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TRISOMY 21

Down syndrome (DS) or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21 Down syndrome is a genetic condition that causes delays in physical and intellectual development. It occurs in approximately one in every 800 live births. Individuals with Down syndrome have 47 chromosomes instead of the usual 46. It is the most frequently occurring chromosomal disorder. Down syndrome is not related to race, nationality, religion or socioeconomic status. The most important fact to know about individuals with Down syndrome is that they are more like others than they are different.

TRISOMY 21

Trisomy 21 (nondisjunction) is caused by a faulty cell division that results in the baby having three #21 chromosomes instead of two. Prior to or at conception, a pair of #21 chromosomes in either the egg or the sperm fails to separate properly. The extra chromosome is replicated in every cell of the body. Ninety five percent of all people with Down syndrome have Trisomy 21. It is not known why this occurs. However, it is known that the error occurs at conception and is not related to anything the mother did during pregnancy. It has been known for some time that the incidence of Down syndrome increases with advancing maternal age. However, 80% of children with Down syndrome are born to women under 35 years of age.

CAUSE

Extra and loose neck skin Single creases on the palmsClinodactyly (curved fifth finger)Broad space between the first and second toesA deep plantar crease The tongue often protrudes, more because of low muscle tone than true enlargement.

CLINICAL FEATURES

CLINICAL FEATURES

Central hair whorl (cowlick) Flat occiput (back of the head) Upslanting eyes Epicanthal folds (folds around

the corner of the eye) White spots in the iris of the

eye (Brushfield spots) Upturned nose

Children may also have delayed mental and social development. Common problems may include: Impulsive behavior,Poor judgment,Short attention span,Slow learning

CLINICAL FEATURES

As with all syndromes, the pattern of minor and major defects in Down syndrome varies from individual to individual.

None of the anomalies taken alone are specific or diagnostic for Down syndrome.

When Down syndrome is suspected, a chromosome study (karyotype) will confirm or exclude the diagnosis

Prognosis: Approximately 2/3 of the fetuses with trisomy 21 die before delivery. A 1/3 if the survivor die

during the 1st year, 50% before the age of 4. The life expectancy of the remainder is shortened. Mental

retardation (moderate to severe) is the rule but in some mosaic case it can almost be normal.

Decreased muscle tone.

Some anomalies will need surgery for correction as heart defects or duodenal atresia

PROGNOSIS

RELATED CONDITIONS

ObesitySleep ApneaDevelopmental

DelaysBlood DiseaseEndocrine DiseaseBlood DisordersAlzheimer’s

Disease

Two types of procedures are available to pregnant women: screening tests and diagnostic tests.

Screening tests estimate the risk of the baby having Down syndrome.

Diagnostic tests tell whether or not the baby actually has Down syndrome.

PRENATAL DIAGNOSIS

•At this time the most commonly used screening test is “The Triple Screen.” This is a combination of three tests that measure quantities of various substances in the blood. These tests are usually done between 15 and 20 weeks of gestation.

•Sonograms (ultrasounds) are usually performed in conjunction with other screenings. These can show some physical traits that are helpful in calculating the risk of Down syndrome.

•Screening tests do not accurately confirm the diagnosis of Down syndrome. In fact, false positives and false negatives frequently occur.

SCREENING TESTS

• Full fetal survey: ~18 weeks gestation (structural anomalies can be detected)

• Early ultrasound: 10-14 weeks gestation– Nuchal translucency measurements

PARENTAL ULTRASOUND

Structural anomaliesNB: found in less than 20% fetuses with Trisomy 21• Cardiac defects: e.g. VSD, ASD• Duodenal atresia• Ventriculomegaly Sonographic markers• Shortened proximal long bones (humerus/femur)• Echogenic intracardiac focus• Echogenic bowel• Absent nasal bone• Nuchal thickening• Pyelectasis (renal pelvis dilation)

ULTRASOUND DETECTION OF DS

SAGITTAL ULTRASOUND

Demonstrating Thickened NT of 4.2 mm

Landmark: skin

COMPANION IMAGES OF NUCHAL TRANSLUCENCY

Thickened Nuchal Translucency

image of embryo & 12-week sagittal ultrasound

ULTRASOUND

Thickened nuchal translucency, (10-14 wks), when combined with maternal age and biochemical markers, can detect Trisomy 21 to 90%.Structural anomalies, e.g. endocardial cushion defects, found in less than 20% of fetuses with Trisomy 21, may be seen in the full fetal scan.Sonographic markers, e.g. shortened femur, echogenic intracardiac focus, and echogenic bowel can be normal variants but also seen frequently in Trisomy 21 fetuses.

NUCHAL TRANSLUCENCY

Three diagnostic tests are currently available:

•Amniocentesis is performed between 12 and 20 weeks gestation.

•Chorionic Villus Sampling (CVS) is conducted between 8 and 12 weeks.

•Percutaneous Umbilical Blood Sampling (PUBS) is performed after 20 weeks.

DIAGNOSTIC TESTS

AMNIOCENTESIS

CHORIONIC VILLUS SAMPLING

PERCUTANEOUS UMBILICAL BLOOD SAMPLING

Women with DS are fertile and may become pregnant.

Nearly all males with DS are infertile. The mechanism is impairment of spermatogenesis

REPRODUCTION

There is no specific treatment for Down syndrome. A child born with a gastrointestinal blockage may need major surgery immediately after birth. Certain heart defects may also require surgery.

When breast-feeding, the baby should be well supported and fully awake. The baby may have some leakage because of poor tongue control. However, many infants with Down syndrome can successfully breast-feed.

Obesity can become a problem for older children and adults. Getting plenty of activity and avoiding high-calorie foods are important. Before beginning sports activit ies, the child's neck and hips should be examined.

Behavioral training can help people with Down syndrome and their families deal with the frustration, anger, and compulsive behavior that often occur. Parents and caregivers should learn to help a person with Down syndrome deal with frustration. At the same time, it is important to encourage independence.

TREATMENT

Experts recommend genetic counseling for persons with a family history of Down syndrome who wish to have a baby.

A woman's risk of having a child with Down syndrome increases as she gets older. The risk is significantly higher among women age 35 and older.

Couples who already have a baby with Down syndrome have an increased risk of having another baby with the condition.

Tests such as nuchal translucency ultrasound, amniocentesis, or chorionic villus sampling can be done on a fetus during the first few months of pregnancy to check for Down syndrome.

PREVENTION