Neurodevelopmental and Behavioral Pediatrics

Down Syndrome - General Overview


Down syndrome (DS) is genetic condition in which a person has 47 chromosomes instead of the usual 46.  The extra chromosome causes problems with the way the body and brain develop.

Causes of Down Syndrome

Down Syndrome is caused by having either an extra copy of chromosome 21 or by part of chromosome 21 attaching to another chromosome. Having an extra copy of chromosome 21 is the most common cause, and is called Trisomy 21. People with Trisomy 21 have 3 copies of this chromosome rather than 2. For most people with Trisomy 21, 3 copies of the chromosome are present in all cells. Very rarely, people with Down syndrome have an extra copy of chromosome 21 in only some cells. This is called mosaic Down syndrome and may be associated with higher cognitive abilities. 

Translocation of chromosome 21 is a less common cause. It occurs when part of chromosome 21 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) in a parent or very early in fetal development. Sometimes translocation Down Syndrome is inherited from an unaffected parent. 

Usually Down syndrome is not inherited. Trisomy 21 and mosaic Down Syndrome are random events. Trisomy 21 occurs during the formation of reproductive cells in a parent. The abnormality usually occurs in egg cells, but it occasionally occurs in sperm cells. Mosaic Down Syndrome occurs as a random event during cell division early in fetal development. As a result, some of the body's cells have the usual two copies of chromosome 21, and other cells have three copies.


A doctor can often make an initial diagnosis of Down syndrome at birth based on how the baby looks. A blood test can be done to check for the extra chromosome and confirm the diagnosis. 

Certain tests, including nuchal translucency ultrasound, amniocentesis, or chorionic villus sampling can allow fetal diagnosis during the first few months of pregnancy  

How Many People Have Down Syndrome

Down syndrome occurs in about 1 in 691 newborns. An estimated 400,000 people in the United States are living with Down Syndrome. Although women of any age can have a child with Down syndrome, the chance of having a child with this condition increases as a woman gets older.  However, because younger women are more likely to have babies than older women, 80% of babies with Down syndrome are born to women younger than 35 years of age.

Associated Developmental and Learning Issues

  • Learning: Most children with Down syndrome usually have mild to moderate intellectual disabilities. This means they learn all skills at a slower pace than other children.  They continue to grow and learn throughout childhood and into adulthood, however. 
  • Movement/mobility difficulties: Most children with Down Syndrome have low muscle tone and trouble with coordination and strength. Young children may be slow in developing motor skills. 
  • Communication difficulties: Most children with Down Syndrome develop speech skills later than other children their age. With intervention, most children develop the ability to communicate wants and needs effectively. Articulation can be a continued difficulty for many.
  • Social Skills: Many children with Down Syndrome learn to play and interact with other children more slowly than other children. With time and support, however, most children develop play and friendship skills.
  • Adaptive Skills: Many children with Down Syndrome learn skills like feeding him/herself, toileting and dressing more slowly than other children. These are very important skills, however, and with family teaching and support can be learned by most children. 

Associated Medical Conditions

Many children with Down Syndrome have other health complications. It is important that these are recognized and treated. A child with Down Syndrome who is physically healthy is more likely to do well in school and in other treatments. This contributes to more independence, better daily functioning, and better quality of life. 

Because of this, it is important for a child’s primary care provider to know about Down Syndrome and how to look for common problems. Guidance for primary care providers is available through the American Academy of Pediatrics, entitled Health Supervision for Children with Down Syndrome Syndrome.

Common Health Problems Include

  • Heart defects (atrial septal defect, ventral septal defect, endocardial cushion defect)
  • Eye & vision problems, including refractive error (trouble seeing close or far), strabismus (lazy eye), nystagmus (involuntary rhythmic eye movement), blepharitis (eye lid inflammation), tear duct obstruction, ptosis (eyelid drooping), cataracts (clouding of the lens of the eye).
  • Hearing loss
  • Sleep apnea
  • Low muscle tone (hypotonia)
  • Feeding problems and slow growth in infancy
  • Obesity in childhood and adulthood
  • Hypothyroidism
  • Diabetes
  • Short stature
  • Bone and muscle problems including atlanto-axial instability (a loosening of the ligament holding cervical vertebrae 1 and 2 in place leading to nerve pinching/damage, which causes weakness in the arms and legs).
  • Structural problems of the GI system, including  imperforate anus (lack of anal opening), duodenal stenosis or atresia (narrowing or closure of the small intestine), hirschsprungs (lack of nerves in the large intestine that cause muscle movement), pyloric stenosis (narrowing of stomach going into the small intestine), trachea-esophageal fistula (incomplete separation of trachea and esophagus). 
  • GI problems including Gastroesophageal reflux disease and constipation
  • Celiac disease (an intolerance of a wheat protein called gluten) are also more common in those with DS.
  • Dental problems
  • Seizures
  • Leukemia
  • Dementia (memory loss) in adulthood & Alzheimer’s disease

Despite many chronic health problems, Life expectancy for people with Down syndrome has increased dramatically in recent decades from 25 in 1983 to 60 today.  Down Syndrome itself does not get worse over time.  However, how it affects a person may be different at different points in the person’s life. 

Associated Behavioral Conditions

  • Autism spectrum disorder
  • ADHD
  • Anxiety
  • Feeding problems
  • Self-injurious behavior
  • Mood and social withdrawal in older school age children, teens and young adults
  • Mood and behavior changes in older adults, related to separation from or loss of a key friend or family member.

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