Trisomy 13 is a chromosomal anomaly caused by the presence of an extra chromosome 13 and is characterized by brain malformations (holoprosencephaly), facial dysmorphism, ocular anomalies, postaxial polydactyly, visceral malformations (cardiopathy) and severe psychomotor retardation. Its incidence is estimated at between 1/8,000 and 1/15,000 births. In utero death occurs in over 95% of fetuses with this chromosomal anomaly. The neurological manifestations are severe with hypotonia and hyporeactivity with an apparent lack of awareness of surroundings. Holoprosencephaly (resulting from a defect in the division of the brain into two hemispheres) is present in 70% of cases and can be observed as variable degrees of hemisphere fusion on MRI. Facial anomalies are variable and may range in severity from hypertelorism and premaxillary agenesis (80% of cases) to cebocephaly or cyclopia with absence of the nasal skeleton. Cleft lip/palate, micro- or anophthalmia, coloboma (even in the absence of major brain malformations), regions of occipital cutaneous aplasia, postaxial polydactyly, cardiac malformations (80% of cases) and urogenital malformations may also be present. Free trisomy 13 is found in around 75% of cases. In 20% of cases, trisomy 13 is associated with a Robertsonian translocation in which the supernumerary chromosome 13 becomes attached to another acrocentric chromosome (chromosomes 13, 14, 15, 21 or 22). In rare cases, the syndrome is caused by reciprocal translocation between chromosome 13 and a nonacrocentric chromosome. Mosaic trisomy 13 (in which there is both trisomic and normal cell types) has been reported in a few patients with a clinical picture that varies between a normal phenotype and that of classical trisomy 13 according to the number of trisomic cells present in the tissues. The risk of recurrence of trisomy (21, 13 or 18) in families of an index case with trisomy 13 is around 1%. However, in families in which trisomy 13 is associated with translocation (Robertsonian or balanced) the risk of recurrence is higher if one of the parents is a carrier of a balanced translocation. Trisomy 13 may be suspected during pregnancy from ultrasound findings (holoprosencephaly, polydactyly) and can be confirmed by karyotype analysis of the fetus. Management is supportive only. Surgical treatment of the malformations does little to improve the poor prognosis associated with this syndrome: half of the infants die within the first month of life and 90% die before 1 year of age from cardiac, renal or neurologic complications. Prolonged survival (in some cases into adulthood) has been reported and is more common in cases of mosaic or partial trisomy and in the absence of severe brain malformations. In general, non-mosaic patients develop only limited autonomy (absence of speech and ambulation).
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