• Clinical science

Down syndrome

Abstract

Down syndrome, also known as trisomy 21, is the most common autosomal chromosome aberration, occurring in approximately 1:700 live births. The risk of a trisomy 21 pregnancy rises with increasing maternal age. Clinically, trisomy 21 manifests as a syndrome involving a characteristic appearance, organ malformations, and mental disability. Typical facial features include upward slanting palpebral fissures, epicanthus, a small oral cavity with a large tongue, a flattened nose. Other typical features are disproportionate short stature, a single crease across the palm of the hand (transverse palmar crease), and an enlarged gap between the first two toes (sandal gap). Common organ malformations include heart defects, duodenal stenosis, and a hypoplastic thyroid. Further associated conditions, such as diabetes mellitus, susceptibility to infections, and a higher incidence of leukemia may shorten life expectancy. The degree of intellectual disability varies greatly and may range from mild manifestations to severe impairment with dependence on caregivers. Children with Down syndrome present with a typical appearance at birth although the disease is often detected in prenatal tests. These include ultrasound measurement of nuchal translucency and maternal blood tests for human gonadotropin, estriol, and alpha-fetoprotein. Fetal karyotyping through chorionic villus sampling and amniocentesis confirm the diagnosis but are associated with an increased risk of fetal injury or loss due to the invasive nature of these tests.

Epidemiology

  • Most common autosomal chromosome aberration (∼ 1:700 live births)
  • The risk of a Down syndrome pregnancy rises with maternal age
Maternal age (years) Incidence of trisomy 21
20 ∼ 1:1500
30 ∼ 1:900
40 ∼ 1:100
45 ∼ 1:30
50 ∼ 1:6

The general risk of trisomy 21 rises with maternal age. This does not, however, apply to translocation trisomies!

References:[1][2][3][4][5][6][7][8]

Epidemiological data refers to the US, unless otherwise specified.

Etiology

Full trisomy 21 (∼ 95% of cases)

  • Full trisomy 21 is not a hereditary disease, as the chromosomal aberration occurs spontaneously.
  • Meiotic non-disjunction
    • Non-disjunction occurs in approx. 70% of cases in meiosis I and in approx. 20% of cases in meiosis II.
    • May occur due to paternal non-disjunction during spermatogenesis (approx. 5% of cases)
  • Karyogram

Translocation trisomy 21 (3–4% of cases)

Mosaicism (1–2%)

  • Mosaic trisomy is caused by non-disjunction during mitosis; . In these cases two cell lines are present, the trisomy 21 cell line and the normal cell line. The resulting karyotypes are either : 46,XX/47,XX,+21 or : 46,XY/47,XY,+21.
  • Depending on the timing of the mitotic error, there is a variable proportion of trisomic and normal cells
    • Phenotypic expression may vary according to the ratio of healthy to trisomic cells

References:[1][9][10]

Clinical features

Facial and cranial features (craniofacial dysmorphia)

Extremities, soft tissue, and skeletal features

  • Extremities
    • Single transverse palmar crease: crease across the palm, which runs along the metacarpophalangeal joints perpendicular to the fingers
    • Sandal gap: large space between the first and second toe
    • Clinodactyly, camptodactyly, brachydactyly
  • Soft tissue: connective tissue deficiency → ↑ increased risk of umbilical and inguinal hernias; marked hyperextension of joints may occur
  • Atlantoaxial instability

Organ malformations and associated conditions

Height and development

  • Motor skills
    • Delayed motor development
    • Muscle hypotonia
  • Height: reduced growth with shortening of long bones → average adult height 150 cm (4 ft 11 in)
  • Obesity: prevalence approx. 50% higher than in general population
  • Intelligence
    • Varying levels of intellectual disability (average IQ: 50)
    • Apparent within first 12 months: developmental milestones (e.g., sitting, walking, talking) achieved at approx. twice the age of healthy children

Although symptoms may be less severe in mosaic trisomies, the clinical manifestation generally provides no indication of the underlying genetic mutation!

References:[11][10][12][13]

Diagnostics

Prenatal screening

Postnatal diagnostics

Typical features and malformations are an important indicator, but, on their own, are insufficient for diagnosis!

References:[11][10][14][7][15][16]

Differential diagnoses

The differential diagnoses listed here are not exhaustive.

Treatment

  • Treatment of the symptom complex and malformations as necessary (e.g., heart surgery for cardiac malformations)
  • Early, targeted intervention, educational programs and support (e.g., encourage physical activity to decrease risk of obesity and improve motor control)

References:[13]

Prognosis

Decreased life expectancy

  • Average life expectancy: approx. 50 years
  • Mostly due to organ malformations (especially cardiac) and ↑ susceptibility to infections (immunodeficiency)

References:[10]