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Down syndrome is one of the most commonly encountered chromosomal disorders in pediatric and obstetric practice. Every medical student, resident, and clinician must be comfortable with the screening, diagnosis, and clinical management.
The approach to Down syndrome is not limited to just making the diagnosis. It involves proper antenatal screening, confirmatory testing, postnatal evaluation, and careful assessment before any surgical procedure.
This article explains Down syndrome in a clear, practical, and exam-oriented manner.
What is Down Syndrome?
Down syndrome is a genetic condition caused by the presence of an extra copy of chromosome 21. This is known as Trisomy 21.
Children with Down syndrome usually present with:
- Developmental delay
- Characteristic facial features
- Intellectual disability
- Reduced muscle tone (hypotonia)
- Multiple associated congenital anomalies
One important point to remember is that Down syndrome is associated with hypotonia, not hypertonia.
Antenatal Screening for Down Syndrome
Screening during pregnancy helps identify pregnancies that have a higher risk of Down syndrome.
First Trimester Screening (11–13 weeks)
The main components are biochemical markers and ultrasound.
Biochemical Markers
In Down syndrome:
- Beta-hCG is increased
- Inhibin A is increased
- Alpha-fetoprotein (AFP) is reduced
These markers form the basis of:
- Triple test – AFP, beta-hCG, estriol
- Quadruple test – AFP, beta-hCG, estriol, inhibin A
AFP is the marker that is classically reduced in Down syndrome.
Ultrasound Marker
The most important ultrasound finding is:
- Increased nuchal translucency thickness (>3 mm)
This is the most sensitive ultrasound marker for Trisomy 21.
Diagnostic Tests for Down Syndrome
Screening only gives probability. A definite diagnosis requires invasive testing.
Chorionic Villus Sampling (CVS)
- Done between 11–13 weeks
- Sample taken from placenta (trophoblast)
- Can be done by:
- Transcervical route
- Transabdominal route
Amniocentesis
- Done between 14–18 weeks
- Sample taken from amniotic fluid
- Only transabdominal route
- Cells studied: fibroblasts and amniocytes
- Considered safer than CVS
Confirmatory Test: Karyotyping
The final diagnosis of Down syndrome is made by karyotyping.
After birth, blood is collected on a DBS (dried blood spot) card for chromosomal analysis.
Karyotyping is mandatory in every suspected case.
Clinical Features of Down Syndrome
Common clinical findings include:
- Hypotonia
- Developmental delay
- Intellectual disability
- Characteristic facial features
- Feeding difficulty due to poor gut motility
- Single palmar crease (may be present but is not specific)
Head Shape in Down Syndrome
The most common skull shape is brachycephaly.
This occurs due to early fusion of the coronal suture, leading to a short and broad head.
Most Sensitive Ultrasound Marker of Trisomy 21
The most sensitive ultrasound finding is:
👉 Increased nuchal translucency thickness (>3 mm)
Other associated findings may include:
- Absent nasal bone
- Short femur
- Echogenic bowel
Common Associated Anomalies
Cardiac Defects
The most common congenital heart disease in Down syndrome is:
Endocardial cushion defect (AV canal defect)
Every child with Down syndrome must undergo echocardiography.
Gastrointestinal Defects
Common gastrointestinal problems include:
- Duodenal atresia (double bubble sign)
- Hirschsprung disease
- Poor gut motility
Preoperative Evaluation in Down Syndrome
Before any surgery, the following must be assessed:
- Cardiac status (ECHO is mandatory)
- Airway anatomy
- Cervical spine stability
- Associated anomalies
This ensures safe anesthesia and surgery.
Conclusion:
Down syndrome is a multisystem disorder that requires a structured and systematic approach. Early screening, confirmatory diagnosis, cardiac evaluation, and developmental support form the backbone of management.
For students and residents, understanding these basics is essential not only for exams but also for day-to-day clinical practice.
A clear concept today makes a confident doctor tomorrow.
