18-ші хромосоманың үштен келу синдромы

Chromosomal disorder in which there are three copies of chromosome 18
Trisomy 18 occurs in around 1 in 5,000 live births. Survival beyond a year of life is around 5–10%. It is named after the English geneticist John Hilton Edwards, who first described the syndrome in 1960.

Children born with Edwards' syndrome may have some or all of these characteristics: kidney malformations, structural heart defects at birth (i. e., ventricular septal defect, atrial septal defect, patent ductus arteriosus), intestines protruding outside the body (omphalocele), esophageal atresia, intellectual disability, developmental delays, growth deficiency, feeding difficulties, breathing difficulties, and arthrogryposis (a muscle disorder that causes multiple joint contractures at birth). Some physical malformations associated with Edwards' syndrome include small head (microcephaly) accompanied by a prominent back portion of the head (occiput), low set, malformed ears, abnormally small jaw (micrognathia), cleft lip/cleft palate, upturned nose, narrow eyelid openings (blepharophimosis), widely spaced eyes (ocular hypertelorism), drooping of the upper eyelids (ptosis), a short breast bone, clenched hands, choroid plexus cysts, underdeveloped thumbs and/or nails, absent radius, webbing of the second and third toes, clubfoot or rocker bottom feet, and in males, undescended testicles. Sometimes, excess amniotic fluid or polyhydramnios is exhibited.

Trisomy 18 is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the 18th chromosome, either in whole (trisomy 18) or in part (such as due to translocations). The additional chromosome usually occurs before conception. The effects of the extra copy vary greatly, depending on the extent of the extra copy, genetic history, and chance. Trisomy 18 occurs in all human populations, but is more prevalent in female offspring. A typical egg or sperm cell contains individual chromosomes, each of which contributes to the 23 pairs of chromosomes needed to form a normal cell with a typical human karyotype of 46 chromosomes. Numerical errors can arise at either of the two meiotic divisions and cause the failure of a chromosome to segregate into the daughter cells (nondisjunction). This results in an extra chromosome, making the haploid number 24 rather than 23. Fertilization of eggs or insemination by sperm that contain an extra chromosome results in trisomy, or three copies of a chromosome rather than two. Trisomy 18 (47,XX,+18) is caused by a meiotic nondisjunction event. In nondisjunction, a pair of chromosomes fails to separate during cell division; thus, a gamete (i. e., a sperm or egg cell) is produced with an extra copy of chromosome (for a total of 24 chromosomes). When combined with a normal gamete from the other parent, the resulting embryo has 47 chromosomes, with three copies of the problematic chromosome (in this case, chromosome 18). (Although an embryo could inherit a trisomy from both parents, it is, as a rule, extremely rare, and worse in terms of clinical perspective and prognosis.) A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic trisomy 18. Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception. Affected individuals have two copies of chromosome 18 plus extra material from chromosome 18 attached to another chromosome. With a translocation, a person has a partial trisomy for chromosome 18, and the abnormalities are often less severe than for the typical trisomy 18.

Ultrasound can increase suspicion for the condition, which can be confirmed by CVS or amniocentesis.

About 95% of pregnancies that are affected do not result in a live birth. The median lifespan is five to 15 days. About 8–12% of infants survive longer than 1 year. One percent of children live to age 10.

Trisomy 18 occurs in about 1 in 5,000 live births, but more pregnancies are affected by the syndrome as the majority of those diagnosed with the condition prenatally will not survive to birth.

Trisomy 18 was first identified by John Hilton Edwards in 1960, although he originally believed it to be caused by a trisomy of chromosome 17. Klaus Patau and Eeva Therman reported another two cases shortly thereafter. They identified the extra chromosome as being part of what Patau's lab called "group E", containing chromosomes 16, 17, and 18, but were unable to determine which chromosome was responsible at the time. Analyzing 5 more cases, they were able to determine that the extra chromosome was in fact chromosome 18.