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III./1.2.: Atrial septal defect (ASD)
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Atrial septal defect (ASD) appears in the weeks 3-7 of embryonic age, with an incidence of 1:1500. This malformation constitutes ca. 10 % of all cardiac disorders.
In weeks 4-5 of embryonic development, the common atrium is separated by a fold (septum primum) into right and left atria. Due to programmed cell death (apoptosis), around the time of complete closure of the septum primum, septal perforations appear, the fusion of which forms the ostium secundum. Another fold beside the septum primum, the septum secundum is generated, rather like a 'cover plate' of the ostium secundum, forming the so called foramen ovale. This opening passes blood freely between the right and left atria. At birth, with the onset of pulmonary circulation, blood pressure is elevated in the left atrium, enabling closure of the 'cover plate' of foramen ovale, and blocking the interatrial passage of blood (image 3).
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Take a look at the images!
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image 3: Foramen ovale apertum. The open hole is an important part of the fetal circulation, but – like in the case of Botallo’s duct – it is unnecessary after birth, therefore, due to the fusion of the septum primum and secundum, the opening disappears and only a bight (fossa ovalis) remains marking the place of the original opening (from the archive of Semmelweis University,2nd Department of Pathology –collected by Attila Kovács and István Kenessey).
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An ostium secundum defect may be due either to abnormal development of septum secundum, of to a larger than normal interatrial opening, following an overshoot of apoptosis. Depending on the diameter of the opening, an intracardial left-to-right shunt (shunt: blood flow of abnormal direction in atria, ventricles or the principal vessels) is likely to form. Through the shunt evoked by the ASD blood steadily flows from the left atrium into the right atrium, rather than into the aorta via the left ventricle. The shunt leads to overloading of the right atrium and subsequent cardiac insufficiency, In the most severe cases, a condition known as common atrium (cor triloculare biatriale) may develop, with a total lack of interatrial septum.
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