Down syndrome is a genetic condition caused by an additional chromosome 21 (trisomy 21). Symptoms include mental retardation and characteristic facial features. The syndrome occurs in an average of 1:650 newborns, with the risk increasing with the age of the mother.
Also called
Down syndrome is also known as:
- Trisomy 21
- Down’s syndrome
Symptoms
Symptoms range in severity between individuals; however, all share characteristic facial features:
- Small, abnormally shaped head
- Flattened nose bridge
- Eyes that slant up
- Protruding tongue
- Small ears and mouth
Other symptoms include:
- Excess skin at nape of neck
- Delayed physical development and short stature
- Small hands with short fingers
- Single crease across palm (transverse palmar crease)
- Poor muscle tone/loose joints
- Intellectual disability, delayed mental and social development
- Small feet with gap between first and second toes (sandal gap)
- Tiny white spots on the iris of the eye (Brushfield spots)
There are many medical conditions associated with Down syndrome, including
- Cardiac abnormalities
- Infertility in males
- Increased susceptibility to infections, especially ear infections, affecting hearing
- Slightly increased risk of childhood leukemia
- Eye problems
- Dementia
- Gastrointestinal/digestive problems
- Sleep apnea
- Underactive thyroid
- Obesity
Frequency
Down syndrome occurs with an average incidence of 1:650 newborns, although there is a maternal age effect (incidence 1:1,250 in a 20-year-old woman and 1:90 in a 40-year-old woman).
Causes
It is caused by an additional chromosome 21 (trisomy 21), meaning that instead of the usual 46 chromosomes, the person has 47. In 92%, an additional chromosome 21 is present (free trisomy 21), 3% show a mosaic trisomy 21 (while some cells have three copies of chromosome 21, others have two), and in 5% there is a Robertsonian translocation between a third chromosome 21 and an acrocentric chromosome (so-called hereditary variant of Down syndrome). Partial trisomies due to other translocations are very rare.
Inheritance
A few cases of Down syndrome caused by a translocation have a hereditary component; most cases of Down syndrome are not inherited.
Screening and diagnosis
Non-invasive prenatal tests (NIPT) can check for genetic conditions of the fetus, including Down syndrome, before birth with no risk of miscarriage. If a prenatal screening test indicates that the fetus has a high risk of having Down syndrome, an invasive test such as amniocentesis or chorionic villus sampling is recommended.
Treatment
There are very good support options available for those with Down syndrome to help them achieve a high degree of independence in adulthood. Nevertheless, most adults with Down syndrome are reliant on support in their daily lives. Heart defects and gastrointestinal blockages may require surgery.
Although life expectancy has increased significantly in recent years, it is reduced by about 20 years compared to the general population.
References
Antonorakis et al. 2004, Nature Rev Genet 5:725 / Stripoli et al. 1999, Genomics 64:252 / Yamakawa et al. 1998, Hum Molec Genet 7:227 / Baird and Sadovnick 1988, Lancet 2:1354 / Pellisier et al. 1988, Hum Genet 80: 277
A.D.A.M. Medical Encyclopedia [Internet]. Johns Creek (GA): Ebix, Inc., A.D.A.M.; c1997-2020. Down syndrome; [updated 28 Apr. 2023]. Available from: https://medlineplus.gov/ency/article/003247.htm. Accessed 29 Jan. 2025.
National Down Syndrome Society. “About down Syndrome.” Ndss.org, National Down Syndrome Society, 2022, https://ndss.org/about. Accessed 29 Jan. 2025.
Cleveland Clinic. “Down Syndrome.” Cleveland Clinic, 31 Jan. 2023, https://my.clevelandclinic.org/health/diseases/17818-down-syndrome. Accessed 29 Jan. 2025.
