Instituto Valenciano de Microbiología

Masía El Romeral
Ctra. de Bétera a San Antonio Km. 0.3
46117 Bétera (Valencia)
Phone. 96 169 17 02
Fax 96 169 16 37
CIF B-96337217


Trisomy 18 (Trisomy 18) - Chromosome 18.

Trisomy 18, also called Edwards syndrome, is a chromosomal abnormality associated with abnormalities in various parts of the body. People usually have intrauterine growth retardation and low birth weight. In addition, those affected may have heart defects and abnormalities of other organs that develop before birth. Other features of trisomy 18 include a small head abnormally, a small mouth and jaw and fists overlapping fingers. Due to the presence of several medical problems that threaten life, many people with trisomy 18 die before birth or within the first month. 5% to 10% of children with this condition live beyond their first year, and these children often have severe intellectual disabilities.

Most cases of trisomy 18 are the result of having three copies of chromosome 18 in every cell in the body rather than the normal two copies. The extra genetic material disrupts the normal course of development, resulting in the characteristic features of trisomy 18. About 5% of people with trisomy 18 have an extra copy of chromosome 18 in only part of the body 's cells. In these people, the alteration is called trisomy 18 mosaic. The severity of trisomy 18 mosaic depends on the type and the number of cells having the extra chromosome. The development of people with this form of trisomy 18 may vary from normal to severely affected.

Very rarely, part of the long arm of chromosome 18 becomes attached to another chromosome during the formation of reproductive cells or early embryonic development. Affected individuals have two copies of chromosome 18, plus extra material from chromosome 18 attached to another chromosome. People with this genetic change are said to have partial trisomy 18. If only part of the long arm is present in three copies, the physical signs of partial trisomy 18 may be less severe than those of trisomy 18. If all the long arm is present in three copies, people can be so badly affected as having three complete copies of chromosome 18.

Most cases of trisomy 18 are not inherited but occur in random events during the formation of eggs and sperm. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm can get an extra copy of chromosome 18. If one of these atypical reproductive cells contributes to the genetic composition of a child, the child will have an extra chromosome 18 in each of the cells of the organism. Meanwhile, mosaic trisomy 18 are not inherited. It is presented as a random event at the beginning of cell division in embryonic development. As a result, some of the cells of the body are the usual two copies of chromosome 18, and other cells have three copies of this chromosome. The partial trisomy 18, it can be inherited. An affected person may have a rearrangement of genetic material between chromosomes 18 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 18. Although they have no signs of trisomy 18, people with this type of balanced translocation have an increased risk of having children with the disease.

Tests in IVAMI: in IVAMI perform detection of mutations associated with trisomy 18, by detecting the additional chromosome.

Samples recommended: EDTA blood collected for separation of blood leukocytes.