Pathology of trisomy 21--with particular reference to persistent common atrioventricular canal of the heart.

While certain external anomalies are specific and almost constant features in Down's syndrome, internal anomalies seem to be more variable in terms of frequency and severity. They may affect any organ system and are more often of minor clinical significance. However, severe malformations may occur. When they affect the cardiovascular system, postnatal survival is impaired, which is responsible for a distinctly higher incidence of cardiac and other malformations in younger children or neonates with Down's syndrome. Fetuses with trisomy 21 at midterm pregnancy show even more frequent manifestation of developmental disorders suggesting an increased spontaneous abortion rate in the second half of pregnancy. The analysis of malformations and minor anomalies in Down's syndrome compared to those of other chromosomal aberrations shows no absolute specificity but a tendency for certain developmental disturbances. These are characterized not so much by the organ system involved, but much more by the time in which the disturbance of a developmental process becomes evident, thus influencing type and localization of an anomaly. Particular reference is made to anomalies of the cardiovascular and cerebral system.