Trisomy 18, also known as Edwards syndrome, from the name of the physiologist who described its characteristics, is a chromosomal abnormality that affects approx one child for every 6-8 live births. Unfortunately, this anomaly has serious consequences on prenatal development, causing in most cases the stillbirth of the fetus. Let's see in detail what it is, what are the causes and how it can be diagnosed.
In this article
- trisomy 18
- the causes
- how it manifests itself
- diagnosis during pregnancy
- the prognosis of the disease
Trisomy 18 and other chromosomal abnormalities
Chromosomal diseases are caused by abnormalities of the chromosomes, which may have undergone one change in number or structure. Chromosomes are rod-shaped structures contained in the nucleus of our cells and have the task of storing and transmitting genetic information from parents to children.
In the human species the number of chromosomes is 46:
- 22 pairs they form autosomes (chromosomes that do not participate in sex determination);
- a pair it is formed by the sex chromosomes (X and Y). In the male the pair of chromosomes consists of an X chromosome and a Y chromosome, while in the female the two chromosomes are both X.
La trisomy 18 it is a numerical anomaly, in the sense that illness derives from having three copies of chromosome 18 in each cell of the body instead of two. The presence of "extra" genetic material unfortunately causes serious problems in the development of the fetus, to the point that most pregnancies are interrupted before birth.
Among the trisomies, trisomy 18 and trisomy 13 (Patau syndrome) are the second and third most frequent polyploidy at birth after trisomy 21 (Down syndrome).Read also: Chromosomal anomalies, what are they?
What causes trisomy 18
In Edwards Syndrome the extra chromosome 18 is present for one chromosome separation error, the cause of which is still little known today. As specified by the Istituto Superiore di Sanità, in most cases the anomaly is of maternal origin: the paired chromosomes did not separate correctly during meiosis of the maternal gamete (oocyte). For this reason, when the maternal and paternal gamete merge in conception, the embryo will have an excess of chromosome 18.
The frequency of maternal nondisjunction defects responsible for trisomy 18 and other chromosomal abnormalities correlates with the age of the woman and increase with age.
How trisomy manifests itself 18
Trisomy 18 is a disease characterized by severe mental retardation and growth retardation both before and after birth, malformations of the skull, limbs and internal organs.
In particular, we find:
- very small head (microcephaly), with elongated and narrow shape (dolichocephaly);
- small and receding chin;
- low set ears;
- eye malformations (wide apart and small eyes);
- small mouth and jaw;
- long fingers overlapping, closed fist with index superimposed on middle finger;
- club foot, i.e. with the toe pointing downwards;
- heart defects (in 90% of cases);
- gastrointestinal malformations;
- genital and urinary malformations;
- hypotonia and psychomotor retardation.
Others signs and symptoms that may occur are:
- malformations of the palate (cleft lip and palate);
- difficulty in breathing;
- bone abnormalities;
- lung and urinary tract infections;
- malformations of the brain and spinal cord.
La severity of signs and symptoms of the disease also depends on the type of form of trisomy 18. Edwards syndrome, in fact, can manifest itself in three different forms.
The different types of Edwards syndrome
There are 3 types of trisomy 18 and are classified according to the severity of symptoms:
- full form: Most (94%) children with Edwards syndrome have an extra chromosome 18 present in each cell. In the case of the complete form, most children die before they are born.
- Mosaic trisomy: About 5% of children with Edwards syndrome have the extra copy of chromosome 18 in only certain cells of the body. It is about a usually less severe form of Edwards syndrome in which signs and symptoms are highly dependent on the type and number of cells that have the extra chromosome. Some children may be only mildly affected by the disease, while others are very severely. In this case, about 7 children with mosaic trisomy out of 10 live births reach at least the first year of life and, in rare cases, could live to adulthood.
- Partial form: it is the least frequent form that affects a very small number of children (about 1 in 100). In the partial form, the baby has only an extra section of chromosome 18 in the cells, rather than an extra whole chromosome. How the disease affects the child depends on the tract and extent of the affected segment.
A possible trisomy 18 can be suspected already during the first control ultrasounds since the fetus has very evident anomalies. For diagnosis, it is necessary to perform the combined test, which is usually done between the XNUMXth and XNUMXth week of pregnancy and which calculate the possibility that the child has this syndrome. If the examination shows that the possibilities are high, the doctor will recommend performing a more in-depth examination that involves the analysis of a sample of the baby's cells, then a CVS or an amniocentesis, traditional invasive investigations.
Today the fetal DNA test is also widely used, a non-invasive screening test that analyzes a sample of the fetus DNA obtained from a simple maternal blood sample. This type of examination is performed starting from the tenth week of gestation, but it is always a probabilistic test. As all the main scientific societies point out, every positive result must be confirmed with an invasive investigation (amniocentesis or CVS).Read also: Combined test (duo test or bi-test): what it is, when to do it, how to read the results
The prognosis of Edwards syndrome is unfortunately not favorable, although there are cases of children with trisomy 18 who survived to adolescence. Studies have shown that over 95% of affected fetuses die in utero. Of those born with trisomy 18, more than 50% die in the first week of life and less than 5% reach the year of life; only 1% reaches the age of 10. For reasons not yet known, the survival rate is higher in females than in males.
There is no specific therapy for trisomy 18. If the baby is born alive, in fact, what doctors can do is try to reduce the symptoms and manifestations of the pathology, such as infections and heart defects. Also, if the baby has difficulty feeding, artificial feeding will be necessary. Movement difficulties and changes in muscle tone are mainly treated with physiotherapy.
After discharge, the child must be followed up at the Follow-up Service (Society of the Paesena of Neonatology) to periodically check their health conditions, to support neuromotor and neurodevelopmental development as far as possible and to offer any psychological and social support. to the family. In aid of families with children suffering from Edwards syndrome there is also the Association of Families (SOFT Association of the country).
Other sources used for the article: information material from the Istituto Superiore di Sanità; British Health System information material; country SOFT Association material.