eMedicine Specialties > Pediatrics: Genetics and Metabolic Disease > Genetics

Down Syndrome

Author: Harold Chen, MD, MS, FAAP, FACMG, Professor, Department of Pediatrics, Chief, Genetic Laboratory Services, Louisiana State University Medical Center
Contributor Information and Disclosures

Updated: Aug 13, 2007

Introduction

Background

In 1866, Down described clinical characteristics of the syndrome that now bears his name. In 1959, Lejeune and Jacobs et al independently determined that trisomy 21 is the cause.1,2 Down syndrome is by far the most common and best known chromosomal disorder in humans. Mental retardation, dysmorphic facial features, and other distinctive phenotypic traits characterize the syndrome.

Pathophysiology

The extra chromosome 21 affects almost every organ system and results in a wide spectrum of phenotypic consequences. These include life-threatening complications, clinically significant alteration of life course (eg, mental retardation), and dysmorphic physical features. Down syndrome decreases prenatal viability and increases prenatal and postnatal morbidity. Affected children have delayed physical growth, maturation, bone development, and dental eruption.

The extra copy of the proximal part of 21q22.3 appears to result in the typical physical phenotype: mental retardation, characteristic facial features, hand anomalies, and congenital heart defects. Molecular analysis reveals that the 21q22.1-q22.3 region, or Down syndrome critical region (DSCR), appears to contain the gene or genes responsible for the congenital heart disease observed in Down syndrome. A new gene, DSCR1, identified in region 21q22.1-q22.2, is highly expressed in the brain and the heart and is a candidate for involvement in the pathogenesis of Down syndrome, particularly, in the mental retardation and/or cardiac defects.

Abnormal physiologic functioning affects thyroid metabolism and intestinal malabsorption. Frequent infections are presumably due to impaired immune responses, and the incidence of autoimmunity, including hypothyroidism and rare Hashimoto thyroiditis, is increased.

Patients with Down syndrome have decreased buffering of physiologic reactions, resulting in hypersensitivity to pilocarpine and abnormal responses on sensory-evoked electroencephalographic tracings. Children with leukemic Down syndrome also have hyperreactivity to methotrexate. Decreased buffering of metabolic processes results in a predisposition to hyperuricemia and increased insulin resistance. Diabetes mellitus develops in many affected patients. Premature senescence causes cataracts and Alzheimer disease. Leukemoid reactions of infancy and an increased risk of acute leukemia indicate bone-marrow dysfunction.

Children with Down syndrome are predisposed to developing leukemia, particularly transient myeloproliferative disorder and acute megakaryocytic leukemia. Nearly all children with Down syndrome who develop these types of leukemia have mutations in the hematopoietic transcription factor gene, GATA1. Leukemia in children with Down syndrome requires at least 3 cooperating events: trisomy 21, a GATA1 mutation, and a third undefined genetic alteration.

Frequency

United States

The frequency is 1 case in 800 live births. Each year, approximately 6000 children are born with Down syndrome.

Mortality/Morbidity

  • Approximately 75% of concepti with trisomy 21 die in embryonic or fetal life. Approximately 85% of infants survive to age 1 year, and 50% can be expected to live longer than age 50 years. Congenital heart disease is the most important factor that determines survival. In addition, esophageal atresia with or without transesophageal (TE) fistula, Hirschsprung disease, duodenal atresia, and leukemia contribute to mortality. The high mortality rate later in life may be the result of premature aging.
  • Individuals with Down syndrome have a greatly increased morbidity rate, primarily because of infections involving impaired immune response. Large tonsils and adenoids, lingual tonsils, choanal stenosis, or glossoptosis can obstruct the upper airway. Airway obstruction can cause serous otitis media, alveolar hypoventilation, arterial hypoxemia, cerebral hypoxia, and pulmonary arterial hypertension with resulting cor pulmonale and heart failure.
  • A delay in recognizing atlantoaxial and atlanto-occipital instability may result in irreversible spinal-cord damage. Visual and hearing impairments in addition to mental retardation may further limit the child's overall function and may prevent him or her from participating in important learning processes and developing appropriate language and interpersonal skills. Unrecognized thyroid dysfunction may further compromise CNS function.

Race

No racial predilection is known.

Sex

The male-to-female ratio is increased (approximately 1.15:1) in newborns with Down syndrome. This effect is restricted to free trisomy 21.

Age

  • Down syndrome can be diagnosed prenatally with amniocentesis, percutaneous umbilical blood sampling (PUBS), chorionic villus sampling (CVS), and extraction of fetal cells from the maternal circulation.
  • Shortly after birth, Down syndrome is diagnosed by recognizing dysmorphic features and the distinctive phenotype.

Clinical

History

When recording the history from the parents of a child with Down syndrome, the clinician should include the following:3

  • Parental concern about hearing, vision, developmental delay, respiratory infections, and other problems
  • Feeding history to ensure adequate caloric intake
  • Prenatal diagnosis of Down syndrome
  • Vomiting secondary to GI tract blockage by duodenal web or atresia
  • Absence of stools secondary to Hirschsprung disease
  • Delay in cognitive abilities, motor development, language development (specifically expressive skills), and social competence
  • Arrhythmia, fainting episodes, palpitations, or chest pain secondary to heart lesion
  • Symptoms of sleep apnea, including snoring, restlessness during sleep, difficulty awaking, daytime somnolence, behavioral changes, and school problems
  • Symptoms of atlantoaxial instability
    • About 13-14% of patients have radiographic evidence of atlantoaxial instability but no symptoms.
    • Only 1-2% of patients have symptoms that require treatment.
    • Symptoms include easy fatigability, neck pain, limited neck mobility or head tilt, torticollis, difficulty walking, change in gait pattern, loss of motor skills, incoordination, clumsiness, sensory deficits, spasticity, hyperreflexia, clonus, extensor-plantar reflex, loss of upper-body strength, abnormal neurologic reflexes, change in bowel and bladder function, increased muscle tone in the legs, and changes in sensation in the hands and feet.
    • These symptoms often remain relatively stable for months or years.
    • In rare cases, the symptoms progress to paraplegia, hemiplegia, quadriplegia, or death.

Physical

  • Growth: Short stature and obesity occurs during adolescence.
  • CNS: Moderate-to-severe mental retardation occurs, with an intelligence quotient (IQ) of 20-85 (mean, approximately 50). Hypotonia improves with age. Articulatory problems are present. Sleep apnea occurs when inspiratory airflow from the upper airway to the lungs is impeded for 10 seconds or longer; it often results in hypoxemia or hypercarbia.
  • Behavior: Natural spontaneity, genuine warmth, cheerful, gentleness, patience, and tolerance are characteristics. A few patients exhibit anxiety and stubbornness.
  • Seizure disorder (5-10%): Infantile spasms are the most common seizures observed in infancy, whereas tonic-clonic seizures are most common in older patients.
  • Premature aging: Decreased skin tone, early graying or loss of hair, hypogonadism, cataracts, hearing loss, age-related increase in hypothyroidism, seizures, neoplasms, degenerative vascular disease, loss of adaptive abilities, and increased risk of senile dementia of Alzheimer type are observed.
  • Skull: Brachycephaly, microcephaly, a sloping forehead, a flat occiput, large fontanels with late closure, a patent metopic suture, absent frontal and sphenoid sinuses, and hypoplasia of the maxillary sinuses occur.
  • Eyes: Up-slanting palpebral fissures, bilateral epicanthal folds, Brushfield spots (speckled iris), refractive errors (50%), strabismus (44%), nystagmus (20%), blepharitis (33%), conjunctivitis, tearing from stenotic nasolacrimal ducts, congenital cataracts (3%), pseudopapilledema, spasm nutans, acquired lens opacity (30-60%), and keratoconus in adults are observed.
  • Nose: Hypoplastic nasal bone and flat nasal bridge are typical characteristics.
  • Mouth and teeth: An open mouth with a tendency of tongue protrusion, a fissured and furrowed tongue, mouth breathing with drooling, a chapped lower lip, angular cheilitis, partial anodontia (50%), tooth agenesis, malformed teeth, delayed tooth eruption, microdontia (35-50%) in both the primary and secondary dentition, hypoplastic and hypocalcified teeth, malocclusion, taurodontism (0.54-5.6%), and increased periodontal destruction are noted.
  • Ears: The ears are small with an overfolded helix. Chronic otitis media and hearing loss are common. About 66-89% of children have a hearing loss of greater than 15-20 dB in at least 1 ear, as assessed by means of the auditory brainstem response (ABR).
  • Neck: Atlantoaxial instability (14%) can result from laxity of transverse ligaments that ordinarily hold the odontoid process close to the anterior arch of the atlas. Laxity can cause backward displacement of the odontoid process, leading to spinal cord compression in about 2% of children with Down syndrome.
  • Chest: The internipple distance is decreased.
  • Congenital heart defects: Congenital heart defects are common (40-50%); they are frequently observed in patients with Down syndrome who are hospitalized (62%), and they are a common cause of death in this aneuploidy in the first 2 years of life. The most common congenital heart defects are endocardial cushion defect (43%), ventricular septal defect (32%), secundum atrial septal defect (10%), tetralogy of Fallot (6%), and isolated patent ductus arteriosus (4%). About 30% of patients have several cardiac defects. The most common lesions are patent ductus arteriosus (16%) and pulmonic stenosis (9%). About 70% of all endocardial cushion defects are associated with Down syndrome.
  • Abdomen: Diastasis recti and umbilical hernia occur.
  • GI system (12%): Duodenal atresia or stenosis, Hirschsprung disease (<1%), TE fistula, Meckel diverticulum, imperforate anus, and omphalocele are observed. The prevalence rate of celiac disease in individuals with Down syndrome is reportedly 5-15% in different European and US studies. Celiac disease occurs in genetically susceptible individuals, specifically those who have the HLA heterodimers DQ2 (observed in 86-100% of individuals with celiac disease) and DQ8. These are strong linkages with high sensitivity and poor specificity.
  • Genitourinary tract: Renal malformations, hypospadias, micropenis, and cryptorchidism occur.
  • Skeleton: Short and broad hands, clinodactyly of the fifth fingers with a single flexion crease (20%), hyperextensible finger joints, increased space between the great toe and the second toe, and acquired hip dislocation (6%) are typical presentations.
  • Endocrine system: Hypothyroidism (16-20% of young patients), diabetes, and decreased fertility occur. Thyroid disorders, such as congenital hypothyroidism, primary hypothyroidism, autoimmune thyroiditis, and compensated hypothyroidism or hyperthyrotropinemia, have been reported to have a prevalence rate of 3-54% in individuals with Down syndrome and increase in frequency with increasing age.
  • Hematologic system
    • The relative risk of acute leukemia in the first 5 years of life is 56 times that of individuals without Down syndrome. Approximately one in 150 patients develops leukemia. Neonatal leukemoid reactions (ie, pseudoleukemia) are common, and distinguishing this from true leukemia frequently poses a diagnostic challenge.
    • Transient myeloproliferative disorder (TMD) associated with pancytopenia, hepatosplenomegaly, and circulating immature WBCs, is found almost exclusively in infants who have Down syndrome, with an incidence rate of approximately 10%. TMD spontaneously regresses within the first 3 months of life. However, in some children, it can be life threatening. Despite the high rate of spontaneous regression, TMD can be a preleukemic disorder in 20-30% of children with Down syndrome.
    • Acute myeloid leukemia (AML) is as common in these individuals as acute lymphoid leukemia (ALL). Acute megakaryocytic leukemia (AMKL) is the most common form of AML in affected children and is uncommon in children who do not have Down syndrome.
    • Although the risk for leukemia is higher in individuals with Down syndrome, these patients have a lower risk of developing solid tumors, with the exception of germ cell tumors and, perhaps, retinoblastomas and lymphomas.
    • The patient's risk of carrying hepatitis B is increased if previously institutionalized.
  • Immunodeficiency: Patients have about a 12-fold increased risk of infectious diseases, especially pneumonia, because of impaired cellular immunity.
  • Skin: Xerosis, localized hyperkeratotic lesions, elastosis serpiginosa, alopecia areata (<10%), vitiligo, folliculitis, abscess formation, and recurrent skin infections are observed.
  • Dermatoglyphics: Distal axial triradius in the palms, transverse palmar creases, a single flexion crease in the fifth finger, ulnar loops (often 10), a pattern in hypothenar, and interdigital III regions are observed.
  • Neurobehavioral disorders: Most children with Down syndrome do not have a coexisting psychiatric or behavioral disorder. The available estimates of psychiatric comorbidity range from 18-38%. The disorders include attention deficit hyperactivity disorder, oppositional defiant disorder, nonspecific disruptive disorder, autism spectrum disorders, and stereotypical movement disorder in prepubertal children with Down syndrome and depressive illness, obsessive-compulsive disorder, and psychoticlike disorder in adolescents and adults with Down syndrome.

Causes

  • The cause of Down syndrome is full trisomy 21 in 94% of patients. Mosaicism (2.4%) and translocations (3.3%) account for the rest. Approximately 75% of the unbalanced translocations are de novo, and approximately 25% result from familial translocation.
  • The most common error is maternal nondisjunction in the first meiotic division, with meiosis I errors occurring 3 times as frequently as meiosis II errors. The remaining cases are paternal in origin, and meiosis II errors predominate.
  • Most mosaic cases result from a trisomic zygote with mitotic loss of 1 chromosome.
  • Advanced maternal age remains the only well-documented risk factor for maternal meiotic nondisjunction. However, understanding of the basic mechanism behind the maternal age effect is lacking.
    • With a maternal age of 35 years, the risk is 1 in 385.
    • With a maternal age of 40 years, the risk is 1 in 106.
    • With a maternal age of 45 years, the risk is 1 in 30.
  • Cytogenetic and molecular studies suggest that dup21(q22.1-22.2) is sufficient to cause Down syndrome. The DSCR contains genes coding for enzymes, such as superoxide dismutase 1 (SOD1), cystathionine beta-synthase (CBS), glycinamide ribonucleotide synthase-aminoimidazole ribonucleotide synthase-glycinamide formyl transferase (GARS-AIRS-GART).

More on Down Syndrome

Overview: Down Syndrome
Differential Diagnoses & Workup: Down Syndrome
Treatment & Medication: Down Syndrome
Follow-up: Down Syndrome
Multimedia: Down Syndrome
References
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Further Reading

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Keywords

Down syndrome, Down's syndrome, mongolism, trisomy 21, mental retardation, Down syndrome critical region, DSCR, DSCR1, Hirschsprung disease, Hirschsprung's disease, duodenal atresia, leukemia, Robertsonian translocation

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Contributor Information and Disclosures

Author

Harold Chen, MD, MS, FAAP, FACMG, Professor, Department of Pediatrics, Chief, Genetic Laboratory Services, Louisiana State University Medical Center
Harold Chen, MD, MS, FAAP, FACMG is a member of the following medical societies: American Academy of Pediatrics, American College of Medical Genetics, American Medical Association, and American Society of Human Genetics
Disclosure: Nothing to disclose.

Medical Editor

James Bowman, MD, Senior Scholar of Maclean Center for Clinical Medical Ethics, Professor Emeritus, Department of Pathology, University of Chicago
Disclosure: Nothing to disclose.

Pharmacy Editor

Mary L Windle, PharmD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy, Pharmacy Editor, eMedicine.com, Inc
Disclosure: Nothing to disclose.

Managing Editor

David Flannery, MD, FAAP, FACMG, Vice Chair of Education, Chief, Section of Medical Genetics, Professor, Department of Pediatrics, Medical College of Georgia
David Flannery, MD, FAAP, FACMG is a member of the following medical societies: American Academy of Pediatrics and American College of Medical Genetics
Disclosure: Nothing to disclose.

CME Editor

Paul D Petry, DO, FACOP, FAAP, Clinical Assistant Professor of Pediatrics, University of North Dakota, School of Medicine and Health Sciences; Consulting Staff, Altru Health System
Paul D Petry, DO, FACOP, FAAP is a member of the following medical societies: American Academy of Osteopathy, American Academy of Pediatrics, American College of Osteopathic Pediatricians, and American Osteopathic Association
Disclosure: Nothing to disclose.

Chief Editor

Bruce A Buehler, MD, Professor, Department of Pathology and Microbiology, Director, Hattie B Munroe Center for Human Genetics, Chairman, Department of Pediatrics, University of Nebraska Medical Center
Bruce A Buehler, MD is a member of the following medical societies: American Academy for Cerebral Palsy and Developmental Medicine, American Academy of Pediatrics, American Association on Mental Retardation, American College of Medical Genetics, American College of Physician Executives, American Medical Association, and Nebraska Medical Association
Disclosure: Nothing to disclose.

 
 
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