Down Syndrome: Key Concepts, Characteristics, and Prenatal Testing
What is Down Syndrome?
- A chromosomal condition caused by having an extra copy of chromosome 21, typically due to trisomy 21.
- Genetic basis:
- Nondisjunction (the most common cause) accounts for about 95\% of cases.
- Translocation Down Syndrome accounts for about 3\% of cases.
- Mosaic Down Syndrome accounts for about 2\% of cases.
- Karyotype examples:
- Trisomy 21 (most common): 47,XY,+21 or 47,XX,+21
- Translocation: 46,XX,t(14;21) or 46,XY,t(14;21)
- Mosaic/other variations may show a mix of normal and trisomy cells.
- Note on terminology: Down Syndrome is named after John Langdon Down, who first described the condition.
Origin
- John Langdon Down: English physician who observed facial features and developmental similarities among individuals with Down Syndrome.
- Contributions:
- Helped found Normansfield, a care facility for people with developmental disabilities, including Down Syndrome.
- Created the Langdon Down Centre, a theater where people with Down Syndrome could be entertained and perform.
- Published findings in medical journals in 1866 and coined the term "Down Syndrome".
Origin of the name and historical context
- The condition was identified in the 19th century and linked to a distinct set of facial features and developmental profiles.
- The name persists as Down Syndrome, recognizing John Langdon Down's early work.
Common characteristics
- A set of physical traits and developmental features commonly associated with Down Syndrome:
- Low muscle tone (hypotonia)
- Short stature
- Almond-shaped eyes
- Small airway
- Single transverse palmar crease
- Wide gap between the first toe (big toe) and second toe
- Flattened nasal bridge
- Large tongue (macroglossia)
- Low-set ears
Medical concerns and risks
- Congenital cardiac defects (CHD): up to 50\% of individuals
- Gastrointestinal tract abnormalities
- Hematologic disorders: about a 10\x{a0}times increased risk for leukemia
- Neurologic disorders: about 70\% experience early-onset Alzheimer's disease
- Endocrine disorders
- Musculoskeletal disorders
- Refractive errors and visual abnormalities
- Otorhinolaryngological (ENT) disorders, including hearing loss
- Diabetes (less specific prevalence provided in the source)
Prenatal testing
- Screening tests:
- Triple screen or Quad screen: most common; not always accurate; false positives and false negatives possible.
- Non-Invasive Prenatal Testing (NIPT):
- Uses the mother’s blood to test fetal genetic material; more accurate than basic screens; increasingly standard.
- Ultrasound screening
- Diagnostic tests (invasive):
- Chorionic Villus Sampling (CVS): small piece of placenta tested.
- Amniocentesis: small sample of amniotic fluid tested.
- Percutaneous Umbilical Blood Sampling (PUBS): examines blood from the umbilical cord.
Prevalence and demographics
- Prevalence:
- According to the Centers for Disease Control and Prevention (CDC), approximately \frac{1}{775} of US births are babies with Down Syndrome.
- This makes Down Syndrome the most common chromosomal condition.
- Births per year in the US: about 5{,}000 babies with Down Syndrome are born annually.
- Age and risk:
- Age is the only factor historically linked to increased risk for having a baby with Down Syndrome.
- However, due to higher birth rates among younger women, about 51\% of children with Down Syndrome are born to women under 35\{} years of age.
- Source context: De Graaf et al. (2022) and the 2024 update from De Graaf et al.; birth population facts from the CDC/Down Syndrome Population factsheet.
Our story
- A slide titled "OUR STORY DOWN SYNDROME SPOTLIGHT" (content specifics not provided in the transcript).
- Purpose: to share lived experiences, personal perspectives, and real-world context around Down Syndrome.
Notable references and data sources
- CDC public health statistics on Down Syndrome prevalence in the United States.
- De Graaf G., Buckley F., & Skotko B. (2024, May 3). People Living with Down Syndrome in the USA: Births and Population. https://go.downsyndromepopulation.org/USA-FACTSHEET
- Additional data points cited: 2022 update by De Graaf et al. on maternal age and Down Syndrome demographics.
- Prevalence per birth: \frac{1}{775}
- Annual US births with Down Syndrome: 5{,}000
- Case distribution by cause:
- Nondisjunction: 95\%
- Translocation: 3\%
- Mosaicism: 2\%
- CHD prevalence: 50\%
- Leukemia risk multiplier: 10\times (increased risk)
- Early-onset Alzheimer’s: 70\%
- Percentage of Down Syndrome births to women under 35: 51\%
Connections to foundational concepts
- Genetic basis of aneuploidy: Trisomy 21 as a classic example of aneuploidy due to nondisjunction during meiosis.
- The difference between screening and diagnostic tests in prenatal care:
- Screening tests (Triple/Quad screens, NIPT, ultrasounds) estimate risk.
- Diagnostic tests (CVS, Amniocentesis, PUBS) provide definitive genetic information.
- Ethical and practical implications:
- Decisions following positive screening results.
- Accessibility and equity of prenatal testing options.
- Implications for education, healthcare planning, and social inclusion for individuals with Down Syndrome.
Summary takeaways
- Down Syndrome is the most common chromosomal condition, largely caused by trisomy 21 due to nondisjunction (about 95\%) with minority due to translocations and mosaicism.
- Karyotype examples reflect the main patterns: 47,XY,+21/47,XX,+21, 46,XX,t(14;21), and mosaic combinations.
- John Langdon Down played a pivotal historical role in describing and naming the condition.
- People with Down Syndrome have characteristic physical features and face a range of medical concerns, especially CHD and potential early-onset neurodegenerative processes.
- Prenatal testing combines screening and diagnostic approaches to inform prospective parents, with important ethical considerations.
- Population data show that while age is a factor for risk, most births with Down Syndrome occur to women under 35 due to base-rate effects in the population.
