What is Down syndrome?
Down syndrome is a genetic disorder that occurs when an individual has an extra full or partial copy of chromosome 21. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome [1].
Down syndrome is also known as trisomy 21, indicating the presence of an extra chromosome 21. This condition is the most prevalent genetic disease worldwide and the common genetic cause of intellectual disabilities [2].
The extra chromosome or piece of a chromosome changes how a baby's body and brain develop, leading to mental and physical challenges throughout their lifetime. Despite these challenges, each person with Down syndrome has different abilities [3].
Charecteristics of Down syndrome
One of the most common features of Down syndrome is intellectual disability, which can range from mild to severe [4]. This is often accompanied by specific cognitive deficits, particularly in expressive language, syntactic/morphosyntactic processing, and verbal working memory [4].
Physical characteristics of Down syndrome include craniofacial dysmorphisms, such as a flattened face, ocular hypotelorism, and mandibular prognathism [5]. Other standard features include brachycephaly (short, broad head), short and broad hands, and a short neck [6].
Individuals with Down syndrome also exhibit certain personality traits. They are often perceived as more affectionate, outgoing, and cooperative than individuals without the syndrome [7, 8]. However, they may also show less persistence and over-reliance on social behaviours during cognitively challenging tasks [9].
Why do people with Down syndrome look similar?
...
In addition to facial features, individuals with Down syndrome often have broad, short hands with a single crease in the palm, relatively short fingers and small hands and feet, and excessive flexibility [1]. Dermatoglyphic traits, such as fingerprint patterns and simian creases, also show a higher level of bilateral symmetry in individuals with Down syndrome than those without the condition [10].
It's important to note that while these features are standard, they do not appear in all individuals with Down syndrome. The degree to which these features are expressed can vary widely among individuals, and some may not exhibit certain features at all.
Types of Down Syndrome
Trisomy 21: Down Syndrome, as discussed, is primarily caused by an extra copy of chromosome 21 in every cell of a patient's body. This condition is known as Trisomy 21 and is the most common form of Down Syndrome [11, 12, 13].
Mosaic Down Syndrome: This is another type of Down Syndrome. In this case, not all cells have an extra copy of chromosome 21. Some cells have the usual two copies, while others have three. This form of Down Syndrome tends to have fewer symptoms than Trisomy 21 and occurs in about 2 per cent of all Down Syndrome cases [11, 14].
Translocation Down syndrome: The third type of Down Syndrome is Translocation Down Syndrome. In this case, children have only an extra part of chromosome 21. There are 46 total chromosomes, but one has an additional piece of chromosome 21 attached. This type of Down Syndrome is less common than Trisomy 21 and Mosaic Down Syndrome [11, 12, 13].
How is Down syndrome diagnosed before birth?
Down syndrome can be diagnosed before birth through a series of prenatal screening and diagnostic tests. These tests are typically offered as part of routine prenatal care, especially for women over 35, those with a family history of Down syndrome, or when the baby's father is over 40 [11].
Prenatal screening tests can indicate whether there's an increased chance your baby may have Down syndrome. These tests include ultrasound evaluations and blood tests, usually provided during pregnancy's first and second trimesters. They measure hormone levels in the blood to detect abnormalities and use an ultrasound to look for irregular fluid buildup in the baby's neck [11, 14].
However, these screening tests cannot confirm a Down syndrome diagnosis. They only provide the likelihood of a baby developing Down syndrome. If these tests show a higher chance of Down syndrome, more definitive diagnostic tests are needed to confirm the diagnosis [14, 3].
Diagnostic tests can confirm if your baby has Down syndrome before it's born. Chorionic villus sampling (CVS) and amniocentesis are the most common diagnostic tests. Both tests take samples from the uterus to analyze chromosomes. CVS uses a placenta sample, which can be completed in the first trimester. In contrast, amniocentesis analyzes an amniotic fluid sample surrounding the growing fetus and is typically performed in the second trimester [11, 14].
These diagnostic tests are more invasive than screening tests and may slightly increase the chances of complications, such as miscarriage. However, they are crucial for a definitive diagnosis as they check for extra genetic material from chromosome 21, which is indicative of Down syndrome [15, 16].