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ETIOLOGY OF ATRIOVENTRICULAR
SEPTAL DEFECTS (AVSD) |
In the primitive heart the common atrium and ventricle communicate via the atrioventricular (AV) canal. As the endocardial cushion begins to develop, the single large AV canal divides into two separate orifices, separating the atria from the ventricles. The interatrial and interventricular septae develop concurrently dividing the single atrium and ventricle into left and right portions. When the endocardial cushions fail to fuse properly, normal development of the mitral and tricuspid valves cannot occur and an atrioseptal defect results.
AVSD's are divided into:
In both types the AV canal has a single large valve with five leaflets. In the complete form the five leaflets are separate, whereas in the incomplete form, two of the leaflets are connected by a narrow strip of tissue (bridging leaflets).
The incomplete form is associated with an ostium primum ASD and a cleft in the anterior mitral leaflet of the single AV valve.
The fetal incidence of AVSD is four times greater than the live born
population suggesting a high incidence of in utero
demise (2).
Embryologically, the endocardial cushions form the atrial
surface of the AV valves and the AV portion of the membranous septum(3,4), while the membranous part of the ventricular
septum develops in the last part of the first trimester of pregnancy after the septal leaflet of the tricuspid valve has delaminated from
the muscular part of the ventricular septum (3).
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In normal fetuses, the septal
leaflet of the tricuspid valve is inserted more apically
than the septal leaflet of the mitral
valve, and a nomogram of
this differential insertion or AV valve offsetting was recently reported in
fetuses.
·
Absence of AV valve offsetting is considered to
be an important criterion for the prenatal diagnosis of
·
AV canal (5)
·
isolated perimembranous
inlet VSD,
·
AV discordance and straddling atrioventricular valves (6). AV canal has been proposed to
have a wide anatomical spectrum (7). One end of this spectrum is represented by
the complete form of AV canal, which is associated with trisomy
21 in as many as 79% of the cases (23), while at the other end is the absence
of AV valve offsetting without septal defects (8)].
Indeed, Fredouille et al. reported that two-thirds of trisomy
21 fetuses had absence of AV valve offsetting without a septal
defect during autopsy (24). Although absence of AV valve offsetting was seen in
the cases presented here, other features of AV canal such as a trileaflet left AV valve (cleft mitral
valve) and common AV junction were not seen pre- or postnatally.
Since there was AV concordance, the most likely explanation is that these cases
may represent spontaneously closed VSDs originally
extended into the inlet.
The membranous septum and AV valves are embryologically
related and it is possible that early insults may affect the normal development
of both cardiac structures. Indeed, studies with transillumination
of the ventricular septum showed that trisomy 21
cases without open septal defects have a larger
membranous septum compared to neonates with normal karyotype
(9). Furthermore, membranous septal aneurysms have
been described in association with endocardial
cushion defects (10).
REFERENCES |