Down syndrome is a disorder caused by a problem with the chromosomes — the pieces of DNA that have the blueprint for the human body. Normally a person has two copies of each chromosome, but a person with Down syndrome has three copies of chromosome 21. The condition also is called trisomy 21.
In a few cases, the extra copy is part of another chromosome (translocation), or found in only some of the person’s cells (mosaicism). Down syndrome is a genetic disorder caused when abnormal cell division results in an extra full or partial copy of chromosome 21. This extra genetic material causes the developmental changes and physical features of Down syndrome.
Other than physical attributes and mental retardedness, they are also susceptible to various diseases like leukaemia and Alzheimer’s. There is no cure, only the quality of life can be improved by taking extra care and training the individual to perform daily essential activities. Down syndrome can be diagnosed during early screening in pregnancy, which can decrease the occurrence of disease. Down syndrome varies in severity among individuals, causing lifelong intellectual disability and developmental delays. It’s the most common genetic chromosomal disorder and cause of learning disabilities in children. It also commonly causes other medical abnormalities, including heart and gastrointestinal disorders.
Infants with Down syndrome may be average size, but typically they grow slowly and remain shorter than other children the same age.
Most children with Down syndrome have mild to moderate cognitive impairment. Language is delayed, and both short and long-term memory is affected.
Types of down syndrome
1. Trisomy is the most common type of Down syndrome. It accounts for 95% of cases of Down syndrome.
There is one extra chromosome 21. The total number of chromosome present is 47 instead of the normal 46 chromosomes. The main cause of trisomy is nondisjunction of chromosome 21 during meiosis at the time of gamete formation. The abnormal cell with trisomy of chromosome 21 is fertilised giving rise to trisomy in all the cell of the foetus.
2. Mosaicism: This is the rare form of Down syndrome, accounting for only 1% of the total cases.
In this type of Down Syndrome, some cells are normal having 46 chromosomes and some cells have abnormal 47 chromosomes. Symptoms may be less prominent in mosaicism.
Mosaicism is caused when nondisjunction occurs during mitotic division in the zygote after the fertilization. It results in some normal cells and some cells with trisomy of 21.
3. Translocation Down Syndrome: This type of Down syndrome accounts for 4% of the total cases.
Here an extra chromosome 21 is not present but there is an extra part of the chromosome 21 present attached to a different chromosome. Total 46 chromosomes are present of which one is abnormal.
Symptoms for down syndrome
Each person with Down syndrome is an individual — intellectual and developmental problems may be mild, moderate or severe. Some people are healthy while others have significant health problems such as serious heart defects.
Children and adults with Down syndrome have distinct facial features. Though not all people with Down syndrome have the same features, some of the more common features include:
• Flattened face
• Small head
• Short neck
• Protruding tongue
• Upward slanting eye lids (palpebral fissures)
• Unusually shaped or small ears
• Poor muscle tone
• Broad, short hands with a single crease in the palm
• Relatively short fingers and small hands and feet
• Excessive flexibility
• Tiny white spots on the colored part (iris) of the eye called Brushfield’s spots
• Short height
• Single transverse palm crease and hand is broad and short
• Poor muscle tone and excessive flexibility
• Small head, short neck and abnormal teeth
• Delay in language development
• Cognitive impairment may be mild to moderate
Down Syndrome Causes
Down syndrome is a chromosomal disorder. It is due to aneuploidy of the autosome. There is one extra chromosome 21 or part of the chromosome present in all the cells or some cells.
The occurrence of Down syndrome is 1:800 live births. The major risk factor is the age of the mother. Most of the trisomy cases occur in the mother having age more than 35.
Down syndrome is caused due to abnormal cell division. During mitotic and meiotic cell division the chromosome pair separate so that each cell gets the copy of each chromosome. In down syndrome, the chromosomes are not able to separate, giving rise to cells with an unequal number of chromosomes. This phenomenon is called nondisjunction.
Nondisjunction happens, when chromosome segregates in anaphase before all of the replicated chromosomes’ kinetochores are attached to microtubules from opposite poles during metaphase. It results in one daughter cell having less chromosome and another with one extra chromosome.
Down Syndrome Diagnosis
Children with Down syndrome can be diagnosed before or after birth.
Due to peculiar facial features, they are easily identifiable. If diagnosed after birth, there is no cure available and only quality of life can be improved by training, education and extra care.
Down syndrome can be diagnosed during pregnancy and if the foetus is found to have Down syndrome, pregnancy is terminated.
Down syndrome can be diagnosed by amniocentesis technique. In amniocentesis, the amniotic fluid is taken out with the help of a needle and karyotype of the dividing cells are done in the lab.
Treatment throughout the course of an individual’s life will involve regular medical check-ups, physical therapy, speech therapy, occupational therapy, nutritional needs and counselling. Regular medical check-ups are important for identifying, managing and monitoring an individual with Down syndrome’s overall health condition.
There’s no way to prevent Down syndrome. If you’re at high risk of having a child with Down syndrome or you already have one child with Down syndrome, you may want to consult a genetic counselor before becoming pregnant.
A genetic counselor can help you understand your chances of having a child with Down syndrome. He or she can also explain the prenatal tests that are available and help explain the pros and cons of testing.
People with Down syndrome can have a variety of complications, some of which become more prominent as they get older. These complications can include:
• Heart defects. About half the children with Down syndrome are born with some type of congenital heart defect. These heart problems can be life-threatening and may require surgery in early infancy.
• Gastrointestinal (GI) defects. GI abnormalities occur in some children with Down syndrome and may include abnormalities of the intestines, esophagus, trachea and anus. The risk of developing digestive problems, such as GI blockage, heartburn (gastroesophageal reflux) or celiac disease, may be increased.
• Immune disorders. Because of abnormalities in their immune systems, people with Down syndrome are at increased risk of developing autoimmune disorders, some forms of cancer, and infectious diseases, such as pneumonia.
• Sleep apnea. Because of soft tissue and skeletal changes that lead to the obstruction of their airways, children and adults with Down syndrome are at greater risk of obstructive sleep apnea.
• Obesity. People with Down syndrome have a greater tendency to be obese compared with the general population.
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