GENERAL FEATURES OF TRISOMY 13

• Incidence = 1 in 5,000 to 1 in 20,000 births.
• Usually, but not always lethal (1).
• 28% of trisomy 13 newborns die by 1 week of age.
• 44% die by 1 month of age.
• 73% die by 4 months of age.
• 86% die by 1 year of age.
• Survival beyond three years has been reported but is exceptional (2).
• Compared with the other trisomies they tend to have more severe craniofacial and brain abnormalities. About 50% of fetuses with alobar holoprosencephaly will have trisomy 13.
• The number and severity of congenital abnormalities associated with trisomy 13 are highly variable. Monozygotic trisomy 13 twins with discordant major anomalies have been reported (3).
• Fetuses with trisomy 13 mosaicism may have a normal phenotype or the full pattern of congenital anomalies seen with trisomy 13 (3).
• Table of age specific risk for trisomies 13, 18 and 21.

DIFFERENTIAL DIAGNOSIS

The three primary lesions seen in Meckel-Gruber syndrome (occipital encephalocele, postaxial polydactyly and multicystic renal dysplasia) are also characteristic of trisomy 13. Fetal karyotyping is therefore required to differentiate (4,5).

REFERENCES

1. Magenis RE, Hecht F, Milham S Jr. Trisomy 13 (D) syndrome: studies on parental age, sex ratio and survival. J Pediatr 1969;73:222-228.
2. Redheendran R, Neu RL, Bannerman RM. Long term survival in trisomy 13 -syndrome: 21 cases including prolonged survival in two patients 11 and 19 years old. Am J Med Genet 1981;8:167-172.
3. Jones KL. Smith's recognizable patterns of human malformations. 4th ed. Philadelphia WB Saunders Co 1988:16-25.
4. Moerman P, Fryns J-P, van der Steen K et.al. The pathology of trisomy 13 syndrome. A study of 12 cases. Hum Genet 1988;8:349-356.
5. Lehman CD, Nyberg DA, Winter TC et.al. Trisomy 13 syndrome: prenatal ultrasound findings in a review of 33 cases. Radiology 1995;194:217-222.