According to the World Health Organisation, Down syndrome occurs in around 1 in 700 live births worldwide. It is determined by many factors, research suggests that there is a higher chance if the mother is older than 35 years of age. Before the age of 30, Down syndrome occurs in less than 1 in 1,000 pregnancies. After the age of 40, this figure rises to about 12 in 1,000*

*Medically reviewed by University of Illinois on December 6, 2017 by Peter Crosta.

It is believed that people with Down syndrome have always existed. However, it was not until 1866 that John Langdon Down, an English doctor, published an accurate description of the condition, which subsequently took his name.

In 1959, Professor Jérôme Lejune identified Down syndrome as a chromosomal condition. Instead of the usual 46 chromosomes present in each cell, he observed that there were 47 chromosomes in the cells of individuals with Down syndrome. It was later determined that this additional chromosomal material results in the varying degrees of intellectual disability and physical characteristics associated with the condition.

Types of Down syndrome

1. Trisomy 21 (Nondisjunction)

Down syndrome is usually caused by an error in cell division called “nondisjunction”. Nondisjunction results in a cell with three copies of chromosome 21 instead of the usual two. Trisomy 21 is the most common form of Down syndrome and it accounts for 95% of cases.

2. Mosaicism

Mosaicism (or mosaic Down syndrome) is diagnosed when there is a mixture of two types of cells, some with the usual 46 chromosomes and some with 47 chromosomes. Those cells with 47 chromosomes contain an extra chromosome 21. Individuals who has this condition may have fewer characteristics of Down syndrome. It is the least common form of Down syndrome and it accounts for 1% of cases.

3. Translocation

Translocation Down syndrome is caused when one chromosome breaks off and attaches to another chromosome. In this case, there are three chromosome 21 but one of the chromosome 21 is attached to another chromosome (chromosome 14). This type of Down syndrome accounts for 4% of cases.

Persons with Down syndrome have unique health conditions that can affect their physical growth, cognitive ability and cause mild to moderate developmental issues. Other higher risks health conditions include Alzheimer’s disease, epilepsy and respiratory problems.

Some physical characteristics of Down syndrome include:

• eyes that slant upward, have oblique fissures, have epicanthic skin folds on the inner corner, and have white spots on the iris
• low muscle tone
• small stature and a short neck
• a flat nasal bridge
• single, deep creases across the center of the palms
• a protrudig tongue
• a large space between large and second toe
• a single flexion furrow of the fifth finger

As each person with Down syndrome is a unique individual, the characteristics may differ.

People who have Down syndrome usually have cognitive development profiles that suggest mild to moderate intellectual disability. However, cognitive development and intellectual ability are highly variable.

Children with Down syndrome often reach developmental milestones a little later than their peers. For example, there may be a delay in learning to talk. A child may need speech therapy to help them gain expressive language. Fine motor skills may also be delayed. They can take time to develop after the child acquires gross motor skills.

On average, a child with Down syndrome will:

• sit at 11 months
• crawl at 17 months
• walk at 26 months

There may also be problems with attention, a tendency to make poor judgments, and impulsive behavior. However, most people with Down syndrome can attend school and become active, valued members of the community.

Prenatal

There are two types of tests for Down syndrome that can be performed before a baby is born: a screening test and a diagnostic test. A screening test can show an increased probability if a fetus has Down syndrome, but it cannot determine that Down syndrome is definitely present. If a screening test shows an increased probability, a diagnostic test can be ordered. A prenatal diagnostic test can determine with almost 100% accuracy that Down syndrome is present. However, diagnostic tests carry a slightly greater risk to the fetus than screening tests.

Most screening tests involve:

• Blood tests that measure protein and hormone levels in pregnant women. Abnormal levels, whether high or low, can indicate a genetic condition.
• Ultrasound can identify congenital heart conditions and other structural changes that may indicate Down syndrome.

Diagnostic tests options include:

• Chorionic villus sampling (CVS), a prenatal diagnostic method in which a small biopsy of the placenta is taken for specific genetic testing. CVS is used to detect any condition that involves specific chromosomal abnormalities, such as Down syndrome. This test is usually done during the first trimester, between 10-14 weeks of gestation.
• Amniocentesis, a prenatal diagnostic method in which a needle is inserted into the amniotic sac that surrounds the fetus. Amniocentesis is most often used to detect Down syndrome and other chromosomal abnormalities. This test is usually done during the second trimester, after 15 weeks of gestation.

After birth

The initial diagnosis of Down syndrome is usually based on the presence of certain physical traits: low muscle tone, a slightly flattened facial profile, an upward slant to the eyes and a single deep crease across the palm. As these features may be present in babies without Down syndrome, a chromosomal analysis called a karyotype is done to confirm the diagnosis. A blood sample is drawn to examine the baby’s cells.

There is no specific treatment for Down syndrome. People who have the condition will receive care for any health problems, as other people do. However, healthcare professionals may recommend additional health screening for issues common to the condition.

Early intervention such as Infant Toddler Program can help a person maximise their potential and prepare them to take up an active role in the community

Children with specific learning and development difficulties may be eligible for educational support, either in a mainstream or specialized school. In recent years, the tendency has been to attend mainstream schools, often with additional support such as Integration Facilitation Support Program to help them integrate and progress

Adults with specific learning and development difficulties may be eligible for Adult Enhancement Program (AEP) and Independent Living and Training Centre to enhance their well-being, independence and employability.

A person with Down syndrome can do many of the things that other people do. Children may take longer to acquire skills such as walking and talking, but with stimulation, they can acquire key life skills and attend school. Depending on how the condition affects a person, it is often possible for someone to work and to live semi-independently with Down syndrome.

The average life expectancy of a person with Down syndrome is around 60 years, but many people with Down syndrome now live into their 70s.