Chromosomal Anomalies

17.1 Trisomy 13 (Patau Syndrome)

Description and Clinical Features

Trisomy 13 is a chromosomal anomaly in which the fetus has an extra chromosome 13 (i.e., it has three, instead of the normal two, chromosomes 13). It is a rare anomaly, occurring in approximately 1 per 5,000 births. The incidence increases with advancing maternal age. Fetuses with trisomy 13 generally have severe structural anomalies involving multiple organ systems. Most die in the neonatal period, and the few long-term survivors are severely neurologically impaired.

Sonography

Many of the common structural anomalies in fetuses with trisomy 13 can be identified by ultrasound. Among these are the following:

Central nervous system
- Holoprosencephaly
- Ventriculomegaly
- Microcephaly
- Agenesis of the corpus callosum
- Dandy–Walker malformation
Face
- Microphthalmia
- Hypotelorism
- Proboscis
- Midline cleft
Extremities
- Polydactyly
- Radial aplasia
- Flexion deformity of fingers
Diaphragmatic defects (hernia/eventration)
Omphalocele
Cardiac anomalies
Echogenic, enlarged, polycystic kidneys

When a constellation of anomalies, including several of the above, is detected by ultrasound (Figures 17.1.1 to 17.1.3), the diagnosis of trisomy 13 should be considered and karyotype testing offered. Furthermore, several anomalies that occur in trisomy 13 carry an increased incidence of aneuploidy even when seen as an isolated finding (e.g., holoprosencephaly, microcephaly, microphthalmia, omphalocele) and, thus, karyotype testing should be considered.

Figure 17.1.1 Trisomy 13 with holoprosencephaly, hypotelorism, echogenic kidneys, and midline facial defect. A: Coronal image of fetal head demonstrating fusion of the ventricles into a large monoventricle (arrows) and absence of the falx, characteristic of alobar holoprosencephaly. B: Coronal image of the face showing the orbits (arrows) abnormally close together. C: Transverse image of fetal abdomen demonstrating enlarged, echogenic kidneys (arrows). D: Coronal image of face showing midline facial defect (arrow).

Figure 17.1.2 Trisomy 13 with holoprosencephaly, proboscis, polydactyly, and echogenic intracardiac focus. A: Coronal image of fetal head showing large monoventricle (*) of alobar holoprosencephaly. B: Coronal image of face showing elevated and upward pointing proboscis (arrow) instead of a normal nose. The arrowhead points to the chin. C: Sagittal image of face demonstrating very abnormal profile with proboscis (arrow) extending superiorly from the upper part of the face. The large monoventricle (*) is visible in the head. D: Image of clenched fist demonstrating four digits together (arrowheads) and an extra digit (arrow) next the little finger, protruding from the side of the hand. E: Transverse image of thorax with four-chamber view of the heart showing a bright echo within the left ventricle (arrowhead), representing an echogenic intracardiac focus.

Figure 17.1.3 Trisomy 13 with Dandy–Walker malformation, micrognathia, midline facial cleft, atrioventricular canal, and polydactyly. A: Axial image of fetal head demonstrating splaying of the cerebellar hemispheres (arrows) in the posterior fossa and absence of the vermis, as well as fluid (*) extending from the region of the fourth ventricle to the cisterna magna, representing a Dandy–Walker cyst. B: 3D image of face in profile showing very small chin (arrow). C: 3D image of front of face demonstrating midline cleft in upper lip (arrow). D: Transverse view of the thorax demonstrating a large defect in the ventricular septum (arrowhead) and abnormal appearing atrioventricular valves, characteristic of an atrioventricular canal cardiac anomaly (LV arrow, left ventricle; RV arrow, right ventricle). E: Image of hand showing thumb and four fingers (arrowheads) in fist with an extra digit (arrow) next to the little finger.

17.2 Trisomy 18 (Edwards Syndrome)

Description and Clinical Features

Trisomy 18 is a chromosomal anomaly in which the fetus has an extra chromosome 18 (i.e., it has three, instead of the normal two, chromosomes 18). It is a rare anomaly, occurring in approximately 3 per 10,000 births. Like other trisomies, the incidence increases with advancing maternal age. Fetuses with trisomy 18 generally have severe structural anomalies involving multiple organ systems. Most die within the first year of life, and the few long-term survivors are severely neurologically impaired.

Sonography

Many of the structural anomalies in fetuses with trisomy 18 can be identified by ultrasound. Among these are the following:

Central nervous system
- Agenesis of the corpus callosum
- Choroid plexus cysts
- Hypoplastic cerebellum with enlarged cisterna magna
- Strawberry-shaped skull
Face
- Micrognathia
- Hypotelorism
- Microphthalmia
Neck
- Cystic hygroma
Extremities
- Clenched hands with overlapping fingers
- Hypoplastic or absent radius
- Limb contractures
- Clubfoot
- Rocker bottom foot
Omphalocele
Diaphragmatic hernia
Cardiac anomalies
Renal anomalies
Neural tube defects
Intrauterine growth restriction

When a constellation of anomalies, including several of the above, is detected by ultrasound (Figures 17.2.1 to 17.2.4), the diagnosis of trisomy 18 should be considered and amniocentesis and karyotype testing offered. Furthermore, several anomalies that occur in trisomy 18 carry an increased incidence of aneuploidy even when seen as an isolated finding (e.g., microphthalmia, clenched hand with overlapping fingers, rocker bottom foot, omphalocele, diaphragmatic hernia) and, thus, karyotype testing should be considered.

Fetuses with trisomy 18 have an increased incidence of choroid plexus cysts (Figure 17.2.5) in the second trimester compared to fetuses with normal chromosomes. Since choroid plexus cysts may be seen in normal fetuses as well as fetuses with trisomy 18, a choroid plexus cyst is considered a “marker” for trisomy 18, not a congenital anomaly. The presence of this “marker” increases the likelihood that the fetus has trisomy 18. The risk of trisomy 18 is even higher in fetuses found to have choroid plexus cysts together with another anomaly (Figures 17.2.6 to 17.2.8), especially one of the anomalies listed above.

Figure 17.2.1 Trisomy 18 with microphthalmia, nuchal cystic hygroma, hypoplastic left heart, ascites, and persistent flexion of the wrists. A: Axial image through upper face showing hypoplastic orbits (arrows). B: Image through head and upper neck demonstrating a fluid collection in soft tissues of neck, representing a cystic hygroma (arrow). C: Image of four-chamber heart showing small, slit-like left ventricle (long arrow) and normal right ventricle (short arrow). D: Axial image of abdomen demonstrating ascites (arrow). E: View of both hands showing abnormal flexion of both wrists (arrows).

Figure 17.2.2 Trisomy 18 with Dandy–Walker malformation, cardiac defect, clubfoot, and single umbilical artery. A: Axial image of posterior fossa showing absence of the cerebellar vermis, with fluid (arrow) between the cerebellar hemispheres (arrowheads) connecting the fourth ventricle to the cistern magna. B: Four-chamber view of the heart showing a large ventricular septal defect (arrow). C: Image of lower extremity demonstrating clubfoot (arrows). D: Axial color Doppler image of pelvis showing single umbilical artery (arrow) on one side of the bladder (*).

Figure 17.2.3 Trisomy 18 with strawberry skull and persistent wrist flexion. A: Axial image of head demonstrating strawberry-shaped cranium. B: 3D image of fetus demonstrating abnormal flexion of both wrists.

Figure 17.2.4 Trisomy 18 with overlapping fingers and rocker bottom feet. A: 3D image of fetal hand at 30 weeks showing a clenched fist with the index finger (arrow) overlapping the middle finger. B: Coronal view of the fetal lower extremities showing rocker bottom foot deformity (arrows) bilaterally.

Figure 17.2.5 Choroid plexus cysts. A: Oblique image of fetal head showing single choroid plexus cyst (arrow). B: Axial image of another fetus showing multiple bilateral choroid plexus cysts (arrows).

Figure 17.2.6 Trisomy 18 with choroid plexus cyst and clenched hands with overlapping digits. A: Axial image of head showing cyst (*) within the choroid plexus of the lateral ventricle (calipers). B: Image of both hands showing abnormal overlapping of index finger over middle finger bilaterally (arrows).

Figure 17.2.7 Trisomy 18 with choroid plexus cysts and clubfoot. A: Axial image of head showing bilateral choroid plexus cysts (arrows). B: Image of clubfoot (arrows).

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