ENDOCARDIAL CUSHION
DEFECTS
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| ENDOCARDIAL CUSHION DEFECTS: ANATOMY AND EMBRYOLOGY |
The group of anomalies known as the endocardial cushion defects (ECDs) is of interest to not only the cardiologist but the embryologist, pathologist, and surgeon as well. All ECD types are primarily caused by a developmental defect of the atrioventricular endocardial cushions. Normally, the endocardial cushions fuse with each other and bend to form an arc, the convexity of which is toward the atrial side. The atrial septum fuses with the apex of the arc, thus dividing it into two approximately equal parts. The right half contributes to the ventricular septum, the atrioventricular septum, and the medial or septal cusp of the tricuspid valve. The left half of the fused cushions forms the aortic or anterior cusp of the mitral valve.
In ECD the cushions partly fuse or
do not fuse, and the arc is usually not formed (see Plate 5-9). This results in the following pathologic features characteristic
of ECDs, shared by all types to varying degree:
1. The aortic cusp of the mitral valve is cleft, and
its origin is concave instead of convex, as in the normal heart.
2. The interventricular septum has a peculiar,
scooped-out appearance.
3. The left ventricular outflow area is narrower
and longer than normal.
4. The superior-inferior diameter of the ventricles
is increased at the base.
5. Imaging may show a large characteristic
interatrial communication, a ventricular communication, or both.
If fusion of the cushions fails
completely, the atrioventricular ostia form a large, single ostium (complete
type of endocardial-cushion defect, also called persistent common
atrioventricular canal), and there is a large, central septal defect that
allows free communication between all four chambers. The common atrioventricular valve consists of the normal left mural
(posterior) mitral valve cusp, the anterior and posterior tricuspid valve
cusps, and two large cusps that cross the defect and have developed from the
unfused endocardial cushions. Either cusp or both these cusps may be attached
to the top of the ventricular septum by short chordae tendineae. The specimen
illustrated in Plate
5-9 also has a persistent left
superior vena cava.
If the cushions fuse only centrally,
there is a division of the atrioventricular canal into right and left atrioventricular
ostia, but the mitral valve (and often the septal cusp of the tricuspid valve)
is cleft (partial ECD). Several types of ECD are distinguished, mainly
depending on whether there is an interventricular or interatrial communication.
The partial form, with only an interatrial communication, is known as the
“ostium primum” type of ASD, as previously discussed. Again, the communication
does not really correspond to the embryonic ostium primum, its position being
similar to that of the atrioventricular septum of the normal heart. It must be
emphasized that the atrial septum in ECDs typically is normally developed and
complete, although associated ASDs do occur. The cleft mitral valve is usually
incompetent. Even in cases of complete ECD, the valve may be competent.
The clinical manifestations of the
ostium primum type of ECD largely resemble those seen in uncomplicated ASD.
Symptoms tend to appear earlier in life, however, and growth retardation,
fatigability, dyspnea, and respiratory infections are often more pronounced.
Pulmonary vascular changes, resulting in right ventricular and pulmonary artery
hypertension, are more common and are likely to occur earlier. A thrill is not
uncommon, and auscultation again finds the systolic murmur at the left upper sternal border and the
fixed splitting of S2, as in ASD. In addition, however, in just over
half the cases, a high-pitched, blowing, systolic murmur of mitral
insufficiency is present at or within the apex and transmitted to the axilla.
On chest radiography the heart tends
to be somewhat larger than in ASDs. The heart may assume a configuration of
left ventricular enlargement, with the apex turned down and out in the presence
of significant mitral insufficiency. Even
with mitral insufficiency, however, there is no left atrial enlargement unless
the interatrial communication is small or absent. Other chest radiograph
features are similar to those seen in primum ASD.
The ECG shows a left deviation of
the QRS axis in the frontal plane, usually between 0 and −60 degrees, but
sometimes more to the left. In the complete type of ECD, the QRS axis may be
located in the right upper quadrant. The precordial leads are similar to those
seen in ASD, but the evidence for right ventricular enlargement tends to be
more pronounced, and the left pre-cordial leads may show a pattern of left
ventricular hypertrophy resulting from mitral incompetence. The left-axis
deviation seen in ECD apparently is not related to possible left ventricular
hypertrophy and seems to be caused by an abnormal anatomic position of the
conduction system. Conduction system abnormalities such as heart block can
occur.
Cardiac ultrasound can confirm the
diagnosis of an ostium primum ASD and easily demonstrates mitral and tricuspid
regurgitation, if present. Cardiac MRI also can define the anatomic
pathophysiology. Cardiac-catheterization findings are similar to those in ASD
and usually are not helpful in differentiating the two entities.
Angiocardiography, on the other hand, is an extremely valuable tool because a
selective left ventricular angiogram shows a configuration not observed in any
other cardiac anomaly. The scooped-out ventricular septum and the long, narrow
left ventricular outflow area are readily apparent during diastole, whereas
during systole the two halves of the cleft mitral valve cusp are seen to bulge
into the left atrium, with a notch indicating the position of the cleft. Mitral
insufficiency, if present, is also readily demonstrated.
The complete type of ECD usually
causes severe problems early in infancy, including repeated respiratory
infections, feeding difficulties, growth retardation or serious failure to
thrive, dyspnea, and congestive heart failure. Most of these children die
within the first 2 years of life. Cyanosis is rare unless there is an
associated obstruction of the right ventricular outflow tract, respiratory
infection, or heart failure. Cardiomegaly develops rapidly after birth. In
general, the larger the ventricular component, the sicker is the child; if this
component is small, the clinical manifestations resemble those of the partial
ostium primum type. A well-documented association exists between ECDs,
particularly the complete type, and Down syndrome, which is seen in 35% to 40%
of patients with a complete ECD.
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| SURGERY FOR OSTIUM PRIMUM AND CLEFT MITRAL VALVE |
The treatment of ECDs consists of
open surgical correction of the malformation, always employing a
cardiopulmonary bypass. Although technically much more difficult than closure
of a simple ASD, the procedure now carries an acceptably low mortality rate if
the anomaly is the partial type. The interatrial communication is accurately
closed by employing a prosthesis of appropriate size. Direct suture should not
be done in most cases, since it may cause distortion of the left atrioventricular ostium and thereby aggravate
mitral insufficiency. Traditionally, the cleft in the anterior cusp of the
mitral valve has been sutured, to create a more or less normal cusp and reduce
insufficiency when present or prevent its development. Although suture of the
cleft in cases with marked mitral incompetence seems justified, the wisdom of
carrying out such a procedure in patients with a competent valve is highly debatable. In fact, suture of a competent cleft
may well be contraindicated, because it will interfere with the ability of the
cusp to open freely and completely and thus produce mitral stenosis (see Plate 5-10).
Correction of the complete forms of endocardial cushion defect is technically more difficult and, in some cases, impossible. In addition, children with ECD are generally smaller and more disabled.
