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Trisomy 21 (Down Syndrome)
Trisomy 21 is a chromosomal anomaly in which the fetus has an extra chromosome 21 (i.e., it has three, instead of the normal two, chromosomes 21). It is the most common chromosomal anomaly in newborns, occurring in one per 700 births. Like other trisomies, the incidence of trisomy 21 increases with advancing maternal age.
Fetuses with trisomy 21 have increased incidence of cardiac defects, duodenal atresia, and other structural anomalies. Trisomy 21 is not usually fatal unless there is a life-threatening cardiac structural defect. Children and adults with trisomy 21 have subnormal intelligence.

Sonography

Many of the structural anomalies in fetuses with trisomy 21 can be identified by ultrasound. Among these are the following:
Cerebral ventriculomegaly
Cardiac
        Atrioventricular canal
        Ventricular septal defect
        Tetralogy of Fallot
Neck
        Nuchal translucency > 2.2 ~ 2.8 mm at 11 ~ 14 weeks
        Nuchal fold > 6 mm at 16 ~ 20 weeks
Duodenal atresia


In addition to the above, there are a number of sonographic findings in the second trimester that appear to be weakly associated with trisomy 21 (and, for some of the findings, other forms of aneuploidy). These “minor markers” of aneuploidy include a short femur and humerus, echogenic bowel (bowel at least as echogenic as bone), echogenic intracardiac focus, and renal pyelectasis. (Note: choroid plexus cysts, another “minor marker” of aneuploidy, are associated with trisomy 18 only). There is some disagreement about whether amniocentesis should be offered when ultrasound demonstrates a single minor marker and no other abnormality. It is generally agreed, however, that the finding of two or more minor markers, or one minor marker in conjunction with another significant fetal abnormality, warrants amniocentesis.



Thick nuchal fold in a fetus with trisomy 21. Angled axial view of the fetal head demonstrates a thick nuchal fold (calipers), measuring 7.2 mm



A
B

Duodenal atresia and hypoplastic left ventricle in a fetus with trisomy 21. A: Transverse view of the fetal abdomen reveals two fluid-filled structures (“double bubble”), representing a distended stomach (ST) and duodenum (DU). There is also polyhydramnios. B: Four-chamber view of the fetal heart reveals a hypoplastic left ventricle (LV arrow), with a normal right ventricle (RV arrow).



A
B

Echogenic bowel and pericardial effusion in a fetus with trisomy 21. A: Transverse view of the fetal abdomen reveals a region of bowel (arrowheads) that is equal in echogenicity to the vertebral ossification centers (arrows). B: Transverse view of the fetal thorax demonstrates a pericardial effusion (arrows).
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