Trisomy 13

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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. Diagnosis is with cytogenetic testing. Treatment is supportive.
(See also Overview of Chromosomal Anomalies.)
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. Hearing loss is common. Scalp defects and dermal sinuses are also common. Loose folds of skin often are present over the back of the neck.
A single transverse 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.