Approach Considerations
The diagnosis of Down syndrome is most commonly made by prenatal screening followed by definitive diagnostic testing. When prenatal diagnosis has not been made, Down syndrome is usually apparent from the clinical examination of the newborn. Diagnosis should be confirmed through chromosomal analysis. Since Down syndrome is associated with multisystem involvement, additional diagnostic studies are performed as appropriate.
Laboratory Studies
A complete blood count (CBC) with differential and bone marrow examination to rule out leukemia is indicated. Thyroid-stimulating hormone (TSH) and thyroxine (T4) levels should be obtained at birth, at 6 and 12 months, and annually thereafter, to rule out hypothyroidism. Perform Papanicolaou smears every 1-3 years in sexually active women starting at the age of first intercourse.
Commonly performed studies in individuals with Down syndrome include the following.
Cytogenetic studies
The clinical diagnosis of trisomy 21 should be confirmed with cytogenetic studies. Karyotyping is essential to determine the risk of recurrence. In translocation Down syndrome, karyotyping of the parents and other relatives is required for proper genetic counseling (see the images below).
G-banded karyotype showing trisomy 21 of isochromosome arm 21q type [46,XY,i(21)(q10)].
Interphase fluorescence in situ hybridization
Fluorescence in situ hybridization (FISH) may be used for rapid diagnosis of trisomy 21. It can be successful in both prenatal diagnosis and diagnosis in the neonatal period. A FISH study will detect the presence of trisomy 21; however, it does not provide information about whether trisomy 21 is secondary to a translocation. Therefore, a FISH test must be confirmed by a complete karyotype analysis.
Occult mosaicism for trisomy 21 may partially explain the association between family history of Down syndrome and risk of Alzheimer disease. Screening for mosaicism with FISH is indicated in selected patients with mild developmental delay and those with early onset Alzheimer disease. [58]
Evaluation of the proportion of cells with trisomy 21 in mosaic trisomy 21 includes the following [59] :
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Lymphocyte preparations
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Buccal mucosa cellular preparations
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FISH
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Scoring frequency of trisomic cells
Measurement of immunoglobulin G
Measurement of immunoglobulin (Ig) G levels focuses on identifying deficiencies of subclasses 2 and 4. Decreased levels of IgG subclass 4 is significantly correlated with bacterial infections. These deficits in cellular immunity have also been documented in individuals with gingivitis and periodontal disease.
Radiography and Ultrasonography
Current evidence does not support performing routine screening radiographs for assessment of potential atlantoaxial instability in asymptomatic children. When obtained, skull series show evidence of flattened facial features (including small or absent nasal bones), hypoplastic sinuses, a flat occiput, microcephaly, and brachycephaly.
Cervical radiography (with lateral flexion and extension views) is required to measure the atlantodens distance and to rule out atlantoaxial instability at the age of 3 years. Radiography is also used before administering anesthesia if signs suggest spinal cord compression. Magnetic resonance imaging (MRI) is also recommended regularly for evaluation.
Reduced iliac and acetabular angles may be present in young infants. Short hands with shortened digits and clinodactyly due to hypoplastic middle phalanx of the fifth finger may be present.
Echocardiography should be performed on all infants suspected of having trisomy 21 to identify congenital heart disease, regardless of findings on physical examination.
Prenatal Screening and Diagnosis
Prenatal screening using a combination of maternal serum biomarkers and ultrasonography can detect up to 95% of pregnancies affected by Down syndrome. [84, 85, 86, 87, 88] The false positive rate is 5%. Recently updated guidelines from the American College of Obstetricians and Gynecologists [16] state the following: (1) all women should be offered screening for aneuploidy before 20 weeks' gestation and (2) all pregnant women, regardless of their age, should have the option of diagnostic testing.
The first prenatal diagnosis of Down syndrome was made in 1968, and screening women with amniocentesis on the basis of advanced maternal age was gradually introduced into medical practice. Low maternal serum alpha fetoprotein (MSAFP) levels were associated with Down syndrome in 1983. Later, elevated human chorionic gonadotropin (hCG) and low unconjugated estriol (uE3) levels were found to be markers for Down syndrome. More recently, elevated inhibin A levels (in the second trimester) and reduced pregnancy-associated plasma protein A (PAPP-A) levels (in the first trimester) have been used to screen for Down syndrome in pregnancy. Maternal serum biomarkers can also be used to detect nonstandard trisomy 21 (translocations and mosaicism); however, detection rates for low-level mosaicism may be low. [89]
A substantial proportion of pregnancies are terminated after a prenatal diagnosis of Down syndrome. [90]
Nuchal translucency scan
The nuchal translucency (NT) scan assesses the amount of fluid in the dorsum of the fetal neck and is best assessed at 11-14 weeks. [91] An increased NT measurement is associated with an increased risk of genetic syndromes and can detect up to 70% of Down syndrome pregnancies. However, some centers may not have personnel with expertise in the scanning procedure, and fetal or maternal variables may lead to difficulties in obtaining an accurate measurement.
First-trimester screening
For pregnant women, for whom an early diagnosis is important, a first-trimester "combined test" performed at 11-14 weeks involving sonographic testing for NT together with testing for PAPP-A and hCG provides a detection rate of 82-87% for Down syndrome.
Second-trimester screening
Tests used for second-trimester screening include the triple and quadruple screens. The triple screen measures serum hCG, AFP and unconjugated estriol to calculate the risk of Down syndrome and can detect up to 69% of Down syndrome pregnancies. Currently, the quadruple test, usually performed at 15-18 weeks' gestation, is the most common screening test performed in the second trimester. This screen measures inhibin A in addition to the biochemical markers measured in the triple screen and provides an 81% detection rate for Down syndrome. In addition, the quadruple test serves as a screening test for open neural tube defects (since it involves measurement of AFP) and can also detect trisomy 18.
Integrated screening
With integrated screening, the pregnant woman undergoes a first-trimester screening (involving NT testing, PAPP-A, hCG) followed by the quadruple screen in the second trimester. This combined screening approach increases the detection rate of Down syndrome to 95%, with a false positive rate of only 5%.
Cell-free fetal DNA screening
Cell-free fetal DNA is composed of fragments of fetal DNA derived from the placenta that can be found in maternal plasma. The fragments can be seen in maternal circulation as early as 7 weeks' gestation and last throughout pregnancy, becoming undetectable in the maternal circulation a few hours after birth. The cell-free fetal DNA screening test can be done at any gestational age after 10 weeks and can detect about 99% of Down syndrome pregnancies. Adoption of cell-free DNA for screening women has been slow because of cost, but it is currently used at many centers for screening women at high risk for offspring with Down syndrome. Studies have shown that this test has a high sensitivity and specificity. [92, 93, 94, 95, 96, 97, 98]
A positive screening result with the above methods only suggests an increased risk for Down syndrome, and definitive testing with chorionic villus sampling or amniocentesis and chromosomal analysis is indicated.
Ultrasonography
Prenatal ultrasonography may reveal the following in a fetus with Down syndrome:
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Ultrasonography soft markers for Down syndrome observed in the second trimester include absent or hypoplastic nasal bone, thickened nuchal fold, echogenic bowel, shortened long bones, and pyelectasis
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Absent or hypoplastic nasal bone is observed in 43-62% of trisomy 21 fetuses, compared with 0.5-1.2% of normal fetuses
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A thickened nuchal fold has been associated with a greatly increased risk of trisomy 21 and may be an early feature of fetal hydrops or cystic hygroma; measurement of NT in the first trimester, as discussed above, can detect up to 70% of fetuses with Down syndrome
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Echogenic bowel has been observed in approximately 15% of fetuses with trisomy 21, compared with 0.6% of normal fetuses; about 35% of fetuses with true echogenic bowel have some underlying pathology, such as first trimester bleeding, fetal infections, and cystic fibrosis due to meconium ileus
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Shortened long bones (humerus and femur) have been associated with an increased risk of chromosomal abnormalities; the humerus is a more reliable discriminator for Down syndrome than the femur and appears to be the next most important marker after nasal bone and nuchal fold
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Pyelectasis has been observed in approximately 17% of fetuses with trisomy 21, and approximately 1 in every 300 fetuses with isolated pyelectasis has aneuploidy; pyelectasis has been associated with an increased risk of hydronephrosis and postnatal urinary reflux
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Other ultrasonographic abnormalities include cystic hygroma, duodenal atresia or stenosis (double-bubble sign), cardiac defects (endocardial cushion defect with atrial and ventricular septal defects and abnormal mitral and tricuspid valves), intracardiac echogenic focus, and prune belly anomaly
Ultrasonography should not be relied on as the primary method of diagnosing Down syndrome; the diagnosis can be missed in affected families. Suggestive prenatal ultrasonographic findings may be followed with amniocentesis and fetal chromosome analysis.
Invasive diagnostic tests
Amniocentesis, routinely performed at 14-16 weeks’ gestation, remains the criterion standard of invasive diagnostic tests. Testing for chromosomal disorders is 99.5% accurate. Rare cases of mosaicism are missed, and results can be inaccurate if maternal-cell contamination occurs. The procedure is associated with a small risk of pregnancy loss (1:200-300).
Chorionic villus sampling (CVS) is performed at 10-13 weeks’ gestation; earlier testing is thought to be associated with a 1 in 300-1000 risk of fetal transverse limb deficiency, a small risk of maternal cell contamination, and a 0.5-1% risk of a fetal loss after the procedure. The accuracy of CVS (96-98%) is less than that of midtrimester amniocentesis, because of confined placental mosaicism and maternal-cell contamination.
Percutaneous umbilical blood sampling (PUBS) is approximately 95% successful in obtaining a blood sample for cytogenetic testing. The pregnancy-loss rate is 3.25% for PUBS done for chromosomal indications, compared with 1.25% and 2.75% for PUBS done for nonchromosomal indications. The indication for the procedure greatly increases the risk of procedure-related pregnancy loss.
The availability of in vitro fertilization has allowed preimplantation diagnosis of single-gene disorders, sex selection for X-linked disorders, and identification of chromosomal aneuploidies. After a biopsy sample is obtained from the first polar body, the blastocyst, or the 6-cell to 8-cell embryo, FISH can then be used to diagnose fetal aneuploidy. However, standard cytogenetic confirmation is not possible for the preimplantation diagnosis.
Other Tests
Postnatal diagnostic tests
The auditory brainstem response (ABR), also known as the brainstem auditory evoked response (BAER), may be tested to demonstrate hearing loss. Evaluation of the ABR in 47 nonselected children with Down syndrome aged 2 months to 3.5 years indicated some hearing loss in 66% (28% unilateral, 38% bilateral). Speech evaluation may also be indicated. A study by Rupela et al found evidence that childhood dysarthria (CD), childhood apraxia of speech (CAS), and motor speech disorder-not otherwise specified (MSD-NOS) are responsible for the motor speech characteristics of children with Down syndrome, with variability and overlapping symptoms occurring in these youngsters. This differs from the previous view that either CD or CAS causes such characteristics in these children. [99]
Infants with Down syndrome should undergo a car safety seat evaluation before discharge because they are at increased risk of apnea, bradycardia, and desaturation in a car seat secondary to hypotonia.
Pediatric ophthalmic examination should be performed for vision screening and for detecting ophthalmologic disorders.
Growth charts are available for children with Down syndrome. [35] A developmental chart for noninstitutionalized children based on a modified Denver Developmental Screening Test is available for assessing developmental milestones.
Rigorous dental hygiene and dental evaluation are indicated beginning after tooth eruption.
Histologic findings
In patients with Down syndrome who have clinical signs of dementia associated with Alzheimer disease, postmortem histopathologic findings of the brains show the typical microscopic findings of Alzheimer disease.
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Infant with Down syndrome. Note up-slanting palpebral fissures, bilateral epicanthal folds, flat nasal bridge, open mouth with tendency for tongue protrusion, and small ear with overfolded helix.
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Child with Down syndrome. Note up-slanting palpebral fissures, bilateral epicanthal folds, small nose with flat nasal bridge, open mouth with tendency for tongue protrusion, and small ears with overfolded helix.
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G-banded karyotype showing trisomy 21 (47,XY,+21).
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G-banded karyotype showing trisomy 21 of isochromosome arm 21q type [46,XY,i(21)(q10)].
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Hand of infant with Down syndrome. Note transverse palmar crease and clinodactyly of fifth finger.
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Ear of infant with Down syndrome. Note characteristic small ear with overfolded helix.
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Characteristic flat facies with hypertelorism, depressed nasal bridge, and protrusion of tongue, as well as single palmar simian crease in 2-year-old girl with Down syndrome. Image courtesy of L. Dourmishev, MD, PhD, DSc.
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Small auricle and anomalies of folds in patient with Down syndrome. Image courtesy of L. Dourmishev, MD, PhD, DSc.
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Palmar simian crease in patient with Down syndrome. Image courtesy of L. Dourmishev, MD, PhD, DSc.
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Patient with Down syndrome with protuberant abdomen and umbilical hernia. Image courtesy of L. Dourmishev, MD, PhD, DSc.
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Wide gap between first and second toes and onychomycosis in patient with Down syndrome. Image courtesy of L. Dourmishev, MD, PhD, DSc.
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Hypodontia in patient with Down syndrome. Image courtesy of L. Dourmishev, MD, PhD, DSc.
