Down syndrome (DS) is the most common genetic chromosomal disorder causing cognitive disabilities in children. Lets see the causes, symptoms, diagnosis and treatment of this genetic disorder.
Last Updated: 22 October 2020
What is Down syndrome?
Each cell has 23 pairs of chromosomes. In Down syndrome, due to abnormal cell division there is an extra or a partial copy of chromosome 21. This extra genetic material causes developmental delays and certain physical changes in children.
Discovery of the disease
In 1866, John Langdon Down, an English physician, published an accurate description of a person with Down syndrome. In recognition of his contribution, the name Down syndrome is used to describe this genetic disorder. Due to the advent of various scientific techniques, scientists could now categorize approximately 329 genes on chromosome 21. This has led to a better understanding of the disease and also the quality of life of these individuals is improving.
Occurrence of the disease
According to the Centers for Disease Control and Prevention, 1 out of 700 babies are born with Down syndrome in the United States, which means ~6000 babies are affected in the US each year. In India, Down syndrome is seen in 23,000-29,000 children every year.
Irrespective of the type of Down syndrome, people with Down syndrome have an extra, critical portion of chromosome 21 in some or all of their cells. The exact cause of Down syndrome is not known. However maternal age has been linked to the development of this disorder. Women who are above 35 years are more likely to have a pregnancy affected by Down syndrome than women who become pregnant at a younger age. This is because older eggs have a greater risk of improper chromosome division.
Is Down syndrome inherited?
Down syndrome is not always inherited. However, in one-third of cases of Down syndrome resulting from translocation there is a hereditary component (transmitted from parent to child). The age of the mother does not increase the risk of translocation. Parents who have one child with Down syndrome and parents who have a translocation themselves are at an increased risk of having another child with Down syndrome.
Different types of Down syndrome Trisomy 21
During reproduction, a child receives 23 chromosomes from the mother and 23 chromosomes from the father. In Down syndrome, due to an error in cell division, a child receives three copies of chromosome 21 instead of the usual two. Prior to or at conception, the pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome keeps on replicating in every cell of the body. This type of Down syndrome is called trisomy 21 and seen in 95% of the cases.
In mosaic Down syndrome, there are mixtures of two types of cells. Some cells contain 46 chromosomes and in some cells 47 chromosomes (extra chromosome 21) are seen. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization. Mosaic DS is the least common form of DS and accounts for 1% of all cases of DS.
In this type of DS, the total number of chromosomes in the cells remains 46; however, an additional full or partial copy of chromosome 21 attaches to another chromosome (chromosome 14). The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome. This type accounts for about 4% of cases of Down syndrome.
Children and adults with DS have certain distinct features. Some of the features are listed here.
- Flat facial features
- Small head and ears
- Short neck
- Protruding tongue
- Upward slanting eye lids
- Atypically shaped ears
- Poor muscle tone
- Relatively short fingers and small hands and feet
- Short height
Children with Down syndrome have some form of intellectual and developmental disabilities. This could be mild or moderate.
Diagnosis of Down syndrome
There are prenatal and diagnostic tests for identifying Down syndrome.
Prenatal and diagnostic test
Prenatal tests estimate the probability whether your fetus is at risk of developing DS. If these prenatal screens turn out to be positive, diagnostic tests are performed which has 100% accuracy. Prenatal tests are a combination of ultrasound and hormone measurements in mother’s blood. If the prenatal results seem to be abnormal, diagnostic tests like amniocentesis or CVS will be performed.
Down syndrome is identified during birth by the presence of certain physical features. Chromosomal analysis called karyotyping is performed to confirm the diagnosis. To obtain a karyotype, doctors draw a blood sample to examine the baby’s cells. The chromosomes are then grouped by size, number, and shape and structural chromosomal abnormalities can be detected. Another genetic test called FISH can apply similar principles and confirm a diagnosis in a shorter period of time.
Complications of DS
People with DS have a lot of complications including heart problems, gastrointestinal disorders, risk of developing infections, sleep disorders; spinal, endocrine and dental problems.
Management of DS
The survival rate of patients with DS has increased dramatically. With the advent of corrective heart surgeries, antibiotics, as many as 80% of adults reach age 60 and many live even longer.
Children with DS require special care not only from doctors but also from speech and occupational therapists. It is the responsibility of the entire society to help and uplift the lives of children with DS.