|Trisomy 13 (Patau Syndrome)|
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 one per 5000 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.|
Many of the common structural anomalies in fetuses with trisomy 13 can be identified by ultrasound. Among these are the following:
Central nervous system
Agenesis of the corpus callosum
Flexion deformity of fingers
Diaphragmatic defects (hernia/eventration)
Echogenic enlarged kidneys
When a constellation of anomalies, including several of the above, is detected by ultrasound, the diagnosis of trisomy 13 should be considered and amniocentesis should be offered. Furthermore, several anomalies that occur in trisomy 13 are associated with an increased incidence of aneuploidy even when seen as an isolated finding (e.g., holoprosencephaly, microcephaly, microphthalmia, omphalocele) and thus are indications for amniocentesis.
Trisomy 13 with diaphragmatic hernia: Transverse view through the fetal thorax demonstrates a left-sided diaphragmatic hernia with an intrathoracic fetal stomach (long arrow) deviating the heart (short arrow) to the right.
Trisomy 13 with micrognathia: Sagittal midline view of the face demonstrates an abnormally small jaw (arrow), representing micrognathia.
Trisomy 13 with abnormal kidneys: Coronal view of the abdomen shows both kidneys (arrowheads) to be hydronephrotic and hyperechoic.
Trisomy 13 with a Dandy-Walker malformation: Transverse view through the head demonstrates a cyst (arrow) in the posterior fossa replacing most of the cerebellar vermis, representing a Dandy-Walker malformation.
Trisomy 13 with polydactyly: The left fetal hand has six digits, including the thumb (arrow) and five other fingers (arrowheads), representing polydactyly.
Trisomy 13 with hypoplastic left heart: The left ventricle (LV arrow) is hypoplastic and has echogenic foci (arrowhead). The right ventricle (RV arrow) appears enlarged.
Trisomy 13 with cleft lip: Coronal view through the lower face reveals bilateral clefts (arrows) in the upper lip extending to the nostrils.
Trisomy 13 with cerebellar hypoplasia: Axial view of the head demonstrates a small posterior fossa with a hypoplastic cerebellum (arrow) and no fluid in the cisterna magna.
Trisomy 13 with ventricular septal defect: Four-chamber view of the heart demonstrates an opening (arrow) in the ventricular septum, forming a communication between the left ventricle (LV arrowhead) and right ventricle (RV arrowhead).
Trisomy 13 with abnormal kidneys: Longitudinal view of a kidney (arrows) demonstrates it to be enlarged and echogenic.