Down syndrome is a genetic disorder. It is also called trisomy 21.
• Down syndrome is the most common viable chromosomal disorder. Each year, about 6,000 babies are born with Down syndrome, which is about 1 in every 700 babies born.
• The Most common cause of genetic intellectual disability.
• Older mothers are more likely to have a baby affected by Down syndrome than younger mothers. In other words, the prevalence of Down syndrome increases as the mother’s age increases.
Types of Down Syndrome
There are three types of Down syndrome. People often can’t tell the difference between each type without looking at the chromosomes because the physical features and behaviors are similar.
• Trisomy 21: About 95% of people with Down syndrome have Trisomy 21.2 With this type of Down syndrome, each cell in the body has 3 separate copies of chromosome 21 instead of the usual 2 copies. The most common type of Down syndrome was standard trisomy 21, with the highest occurrence in parents from 30 to 39 years old.
• Translocation Down syndrome: This type accounts for a small percentage of people with Down syndrome (about 3%).2 This occurs when an extra part or a whole extra chromosome 21 is present, but it is attached or “translocated” to a different chromosome rather than being a separate chromosome 21. Parents were the youngest in the translocation group.
• Mosaic Down syndrome: This type affects about 2% of the people with Down syndrome.2 Mosaic means mixture or combination. For children with mosaic Down syndrome, some of their cells have 3 copies of chromosome 21, but other cells have the typical two copies of chromosome 21. Children with mosaic Down syndrome may have the same features as other children with Down syndrome. However, they may have fewer features of the condition due to the presence of some (or many) cells with a typical number of chromosomes.
Even though people with Down syndrome might act and look similar, each person has different abilities. People with Down syndrome usually have an IQ (a measure of intelligence) in the mildly-to-moderately low range and are slower to speak than other children.
Some common findings of Down syndrome include:
• A flattened face, especially the bridge of the nose
• Almond-shaped eyes that slant up
• A short neck, Small ears • A tongue that tends to stick out of the mouth
• Tiny white spots on the iris (colored part) of the eye-Brushfield spots.
• Small hands and feet
• A single line across the palm of the hand (palmar crease)
• Shorter in height as children and adults
• Congenital heart diseases, Duodenal atresia, Hirschsprung disease.
ASSOCIATED WITH
• Early-onset Alzheimer disease(chromosome 21 codes for amyloid precursor protein)
• Increased risk of Acute Lymphoblastic leukemia(ALL) and Acute Myeloid Leukemia(AML)
What does wrong in the brain that leads to intellectual disability?
• The brain of a child with Down syndrome develops differently from a normal one, attaining a form reduced in size and altered in the configuration.
• Directly related to the mental retardation are neuronal modifications manifest as alterations of cortical lamination, reduced dendritic ramifications, and diminished synaptic formation.
• At 4 months of age, the neurons show a relatively expanded dendritic tree, but during the first year the dendrites stop growing and become atrophic relative to control neurons.
• Oligodendrogliocyte dysfunction is reflected in delayed myelination in pathways of frontal and temporal lobes.
WHAT CAN WE DO TO PREVENT IT?
Screening Tests
Screening tests often include a combination of a blood test, which measures the number of various substances in the mother’s blood ( high hCG and Inhibin are the markers for Down syndrome )
First-trimester Ultrasound shows increased nuchal translucency and hypoplastic nasal bone which creates a picture of the baby. During an ultrasound, one of the things the technician looks at is the fluid behind the baby’s neck. Extra fluid in this region could indicate a genetic problem. These screening tests can help determine the baby’s risk of Down syndrome.
Diagnostic Tests
Diagnostic tests are usually performed after a positive screening test in order to confirm a Down syndrome diagnosis. Types of diagnostic tests include:
• Chorionic villus sampling (CVS)—examines material from the placenta
• Amniocentesis—examines the amniotic fluid (the fluid from the sac surrounding the baby)
• Percutaneous umbilical blood sampling (PUBS)—examines blood from the umbilical cord.
These tests look for changes in the chromosomes that would indicate a Down syndrome diagnosis.
REFERENCES
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