Pathology and embryology of common atrioventricular canal

This paper focuses on the terminology, classification, pathologic anatomy, embryology, and etiology of common atrioventricular (AV) canal. The designa tion common AV canal is preferred because it includes both the characterist ic septal and leaflet defects; i.e. common AV canal is much more than an AV septal defect, as the cleft in the anterior mitral leaflet indicates. This cleft is indeed a cleft (not a commissure), and the left-sided AV valve is a malformed mitral valve (not a trileaflet non-mitral valve). Common AV ca nal is classified as complete, partial, transitional, intermediate, balance d, left ventricular type, and right ventricular type. The complete form is subclassified as Rastelli types A, B, and C. The normal embryology of AV ca nal division is presented in man and in the rat. In humans, the superior an d inferior endocardial cushions of the AV canal normally fuse during Street er's horizon XVII, i.e. 34-36 days of age. Al understanding of the spatial geometry of the endocardial cushions of the AV canal relative to the ventri cular and atrial septa helps to explain normal and abnormal development and the classification of common AV canal. Normal morphogenesis is a three-ste p process. An increase in fibroblast cell-surface adhesiveness may well be important in the morphogenesis of common RV canal in Down syndrome. Etiolog ically, a single gene stochastic model is now thought more likely than the polygenic multifactorial hypothesis. The Down syndrome congenital heart dis ease gene (or genes) is tare) now thought to be located on chromosome 21 fr om q22.1 to q22.3. Common AV canal may be non-syndromic or syndromic, the l atter including Down syndrome and the heterotaxy syndromes with asplenia an d polysplenia. There are statistically highly significant differences betwe en Down and non-Down canals, and between asplenic and polysplenic canals. ( C) 1999 Elsevier Science Ireland Ltd. All rights reserved.