Chromosomal Defects Flashcards
Trisomy 21 Features
Nasal hypoplasia, increased prenasal and nuchal fold thickness, cardiac defects, intracardiac echogenic foci, duodenal atresia and echogenic bowel, mild hydronephrosis, shortening of the femur, sandal gap and clinodactyly or mid-phalanx hypoplasia of the fifth finger.
Trisomy 18 Features
Strawberry-shaped head, choroid plexus cysts, absent corpus callosum, enlarged cisterna magna, facial cleft, micrognathia, nuchal edema, heart defects, diaphragmatic hernia, esophageal atresia, exomphalos, single umbilical artery, renal abnormalities, echogenic bowel, myelomeningocoele, growth restriction and shortening of the limbs, radial aplasia, overlapping fingers and talipes or rocker bottom feet.
Trisomy 13 Features
Holoprosencephaly, microcephaly, facial abnormalities, cardiac abnormalities, enlarged and echogenic kidneys, exomphalos and post axial polydactyly.
Tripoidy
When there is double paternal contribution (diandric) there is a molar placenta and the pregnancy rarely persists beyond 20 weeks. When there is a double maternal contribution (digynic) the placenta is thin but of normal consistency and the pregnancy may persist into the third trimester; the fetus demonstrates severe asymmetrical growth restriction, mild ventriculomegaly, micrognathia, cardiac abnormalities, myelomeningocoele, syndactyly, and ‘hitch-hiker’ toe deformity.
Turner Syndrome Features
Large cystic hygromas, generalised edema, mild pleural effusions and ascites, cardiac abnormalities and horseshoe kidneys.
47,XXX, 47,XXY or 47,XYY
These are not associated with an increased prevalence of sonographically detectable defects.
Management of Multiple abnormalities
The risk for chromosomal defects is substantially increased and fetal karyotyping should be considered.
Management of Isolated abnormalities
The decision of whether to carry out an invasive test depends on whether the abnormality is major or minor.
If there is a major abnormality it is advisable to offer fetal karyotyping, even if these abnormalities are apparently isolated. This is because: first, if the abnormalities are either lethal or they are associated with severe handicap (e.g. holoprosencephaly), fetal karyotyping constitutes one of a series of investigations to determine the possible cause and thus the risk of recurrence, and second, if the abnormality is potentially correctable by intrauterine or postnatal surgery (e.g. diaphragmatic hernia), it is important to exclude an underlying chromosomal defect – especially because, for many of these conditions, the usual defect is trisomy 18 or 13.
If there is a minor abnormality the estimated risk can be derived by multiplying the a priori risk (based on the results of previous screening) by the likelihood ratio of the specific abnormality or marker. The likelihood ratio for trisomy 21 is about 1 (therefore the a priori risk is not increased in the case of choroid plexus cysts, echogenic endocardiac focii, mild hydronephrosis and short femur) and about 10 (therefore there is a 10-fold increase in the a priori risk for nuchal or prenasal edema and hypoplastic nasal bone).
