Mosaic Down Syndrome (Mosaicism)

Mosaic Down syndrome is a very rare type of Down syndrome. It occurs when some of your child’s cells contain three copies of chromosome 21. Other cells have the usual two copies. So, your child has a combination, or a mixture, of cells. That’s what “mosaic” means. This is also called mosaicism (pronounced “moh-ZAY-iss-i-zuhm”).

People with this type of Down syndrome may have fewer distinct features than people with the more common version of the condition, trisomy 21. That’s because some of their cells contain the usual two copies of chromosome 21.

Mosaic Down syndrome symptoms

Symptoms of mosaic Down syndrome mirror those of classical Down syndrome. But they may vary significantly in number due to the mosaic nature of the condition. If your child does have symptoms, sometimes they’re less noticeable.

Physical signs of mosaic Down syndrome may include:

• Flattened back of their head
• Excess skin around their neck
• Small ears
• Fine, straight hair
• Epicanthal folds
• Almond-shaped eyes
• White flecks on the colored part of their eyes
• Flat nose bridge
• A tongue that sticks out
• A crease across one or both of their palms
• Short fingers and toes
• Short stature
• Very dry skin
• Weak muscle tone
• Loose joints

Children with mosaic Down syndrome may have higher IQs than children with typical (nonmosaic) Down syndrome. They may also reach certain developmental milestones — like crawling and walking — earlier. But children with mosaicism often still have some delays. They may develop skills more slowly than typically developing children.

Mosaic Down syndrome causes

An extra chromosome 21 in some of your baby’s cells causes mosaic Down syndrome (mosaicism). Some of their cells contain three copies, and others contain two. This can happen in two ways:

• Trisomy rescue: At first, the fertilized egg had three copies of chromosome 21. But at some point during cell division, genetic mechanisms led to the loss of one chromosome from certain cell lines.
• Trisomy: At first, the fertilized egg had two copies of chromosome 21. But at some point during cell division, one copy was duplicated.

This change happens randomly as the fetus’s cells divide. It isn’t caused by anything you did before or during pregnancy.

Risk factors

Risk factors for having a baby with mosaicism include:

• Age: Females who are 35 or older have a higher risk of having a baby with a genetic condition, including mosaic Down syndrome.
• Other children with Down syndrome: If you already have one child with Down syndrome, your chances of having another one may be higher.

Complications of mosaicism

People with mosaic Down syndrome are more likely to develop certain cognitive conditions, including:

• Autism
• ADHD
• Alzheimer’s disease

How doctors diagnose mosaic Down syndrome

When your baby is born, their healthcare provider may notice physical signs of the condition. But as mosaicism doesn’t affect all of your baby’s cells, they may not have any symptoms.

To diagnose mosaic Down syndrome, your baby’s provider may perform a karyotype test. This is a genetic test that examines your baby’s chromosomes. Your baby’s provider will take a small sample of your baby’s blood. They’ll send it to a lab where a pathologist will analyze up to 50 cells. If some cells have trisomy 21 and some don’t, your baby’s provider can make a mosaic Down syndrome diagnosis.

How is mosaic Down syndrome treated?

Treatment of mosaic Down syndrome depends on the severity of your child’s condition. Providers focus on treating birth defects and developmental challenges related to genetic differences to improve health and quality of life.

Most children with mosaic Down syndrome have some developmental delays. Your child may not reach developmental milestones at the same time as typically developing children. They may have challenges with fine motor skills and gross motor skills. Your child may benefit from various kinds of therapy, including:

• Speech therapy
• Physical therapy
• Occupational therapy

Some children may also benefit from special education programs and other early interventions.

What is the outlook (prognosis) for my child?

Mosaic Down syndrome (mosaicism) affects every child differently. The severity of your child’s condition depends on how many of their cells have three copies of chromosome 21. You may not be able to tell right away. But with time, you may start to notice certain physical features or developmental delays.

Children with mosaic Down syndrome have the same outlook as those with typical Down syndrome. They may need certain treatments or therapies. But with the right support, many children go on to lead full, active lives, including forming relationships, pursuing education and enjoying careers.

What is the survival rate for mosaic Down syndrome?

There’s no current research that suggests people with mosaic Down syndrome live any longer or shorter than people with typical Down syndrome. Both groups have an average life expectancy of 60 years. Studies in genetics are helping to understand the various factors associated with lifespan and health outcomes.