Down Syndrome (Trisomy 21): Causes, Symptoms, Risk Factors, Diagnosis, Treatment & Current Interventions

Introduction to Down Syndrome
What causes Down syndrome?
Symptoms of Down Syndrome
Major Risk Factor of Down Syndrome
Diagnosis of Down Syndrome
Treatment of Down Syndrome
Current Interventions for Down Syndrome
References

Introduction to Down Syndrome

Down syndrome (DS), also known as Down’s syndrome, is a genetic disorder caused by the presence of an extra copy of chromosome 21, a condition known as trisomy 21.
According to the World Health Organization (WHO), approximately 1 in every 1,000 babies is born with Down syndrome each year worldwide.
Common characteristics of Down syndrome include:

Weak muscle tone (hypotonia), particularly noticeable in infants.
Intellectual disability, which can vary in severity.
Characteristic facial features associated with the condition.

Individuals with Down syndrome may have a shorter life expectancy compared with the general population. However, with appropriate healthcare, education, social support, and opportunities for participation, many people with Down syndrome can live long, healthy, and fulfilling lives.
Evidence suggests that Down syndrome has existed in human populations for thousands of years. However, the condition was first fully described in 1866 by John Langdon Down, after whom the syndrome was named.
In 2015, Down syndrome was estimated to affect approximately 5.4 million people worldwide and was associated with approximately 27,000 deaths globally.
Down syndrome is one of the most common chromosomal conditions, resulting from an alteration in chromosome number rather than a change in a single gene.

What causes Down syndrome?

Down syndrome is caused by the presence of an additional copy of chromosome 21, the smallest human autosome. This results in an extra amount of genetic material from chromosome 21 and is known as trisomy 21.
The additional chromosome 21 can be of maternal or paternal origin, although most cases arise from errors during the formation of the mother's egg cell.
Down syndrome can occur through three main chromosomal mechanisms:

Meiotic nondisjunction
Robertsonian translocation
Mosaicism

1. Nondisjunction During Meiosis

Meiotic nondisjunction accounts for approximately 95% of Down syndrome cases.
During normal meiosis, homologous chromosomes and, subsequently, sister chromatids are separated accurately so that the resulting gametes contain the correct haploid number of chromosomes (23).
Nondisjunction occurs when chromosomes fail to separate properly during meiosis. This produces gametes with an abnormal chromosome number, such as n + 1 or n − 1.
Nondisjunction can occur during either meiosis I or meiosis II:

During meiosis I, homologous chromosomes fail to separate during anaphase I. This can result in all four resulting gametes having an abnormal chromosome number.
During meiosis II, sister chromatids fail to separate during anaphase II. This generally produces two normal gametes and two abnormal gametes.

If a gamete containing an extra chromosome 21 (n + 1) fuses with a normal gamete during fertilization, the resulting zygote contains three copies of chromosome 21, producing trisomy 21 and, consequently, Down syndrome.

2. Robertsonian Translocation

Robertsonian translocation accounts for approximately 4% of Down syndrome cases.
This form occurs when chromosome 21 becomes attached to another acrocentric chromosome, most commonly chromosome 14, through a chromosomal rearrangement.
Acrocentric chromosomes have very short p arms and long q arms. During a Robertsonian translocation, the long arms of two acrocentric chromosomes become joined, while the short-arm material is usually lost.
A person carrying a Robertsonian translocation involving chromosome 21 may have 45 chromosomes instead of 46, but can remain phenotypically normal because the genetic material lost from the short arms is largely redundant.
During the formation of gametes, the translocated chromosome can be inherited along with a normal chromosome 21. If fertilization produces a zygote with three copies of chromosome 21 material, the child develops translocation Down syndrome.
Unlike nondisjunction, this form of Down syndrome can sometimes be inherited from a parent who carries a balanced Robertsonian translocation.

3. Mosaic Down Syndrome

Mosaic Down syndrome accounts for approximately 1–2% of Down syndrome cases. In this form, an individual has a mixture of:

Normal cells with 46 chromosomes
Cells containing an extra chromosome 21, giving 47 chromosomes

Mosaicism usually results from nondisjunction occurring during an early mitotic cell division after fertilization, rather than during the formation of the egg or sperm.
The timing of the chromosomal error influences how many cells become trisomic. If nondisjunction occurs early in embryonic development, a larger proportion of cells may contain the extra chromosome 21.
If the error occurs later, fewer cells may carry trisomy 21, resulting in a lower proportion of trisomic cells.
Because different tissues may contain different proportions of normal and trisomic cells, the clinical features of mosaic Down syndrome can vary considerably among individuals.

Symptoms of Down Syndrome

The features of Down syndrome (DS) can vary considerably from person to person and may range from mild to more pronounced. Some individuals may experience significant health complications, while others may have relatively few medical problems.

Common Physical Features

Common physical characteristics associated with Down syndrome include:

Flattened facial profile and a short neck.
Almond-shaped eyes that typically slant upward.
Small ears, hands, and feet.
Shorter stature, with children and adults generally being shorter than average.
A single transverse palmar crease, commonly called a single palmar crease or “simian crease.”
Small fifth (pinky) fingers that may curve inward toward the other fingers, a feature known as clinodactyly.
A relatively wide gap between the first and second toes, sometimes called a sandal gap.
Reduced muscle tone (hypotonia) and increased joint laxity, which can affect movement and physical development.

Cognitive and Developmental Features

Individuals with Down syndrome commonly experience developmental delays and intellectual disability, although the degree varies considerably between individuals.
Speech and language development may be delayed, and children may develop spoken language more slowly than their peers.
Cognitive abilities generally fall within the mild-to-moderate range of intellectual disability, although individual abilities vary widely.
Children may also experience delays in motor skills, learning, and social development.

Health Complications Associated with Down Syndrome

The extra copy of chromosome 21 increases the expression of genes located on chromosome 21. This altered gene dosage can affect development and function in multiple organs and body systems, contributing to the characteristic features and associated medical conditions of Down syndrome.

1. Eyes and Hearing

People with Down syndrome have an increased risk of hearing and vision problems.
Eye conditions may include strabismus (crossed eyes), refractive errors, and cataracts.
Hearing impairment can occur because of problems involving the middle ear or other parts of the auditory system.

2. Heart

Congenital heart defects occur in approximately half of babies born with Down syndrome.
Common defects include septal defects, in which there is an abnormal opening between the chambers of the heart.
Atrioventricular septal defects are particularly associated with Down syndrome.
Some congenital heart defects may require surgical or other medical treatment.

3. Gastrointestinal System

Individuals with Down syndrome have an increased risk of gastrointestinal abnormalities, including intestinal atresia, in which part of the intestine is narrowed or completely closed.
Other gastrointestinal conditions can also occur and may require medical or surgical management.

4. Blood and Cancer

People with Down syndrome have an increased risk of certain blood disorders and leukemias, particularly during childhood.
Some blood abnormalities may occur even in the absence of leukemia and therefore require appropriate medical monitoring.

5. Brain and Nervous System

Intellectual disability and developmental delays are common features of Down syndrome.
Individuals may also have an increased risk of sleep apnea, a condition in which breathing repeatedly stops and starts during sleep.
Adults with Down syndrome have a substantially increased risk of developing Alzheimer's disease, partly because the APP gene, which is involved in Alzheimer's disease pathology, is located on chromosome 21.

6. Reproductive System

Male infertility or reduced fertility is common among males with Down syndrome.
Female fertility can also be affected, although women with Down syndrome can become pregnant.

7. Immune System

Down syndrome is associated with differences in immune-system function.
Individuals have an increased susceptibility to certain infectious diseases, including respiratory infections such as pneumonia.
They also have an increased risk of some autoimmune disorders and certain cancers.

Major Risk Factor of Down Syndrome

Advanced maternal age is one of the most important known risk factors for having a baby with Down syndrome (trisomy 21).
The likelihood of having a child with Down syndrome increases as maternal age increases, particularly after the age of 35. However, Down syndrome can occur in pregnancies at any maternal age.
The risk increases progressively with maternal age. Approximate risk estimates include:

Age 35: about 1 in 350 pregnancies.
Age 40: about 1 in 100 pregnancies.
Age 45: about 1 in 30 pregnancies.

The increase in risk is associated with chromosomal nondisjunction, an error in which chromosomes fail to separate properly during the formation of egg cells.
One proposed explanation is that older oocytes (egg cells) are more susceptible to errors in chromosome separation during meiosis. However, the precise biological mechanisms responsible for the age-related increase in Down syndrome risk are complex and continue to be investigated.
Although advanced maternal age significantly increases the individual pregnancy risk, a substantial proportion of babies with Down syndrome are born to younger mothers because younger women have more pregnancies overall.

Diagnosis of Down Syndrome

Down syndrome can be suspected during pregnancy or after birth. After birth, a healthcare provider may recognize characteristic physical features of Down syndrome, but a definitive diagnosis requires chromosomal testing.
During pregnancy, prenatal evaluation is broadly divided into two categories:

Screening tests, which estimate the likelihood that the fetus has Down syndrome.
Diagnostic tests, which can confirm whether the fetus has an extra copy of chromosome 21.

Prenatal Screening Tests

Blood tests: Maternal blood tests measure specific substances associated with chromosomal abnormalities.

During the first trimester, screening may measure pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG).
During the second trimester, the triple screen or quadruple (quad) screen measures markers such as alpha-fetoprotein (AFP) and estriol, along with other markers depending on the test.
Abnormal marker levels can indicate an increased likelihood of Down syndrome, but they do not confirm the diagnosis.
Modern prenatal screening may also include cell-free DNA (cfDNA) or non-invasive prenatal testing (NIPT), which analyzes fetal DNA fragments circulating in the mother's blood and provides highly accurate screening for trisomy 21.

Ultrasound: Ultrasound examination can identify certain structural abnormalities or soft markers associated with Down syndrome, such as increased nuchal translucency. However, ultrasound findings alone cannot definitively diagnose Down syndrome.

Prenatal Diagnostic Tests

Chorionic villus sampling (CVS):

CVS involves collecting a small sample of chorionic villi, which are tissues that contribute to the placenta and contain the fetus's genetic material.
The cells are analyzed for chromosomal abnormalities, including an extra copy of chromosome 21.
CVS is generally performed during the first trimester, typically around 10–13 weeks of pregnancy.

Amniocentesis:

In this procedure, a small amount of amniotic fluid is collected using a thin needle.
The fluid contains fetal cells that can be analyzed for chromosomal abnormalities.
Amniocentesis is generally performed during the second trimester, commonly from around 15 weeks of pregnancy onward.
Chromosome analysis can confirm whether the fetus has trisomy 21.

Percutaneous umbilical blood sampling (PUBS):

Also known as cordocentesis, PUBS involves obtaining a blood sample from the fetal umbilical cord.
The sample can be analyzed for chromosomal abnormalities.
Because PUBS carries a higher risk of complications than CVS or amniocentesis, it is not routinely used to diagnose Down syndrome and is generally reserved for specific clinical situations.
It is typically performed later in pregnancy, usually after 18 weeks.

Screening vs. Diagnostic Testing

Screening tests such as blood tests, ultrasound, and NIPT estimate the probability of Down syndrome but cannot establish a definitive diagnosis.
Diagnostic tests, particularly CVS and amniocentesis, analyze fetal or placental cells and can provide a definitive chromosomal diagnosis.
If a screening test indicates a high probability of Down syndrome, healthcare providers may offer diagnostic testing to confirm the result.

Treatment of Down Syndrome

There is currently no cure for Down syndrome, as it is a genetic condition caused by the presence of an extra copy of chromosome 21.
However, various therapies, educational services, medical care, and developmental interventions can help individuals with Down syndrome reach their full potential and improve their quality of life.
These interventions are generally more beneficial when started early, particularly during infancy and early childhood.
Physical therapy can help improve muscle strength, balance, coordination, posture, and motor development.
Occupational therapy helps individuals develop skills needed for daily activities, such as eating, dressing, writing, and other self-care tasks.
Speech and language therapy can support the development of speech, communication, language, and, when needed, feeding and swallowing skills.
Social and recreational activities, including sports, hobbies, and community programs, can promote physical fitness, social interaction, confidence, and independence.
Specialized education services can provide individualized learning support to help children develop academic, cognitive, communication, and social skills according to their abilities and needs.
Job training and vocational programs can help adolescents and adults develop workplace skills and prepare for employment.
Self-care and independent living programs can help individuals develop practical skills for everyday life and increase their independence.
Regular medical care and monitoring are also important because people with Down syndrome may develop associated health conditions, including heart problems, hearing and vision disorders, thyroid disorders, and sleep apnea.
With appropriate healthcare, early intervention, education, family support, and social inclusion, many individuals with Down syndrome can lead healthy, productive, and fulfilling lives.

Current Interventions for Down Syndrome

Advances in medical care, surgical techniques, early intervention, and supportive therapies have significantly improved the health, quality of life, and life expectancy of individuals with Down syndrome.
In the 1960s, the average life expectancy of a person with Down syndrome was approximately 10 years. Today, many individuals live into their 50s and 60s or beyond, although life expectancy varies depending on associated health conditions and access to appropriate medical care.
Improvements in the diagnosis and treatment of congenital heart defects, including corrective heart surgery, have contributed substantially to increased survival among individuals with Down syndrome.
Early intervention programs, including physical, occupational, speech and language therapies, specialized education, and social support, can help children with Down syndrome develop communication, motor, cognitive, and adaptive skills and participate more fully in daily life.
Traditional invasive prenatal diagnostic procedures, such as chorionic villus sampling (CVS) and amniocentesis, provide diagnostic information but carry a small risk of pregnancy complications, including miscarriage.
Advances in genomic medicine have led to non-invasive prenatal screening (NIPS/NIPT), which analyzes fetal genetic material circulating in maternal blood. It provides highly accurate screening for trisomy 21 without the procedure-related miscarriage risk associated with invasive testing.
However, NIPS/NIPT is a screening test rather than a definitive diagnostic test. A positive screening result should generally be followed by diagnostic testing such as CVS or amniocentesis, while a negative result substantially lowers the likelihood of Down syndrome but does not completely exclude all chromosomal or other fetal abnormalities.
Cell-free fetal DNA (cffDNA) analysis forms the basis of many modern non-invasive prenatal screening approaches. Although highly accurate for detecting trisomy 21, the cost and availability of these tests can vary between healthcare systems.
Advances in genomics, cytogenetics, and molecular biology have enabled researchers to investigate relationships between specific genetic changes and the physical, developmental, and clinical features (phenotypes) associated with Down syndrome.
Researchers have identified associations between increased dosage or altered expression of particular genes and certain features of Down syndrome. However, the condition involves complex interactions among multiple genes and biological pathways.
Animal models, particularly mouse models carrying additional copies of chromosome 21-related genes, are being used to investigate how trisomy affects the development and function of the brain, heart, immune system, and other organs.
Ongoing cytogenetic and molecular research is helping scientists understand abnormal developmental pathways, changes in gene expression, and the biological mechanisms underlying Down syndrome.
Continued research in genomics, developmental biology, and targeted therapies may provide further insights into Down syndrome and potentially lead to improved interventions for specific associated health and developmental conditions.

References

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