Define Down Syndrome. Describe its types, clinical features, diagnosis and management.

Definition

Down syndrome is the commonest chromosomal disorder and the commonest genetic cause of intellectual disability, resulting from an extra copy of chromosome 21 (trisomy 21).

Cytogenetic Types

Clinical Features

• Craniofacial — brachycephaly with a flat occiput, upslanting palpebral fissures, epicanthic folds, Brushfield spots (iris), flat nasal bridge, small ears, and a protruding tongue.
• Hands/feet — a single transverse palmar (simian) crease, clinodactyly of the 5th finger, and a wide 'sandal gap' between the 1st and 2nd toes.
• Generalised hypotonia, short stature and intellectual disability.

Associated Anomalies (must Be Actively Screened For)

• Congenital heart disease — atrioventricular (endocardial cushion) defect is characteristic; also VSD.
• Gastrointestinal — duodenal atresia (double-bubble), Hirschsprung disease.
• Hypothyroidism, atlantoaxial instability, an increased risk of leukaemia (all/AML), hearing and visual defects, obstructive sleep apnoea, and early Alzheimer changes.

Diagnosis

• Clinical suspicion confirmed by karyotype (also identifies the type — essential for recurrence-risk counselling).
• Antenatal screening — combined first-trimester test (nuchal translucency + PAPP-A + β-hCG), maternal serum markers, and NIPT (cell-free fetal DNA); confirmed by CVS/amniocentesis.

Management

• Multidisciplinary care — treat associated conditions (cardiac surgery, thyroid, etc.).
• Early intervention/therapy (physiotherapy, speech, special education) to optimise development.
• Routine health surveillance — thyroid, hearing, vision, cardiac and (as indicated) atlantoaxial screening.
• Family support and genetic counselling (recurrence risk depends on the cytogenetic type).

Epidemiology & Risk

• The commonest autosomal trisomy compatible with survival; incidence rises with maternal age.
• Recurrence risk is low for non-disjunction but higher for the inherited translocation form.

Development & Cognition

• Global developmental delay with mild-to-moderate intellectual disability; language is often more affected than social skills.
• Early intervention improves functional outcomes; many achieve semi-independent living.

Health-surveillance Schedule

• Birth — echocardiogram, red-reflex, hearing screen, thyroid function.
• Ongoing — periodic thyroid, vision and hearing checks; growth on Down-specific charts; atlantoaxial assessment before sports.

Differential Diagnosis

• Other trisomies and syndromes with hypotonia and facial anomalies; congenital hypothyroidism (also hypotonia, macroglossia).
• Karyotype settles the diagnosis.

Prognosis

Life expectancy has improved greatly (into adulthood) with treatment of heart defects and infections; the main determinants are cardiac disease and, later, early-onset Alzheimer disease.

Mechanism of Non-disjunction

• Most cases arise from meiotic non-disjunction (failure of chromosome-21 pairs to separate), usually in maternal meiosis — the basis of the maternal-age effect.
• Translocation cases involve fusion of chromosome 21 with another acrocentric chromosome (commonly 14).

Growth & Feeding

• Short stature and feeding difficulties (hypotonia) are common in infancy; plot growth on Down-specific charts.
• Obesity, constipation and dental problems need attention in later childhood.

Antenatal Screening in Detail

• First-trimester combined test — increased nuchal translucency, low PAPP-A, raised free β-hCG.
• Second-trimester quadruple marker and, increasingly, NIPT; ultrasound soft markers support risk assessment.
• A high-risk screen is confirmed by CVS/amniocentesis karyotype.

Family Support

Parents benefit from early, balanced counselling, contact with support groups, and a clear surveillance plan; emphasising the child's potential and needs (not just the diagnosis) is important.