DISCORDANT ANOMALIES IN MONOZYGOTIC TWINS

Abstract

Monozygotic twins, as a rule, have the same genotype and phenotype. Cases, where disturbances develop in only one of two twins are very rare. Such cases are called discordant anomalies. The purpose of the work is to systematize the data on the causes of discordant anomalies, their phenotypic manifestations and methods of managing pregnancy in the case of discordant anomalies, search for articles on this topic in the PubMed database and analyze the found literature. Data analysis showed following factors to be able to serve as causes of discordant anomalies: mutations occurred both before and after the separation of the blastomeres; inactivation of one of the X chromosomes in women; variations in the number of copies of genes; different embryo attachment to the placenta; epigenetic factors. Most often there are discordant anomalies of the central nervous system, the cardiovascular system, and defects of the abdominal wall. There are less common chromosomal abnormalities - Down syndrome, Patau syndrome, Klinefelter syndrome. Isolated cases are cystic hygroma and autism spectrum disorders. The average gestational age at which discordant anomalies are detected is 17 weeks in monochorial twins, and 21 weeks in bichorial twins. Malformations of one of the fetuses increase the risk of the miscarriage or premature birth. When detecting twins, discordant for a large anomaly, selective reduction of embryos is appropriate. For bichorial twins, an intracardiac injection of potassium chloride is used. In monochorial twins, umbilical cord coagulation is used, because often monochorial fetuses have vascular anastomoses, and the cardiotoxic agent can enter into a healthy fetus. Discordant anomalies are rare, poorly understood pathology. Anomalies can affect any organ system, often there are multiple malformations. Large anomalies can be an indication for the selective reduction of the fetus.