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Down Syndrome (Trisomy 21)

(Down's Syndrome; Trisomy G)

By

Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Last full review/revision Jun 2020| Content last modified Jun 2020
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Down syndrome is an anomaly of chromosome 21 that can cause intellectual disability Intellectual Disability Intellectual disability is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient 70 to 75) combined with limitations of adaptive functioning... read more , microcephaly, short stature, and characteristic facies. Diagnosis is suggested by physical anomalies and abnormal development and confirmed by cytogenetic analysis. Management depends on specific manifestations and anomalies.

Overall incidence among live births is about 1/700, and the risk increases gradually with increasing maternal age. At 20 years of maternal age, the risk is 1/2000 births; at 35, it is 1/365; and at 40, it is 1/100. However, because most births occur among younger women, the majority of children with Down syndrome are born to women < 35 years; only about 20% of infants with Down syndrome are born to mothers > 35 years.

Etiology of Down Syndrome

In about 95% of cases, there is an extra separate chromosome 21 (trisomy 21), which is typically maternally derived. Such people have 47 chromosomes instead of the normal 46.

About 3% of people with Down syndrome have the normal count of 46 chromosomes but have an extra chromosome 21 translocated to another chromosome (the resulting abnormal chromosome is still counted only as 1).

The most common translocation is t(14;21), in which a piece of an additional chromosome 21 is attached to chromosome 14. In about half of people with the t(14;21) translocation, both parents have normal karyotypes, indicating a de novo translocation. In the other half, one parent (almost always the mother), although phenotypically normal, has only 45 chromosomes, one of which is t(14;21). Theoretically, the chance that a carrier mother will have a child with Down syndrome is 1:3, but the actual risk is lower (about 1:10). If the father is the carrier, the risk is only 1:20.

The next most common translocation is t(21;22). In these cases, carrier mothers have about a 1:10 risk of having a child with Down syndrome; the risk is smaller for carrier fathers.

A 21q21q translocation chromosome, which occurs when the extra chromosome 21 is attached to another chromosome 21, is much less common. It is particularly important to determine whether a parent is a carrier of, or mosaic for, translocation 21q21q (such mosaics have some normal cells and some 45 chromosome cells with the 21q21q translocation). In such cases, each offspring of a carrier of the translocation will either have Down syndrome or monosomy 21 (the latter is not typically compatible with life). If the parent is mosaic, the risk is similar, although these people may also have offspring with normal chromosomes.

Down syndrome mosaicism presumably results from nondisjunction (when chromosomes fail to pass to separate cells) during cell division in the embryo. People with mosaic Down syndrome have two cell lines, one with the normal 46 chromosomes and another with 47 chromosomes, including an extra chromosome 21. The prognosis for intelligence and risk of medical complications probably depends on the proportion of trisomy 21 cells in each different tissue, including the brain. However, in practice, risk cannot be predicted because it is not feasible to determine the karyotype in every single cell in the body. Some people with mosaic Down syndrome have very subtle clinical signs and may have normal intelligence; however, even people with no detectable mosaicism can have very variable findings. If a parent has germline mosaicism for trisomy 21, an increased risk, above the maternal age-based risk, exists for a second affected child.

Pathophysiology of Down Syndrome

As with most conditions that result from chromosome imbalance, Down syndrome affects multiple systems and causes both structural and functional defects (see Table: Some Complications of Down Syndrome* Some Complications of Down Syndrome* Down syndrome is an anomaly of chromosome 21 that can cause intellectual disability, microcephaly, short stature, and characteristic facies. Diagnosis is suggested by physical anomalies and... read more Some Complications of Down Syndrome* ). Not all defects are present in each person.

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Most affected people have some degree of cognitive impairment, ranging from severe (IQ 20 to 35) to mild (IQ 50 to 75). Gross motor and language delays also are evident early in life. Height is often reduced, and there is an increased risk of obesity.

About 5% of affected people have gastrointestinal anomalies, particularly duodenal atresia, sometimes along with annular pancreas Annular pancreas The duodenum can be obstructed by atresia, stenosis, and pressure due to an extrinsic mass. (See also Overview of Congenital Gastrointestinal Anomalies.) This anomaly is the 2nd most common... read more Annular pancreas . Hirschsprung disease Hirschsprung Disease Hirschsprung disease is a congenital anomaly of innervation of the lower intestine, usually limited to the colon, resulting in partial or total functional obstruction. Symptoms are obstipation... read more Hirschsprung Disease and celiac disease Celiac Disease Celiac disease is an immunologically mediated disease in genetically susceptible people caused by intolerance to gluten, resulting in mucosal inflammation and villous atrophy, which causes malabsorption... read more Celiac Disease also are more common. Many people develop endocrinopathies, including thyroid disease (most often hypothyroidism Hypothyroidism in Infants and Children Hypothyroidism is thyroid hormone deficiency. Symptoms in infants include poor feeding and growth failure; symptoms in older children and adolescents are similar to those of adults but also... read more ) and diabetes Diabetes Mellitus (DM) Diabetes mellitus is impaired insulin secretion and variable degrees of peripheral insulin resistance leading to hyperglycemia. Early symptoms are related to hyperglycemia and include polydipsia... read more . Atlanto-occipital and atlantoaxial hypermobility, as well as bony anomalies of the cervical spine, can cause atlanto-occipital and cervical instability; weakness and paralysis may result. About 60% of people have eye problems, including congenital cataracts Congenital Cataract Congenital cataract is a lens opacity that is present at birth or shortly after birth. (See also Cataracts in adults.) Congenital cataracts may be sporadic, or they may be caused by chromosomal... read more , glaucoma, strabismus Strabismus Strabismus is misalignment of the eyes, which causes deviation from the parallelism of normal gaze. Diagnosis is clinical, including observation of the corneal light reflex and use of a cover... read more Strabismus , and refractive errors Overview of Refractive Error In the emmetropic (normally refracted) eye, entering light rays are focused on the retina by the cornea and the lens, creating a sharp image that is transmitted to the brain. The lens is elastic... read more . Most people have hearing loss, and ear infections are very common.

The aging process seems to be accelerated. In recent decades, the median life expectancy has increased to about 60 years, and some affected people live into their 80s. Comorbidities contributing to decreased life expectancy include heart disease, increased susceptibility to infections, and acute myelogenous leukemia Acute Myeloid Leukemia (AML) In acute myeloid leukemia (AML), malignant transformation and uncontrolled proliferation of an abnormally differentiated, long-lived myeloid progenitor cell results in high circulating numbers... read more Acute Myeloid Leukemia (AML) . There is an increased risk of Alzheimer disease Alzheimer Disease Alzheimer disease causes progressive cognitive deterioration and is characterized by beta-amyloid deposits and neurofibrillary tangles in the cerebral cortex and subcortical gray matter. Diagnosis... read more at an early age, and at autopsy, brains of adults with Down syndrome show typical microscopic findings. The results of recent research indicate that blacks with Down syndrome have a substantially shorter life span than whites. This finding may be the result of poor access to medical, educational, and other support services.

Affected women have a 50% chance of having a fetus that also has Down syndrome; however, many pregnancies are spontaneously lost. Men with Down syndrome are infertile, except for those with mosaicism.

Symptoms and Signs of Down Syndrome

General appearance

Affected neonates tend to be placid, rarely cry, and have hypotonia. Most have a flat facial profile (particularly flattening of the bridge of the nose), but some do not have obviously unusual physical characteristics at birth and then develop more noticeable characteristic facial features during infancy. A flattened occiput, microcephaly, and extra skin around the back of the neck are common. The eyes are slanted upward, and epicanthal folds at the inner corners usually are present. Brushfield spots (gray to white spots resembling grains of salt around the periphery of the iris) may be visible. The mouth is often held open with a protruding, furrowed tongue that may lack the central fissure. The ears are often small and rounded.

The hands are often short and broad and often have a single transverse palmar crease. The fingers are often short, with clinodactyly (incurving) of the 5th digit, which often has only 2 phalanges. The feet may have a wide gap between the 1st and 2nd toes (sandal-gap toes), and a plantar furrow often extends backward on the foot.

Characteristic Physical Features of Down Syndrome

Growth and development

Cardiac manifestations

Symptoms of heart disease are determined by the type and extent of the cardiac anomaly.

Murmurs may not be appreciated; however, a number of different murmurs are possible.

Gastrointestinal manifestations

Infants with Hirschsprung disease Hirschsprung Disease Hirschsprung disease is a congenital anomaly of innervation of the lower intestine, usually limited to the colon, resulting in partial or total functional obstruction. Symptoms are obstipation... read more Hirschsprung Disease usually have delay in passage of meconium. Severely affected infants may have signs of intestinal obstruction (eg, bilious vomiting, failure to pass stool, abdominal distention).

Duodenal atresia or stenosis Duodenal Obstruction The duodenum can be obstructed by atresia, stenosis, and pressure due to an extrinsic mass. (See also Overview of Congenital Gastrointestinal Anomalies.) This anomaly is the 2nd most common... read more Duodenal Obstruction can manifest with bilious vomiting or with no symptoms, depending on the extent of the stenosis. These defects may be detected by prenatal ultrasonography (double-bubble sign).

Diagnosis of Down Syndrome

  • Prenatal chorionic villus sampling and/or amniocentesis with karyotyping

  • Postnatal karyotyping (if prenatal karyotyping not done)

Diagnosis of Down syndrome may be suspected prenatally based on physical anomalies detected by fetal ultrasonography (eg, increased nuchal translucency, atrioventricular canal defect, duodenal atresia) or based on abnormal levels of plasma protein A in late 1st trimester and alpha-fetoprotein, beta-hCG (human chorionic gonadotropin), unconjugated estriol, and inhibin in early 2nd trimester (15 to 16 weeks gestation) on maternal serum screening. More recently, noninvasive prenatal screening Screening Chromosomal anomalies cause various disorders. Anomalies that affect autosomes (the 22 paired chromosomes that are alike in males and females) are more common than those that affect sex chromosomes... read more (NIPS), in which fetal DNA obtained from the maternal circulation is tested, has become a screening option for trisomy 21 because it has good sensitivity and specificity.

If Down syndrome was suspected based on maternal serum screening tests or ultrasonography, fetal or postnatal confirmatory testing is recommended. Fetal confirmatory methods include chorionic villus sampling Chorionic Villus Sampling Genetic evaluation is part of routine prenatal care and is ideally done before conception. The extent of genetic evaluation a woman chooses is related to how the woman weighs factors such as... read more Chorionic Villus Sampling and/or amniocentesis Amniocentesis Genetic evaluation is part of routine prenatal care and is ideally done before conception. The extent of genetic evaluation a woman chooses is related to how the woman weighs factors such as... read more Amniocentesis with testing by karyotype analysis. Karyotyping is the test of choice to rule out an associated translocation so that parents can receive appropriate genetic counseling regarding recurrence risk. The option of prenatal confirmatory testing is offered to all patients with an abnormal, indeterminate, or unclear NIPS result. Management decisions, including termination of pregnancy, should not be made based on NIPS alone.

Maternal serum screening and diagnostic testing for Down syndrome are options for all women who present for prenatal care before 20 weeks gestation regardless of maternal age.

The American College of Obstetricians and Gynecologists Committee on Genetics and the Society for Maternal–Fetal Medicine practice bulletin advises that cell-free fetal DNA testing be offered to patients at increased risk of aneuploidy. At-risk patients include women ≥ 35 years and in cases where fetal ultrasonographic findings indicate an increased risk. The committee advises that cell-free fetal DNA does not replace the accuracy and diagnostic precision of prenatal diagnosis with chorionic villus sampling or amniocentesis.

If diagnosis is not made prenatally, then neonatal diagnosis is based on physical anomalies and confirmed by cytogenetic testing.

Concomitant medical conditions

Certain age-specific routine screening helps identify conditions associated with Down syndrome (see the 2011 American Academy of Pediatrics Guidelines Health Supervision for Children with Down Syndrome):

  • Echocardiography: At prenatal visit or at birth

  • Thyroid screening (thyroid-stimulating hormone [TSH] levels): At birth, 6 months, 12 months, and annually thereafter

  • Hearing evaluations: At birth, every 6 months thereafter until normal hearing established (about age 4 years), then annually (more frequently if indicated)

  • Ophthalmology evaluation: By 6 months, then annually until age 5; then every 2 years until age 13 and every 3 years until age 21 (more frequently if indicated)

  • Growth: Height, weight, and head circumference plotted at each health supervision visit using a Down syndrome growth chart

  • Sleep study for obstructive sleep apnea: Completed by age 4 years

Routine screening for atlantoaxial instability and celiac disease is no longer recommended; children are tested based on clinical suspicion. It is recommended that patients with a history of neck pain, radicular pain, weakness, or any other neurologic symptoms that suggest myelopathy have x-rays of the cervical spine in the neutral position; if no suspicious abnormalities are seen, they should have x-rays done in flexion and extension positions.

Treatment of Down Syndrome

  • Specific manifestations treated

  • Genetic counseling

The underlying disorder cannot be cured. Management depends on specific manifestations, but surveillance is fairly uniform for all children. Some congenital cardiac anomalies are repaired surgically. Hypothyroidism is treated with thyroid hormone replacement.

Key Points

  • Down syndrome involves an extra chromosome 21, either a separate chromosome or a translocation onto another chromosome.

  • Diagnosis may be suspected prenatally based on anomalies detected by fetal ultrasonography (eg, increased nuchal translucency, heart defect, duodenal atresia) or based on cell-free fetal DNA analysis of maternal blood or maternal multiple marker screening for levels of plasma protein A in late 1st trimester and levels of alpha-fetoprotein, beta-human chorionic gonadotropin (beta-hCG), unconjugated estriol, and inhibin in early 2nd trimester.

  • Karyotype analysis is the confirmatory test of choice and can be done prenatally by chorionic villus sampling in the 1st trimester or amniocentesis in the 2nd trimester, or postnatally on a blood sample.

  • Life expectancy is decreased primarily by heart disease and, to a lesser degree, by increased susceptibility to infections, acute myelocytic leukemia, and early-onset Alzheimer disease; however, it has increased remarkably in recent decades, and some people live into their 80s.

  • Do routine age-specific screening to detect associated medical conditions (eg, cardiac anomalies, hypothyroidism).

  • Treat specific manifestations, and provide social and educational support and genetic counseling.

More Information

The following are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

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