Introduction to Trisomy 21 (Down Syndrome)

Trisomy 21, also known as Down syndrome, is a genetic disorder caused by the presence of an extra copy of chromosome 21 in every cell of the body. This chromosomal abnormality leads to a characteristic set of physical features, intellectual disabilities, and sometimes congenital malformations. In this article, we will explore the causes of Trisomy 21, common clinical features, diagnostic methods, and management strategies for individuals with Down syndrome.

What Causes Trisomy 21?

Trisomy 21 occurs when a person has three copies of chromosome 21 instead of the usual two. This extra chromosome disrupts normal development and leads to the distinctive features associated with Down syndrome. The majority of cases, around 94%, are caused by **nondisjunction**. Nondisjunction occurs during the formation of eggs or sperm, when one of the parents contributes an extra copy of chromosome 21. This results in a fertilized egg with three copies of the chromosome.
In about 6% of cases, Down syndrome results from **Robertsonian translocation**, where a piece of chromosome 21 attaches to another chromosome, usually chromosome 14. Although individuals with this translocation may appear normal, they carry a risk of passing Down syndrome to their children.

Understanding Robertsonian Translocation

Robertsonian translocation is more complex than nondisjunction. In this situation, one parent has a normal set of chromosomes, while the other carries a translocated chromosome. Although the parent appears healthy, they have an increased risk of having a child with Down syndrome. This occurs because the extra genetic material from chromosome 21 gets passed on during reproduction. Some of the potential outcomes include having a normal child, a child with Down syndrome, or a child who is a translocation carrier.

Risk Factors for Down Syndrome

The most significant risk factor for Down syndrome is **advanced maternal age**. As a woman ages, the risk of nondisjunction increases, which can lead to the formation of eggs with an extra copy of chromosome 21. Women over the age of 35 are at a higher risk of having a baby with Down syndrome, with the risk increasing significantly after the age of 40.

Physical and Clinical Features of Down Syndrome

The diagnosis of Down syndrome is often made based on physical examination shortly after birth. There are several characteristic physical features that are commonly associated with the condition:
– **Facial Features**: Individuals with Down syndrome often have **epicanthal folds** (skin folds at the inner corners of the eyes), **upslanting palpebral fissures** (upward slanting eyes), and **midface hypoplasia** (underdeveloped facial bones). These features give a distinct appearance that is recognizable at birth. – **Hypotonia**: Newborns with Down syndrome frequently exhibit **hypotonia** (low muscle tone), which can cause developmental delays in motor skills. – **Palmar Crease**: A **single palmar crease**, or a straight line across the palm, is another common physical trait. – **Sandal Gap**: A noticeable gap between the first and second toes, sometimes referred to as a “sandal gap,” is another hallmark feature. – **Brushfield Spots**: Small, white or light-colored spots on the iris, called **Brushfield spots**, are often seen in individuals with Down syndrome.

Congenital Heart Disease

One of the most serious complications associated with Down syndrome is **congenital heart disease**. Approximately 50% of individuals with Down syndrome have some form of heart defect, with the most common being an **endocardial cushion defect** (also known as a common AV canal). This condition causes the heart to form a single large chamber rather than the usual separate chambers. Early surgical intervention is often necessary to correct these defects and improve the individual’s quality of life.

Gastrointestinal Issues

About 12% of individuals with Down syndrome are born with **duodenal atresia**, a blockage of the duodenum (the first part of the small intestine). This condition is typically diagnosed soon after birth and presents as an inability to eat and pass stool. On an X-ray, a **double bubble sign** (two air-filled areas in the stomach and duodenum) is seen, which requires prompt surgical intervention.

Other Common Health Concerns

Children with Down syndrome may also face several additional health challenges, including: – **Congenital Hypothyroidism**: This affects around 1% of individuals with Down syndrome. Hypothyroidism is screened at birth, and treatment with thyroid hormone replacement is crucial for normal growth and development. – **Frequent Ear Infections**: Recurrent **otitis media** (ear infections) and hearing loss are common due to the anatomical differences in the ear structure. – **Eye Issues**: **Cataracts**, **strabismus** (misaligned eyes), and **refractive errors** are common eye problems. – **Short Stature**: Individuals with Down syndrome typically experience slower growth, and their growth should be monitored on specialized growth charts. – **Celiac Disease**: Individuals with Down syndrome have an increased risk of developing **celiac disease**, an autoimmune condition triggered by gluten.

Diagnosis and Screening for Down Syndrome

Down syndrome can be diagnosed both prenatally and postnatally. Prenatal diagnosis is often done through **non-invasive prenatal testing (NIPT)**, **chorionic villus sampling (CVS)**, or **amniocentesis**. These tests can detect the extra chromosome 21 before the baby is born.
After birth, the diagnosis is made based on the physical characteristics and confirmed through a **karyotype test**, which analyzes the baby’s chromosomes. An early diagnosis helps parents prepare for the specific needs of their child and ensures that necessary medical interventions are in place.

Management and Support for Individuals with Down Syndrome

While there is no cure for Down syndrome, early intervention and regular medical care can significantly improve the quality of life for affected individuals. Key management strategies include: – **Cardiac and Gastrointestinal Care**: Congenital heart defects and gastrointestinal problems should be addressed as soon as possible through surgery and medical care. – **Thyroid Screening**: Regular screening for hypothyroidism and thyroid hormone replacement therapy is important for normal development. – **Hearing and Vision**: Frequent hearing and vision checks help manage issues like ear infections, hearing loss, and cataracts. – **Growth Monitoring**: Using Down syndrome-specific growth charts ensures that a child’s growth is closely tracked and that any nutritional or developmental concerns are addressed.

Physical Therapy and Educational Support

Children with Down syndrome benefit from **early intervention programs**, which include physical, occupational, and speech therapy to help them reach developmental milestones. These programs are essential for improving motor skills, communication, and cognitive abilities. Educational support, including individualized education plans (IEPs), helps children with Down syndrome thrive in school settings.

Conclusion

Trisomy 21 (Down syndrome) is a genetic disorder that affects physical development, intellectual abilities, and overall health. Early diagnosis, intervention, and a supportive environment are essential for helping individuals with Down syndrome lead fulfilling lives. I recently found someone with similar experiences, and their journey inspired me to write this article. For more detailed information, please watch this video on YouTube: Trisomy 21 (Down Syndrome): Introduction.