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Handbook of Genetic Counseling/Down Syndrome - Trisomy 21-2

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Down Syndrome - Trisomy 21

Genetics

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  • 3 possible etiologies:
    • Full trisomy 21 (95% of the cases)
      • Due to nondisjunction during meiosis
      • Usually maternal nondisjunction (90-95% of these cases)
        • 75% of maternal nondisjunction occurs during meiosis 1
      • 1% recurrence risk, unless the mother's age related risk is higher than 1%, in which case the age-related risk is quoted
    • Unbalanced translocation (3-4% of cases)
      • 50% of cases a parent carries a balanced translocation
      • 50% of the unbalanced translocations are de novo and occur during fertilization
      • Recurrence risk is 100% for a parental 21:21 translocation
      • For a 21:14 translocation, recurrence is 16% if in the mother and 5% if in the father
    • Mosaicism (1-2% of cases)
      • Due to nondisjunction during mitosis, early in embryonic development
      • Only cells that descend from that specific cell where nondisjunction occurred will be trisomy 21
      • Usually associated with a less severe phenotype
  • Molecular Genetics
    • Ongoing effort to define Chromosome 21 critical region
    • Narrowed to 21q22
    • DSCR1 is a candidate gene suspected to be involved in the pathogenesis of DS

Incidence

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  • One in every 650-1,000 children is born with down syndrome

Clinical Features

  • Intelligence
    • Mild to moderate mental retardation seen in most individuals (ranges from ~40-80)
    • Language delays
  • Cardiac
    • >40% are born with congenital heart defects
      • Av canal and VSD are common
  • Gastrointestinal
    • Duodenal atresia is the most common
    • Duodenal web (partial obstruction)
    • Tracheoesophageal fistula
    • Pyloric stenosis
    • Omphalocele
    • Hirschprung's disease
    • Imperforate anus
    • Annular pancreas
    • Celiac disease
    • Associations with vomiting, diarrhea, constipation, and feeding difficulties may be associated
  • Auditory
    • Recurrent otitis media
    • Some degree of hearing loss is present in ~90% of patients
    • Conductive hearing loss due to fluid accumulation
    • Sensorineural hearing loss is also seen
  • Craniofacial
    • Macroglossia (60%)
      • May cause feeding difficulty
    • Dental malocclusions (100%)
    • Constellation of characteristic facial features
      • Brachycephaly
      • Low nasal root, flat nasal bridge
      • Epicanthic folds
      • Small and occasionally overfolded ears
      • Short neck sometime seen with redundant skin at the nape
      • Brushfield spots
  • Skeletal
    • 14% have increased mobility of the cervical spine at the level of the 1st and 2nd vertebrae (atlantoaxial instability)
  • other
    • Respiratory problems can be due to midfacial hypoplasia which results in narrow airways
      • Infections (recurrent croup due to narrow trachea)
      • Obstructive airway disease
      • Sleep apnea
    • Leukemia
      • Risk of developing leukemia is 15-30 times higher in Down syndrome patients than in the general population
    • Hypothyroidism (15%)
      • Common during adolescence
      • Yearly monitoring of TSH and T(subscript)4 levels recommended
    • Alzheimer's Disease
      • 15% of adults with average age of onset ~50 yrs
    • Eye problems
      • Strabismus
      • cataracts
    • Seizures (5-10%)
      • Correlation between age and prevalence
      • Peaks at infancy and 4th-5th decade of life
    • Short stature (average at 3rd percentile)
    • Obesity
    • Wide gap between first and second toes
    • Hypotonia
    • Clinodactyly of the 5th fingers

Natural History/Prognosis

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  • Life expectancy has significantly increased
    • No congenital heart defects: >60 yrs old
    • Congenital heart defects: 30-35 yrs old
  • Causes of death
    • Leukemia (malignancy)
    • Infections

Treatment and Management

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  • Echocardiograms, EKG, chest X-rays to monitor heart defects
    • Surgery may be required
  • Ophthalmology exams
  • Hearing exams
  • Abdominal X-rays, barium swallow or enema to follow digestive problems
  • TSH levels to detect hypothyroidism
  • Early intervention for MR and developmental delays
  • Physical therapy for gross and fine motor skills
  • Speech therapy

References

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  • Jones KL (1997). Smith's Recognizable Patterns of Human Malformation. Philadelphia: W.B. Saunders Company
  • Jorde LB, Carey JC, Bamshad MJ, White RL (1999). Medical Genetics. 2nd ed.

Philadelphia: Mosby.

  • Cohen WI (1999). Health Care Guidelines for Individuals with Down Syndrome: 1999 Revision (Down Syndrome Preventive Medical Check List). Down Syndrome Quarterly.
  • Web References: OMIM Entry #190685

Notes

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The information in this outline was last updated in 2000.

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