Trisomy 21 or Down’s Syndrome: symptoms, causes, definition, what really is it?
Trisomy 21 or Down’s Syndrome: symptoms, causes, definition, what really is it?

Trisomy 21 or Down’s Syndrome: symptoms, causes, definition, what really is it?

Down’s syndrome remains today to be one of the most common genetic anomalies and the first cause of genetically caused mental disabilities. Moreover, the understanding of the diagnosis of the disease have been greatly improved over recent decades. Here are the symptoms, causes and screenings associated with this disease.

Definition, what is Down’s Syndrome?

Also known as Trisomy 21, Down’s Syndrome is a genetic anomaly related to the number of chromosomes. In fact, sufferers carry three chromosomes 21 instead of only 2.

This deformity is generally caused by an anomaly in the distribution of chromosomes in the parent’s sex cells: that’s to say one of the two parents brings two chromosomes 21 instead of a single one.

According to calculations, around 8 million people in the world are affected by Down’s Syndrome. Nevertheless, the rate tends to grow as a consequence of those affected living longer.

Symptoms of Down’s Syndrome

Down’s Syndrome is responsible for detectable disabilities in an ultrasound even before birth. The increase of nuchal translucence (small liquid space situated in the back of the neck) measured towards the 11th week of pregnancy, of abnormalities in the neck but also in the heart, are symptoms allowing the diagnosis to be detected quite early.

Upon birth, muscle weakness is noticeable, as well as specific morphological characteristics:

- a flat neck

- a round face

- a small nose

- wide-set eyes

- slanted eyelids

- feet with generally toes and more stubby hands

There are visceral disabilities such as cardiac and digestive disabilities which cause more problems.

In addition to this, Down’s Syndrome is generally associated to a late cognitive development in its most general form, and therefore associated to an intellectual deficiency, with those affected having an average IQ of 50. However, the disabilities can vary in each individual and some sufferers are capable of being almost independent whilst others are less so.

Causes and risk factors of Down’s Syndrome

Down’s Syndrome is a chromosome aberration. It therefore results in a mechanic accident suffered by chromosomes during cell division. The biggest factor of Down’s Syndrome is the mother’s age. Some research has shown that the older the mother, the more the risk of Down’s Syndrome increases: at 40 years old, the risk is far greater than at 30 years old.

A woman who has already given birth to a child with Down’s Syndrome, has a higher chance of having future children who will suffer. Down’s Syndrome isn’t generally hereditary and is not transmitted from parents to their children. Nevertheless, researchers have found evidence that a certain type of Down’s Syndrome is more likely due to the presence of a particular gene, common to many members of the same family.

Detection and diagnosis of Down’s Syndrome

Two sorts of tests exist to detect Down’s Syndrome before birth: the screening test which assesses the extent of the risk that the unborn baby has of being affected. Nowadays, more effective and earlier screening can be carried out from the first trimester of pregnancy.

They rely on various methods including blood tests and ultrasounds. These tests are done generally between the 15th and 20th week of pregnancy. From a diagnostic point of view, there are two methods: amniocentesis and a biopsy or the trophoblast.

- Amniocentesis is carried out by puncturing amniotic fluid with a needle through the belly. It is suggested to women when their blood tests reveal a disruption of hormone and protein levels which could suggest the presence of a fetal trisomy 21, and the ultrasound from the 1st trimester supposes an irregularity in morphological proportions. If the results take a while to arrive, the technique is 100% reliable. However, it can cause a miscarriage in 0.5% to 1% of cases.

- The other method, the biopsy of a trophoblast involves collecting a piece of placenta passing by the abdominal passage or by the cervix. This sampling will make it possible to then carry out a karyotype (a photograph taken of all the chromosomes) of the child which will confirm the presence of an anomaly. If the results are faster than they are for amniocentesis, the risk for the fetus is higher.

Today, the detection of Down’s Syndrome is not compulsory but it must be offered “to all pregnant women during a medical consultation”, according to bioethical law. Each future mother can therefore decide whether or not they want to do it. Thanks to these tests, the parents can consider their options in advance, among which can include a medical termination of pregnancy.

By Stacey Williams
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