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Down syndrome
Also known as Trisomy 21 · 47,XX,+21 · 47,XY,+21
Down syndrome, also known as Trisomy 21, is a genetic condition affecting around 1 in 700 babies born in the UK. It is caused by the presence of an extra copy of chromosome 21, resulting in distinctive physical features, developmental delays, and an increased risk of certain health issues.
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Overview
Down syndrome is a genetic condition that occurs when a person has an extra full or partial copy of chromosome 21. This additional genetic material affects development, leading to a combination of physical and intellectual differences [PMID:33008906]. While the specific features and their severity can vary greatly among individuals, common characteristics include distinctive facial features, intellectual disability, and an increased likelihood of certain medical conditions. The condition is not inherited in most cases; it typically arises from a random error during cell division.
Down syndrome is one of the most common chromosomal conditions, affecting approximately 1 in every 700 live births in the UK. The impact of Down syndrome on individuals and families has led to extensive research and the development of supportive care pathways, focusing on early intervention and tailored support to help individuals achieve their full potential [PMID:20880145].
Symptoms & clinical features
Individuals with Down syndrome often share some common physical characteristics, though the extent of these features varies. These may include a flattened facial profile, upward-slanted eyes, a single deep crease across the palm of the hand (single palmar crease), and a shorter neck. Muscle tone can also be lower (hypotonia), which might affect feeding in infancy and motor development.
Developmental differences are a key aspect of Down syndrome. There is typically some degree of intellectual disability, ranging from mild to moderate, which can affect learning and cognitive abilities. Delays in speech and language development are common, as are delays in achieving motor milestones such as sitting, crawling, and walking. However, with appropriate early intervention and ongoing support, individuals with Down syndrome can achieve many developmental milestones, and their skills and abilities can continue to develop throughout their lives.
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Affected organs
Down syndrome can affect various body systems and organs. Congenital heart defects are common, with a significant number of individuals born with heart abnormalities that may require surgical correction [PMID:32587747]. Gastrointestinal issues, such as duodenal atresia (a blockage in the small intestine), are also observed more frequently. Vision and hearing problems, including cataracts and hearing loss, are also more common in individuals with Down syndrome.
Other organ systems that can be affected include the thyroid gland, with hypothyroidism being a common occurrence. The immune system can be less robust, leading to an increased susceptibility to infections. Skeletal issues, such as atlantoaxial instability (instability of the neck vertebrae), can also occur. Regular medical check-ups and screenings are important to monitor these potential health concerns.
Risks & severity
The severity of Down syndrome can vary significantly from one individual to another, encompassing a wide spectrum of physical and intellectual challenges. While some individuals may experience mild effects, others may face more profound developmental delays and complex health issues. The presence of specific health complications, such as serious heart defects or gastrointestinal abnormalities, can influence the overall severity and medical needs.
Individuals with Down syndrome have an increased lifetime risk for several medical conditions beyond those typically present at birth. These include an elevated risk of developing leukaemia, particularly acute myeloid leukaemia in infancy. There is also an increased predisposition to early-onset Alzheimer's disease in adulthood, often related to the extra copy of the APP gene located on chromosome 21. Regular health monitoring and early detection are crucial for managing these potential risks effectively.
Genetic causes
Down syndrome is most often caused by an extra full copy of chromosome 21 in every cell of the body, a condition known as Trisomy 21. This usually happens randomly during the formation of reproductive cells (eggs or sperm) or very early in embryonic development. Less commonly, Down syndrome can be caused by a 'translocation', where part of chromosome 21 becomes attached to another chromosome, or by 'mosaicism', where only some cells have the extra chromosome 21.
The presence of an extra copy of chromosome 21 means that there are three copies of genes located on this chromosome, instead of the usual two. This extra genetic material leads to an overexpression of these genes, which is thought to disrupt normal development. Genes like APP (amyloid beta precursor protein), DYRK1A (dual specificity tyrosine phosphorylation regulated kinase 1A), RUNX1 (RUNX family transcription factor 1), and SOD1 (superoxide dismutase 1) are found on chromosome 21. Overexpression of these genes is believed to contribute to some of the features of Down syndrome, such as intellectual disability and an increased risk of Alzheimer's disease.
- APP amyloid beta precursor protein
- DYRK1A dual specificity tyrosine phosphorylation regulated kinase 1A
- RUNX1 RUNX family transcription factor 1
- SOD1 superoxide dismutase 1
Inheritance pattern
In the vast majority of cases (about 95%), Down syndrome is not inherited. It is typically the result of a random event during the formation of egg or sperm cells, or during the very early stages of foetal development. This type of Down syndrome is called Trisomy 21 and means there is an extra copy of chromosome 21 in all cells. It is not passed down through families.
In a smaller number of cases (about 3-4%), Down syndrome can be caused by a 'translocation'. This is where a part of chromosome 21 breaks off and attaches to another chromosome. While most translocations are spontaneous, some can be inherited from a parent who carries a 'balanced translocation'. A parent with a balanced translocation has the correct amount of genetic material, but it is arranged differently, usually without causing health problems for them. However, they have an increased chance of having a child with Down syndrome or other chromosomal imbalances. Genetic counselling can provide specific risk information for families where a translocation is identified.
Diagnosis & testing
Down syndrome can sometimes be suspected during pregnancy through prenatal screening tests, such as blood tests and ultrasound scans. If screening results suggest a higher chance of Down syndrome, diagnostic tests like chorionic villus sampling (CVS) or amniocentesis can be offered to confirm the diagnosis. These tests involve analysing foetal cells to check the chromosomes. For confirmed cases, these diagnostic tests are typically offered under the NHS Genomic Medicine Service.
After birth, Down syndrome is often suspected based on the baby's distinctive physical features. A definitive diagnosis is made through a blood test called a karyotype, which analyses the baby's chromosomes to confirm the presence of an extra chromosome 21. Referral to a clinical genetics service is usually made for diagnosis and to discuss the implications for the family. In the UK, genetic testing for Down syndrome would fall under relevant R-codes for constitutional cytogenomic analysis.
Management & lifestyle
While there is no cure for Down syndrome, management focuses on early intervention, supportive therapies, and addressing associated health conditions. A multidisciplinary team approach is often employed, involving paediatricians, physiotherapists, speech and language therapists, occupational therapists, and educational specialists. Early intervention programmes, starting in infancy, can significantly help with motor, cognitive, and social development.
Regular health surveillance is crucial for individuals with Down syndrome due to the increased risk of certain medical issues. This includes routine checks for heart conditions, thyroid function, hearing, and vision. Any identified health problems are managed according to standard medical protocols. For instance, congenital heart defects may require surgical repair, and hypothyroidism can be managed with medication. Genetic counsellors can provide ongoing support and information to families regarding the condition and available resources, aligning with NHS care pathways for chromosomal disorders.
UK care pathway
In the UK, the care pathway for individuals with Down syndrome typically involves a coordinated approach through the NHS Genomic Medicine Service. If Down syndrome is suspected during pregnancy or at birth, diagnostic genetic testing is usually initiated, often facilitated by a clinical genetics team. Genetic counsellors play a vital role in explaining the condition, its inheritance patterns, and the implications for the family.
Following diagnosis, individuals are typically supported by a multidisciplinary team, including paediatricians, and various therapists. Regular health screenings are integrated into their routine care to monitor for associated conditions. The relevant NHS R-codes for genomic testing would be applied for confirmatory diagnosis, ensuring access to appropriate genetic services and ongoing support.
Frequently asked questions
How common is Down syndrome?
Down syndrome affects approximately 1 in 700 live births in the UK. The likelihood of having a baby with Down syndrome increases with the mother's age, but it can occur in pregnancies at any age.
Can Down syndrome be inherited?
Most cases of Down syndrome (Trisomy 21) are not inherited and result from a random error in cell division. A small percentage of cases, due to a 'translocation', can be inherited if a parent carries a balanced translocation. Genetic counselling can clarify individual risks.
What kind of support is available for individuals with Down syndrome?
Support for individuals with Down syndrome in the UK includes early intervention programmes, special educational support, and various therapies like physiotherapy, speech and language therapy, and occupational therapy. The NHS provides a pathway for managing associated health conditions and accessing specialist services.
Do people with Down syndrome typically have a shorter life expectancy?
In recent decades, the life expectancy for individuals with Down syndrome has significantly increased due to advancements in medical care and early intervention. While it is generally shorter than the average population, many individuals with Down syndrome now live into their 50s and 60s, leading fulfilling lives.
Are there different types of Down syndrome?
Yes, there are three main types: Trisomy 21 (the most common, an extra full chromosome 21), Translocation Down syndrome (part of chromosome 21 attached to another chromosome), and Mosaic Down syndrome (only some cells have an extra chromosome 21). The type is determined by genetic testing.
References
- Rafii MS, Kleschevnikov AM, Sawa M. Down syndrome. Handbook of clinical neurology. 2019. PMID: 31753140
- Papavassiliou P, Charalsawadi C, Rafferty K. Mosaicism for trisomy 21: a review. American journal of medical genetics. Part A. 2015. PMID: 25412855
- Hashizume R, Wakita S, Sawada H. Trisomic rescue via allele-specific multiple chromosome cleavage using CRISPR-Cas9 in trisomy 21 cells. PNAS nexus. 2025. PMID: 39967679
- Earley MA, Sher ET, Hill TL. Otolaryngologic Disease in Down syndrome. Pediatric clinics of North America. 2022. PMID: 35337546
- Ryan C, Vellody K, Belazarian L. Dermatologic conditions in Down syndrome. Pediatric dermatology. 2021. PMID: 34418156
- Head E, Helman AM, Powell D. Down syndrome, beta-amyloid and neuroimaging. Free radical biology & medicine. 2018. PMID: 28935420
- Brás A, Rodrigues AS, Gomes B. Down syndrome and microRNAs. Biomedical reports. 2018. PMID: 29403643
- Oo KT, Abou-Antoun N, Danopoulos S. Advances in Down Syndrome. Advances in pediatrics. 2026. PMID: 42481087