Trisomy 21 (Down Syndrome)

Trisomy 21, commonly known as Down syndrome, is the most common chromosomal condition in humans, affecting about 1 in every 700 babies born in the United States—approximately 5,000 to 6,000 babies each year.

Table of contents

• What is Trisomy 21?
• Types of Down Syndrome
• What Causes Down Syndrome?
• Physical and Developmental Features
• Common Health Problems
• Risk Factors
• Diagnosis and Testing
• Treatment and Support
• Life Expectancy and Quality of Life

What is Trisomy 21?

Trisomy 21 is a genetic condition caused by an extra chromosome. Chromosomes are small packages of genes in the body’s cells that determine how the body forms and functions^[2]. Most babies inherit 23 chromosomes from each parent, for a total of 46 chromosomes. Babies with Down syndrome, however, end up with three chromosomes at position 21 instead of the usual pair^[1].

Down syndrome, Down’s syndrome, Trisomy 21

The condition is named after John Langdon Down, an English physician who first published an accurate description of the syndrome in 1866^[5]. The extra chromosome changes how a baby’s body and brain develop, causing both physical and mental challenges throughout their lifetime^[2].

Down syndrome affects about 350,000 people in the United States^[1]. It is the most common chromosomal condition diagnosed in the country, occurring in approximately 1 in every 700 newborns^[2]. Each person with Down syndrome is unique, having some of the many possible health, learning, and related differences that can occur with this condition^[1].

Types of Down Syndrome

There are three types of Down syndrome. The physical features and behaviors are similar for all three types^[2].

Trisomy 21

With Trisomy 21, each cell in the body has three separate copies of chromosome 21. About 95% of people with Down syndrome have this type^[2]. This is the most common form, where the extra chromosome comes from an unusual cell division^[3].

Translocation Down Syndrome

In this type, an extra part or a whole extra chromosome 21 is present. However, the extra chromosome is attached or “trans-located” to a different chromosome rather than being a separate chromosome 21. This type accounts for about 3% to 4% of people with Down syndrome^[2][16].

Mosaic Down Syndrome

Mosaic means mixture or combination. In this type, some cells have three copies of chromosome 21, but other cells have the typical two copies. People with mosaic Down syndrome may have fewer features of the condition. This type accounts for about 2% of people with Down syndrome^[2].

What Causes Down Syndrome?

Down syndrome occurs because of the extra copy of chromosome 21, which can cause the body and brain to develop differently than a child without the syndrome^[1]. The condition happens by chance, as an error when cells are dividing during early development of the fetus (unborn baby)^[2].

It is important to understand that Down syndrome is not caused by something that parents did before or during pregnancy. The majority of Down syndrome cases happen randomly (sporadically). People don’t usually inherit Down syndrome in an autosomal dominant or recessive pattern during conception when the egg and sperm meet^[4].

The extra chromosome usually comes from the mother^[6]. Researchers believe that having extra copies of genes on chromosome 21 disrupts the normal course of development, causing the characteristics associated with Down syndrome^[8].

Physical and Developmental Features

Each person with Down syndrome is an individual, with problems with intellect and development usually ranging from mild to moderate^[3]. Physical signs of Down syndrome are usually present at birth and become more apparent as your baby grows^[1].

Physical Features

Common physical signs include^[2][4]:

• A flattened face, especially the bridge of the nose
• Almond-shaped eyes that slant upward
• A tongue that tends to stick out of the mouth
• A short neck
• Small ears, hands, and feet
• A single line across the palm of the hand (palmar crease)
• Small pinky fingers that point inward toward the thumb
• Poor muscle tone or loose joints
• Shorter-than-average height

Developmental Features

Children with Down syndrome often have developmental challenges, such as being slower to learn to speak than other children^[2]. Your child’s ability to meet developmental milestones—things that your child can do at a certain age—may differ from other children^[4].

As a result, it may take a child with Down syndrome longer to do the following things^[4]:

• Toilet training
• Speaking their first words
• Taking their first steps
• Eating food independently

People with Down syndrome possess varying degrees of cognitive delays (delays in thinking and learning), from very mild to severe. Most people with Down syndrome have cognitive delays that are mild to moderate^[5].

Common Health Problems

As a child with Down syndrome grows, they are at greater risk for certain medical problems^[1]. Not all people with Down syndrome have all of these problems. Some people are healthy while others have serious health issues^[3].

Heart Problems

About 50% of children with Down syndrome are born with heart defects^[1][8]. The most common heart problems include ventricular septal defect (a hole in the lower heart chambers) and atrioventricular septal defect (a larger hole that can cause heart failure)^[18].

Digestive Problems

About 6% of children have gastrointestinal abnormalities^[1]. Digestive abnormalities, such as a blockage of the intestine, can occur. Common conditions include duodenal atresia and Hirschsprung disease^[8][12].

Hearing and Vision Problems

Most people with Down syndrome have hearing loss, and ear infections are very common^[12]. Up to 75% may be affected by hearing loss^[19]. About 60% of children have eye diseases like cataracts, and 50% have eye issues requiring glasses^[12][19].

Other Health Conditions

People with Down syndrome also have an increased risk of developing several other medical conditions^[8]:

• Obstructive sleep apnea (between 50 and 75% may be affected)
• Hypothyroidism (underactive thyroid), which occurs in about 10% of children and up to 50% of adults with Down syndrome
• Celiac disease (intolerance to wheat protein called gluten)
• Gastroesophageal reflux (backflow of acidic stomach contents)
• Dental problems
• Being more prone to infections or illnesses
• A small percentage of children develop leukemia (cancer of blood-forming cells)

Alzheimer’s Disease

People with Down syndrome often experience a gradual decline in thinking ability as they age, usually starting around age 50. Approximately half of adults with Down syndrome develop Alzheimer’s disease, a brain disorder that results in a gradual loss of memory, judgment, and ability to function. Although Alzheimer’s disease is usually a disorder that occurs in older adults, people with Down syndrome commonly develop this condition earlier, in their fifties or sixties^[8].

Risk Factors

It is not known for sure why Down syndrome occurs or how many different factors play a role^[2]. However, one factor that increases the risk of having a baby with Down syndrome is the age of the mother.

The risk of having a baby with Down syndrome increases as a woman ages. This is especially the case if you are 35 years or older when you get pregnant^[2][4]. A couple’s risk of having a baby with an extra chromosome gradually increases with the mother’s age^[6].

However, the majority of babies with Down syndrome are still born to mothers less than 35 years old. This is because there are many more births among younger women^[2]. About 80% of infants with Down syndrome are born to mothers younger than 35, simply because younger women have more babies overall^[6].

Parents with one child with Down syndrome have a higher chance of having another child with Down syndrome regardless of age^[2].

Diagnosis and Testing

There are two types of tests available to detect Down syndrome during pregnancy: screening tests and diagnostic tests^[2].

Screening Tests During Pregnancy

Prenatal screening tests can show whether your unborn baby has a higher or lower chance of having Down syndrome. These tests cannot tell for sure whether your baby has Down syndrome. If a screening test shows that your baby could have Down syndrome, you’ll need another test to find out for sure^[2].

Screening tests include^[11]:

• Blood tests that measure levels of pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG)
• Nuchal translucency screening test, which uses ultrasound to measure a specific area on the back of your baby’s neck

Diagnostic Tests During Pregnancy

Prenatal diagnostic tests can diagnose or rule out Down syndrome by checking the chromosomes in a sample of cells. These tests have a small risk of causing a miscarriage^[2]. Diagnostic tests include:

• Amniocentesis: a procedure to sample the fluid around the baby
• Chorionic villus sampling: a procedure to sample tissue from the placenta

Diagnosis After Birth

After a baby is born, the provider may make an initial diagnosis of Down syndrome based on the physical signs of the syndrome. The provider can use a karyotype genetic test to confirm the diagnosis. The test can check for extra chromosomes in a sample of the baby’s blood^[14].

Treatment and Support

There is no cure for Down syndrome, but doctors know more about it now than ever. Getting the right care early on can make a big difference in helping a child live a full and meaningful life^[16].

Medical Care

Treatments for children with Down syndrome are based on each person’s physical and intellectual needs, strengths, and limitations. There are no medicines or therapies that are needed by all people with Down syndrome^[22].

Your child needs regular doctor visits and a few special tests. Usual immunizations and well-child care should be performed as recommended. Associated conditions should be monitored periodically as the child grows older^[15].

Regular screening is recommended for^[15]:

• Hearing loss through audiologic evaluation
• Thyroid problems with blood tests
• Heart conditions with echocardiogram
• Vision problems with eye exams

Surgical Treatment

Timely surgical treatment of cardiac abnormalities, detected during the newborn period or early infancy, may be necessary to prevent serious complications and is crucial for optimal survival. Prompt surgical repair is necessary for gastrointestinal abnormalities, most commonly duodenal atresia and Hirschsprung disease^[15].

Early Intervention and Therapy

Services early in life focus on helping children with Down syndrome develop to their full potential. These services include speech, occupational, and physical therapies. They are typically offered through early intervention programs in each state^[14].

Children with Down syndrome may also need extra help or attention in school, although many children are included in regular classes^[14].

Special Considerations

Because low muscle tone is a common physical trait, babies with Down syndrome tend to feel “floppy” when you pick them up. Parents should take hold of them beneath their arms and shoulders and be very careful with the baby’s head and neck^[18].

Many babies with Down syndrome have atlantoaxial instability (AAI), meaning their first two vertebrae are unstable. Care should be taken when sedation and airway management are considered for procedures or for consideration of sports participation^[15].

Life Expectancy and Quality of Life

The life expectancy of people with Down syndrome increased dramatically between 1960 and 2007. In 1960, on average, persons with Down syndrome lived to be about 10 years old. In 2007, on average, persons with Down syndrome lived to be about 47 years old^[19].

Many people diagnosed with Down syndrome live happy and healthy lives. With supportive care, people with Down syndrome can lead fulfilling lives^[2][4].

Better understanding of Down syndrome and early interventions can greatly improve the quality of life for children and adults with this condition. The dramatic improvements in medical care have greatly improved the quality of life for patients and increased their life expectancy^[6].

Like any child, children with Down syndrome will have their own personality, learn at their own pace, and have things that are important and unique to them^[20].