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(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 (eg, scalp defects, dermal sinuses) are characteristic. Holoprosencephaly (failure of the forebrain to divide properly) is common. Concurrent facial anomalies can include cleft lip and cleft palate. Microphthalmia, colobomas (fissures) of the iris, and retinal dysplasia are also common. Supraorbital ridges are shallow, and palpebral fissures usually are slanted. The ears are abnormally shaped and usually low-set. Deafness is 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.
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