestation, the superior and inferior endocardial cushions of the common AV canal fuse and contribute to the formation of the AV valves (ie, mitral and tricuspid valves) and the AV septum (figure 3) [25,26].
Anatomy of atrioventricular canal defects
Endocardial cushion defects — Failure of the endocardial cushions to fuse correctly results in a broad range of AV septal defects and abnormalities of the AV valves (figure 4). In patients with AV canal defects, the AV valve(s) is displaced apically and the inlet portion of the ventricular septum appears "scooped out," resulting in a shorter distance from the crux to the apex of the heart than from apex to the aortic valve [26,27].
●Complete failure of fusion between the superior and inferior endocardial cushions results in both large atrial (primum atrial septal defect [ASD]) and ventricular septal defects (VSD), due to a deficiency of the inlet portion of the interventricular septum, and a single common AV valve.
●Incomplete or partial lack of fusion between the superior and inferior cushions results in a partial AV canal defect with a primum ASD, a common valvular annulus with two separate AV valve orifices, and a cleft in the anterior mitral leaflet.
Atrioventricular valve abnormalities — For both complete and partial AV canal defects, the AV valve(s) is usually abnormal, resulting in regurgitation. Over time, the valve leaflets become more thickened, with prolapse of leaflet tissue and worsening regurgitation. Even after repair, the continued presence of AV valve regurgitation is a significant cause of morbidity and mortality [28].
●Complete AV canal (CAVC) defect – Patients with CAVC defects have a single AV valve, which is often associated with regurgitation. Regurgitation through the common valve may be from left ventricle (LV) to left atrium (LA) or right ventricle (RV) to right atrium (RA). It can exacerbate volume overload and contribute to heart failure.
●Partial AV canal defect – The mitral valve in partial AV canal defects is almost always abnormal. There is usually a cleft of the anterior leaflet, which is often associated with mitral regurgitation. Regurgitation is most common from the LV to LA through the cleft in the anterior mitral valve leaflet. However, regurgitation can also occur from the LV to RA through the AV septum, resulting in an "LV to RA shunt" and RA and RV volume overload.
Double-orifice mitral valve is an uncommon but surgically important abnormality. It is found in approximately 4 to 5 percent of all patients with AV canal defects. The combined effective opening of a double-orifice mitral valve is always less than that of a valve with a single orifice [29]. Double-orifice mitral valve is a risk factor for poor surgical outcome because of significant regurgitation. However, combining the two orifices by incising bridging tissue is not optimal, as it also commonly results in significant left AV valve (mitral) regurgitation.
A parachute mitral valve is associated with papillary muscle abnormalities. The papillary muscles in AV canal defects are usually abnormal in size and location [30,31]. The LV papillary muscles in AV canal defects are typically shifted laterally and closer together compared with their usual location. On occasion, only one LV papillary muscle may be present, resulting in parachute mitral valve deformity. In such a patient, traditional repair with closure of the cleft may result in postoperative mitral valve stenosis or disruption of the mitral valve tensor apparatus [32-34].
Left ventricular outflow tract — The combined changes of valve position and septal deficiency increase the distance between the aorta and the apex of the heart, resulting in an elongation of the LV outflow tract (LVOT). This anatomic change is visualized by echocardiography and angiography as a gooseneck deformity (image 1). Although the LVOT appears elongated and narrow, in most cases, there is no obstruction [35]. When LVOT obstruction occurs, it typically presents in patients with partial AV canal defect because the superior bridging leaflet is frequently bound tightly to the septal crest, causing the LVOT to be longer and more narrow (image 2) [34].
Other anatomical substrates that can be associated with LVOT obstruction include:
●Accessory papillary muscle extending to the LVOT
●Discrete subaortic membrane
●Aneurysm of membranous ventricular septum into the outflow region
●Aortic arch hypoplasia and/or aortic coarctation
Associated cardiac defects — Other cardiac defects are frequently found in patients with AV canal defects and may include [36]:
●Tetralogy of Fallot (see "Tetralogy of Fallot (TOF): Pathophysiology, clinical features, and diagnosis")
●Transposition of the great arteries (see "D-transposition of the great arteries (D-TGA): Anatomy, clinical features, and diagnosis")
●Heterotaxy syndrome (see "Heterotaxy (isomerism of the atrial appendages): Anatomy, clinical features, and diagnosis")
●Patent ductus arteriosus (see "Patent ductus arteriosus (PDA) in term infants, children, and adults: Clinical manifestations and diagnosis")
●Coarctation of the aorta (see "Clinical manifestations and diagnosis of coarctation of the aorta")
●Secundum ASDs (see "Isolated atrial septal defects (ASDs) in children: Classification, clinical features, and diagnosis")
●Absent atrial septum
●Muscular VSDs (see "Isolated ventricular septal defects (VSDs) in infants and children: Anatomy, clinical features, and diagnosis")
●Persistent left superior vena cava
●Anomalous pulmonary venous connection (see "Partial anomalous pulmonary venous return" and "Total anomalous pulmonary venous connection")
Pulmonary and systemic venous abnormalities may also be associated with AV canal defects as a part of heterotaxy syndrome (a rare condition in which the usual position and laterality of the heart and other organs and vessels are disrupted). AV canal defects occur in approximately two-thirds of patients with heterotaxy syndrome [37,38]. The combination of CAVC with double-outlet RV is common in patients with heterotaxy syndrome, especially the asplenia form. Heterotaxy syndrome is discussed in greater detail separately. (See "Heterotaxy (isomerism of the atrial appendages): Anatomy, clinical features, and diagnosis".)
PATHOPHYSIOLOGY — The degree of shunting and risk of pulmonary hypertension depend on the cardiac anatomy and whether the child has Down syndrome.
●Complete AV canal (CAVC) defect – CAVC defects result in increased pulmonary blood flow because of left-to-right shunting at the atrial and ventricular levels (figure 1). As a result, excessive pulmonary blood flow in patients with unrepaired CAVC typically leads to heart failure and, eventually, elevated pulmonary vascular resistance (PVR). Almost all uncorrected patients with CAVC develop heart failure symptoms (eg, failure to thrive, respiratory distress) prior to one year of age. Unrestrictive interventricular communication and significant AV valve regurgitation eventually lead to pulmonary hypertension. If the patient does not develop heart failure symptoms early on, premature development of pulmonary vascular disease (PVD) should be suspected. (See 'Complete atrioventricular canal defect' below.)
●Pulmonary hypertension and Down syndrome – It is uncommon for irreversible PVD to develop in patients with CAVC and other shunting defects before the age of one year. However, patients with Down syndrome are at risk of developing irreversible PVD early in life. In one study, fixed and elevated PVR was noted in 11 percent of infants <1 year old with Down syndrome and CAVC [39]. Patients with Down syndrome often have additional factors other than cardiac disease that contribute to development of pulmonary hypertension (eg, obstructive sleep apnea, hypoventilation, recurrent aspiration), and it is likely that these factors and perhaps genetic predisposition contribute to the accelerated progression of PVD seen in this population. (See "Down syndrome: Clinical features and diagnosis", section on 'Pulmonary disorders' and "Pulmonary hypertension in children: Classification, evaluation, and diagnosis", section on 'Syndromes'.)
●Partial AV canal defect – Partial AV canal defects have left-to-right shunting at the level of the primum atrial septal defect (ASD) (figure 1). This causes volume overload of the right atrium (RA) and right ventricle (RV) and pulmonary overcirculation, but the pulmonary artery pressures are usually normal to mildly elevated. As a result, symptoms may remain minimal until adulthood, similar as in patients with unrepaired ASDs. (See "Clinical manifestations and diagnosis of atrial septal defects in adults".)
If there is significant mitral regurgitation, symptoms of heart failure may occur because of increased atrial shunting. In contrast, symptoms of pulmonary edema may be absent, even in the presence of left atrial (LA) hypertension due to mitral valve regurgitation, because of decompression across the ASD. Patients with partial AV canal defect and severe mitral regurgitation may also present with atrial fibrillation.
●Transitional AV canal defect – Interventricular shunting in transitional AV canal defects is usually minimal due to a small and restrictive ventricular septal defect (VSD). The pathophysiology and clinical presentation of transitional AV canal defects are similar to those of partial AV canal defects. These patients may have a left ventricle (LV) to RA shunt and, rarely, significant AV regurgitation.
●AV valve regurgitation – In patients with complete and partial AV canal defects, the AV valves are usually abnormal and incompetent, resulting in regurgitation. (See 'Anatomy of atrioventricular canal defects' above.)
•In complete defects, regurgitation through the common valve may be from LV to LA or RV to RA.
•In partial defects, most of the regurgitation is from LV to LA through the cleft in the anterior mitral valve leaflet. However, regurgitation can also occur between the LV and RA through the AV septum, resulting in the typical "LV to RA shunt" and RA and RV volume overload.
CLINICAL PRESENTATION
Fetal presentation — Advances in ultrasound technology have enabled accurate diagnosis of AV canal defects as early as the first trimester (image 3). (See 'Fetal diagnosis' below.)
Because AV canal defects are associated with a 40 to 50 percent risk of Down syndrome, genetic evaluation is recommended for the fetus [24]. (See 'Association with Down syndrome' above and "Down syndrome: Clinical features and diagnosis", section on 'Cardiovascular disease' and "Down syndrome: Overview of prenatal screening".)
Most often, AV canal defects are well tolerated in utero, with normal fetal growth and development. AV valve regurgitation, if not severe, usually does not progress in utero [40]. However, AV canal defects with severe AV valve regurgitation, or heart block (which may be associated with heterotaxy syndrome), can lead to nonimmune hydrops fetalis. In such cases, fetal echocardiography may be useful in determining the underlying cause of in utero heart failure and may also be used in monitoring transplacental therapy of heart failure [41]. (See "Congenital heart disease: Prenatal screening, diagnosis, and management".)
Postnatal presentation — The postnatal clinical presentation ranges from heart failure in patients with complete AV canal (CAVC) defects or as an incidental cardiac physical finding in asymptomatic patients with partial or transitional AV canal defects (figure 1).
Complete atrioventricular canal defect
Balanced atrioventricular canal — Most infants with balanced CAVC develop signs and symptoms of heart failure in early infancy. If untreated, pulmonary vascular disease (PVD) and pulmonary hypertension can develop as a consequence of excessive pulmonary blood flow. Infants with Down syndrome are at particularly high risk of developing PVD. In patients who have CAVC associated with tetralogy of Fallot, the development of heart failure and PVD may be mitigated by right ventricular outflow tract (RVOT) obstruction.
●Heart failure – The most common presentation of CAVC in infancy is heart failure due to excessive pulmonary blood flow and biventricular volume overload. Affected infants usually present with symptoms of tachypnea, poor feeding, poor growth, sweating, and pallor around one to two months of age.
The severity of heart failure and timing of onset depends on the size of the interventricular communication, ratio of pulmonary to systemic vascular resistance, and severity of AV valve regurgitation. Almost all uncorrected patients with CAVC develop symptoms by one year of age.
Physical findings of a patient with CAVC and associated heart failure may include:
•Hyperactive precordium with inferior and laterally displaced precordial impulse
•Increased pulmonary component of the second heart sound (S2) due to pulmonary artery hypertension
•Systolic ejection murmur heard at the left upper sternal border because of increased blood flow across the pulmonary valve
●PVD – IF CAVC is not repaired in early infancy, progressive PVD may develop. Infants with Down syndrome are at particularly high risk of developing PVD. The intensity of the pulmonary outflow murmur typically decreases as PVD progresses, a result of diminished blood flow across the pulmonary valve; however, the murmur due to AV valve regurgitation persists. Recurrent respiratory tract infections are another common manifestation of PVD, thought to be due to microatelectasis caused by increased pulmonary blood flow and interstitial pulmonary edema. If heart failure symptoms do not develop by one year of age, premature development of PVD should be suspected.
If CAVC is left unrepaired, PVD progresses, leading to irreversible pulmonary hypertension and, eventually, reversal of direction of shunting to right-to-left (ie, Eisenmenger syndrome). (See "Pulmonary hypertension with congenital heart disease: Clinical manifestations and diagnosis", section on 'Clinical manifestations'.)
●CAVC associated with tetralogy of Fallot – In patients who have CAVC associated with tetralogy of Fallot, the development of heart failure and PVD may be mitigated by RVOT obstruction. If the degree of RV obstruction is moderate, the ratio of pulmonary to systemic blood flow may be fairly balanced. Such infants typically lack heart failure symptoms, and they have good growth and development. However, as the degree of RVOT obstruction increases further and pulmonary blood flow decreases, patients may become more cyanotic. (See "Tetralogy of Fallot (TOF): Pathophysiology, clinical features, and diagnosis".)
Unbalanced atrioventricular canal — If the AV canal defect is unbalanced, one ventricle receives most of the blood flow across the AV valve, resulting in hypoplasia of the other ventricle and, potentially, the corresponding great vessels. Physiologically, an unbalanced AV canal behaves as a single-ventricle defect, with complete mixing of oxygenated and deoxygenated blood at the ventricular level.
Presenting symptoms vary depending on which ventricle is affected and associated lesions:
●Left ventricle (LV) – If LV hypoplasia is severe, the presentation may manifest as a variant of hypoplastic left heart syndrome, or because the aortic valve and aortic arch are also hypoplastic, the presentation may mimic that of other left-sided obstructed lesions such as critical aortic stenosis or aortic coarctation. These typically manifest with signs of low cardiac output (pallor, diaphoresis, dyspnea, weak or absent femoral pulses) as the ductus arteriosus closes. (See "Hypoplastic left heart syndrome: Anatomy, clinical features, and diagnosis" and "Clinical manifestations and diagnosis of coarctation of the aorta", section on 'Neonates'.)
●RV – If the RV is hypoplastic, the presentation is similar to that of hypoplastic right heart conditions such as tricuspid atresia. If there is associated RVOT obstruction (pulmonary valvar or subvalvar stenosis or atresia), cyanosis is the predominant finding once the patent ductus arteriosus closes due to decreased blood flow to the lung. (See "Tricuspid valve atresia".)
Partial and transitional atrioventricular canal defects — Most patients with partial or transitional AV canal defects remain asymptomatic during childhood, and they may be identified based upon detection of a murmur during a routine physical examination. However, symptoms of heart failure can develop during childhood if there is significant mitral valve regurgitation and/or pulmonary overcirculation.
The following physical findings are typically present in patients with a partial or transitional AV canal defect:
●Wide and fixed splitting of the S2 during respiration
●Systolic ejection murmur at the left upper sternal border with radiation to the lung fields due to increased blood flow across the pulmonary valve
●Increased RV precordial impulse
●Diastolic rumble may be present as a result of increased blood flow across the tricuspid valve
●Holosystolic murmur of mitral regurgitation related to a mitral valve cleft
Some patients may not present until adulthood. Adults with uncorrected partial or traditional AV canal defects typically present with exertional dyspnea and/or atrial fibrillation [42].
Imaging and electrocardiogram findings — Many patients undergo initial testing that includes chest radiography and electrocardiography (ECG). However, chest radiograph and ECG findings are nonspecific and echocardiography is required to confirm the diagnosis. (See 'Diagnosis' below.)
●Chest radiograph – In both complete and partial AV canal defects, the cardiac silhouette is usually enlarged secondary to volume overload (image 4).
•In partial AV canal defects, right atrial (RA) and RV enlargement and dilatation of main pulmonary artery are usually present because of increased pulmonary blood flow.
•CAVC defects often demonstrate enlargement in all four cardiac chambers. Increased pulmonary markings are present in patients with a large left-to-right shunt.
●ECG – In patients with AV canal defects, the most characteristic ECG finding is a leftward and superior QRS axis in the frontal plane and counterclockwise depolarization pattern caused by posterior displacement of the AV node and His bundle (waveform 1) [43].
Other potential findings include:
•RA enlargement due to shunting either from the left atrium (LA) to RA or from the LV to RA
•Prolongation of PR interval in some patients with atrial enlargement and increased conduction time [44]
•RV volume or pressure overload, reflected as a rsR' pattern in the right precordial leads (waveform 2)
•LV hypertrophy in the setting of significant left AV valve or common AV valve regurgitation
DIAGNOSIS
Fetal diagnosis — Routine obstetric screening fetal ultrasound with a four-chamber view can make an early in utero diagnosis of AV canal defect (image 3). The presence of a characteristic large defect at the crux of the heart that involves the atrial and ventricular septa and the large common AV valve are key diagnostic features [45]. However, the sensitivity of routine fetal ultrasound is relatively poor, with reported detection rates ranging from 30 to 50 percent [46,47].
When abnormalities are detected or suspected on the screening obstetric evaluation, the mother should be referred for specific fetal echocardiography. Detailed fetal echocardiography provides more precise information regarding the type of AV canal defect, any associated anatomic and hemodynamic abnormalities (especially AV valve regurgitation), and detection of any abnormality of the outflow tracts and the aortic or pulmonic valves. (See "Congenital heart disease: Prenatal screening, diagnosis, and management".)
Postnatal diagnosis — Postnatal diagnosis of AV canal defect is generally confirmed by echocardiography [32,48,49]. Echocardiography is performed based on a strong clinical suspicion for congenital heart disease because of findings from a prenatal echocardiography, postnatal signs and symptoms of heart failure (eg, failure to thrive and tachypnea), and/or physical findings suggestive of cardiac disease (eg, heart murmur). (See 'Clinical presentation' above.)
In most cases, a complete two- and three-dimensional transthoracic echocardiographic evaluation is sufficient to confirm the diagnosis and gather the information necessary for surgical planning [50,51]. Cardiac catheterization is rarely necessary.
Echocardiography — The echocardiographic diagnosis is made by identifying the characteristic AV septal and valvular defects.
Echocardiography assesses the following:
●Presence and size of atrial and ventricular communications
●Direction of shunting
●AV valve morphology
●Orientation of the AV valve
●Relative sizes of the right ventricle (RV) and left ventricle (LV)
●Number and location of papillary muscles
●LV outflow tract (LVOT) obstruction
●RV outflow tract (RVOT) obstruction
●Additional atrial or ventricular septal defects (ASD/VSD)
●Ventricular function
●Associated anomalies, such as abnormalities of systemic and pulmonary venous return or coarctation of the aorta
The apical or subcostal four-chamber views readily provide details of both atrial and ventricular septa using the internal crux as the imaging landmark (image 5). Spectral and color-flow Doppler studies demonstrate the left-to-right shunts at the inferior border of the atrial septum (image 6) and, in complete AV canal (CAVC) defects, the ventricular component. On occasion, an ostium primum defect can be confused with a dilated coronary sinus or an associated coronary sinus septal defect. Visualization of the floor of the left atrial (LA) cavity or roof of the coronary sinus, and the absence of turbulent flow across the coronary sinus, help to differentiate a primum ASD from an enlarged coronary sinus.
In patients with CAVC, the subcostal oblique angle demonstrates the common AV valve annulus (image 7), whereas in patients with partial AV canal defects, short-axis views from the parasternal or subcostal angles best visualize the mitral valve orifice, which is almost always abnormal and usually has a cleft between the superior and inferior leaflets.
The echocardiographer should place particular attention on defining the following:
●AV valve morphology – As noted above, echocardiography detects the characteristic AV valve abnormalities in patients with AV canal defects. Further imaging defines that morphology and assesses the function of the abnormal valve.
•CAVC defects – Patients with CAVC have a common AV valve annulus that is composed of five leaflets. The chordal attachment of the superior bridging leaflet can be evaluated, allowing the noninvasive determination of the Rastelli classification of the common AV valve (see 'Classification' above). In parasternal long-axis view, the valve is seen to open toward the ventricular septum, which is different than the normal appearance of opening toward the apex of the LV. Color-flow Doppler studies show orientation, location, and degree of AV valve regurgitation (image 8).
•Partial AV canal defects – In partial AV canal defects, the typically abnormal mitral valve is best visualized in short-axis views from the parasternal or subcostal views. A cleft in the anterior mitral valve leaflet is best appreciated in a continuous short-axis sweep from the LV apex to the aortic valve. Typically, the cleft in an AV canal defect is oriented toward the ventricular septum. In non-AV canal defects, the cleft is typically oriented toward the aortic valve [31]. The apical and subcostal four-chamber views also confirm that the AV valves are located at the same level in patients with partial AV canal defects, as compared with patients with normal hearts where the tricuspid valve is apically displaced compared with the mitral valve.
Three-dimensional echocardiographic images can be used to evaluate AV valve morphology, with an increased diagnostic accuracy compared with two-dimensional echocardiography [52,53]. En face views by three-dimensional echocardiography from the LV toward the mitral valve can image the length and extent of the cleft. En face views from above or below can detect the relative effective leaflet surface areas of the three components of the mitral valve [54]. These views are similar to the surgeon's view at operation and can aid in surgical planning.
●Ventricular size – Two-dimensional echocardiography can easily diagnose the extreme forms of unbalanced CAVC. However, three-dimensional volume-based echocardiography detects the more subtle forms of unbalanced AV canal defects, which may have an impact on the decision to proceed with one- or two-ventricle repair [55,56]. Echocardiographic parameters that are used to define balanced versus unbalanced CAVC that inform surgical decision-making are discussed in greater detail separately. (See "Management and outcome of atrioventricular (AV) canal defects", section on 'Single-ventricle palliation'.)
●Size of communication and shunting – The degree of ventricular shunting should be assessed from parasternal, apical, and subcostal locations. This is especially important in patients with transitional AV canal defects who typically have a small degree of ventricular shunting. In these patients, it is important to confirm that the VSD is restrictive since surgical repair is performed earlier when unrestrictive ventricular shunting is present. A high pressure gradient from LV to RV confirms the restrictive nature of the VSD. A low-velocity tricuspid regurgitation jet also confirms low RV pressure.
●Papillary muscle – The papillary muscle orientation in AV canal defects is best visualized on short-axis imaging of the LV. As previously discussed, papillary muscles are usually abnormal in size and location and are associated with an increased risk of parachute mitral valve deformities and related mitral valve stenosis [30,31]. (See 'Atrioventricular valve abnormalities' above.)
●LVOT and RVOT – Subcostal imaging demonstrates the typical elongation of the LVOT, referred to as gooseneck deformity (image 1). In most cases, there is no LVOT obstruction. (See 'Left ventricular outflow tract' above.)
Subcostal and parasternal short-axis imaging demonstrate the degree of infundibular narrowing of the RVOT when tetralogy of Fallot is present. In this view, assessment of the pulmonary arteries, including their confluence and their sizes, should also be performed.
Continuous-wave Doppler evaluation reliably estimates combined dynamic and fixed pressure gradient and is used to detect either LVOT or RVOT obstruction (image 2).
●Additional cardiovascular defects – Additional secundum ASDs and VSDs can be identified from subcostal and right parasternal views. Color-Doppler study can provide additional information in evaluating the amount and direction of shunting in such defects.
A patent ductus arteriosus is a common finding associated with an AV canal defect that can be imaged directly from left parasternal, high left parasternal, and suprasternal locations. Color-flow Doppler evaluation effectively identifies the magnitude and direction of shunting. Although direction of shunting is usually left to right across the patent ductus arteriosus, in patients with severe pulmonary hypertension and elevated pulmonary vascular resistance (PVR), right-to-left shunt occurs at the ductal level. In these cases, it may be difficult to detect the presence and patency of the ductus.
Cardiac catheterization — Preoperative cardiac catheterization is rarely needed for routine anatomic delineation. Cardiac catheterization may be required for the assessment of PVR in the older child when pulmonary vascular obstructive disease is suspected. Patients with elevated PVR should be evaluated to determine their response to oxygen and pulmonary artery vasodilators, such as inhaled nitric oxide (iNO). Patients with PVR ≥10 Wood units/m2 or that does not fall to <5 to 7 Wood units/m2 in response to vasodilators are at increased risk for death after surgical repair.
Patients with Down syndrome tend to have higher PVR compared with patients without Down syndrome. The etiology is likely multifactorial and may be related to respiratory comorbidities (eg, obstructive sleep apnea, hypoventilation, recurrent aspiration) and genetic factors. This clinical difference usually resolves with 100% oxygen administration. For patients who have significant upper airway obstruction, endotracheal intubation may be required for the procedure and ventilation to a normal carbon dioxide level may be necessary for accurate measurement of PVR [57,58]. (See "Pulmonary hypertension in children: Classification, evaluation, and diagnosis", section on 'Cardiac catheterization'.)
DIFFERENTIAL DIAGNOSIS — The differential diagnosis of AV canal defects differs depending on the type:
●Complete AV canal (CAVC) defect – CAVC typically presents with signs and symptoms of heart failure in early infancy. The differential diagnosis includes other congenital heart diseases that cause early heart failure, the most common of which is a large ventricular septal defect (VSD). (See "Isolated ventricular septal defects (VSDs) in infants and children: Anatomy, clinical features, and diagnosis".)
Other causes of pediatric heart failure are summarized in the table (table 1) and are discussed in greater detail separately. (See "Heart failure in children: Etiology, clinical manifestations, and diagnosis", section on 'Etiology and pathophysiology'.)
●Partial and transitional AV canal defects – Most children with partial or transitional AV canal defects are asymptomatic, and the defect may come to attention based upon detection of a murmur. The approach to the infant or child with a cardiac murmur is presented separately. (See "Approach to the infant or child with a cardiac murmur".)
AV canal defects are distinguished from other cardiac diseases by echocardiography. (See 'Echocardiography' above.)
SUMMARY AND RECOMMENDATIONS
●Prevalence – Atrioventricular (AV) canal defects are a group of congenital cardiac defects involving the AV septum and AV valves (ie, mitral and tricuspid valves). They occur in approximately 0.4 per 1000 live births, accounting for approximately 5 percent of congenital heart defects. (See 'Epidemiology' above.)
●Anatomy – Failure of the superior and inferior endocardial cushions to fuse correctly during cardiac development results in a broad range of AV septal and valve defects (figure 4 and figure 1). (See 'Anatomy of atrioventricular canal defects' above.)
•Complete AV canal (CAVC) defect is caused by complete failure of fusion between the endocardial cushions, resulting in both large atrial and ventricular septal defects (ASD/VSD) and a common AV valve.
•Partial AV canal defect is caused by incomplete or partial lack of fusion between the endocardial cushions, resulting in a primum ASD, a single-valve annulus with two separate AV valve orifices, and a cleft in the anterior mitral leaflet.
•Other subtypes include transitional (large primum defect, cleft mitral valve, and small-inlet VSD) and intermediate (in which a bridging tongue of tissue divides the common AV valve into two distinct orifices) AV canal defects. The transitional subtype has a similar physiology to that of a partial AV canal defect because there is only a small degree of shunting at the ventricular level.
●Association with Down syndrome – There is a strong association between AV canal defects and Down syndrome (trisomy 21). For this reason, genetic testing should be performed when an antenatal diagnosis of AV canal defect is made. (See 'Association with Down syndrome' above and "Down syndrome: Overview of prenatal screening".)
●Clinical presentation – The clinical presentation of AV canal defects depends of the type of defect (see 'Clinical presentation' above):
•CAVC – Infants with CAVC usually present with symptoms of heart failure (tachypnea, poor feeding, poor growth, sweating, and pallor) in early infancy. If untreated, pulmonary vascular disease (PVD) and pulmonary hypertension can develop as a consequence of excessive pulmonary blood flow. (See 'Complete atrioventricular canal defect' above.)
•Partial and transitional AV canal defects – Most patients with partial or transitional AV canal defects remain asymptomatic during childhood. These defects often come to light through detection of a murmur during a routine physical examination. However, symptoms of heart failure can develop during childhood if there is significant mitral valve regurgitation and/or pulmonary overcirculation. Some patients may present in adulthood with exertional dyspnea and/or atrial fibrillation. (See 'Partial and transitional atrioventricular canal defects' above.)
●Radiographic and ECG findings – Findings on chest radiography and ECG are nonspecific and may include cardiomegaly, increased pulmonary markings, left and superior QRS axis, and a counterclockwise depolarization pattern (image 4 and waveform 1). (See 'Imaging and electrocardiogram findings' above.)
●Diagnosis – AV canal defects are diagnosed by echocardiography with identification of the AV septal and valve defects. Echocardiography delineates the type of AV canal defect and provides information on associated anatomic and hemodynamic abnormalities that may impact surgical management. AV canal defects can be detected prenatally with routine obstetric screening ultrasound, though the sensitivity is fairly poor. (See 'Diagnosis' above.)
●Differential diagnosis – The differential diagnosis of CAVC includes other congenital cardiac conditions that cause early heart failure in infancy, such as a large VSD. For partial AV canal defects, the differential includes other conditions that present with isolated heart murmur. AV canal defects are distinguished from other cardiac conditions by echocardiography. (See 'Differential diagnosis' above.)
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