Down syndrome

Down syndrome is the most commonly diagnosed chromosomal syndrome. Clinically described by Down in 1866

Karyotypically identified in 1959. Children with this syndrome have an extra copy (trisomy) of 21 chromosomes, which leads to the development of mental (reduced intelligence) and physical disorders (various malformations of internal organs).

[1], [2], [3], [4]

Epidemiology

The population frequency does not depend on gender and is 1: 700 without taking into account the age of the parents. With the proliferation of screening programs for pregnant women in the first or second trimester, the number of newborns with Down syndrome began to decrease.

[5], [6], [7], [8], [9], [10], [11], [12]

Causes of the down syndrome

The true causes of this disease are not deciphered, however, mutagenic environmental factors seem to play a role in this.

[13], [14], [15], [16], [17]

Risk factors

The main risk factors for Down syndrome.

• Age of mother over 35 years.
• Regular trisomy on chromosome 21 (47, t21).
• Translocation (exchange of chromosome regions) of chromosomes 14 and 21 (46, tl4 / 21).
• Translocation of chromosomes 21/21 (46, t21 / 21).
• In 2% of cases, there is a mosaic variant, if non-divergence of these chromosomes occurs in the early stages of zygote division. At the same time, the karyotype looks like 47; 21 + / 46.

[18], [19], [20], [21], [22], [23], [24], [25], [26], [27]

Pathogenesis

The pathogenesis of Down syndrome is finally unclear. Probably, the imbalance of chromosomes in somatic cells leads to disruption of the functioning of the genotype. In particular, it has been established that myelinization of nerve fibers and the production of neurotransmitters suffer from trisomy.

Depending on the changes in 21 chromosomes, there are three types of Down syndrome:

1. Trisomy 21: About 95% of people with Down Syndrome have trisomy 21 chromosomes. With this form of Down syndrome, each cell in the body has 3 separate copies of chromosome 21 instead of the usual 2.
2. Chromosome 21 translocation: This type is found in 3% of cases of this syndrome. In this part of the 21st chromosome is attached to the shoulder of the other (most often it is the 14th chromosome).
3. Mosaicism: This type is found in 2% of cases. Characterized by the fact that some of their cells have 3 copies of chromosome 21, and some are typical two copies of chromosomes 21.

[28], [29], [30], [31], [32], [33], [34], [35], [36]

Symptoms of the down syndrome

Children with Down syndrome have a flattened face with a mongoloid eye slit and diminished auricles (<3 cm vertical length in newborns) in combination with generalized muscular hypotonia, clynodactyly of the little fingers of the hand with a single flexion fold on the palms and little fingers of the hand. Of congenital malformations, specific atresia of the duodenum, annular pancreas and defects of the interventricular septum of the heart are specific. Hirschsprung's disease and congenital hypothyroidism are less common.

Shortly after birth, a distinct delay in psychomotor and physical development is noted. Subsequently, pronounced mental retardation and short stature develop. Children with Down syndrome have impaired immunity, which makes them very vulnerable to bacterial infection. They are 20 times more likely to develop acute leukemia. The reasons for this are unclear.

Other diseases that may accompany Down syndrome:

• Hearing loss (75% of cases);
• Obstructive sleep apnea syndrome (50 - 75%);
• Ear infections (50 - 70%);
• Eye diseases, including cataracts (up to 60%).

Other less common diseases:

• Hip dislocation;
• Thyroid disease;
• Anemia and iron deficiency.

Until maturity, few survive. The life expectancy of such patients is limited by the malformations of the heart and large vessels, the gastrointestinal tract, the manifestation of acute leukemia and infection.

[37], [38], [39], [40]

Diagnostics of the down syndrome

Diagnosis does not cause difficulties, however, in newborns with prematurity (previously 34 weeks of gestation) may be delayed.

[41], [42], [43], [44], [45]

Screening tests

Screening tests are a combination of a blood test that measures the amount of various substances in the mother's serum [eg, MS-AFP, prenatal diagnosis (triple screening, quadruple screening)] and fetal ultrasound.

Since 2010, a new genetic test has been available to diagnose Down syndrome, which consists in detecting small pieces of DNA of a developing child that circulate in the mother’s blood. This test is usually performed in the first 3 months of the gestational period.

[46], [47], [48]

Diagnostic tests

Diagnostic tests are carried out after a positive screening test in order to make a final diagnosis:

1. Biopsy Chorion (CVS).
2. Amniocentesis and cordocentesis.
3. Cord blood test (PUBS).

[49], [50], [51]

Treatment of the down syndrome

Down syndrome does not have specific treatment methods. Use courses of nootropic and vascular drugs. Of great importance are educational activities that can significantly socially adapt these patients.

All pregnant women over 45 years old are shown to have fetal karyotyping. This is carried out with the purpose of timely artificial termination of pregnancy in cases of detection of an abnormal karyotype. If a patient has a translocation version of Down syndrome, in order to predict the risk of the occurrence of such a pathology in subsequent children, parents should be karyotyped.

[52], [53]

Forecast

With regular trisomy of chromosome 21, the repeated risk of having a child with a disease such as Down syndrome is low and is not a contraindication to childbearing. In women older than 35, the likelihood of regular trisomy of chromosome 21 increases.

In translocation, the repeated risk of having a child with this disease varies from 1 to 10% and depends on the type of translocation and the gender of the carrier of this balanced rearrangement of the karyotype. With the translocation of 21/21 in one of the parents, the repeated risk of having a baby is 100%.

Sometimes not all cytogenetically studied cells of a child contain an extra chromosome 21. These cases of mosaicism are the most difficult in the process of medical and genetic counseling of parents in terms of predicting the future intellectual development of a child who has Down syndrome.

[54]