Haifa, Israel

Definition - A common atrioventricular junction with a five-leaflet common valve. The incomplete form of Atrioventricular septal defect is comprised of a spectrum of malformations characterized by varying degrees of incomplete development of the inferior portion of the atrial septum, the inflow portion of the ventricular septum and the AV valves (1).

Synonyms - Common AV Canal. Endocardial Cushing Defect.

Prevalence - Atrioventricular septal defect accounts for 3-7% of congenital heart defects in infants; in fetuses the incidence is higher. Allen et al (2) reported on an incidence of 17% in a high-risk population. In our series, which is comprised of mainly low risk pregnancies, the incidence is 11%.
Rastellis classification (3). Type A: The anterosuperior-bridging leaflet is attached to the septum by chordae tendineae. Type B: The leaflet is attached to a septal papillary muscle on the right side of the ventricular septum. Type C: The leaflet is extended to the anterior papillary muscle within the right ventricle.

Prenatal diagnosis
In the normal four-chamber view the atrioventricular valves have a differential insertion on the septum. The tricuspid valve is slightly more apically inserted as compared to the mitral valve. The view of the AV valves with the septum and septum primum provide an image with resembles a cross – the crux of the heart. (Figure A)

Figure A: Normal crux: The differential insertion of the atrioventricular valves .

The classic main sonographic markers of Atrioventricular septal defect are (1,2,4):

1. A large hole in the middle of the heart and the absence of the crux. (Fig. B,C)
2. Linear insertion (non-differential insertion) of the A-V valves (Fig D)
3. Common atrioventricular junction. (Fig. E)
4. VSD or absence of septum primum (Fig. D)
5. Pseudo overriding aorta — "unsprung†from the crux of the heart (gooseneck) Fig. F
   

Figure B :Atrioventricular septal defect  week 12

Figure C :Atrioventricular septal defect  week 15

Figure D :Atrioventricular septal defect  week 15: Absence of septum primum(low arrow), non differential insertion (linear) of the av valves  and vsd (double arrows)

Figure E :Atrioventricular septal defect  week 16 : common insertion of av valves ("Mercedes" sign )

Figure F   : "Gooseneck" deformity of the ascending aorta (pseudo overriding)

Diagnosis in early pregnancy — Atrioventricular septal defect is best imaged in an apical view or a posterior anterior view from the fetal back. The views are easily obtained because there is only minimal ossification of the vertebrae and ribs in early pregnancy. According to our experience there is an additional sonographic marker in early pregnancy — the unstable floating "kicking" crux.

While in normal hearts the point of insertion of the valve on the cardiac wall is clear, the point is not so clear in cases of Atrioventricular septal defect. There is a continuous flutter of the valve making it difficult to clearly define the borders of the valve and the cardiac wall. It seems that in early pregnancy the other classic signs may not be observed in all cases (4,5). The unstable floating "kicking" crux might be the only sign of the anomaly in such cases.(Fig. G,H)

There are three levels of difficulty in the diagnosis of Atrioventricular septal defect. The easiest cases to diagnose are those with a complete absence of the crux of the heart (Fig B,C ). Such an image probably represents the type C defect according to Rastelli.
More difficult to diagnose are the cases with the linear insertion of the valves (Fig D,E). These cases probably represent the type B defect according to Rastelli.
The most difficult diagnosis is in cases where no defect is observed in the septum and the valves seem to have a differential insertion (5) (Fig G). The only clue is usually the floating "kicking†crux. This probably represents the type A defect (Fig. H) according to Rastelli and may be overlooked in many in early pregnancy.

Figure G :Atrioventricular septal defect  week 15. According to the to the classical criteria the crux is "normal"

Figure H :Atrioventricular septal defect  Autopsy (week 20) .The arrow points to the common av valve (Rastelli A) Courtesy S. Egenburg
Differential disgnosis — VSD, dilatation of the coronary sinus in cases of persistent left superior vena cava.(7)
Atrioventricular septal defect and Down's syndrome - About 66% of fetuses with Atrioventricular septal defect have DS. Nearly 25% of fetuses with DS have AVDS (2,6).

Clinical implications:

Associated anomalies 50%, mainly, heterotaxy syndrome

• Chromosomal anomalies — 60%, mainly trisomy 21
• Intrauterine fetal death — 17%
• Neonatal death — 25%
• Infant death — 10%
• Survival rate — 50-60% of those without chromosomal anomalies
  

Management — Reconstruction of AV valve and repair of clefts at the age of 3-6 months. The mortality rate at surgery is less than 10%.

References
1) Perloff JK. The Clinical Recognition of Congenital Heart Disease. 4th edn. WB Saunders Company 1994.
2) Allan L, Hornberger LK, Sharland G. Textbook of Fetal Cardiology.Greenwich Medical Media Publishing 2000.
3) Rastelli GC, Kirklin  JW, Titus JI. Anatomic observation on complete form of common atriventricular canal with special reference to atriovenricular valves. Mayo Clinic Proc 1966 41:296-308.      
4) Bronshtein M, Zimmer EZ. Transvaginal sonography of the normal and abnormal   fetus. Parthenon Publishing 2001.
5) Bronshtein M, Egenburg S, Auslander R, Zimmer EZ. Atrioventricular septal defect in a fetus: a false negative diagnosis in early pregnancy. Ultrasound Obstet Gynecol 2000;16:98-9
6) Paladini D, Calabro R, Palmieri S, D'Andrea T. Prenatal diagnosis of congenital heart disease and fetal karyotyping. Obstet Gynecol 1993;81:679-82.
7) TheFetus.net