Down Syndrome Definition

Down Syndrome is an autosomal trisomy disorder that results from a replication error during cell division in which a gamete (sex cell) ends up with two copies of chromosome 21 instead of the normal single copy (as haploid cells, gametes contain one half the complement of chromosomes).

At conception the zygote thus ends up with three instead of the normal two copies of chromosome 21, which ultimately produces multiple congenital anomalies. When all cells carry the extra chromosome, the resulting anomalies occasionally may be so severe that the disorder is lethal before birth. Sometimes Down syndrome occurs as a mosaic disorder in which some but not all cells contain the extra chromosome 21, which typically produces milder symptoms.

Down syndrome occurs in about 1 in 1,200 live births in the United States and about 350,000 Americans currently live with Down syndrome, many independently. Though the risk for Down syndrome increases dramatically with maternal age, most infants who have Down syndrome are born to younger mothers because the increased rate of conception more than offsets the increase in age-related risk. Down syndrome is the most commonly occurring of the autosomal trisomy disorders.

Symptoms of Down Syndrome and Diagnostic Path

Children born with Down syndrome often have characteristic facial features, which include:

  • flat, upwardly slanting eyes with extra fatty tissue in the lids
  • rounded face with a small nose and mouth
  • small ears
  • broad, short neck
  • short stature with noticeably small hands and short fingers

Other findings of Down syndrome include congenital anomalies affecting the heart, intestines, and other organs. About half of infants born with Down syndrome have atrial or ventricular septal defects (openings or holes in the septum, or wall, between the chambers of the heart). About half also have impaired vision (notably, congenital cataracts and amblyopia) and partial to complete HEARING LOSS. 10 percent have malformations of the intestines that require surgical correction. All individuals with Down syndrome have some intellectual impairment. Mild to moderate intellectual impairment is most common. Some children who have Down syndrome do well in regular classes in school and grow up to be capable of independent living.

Doctors often can diagnose Down syndrome and other autosomal trisomy disorders before birth using prenatal screening methods such as blood tests, ultrasound, amniocentesis, and chorionic villi sampling (cvs). These methods retrieve cells from the fetus from which a geneticist can construct a karyotype, which presents photomicrographic images of the fetus’s or infant’s chromosomes. Advanced maternal age (mother’s age over 40) and the previous conception of a child with Down syndrome or another autosomal trisomy disorder are the leading risks for Down syndrome. Whether done prenatally or after birth the karyotype provides definitive diagnosis.

Down Syndrome Treatment Options and Outlook

There are no specific treatments for Down syndrome. Because children who have Down syndrome are more susceptible to infection, they often need more medical care while growing up. There is a high correlation between early-onset Alzheimer’s disease and adults who have Down syndrome. Typically men who have Down syndrome are sterile, though women who have Down syndrome may be fertile and can become pregnant. Their risk for conceiving a child with Down syndrome is very high, however, and doctors strongly recommend genetic counseling.

About 350,000 Americans live with Down syndrome. Because Down syndrome is the mildest of the survivable trisomy disorders and because it can occur in a mosaic rather than a complete presentation, some people who have it are able to lead relatively independent and productive lives, living into their mid-50s or beyond. Early treatment for the anomalies common with Down syndrome, such as septal defects, and ongoing care for other health conditions, such as hypothyroidism and vision impairment, have greatly improved both health and quality of life for people who have Down syndrome.

Risk Factors and Preventive Measures

Down syndrome occurs as a replication error that does not appear to be preventable, though recent research suggests that folic acid supplementation beginning before conception and extending through pregnancy, such as obstetricians recommend for preventing neural tube defects, may reduce the risk for Down syndrome. Older women, who have a higher risk of conceiving a child with Down syndrome, may opt for genetic testing early in pregnancy to determine whether the fetus has the trisomy 21 genotype. Knowing allows the woman to make appropriate plans and decisions regarding the pregnancy and potential care needs of the child.

Congenital Anomalies Characteristic of Down Syndrome

amblyopia atrial septal defect
bowel atresia broad, short neck
congenital cataracts developmental delays
flat, upwardly slanting eyes HEARING LOSS
intellectual impairment rounded face
short stature small ears
ventricular septal defect
small hands and short fingers small NOSE and MOUTH


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