Etiological heterogeneity in sirenomelia.

Two babies with sirenomelia are described. Case 1, one of twins, showed the full sirenomelia sequence in conjunction with atelencephaly and cebocephaly. Case 2 had malformations consistent with a diagnosis of the VATER association. Review of the literature indicates that the basic defect in sirenomelia and the VATER association lies in the formation and differentiation of mesodermal tissue and that sirenomelia, the VATER association, and monozygotic twinning show a complex etiological interrelationship.