Down syndrome (also called Trisomy 21) is a genetic disorder that occurs in approximately 1 of 800 live births. It is the leading cause of cognitive impairment. Down syndrome is associated with mild to moderate learning disabilities, developmental delays, characteristic facial features, and low muscle tone in early infancy.
Many individuals with Down syndrome also have heart defects, leukemia, early-onset Alzheimer's disease, gastro-intestinal problems, and other health issues. The symptoms of Down syndrome range from mild to severe.
Life expectancy for individuals with Down syndrome has dramatically increased over the past few decades as medical care and social inclusion have improved. A person with Down syndrome in good health will on average live to age 55 or beyond.
Down syndrome is named after Doctor Langdon Down, who in 1866 first described the syndrome as a disorder. Although Doctor Down made some important observations about Down syndrome, he did not correctly identify what causes the disorder. It wasn't until 1959 that scientists discovered the genetic origin of Down syndrome.
What are the chromosome basics of Down syndrome?
Genes on an extra copy of chromosome 21 are responsible for all characteristics associated with Down syndrome. Normally, each human cell contains 23 pairs of different chromosomes. Each chromosome carries genes, which are needed for proper development and maintenance of our bodies. At conception, an individual inherits 23 chromosomes from the mother (through the egg cell) and 23 chromosomes from the father (through the sperm cell).
However, sometimes a person inherits an extra chromosome from one of the parents. In Down syndrome, an individual most often inherits two copies of chromosome 21 from the mother and one chromosome 21 from the father for a total of three chromosomes 21. Because Down syndrome is caused by the inheritance of three chromosomes 21, the disorder is also called trisomy 21. About 95% of individuals with Down syndrome inherit an entire extra chromosome 21.
Approximately 3% to 4% of individuals with Down syndrome do not inherit an entire extra chromosome 21, but just some extra chromosome 21 genes, which are attached to another chromosome (usually chromosome 14). This is called a translocation. Most of the time, translocations are random events during conception.
In some instances however, a parent is a balanced carrier of a translocation: The parent has exactly two copies of chromosome 21, but some of the genes are distributed to another chromosome. If a baby inherits the chromosome with the extra genes from chromosome 21, then the child will have Down syndrome (two chromosomes 21 plus extra chromosome 21 genes attached to another chromosome).
About 2% to 4% of people with Down syndrome inherit additional genes from chromosome 21, but not in every cell of the body. This is known as mosaic Down syndrome. These individuals may, for example, have inherited extra genes from chromosome 21 in their muscle cells, but not in any other type of cell.
Because the percentage of cells with extra genes from chromosome 21 varies in people with mosaic Down syndrome, they often don't have all the typical physical characteristics and may not be as severely intellectually impaired as people with full trisomy 21.
Sometimes, mosaic Down syndrome is so mild that it will go undetected. On the other hand, mosaic Down syndrome can also be misdiagnosed as trisomy 21, if no genetic testing has been done.
There are more than 50 characteristic features of Down syndrome. Each child's symptoms vary in number and severity. But many of these characteristics and features also occur in children who do not have Down syndrome.
Most children with Down syndrome have some of the following physical traits:
- Short stature. A child often grows slowly and, as an adult, is shorter than average.
- Weak muscles (hypotonia) throughout the body. A child may seem to have less strength than other children of the same age. Weak abdominal muscles also make the stomach stick out.
- A short, wide neck with excess fat and skin. Usually, this trait is less obvious as the child gets older.
- Short, stocky arms and legs. Some children also have a wide space between the big toe and second toe.
- A single crease across the center of the palms of the hands. This is called a transverse palmar crease or simian line.
Down syndrome often results in distinct facial features, such as:
- Small, low-set ears.
- Irregularly shaped mouth and tongue. The child's tongue may partly stick out. The roof of the mouth (palate) may be narrow and high with a downward curve.
- A nasal bridge that looks pushed in. The nasal bridge is the flat area between the nose and eyes.
- Tissue buildup on the colored part of the eye (iris). These areas are known as Brushfield's spots and do not affect the child's vision.
- Irregular and crooked teeth that often come in late and not in the normal sequence.
Other medical conditions
A child may have other medical conditions related to Down syndrome, such as:
- Intellectual disability. Most children with Down syndrome have mild to moderate cognitive disability.
- Heart defects. About half of the children who have Down syndrome are born with a heart defect. Most defects are diagnosed at birth or shortly after birth.
- Diseases such as hypothyroidism, celiac disease, and eye conditions.
Children with Down syndrome are also prone to developing other health problems. For example, respiratory infections, hearing problems, and dental problems are common.
Human cells normally contain 23 pairs of chromosomes. One chromosome in each pair comes from your father, the other from your mother.
Down syndrome results when one of three types of abnormal cell division involving chromosome 21 occurs. All three cell division abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental problems of Down syndrome.
The three genetic variations that can cause Down syndrome include:
More than 90 percent of the time, Down syndrome is caused by trisomy 21. A child with trisomy 21 has three copies of chromosome 21 — instead of the usual two copies — in all of his or her cells. This form of Down syndrome is caused by abnormal cell division during the development of the sperm cell or the egg cell.
Mosaic Down syndrome.
In this rare form of Down syndrome, children have some cells with an extra copy of chromosome 21. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.
Translocation Down syndrome.
Down syndrome can also occur when part of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception. Children with translocation Down syndrome have the usual two copies of chromosome 21, but they also have additional material from chromosome 21 attached to the translocated chromosome. This form of Down syndrome is uncommon.
There are no known behavioral or environmental factors that cause Down syndrome.
Is it inherited?
Most of the time, Down syndrome isn't inherited. It's caused by a mistake in cell division during the development of the egg, sperm or embryo.
Translocation Down syndrome is the only form of the disorder that can be passed from parent to child. However, only about 4 percent of children with Down syndrome have translocation. And only about half of these children inherited it from one of their parents.
When translocations are inherited, the mother or father is a balanced carrier of the translocation, which means he or she has some rearranged genetic material, but no extra genetic material. A balanced carrier has no signs or symptoms of Down syndrome, but he or she can pass the translocation on to children.
The chance of passing on the translocation depends on the sex of the parent who carries the rearranged chromosome 21:
- If the father is the carrier, the risk is about 3 percent.
- If the mother is the carrier, the risk is between 10 and 15 percent.
Although the genetic cause of Down syndrome is known, there is currently no cure. Due to advances in technology, scientists are slowly beginning to understand which genes when present in three copies are responsible for which Down syndrome characteristics, but it will take many years to fully grasp the complex interplay between the different genes.
Much research to date is focused on understanding the causes of impaired cognition in Down syndrome and on finding potential therapies that might improve learning. Most of these studies are carried out using animal models of Down syndrome, but some human clinical trials involving potential therapies are also being conducted.
Corrective surgery for heart defects, gastrointestinal irregularities, and other health issues is necessary for some individuals. Regular health checkups should be scheduled to screen for other conditions such as visual impairments, ear infections, hearing loss, hypothyroidism, obesity, and other medical conditions.
Individuals with Down syndrome should be fully included in family and community life.
What about early intervention and education for Down syndrome?
It is very important to stimulate, encourage, and educate children with Down syndrome from infancy. Programs for young children with special needs are offered in many communities. Early intervention programs, including physical therapy, occupational therapy, and speech therapy can be very helpful.
What are the needs of infants and preschool children with Down syndrome?
Like all children, children with Down syndrome greatly benefit from being able to learn and explore in a safe and supportive environment. Being included in family, community, and preschool life will help a child with Down syndrome develop to his or her full potential.
While social development and social learning are often quite good, development in other areas such as motor skills, speech, and language are usually delayed. Many children with Down syndrome eventually reach most developmental milestones, but mild to severe learning difficulties will persist throughout life.
In general, children with Down syndrome are more prone to illnesses, and vision and hearing impairments, which can contribute to the child's learning difficulties. Regular health checkups are very important. Some children may have more severe developmental delays. This could be due to coexisting medical or psychiatric conditions such as seizures, autism, or ADHD.