(Patau Syndrome; Trisomy D)
Trisomy 13 is caused by an extra chromosome 13 and causes abnormal forebrain, midface, and eye development; severe intellectual disability; heart defects; and small birth size.
Trisomy 13 occurs in about 1/10,000 live births; about 80% of cases are complete trisomy 13. Advanced maternal age increases the likelihood, and the extra chromosome is usually maternally derived.
Infants tend to be small for gestational age. Midline anomalies are common and include holoprosencephaly (failure of the forebrain to divide properly), facial anomalies such as cleft lip and cleft palate, microphthalmia, colobomas (fissures) of the iris, and retinal dysplasia. Supraorbital ridges are shallow, and palpebral fissures usually are slanted. The ears are abnormally shaped and usually low-set. Deafness is common. Scalp defects and dermal sinuses are also common. Loose folds of skin often are present over the back of the neck.
Simian crease (a single, palmar crease), polydactyly, and hyperconvex narrow fingernails are also common. About 80% of cases have severe congenital cardiovascular anomalies; dextrocardia is common. Genitals are frequently abnormal in both sexes; cryptorchidism and an abnormal scrotum occur in boys, and a bicornuate uterus occurs in girls. Apneic spells in early infancy are frequent. Intellectual disability is severe.
Diagnosis of trisomy 13 may be suspected postnatally by appearance or prenatally by abnormalities on ultrasonography (eg,intrauterine growth restriction), or by increased risk noted on multiple marker screening or noninvasive prenatal screening (NIPS) using cell-free fetal DNA sequences obtained from a maternal blood sample.
Confirmation in all cases is by cytogenetic testing (karyotyping, fluorescent in situ hybridization [FISH] analysis, and/or chromosomal microarray analysis ) of samples obtained by chorionic villus sampling or amniocentesis. Postnatally, confirmation is by cytogenetic testing of a blood sample.
Confirmatory testing also is done in cases suspected based on NIPS, particularly when the screening result is indeterminate or unclear; in younger women, in whom the positive predictive value of NIPS is lower; and to diagnose other fetal chromosomal disorders. Management decisions, including termination of pregnancy, should not be made based on NIPS testing alone. See also The American College of Obstetricians and Gynecologists Committee on Genetics and the Society for Maternal–Fetal Medicine committee opinionregarding cell-free fetal DNA testing.