<B>atrial septal defect </B>

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atrial septal defect

Etiology:

• Unknown in > 90% of cases. Identifiable chromosomal abnormalities in 5% of cases,i.e., trisomies 18 & 21.
• Clear that etiology not entirely genetic, since only one of monozygotic twins may develop an anomaly, and German measles in the mother during the first trimester leads to CHD and a variety of defects in other systems, including immunodeficiencies.
• Definite evidence of radiation, and chemicals also being etiologic factors.

Pathogenesis:

• Septum primum, septum secundum, and the sinus venosus are involved anlage in 3 types of defects.
• Failure of S. primum to form completely results in a low ASD adjoining AV valve.
• Failure of normally partially formed S. secundum to reach antero-inferiorly far enough, &/or excessive S. primum resorption results in fossa ovalis ASD's.
• Primitive sinus venosus contributes to atrial walls, septum, ostia of S.&I. Vena cavas and pumonary veins.
• Defects form high or low in septum near V. cava ostia.

Epidemiology:

• ASD found in 5% of all cases of congenital heart disease (CHD).
• CHD most common form of heart disease in childhood.
• Incidence is 6-8/1000 live full term births.
• Incidence higher in stillborns and premature births.
• 90% of ASD's are of the septum secundum type, and 5% of the septum primum type.

General Gross Description:

• Septum primum defects are adjacent to AV valve ring, antero-inferior to fossa ovalis.
• Septum secundum defects form within the fossa ovalis or along its superior margin if S. secundum development is incomplete.
• Sinus venosus defects are located immediately adjacent to the ostia of the superior or inferior vena cavas, and are often associated with anomalies of pulmonary venous drainage into the R. atrium or Superior vena cava.

General Microscopic Description:

• Individual layers of involved structures are histologically normal.

Clinical Correlations:

• A left to right atrial shunt increases pulmonary blood flow, but may be asymptomatic throughout life if the defect is much less than 1 cm in diameter.
• Arrhythmias and a murmur developing in the third decade with larger defects prompt evaluations for corrective low risk surgery to prevent later potential pulmonary hypertension.
• Chronic 2-4 X normal pulmonary blood flow may provoke pulmonary hypertension with right to left shunt and heart failure in less than 10% of cases.

References:
•1. Robbins Pathological Basis of Disease, 5th Edition pp. 573-574. •2. Harrison's Principles of Internal Medicine, 13th Edition, pp. 1040-1041. •3. Langman's medical Embryology, 7th edition, pp.188-197.