Down Syndrome

Down syndrome is a genetic disorder that occurs when cells divide abnormally and produce an extra copy of chromosome 21. This alteration affects the physical and behavioral characteristics of those who have it, and the severity can vary from person to person.

Currently, trisomy 21—so named because of the presence of three copies of chromosome 21 instead of the usual two—is the leading cause of intellectual disability in children.

Depending on how the third chromosome is formed, there are three types of Down syndrome:

• Free or simple trisomy: This is the most common form. Three complete copies of chromosome 21 are formed because one of the germ cells carries two joined copies of chromosome 21.
• Translocation: There are two complete copies of chromosome 21 and an additional partial copy attached to a different chromosome, usually chromosome 14. Although each cell still contains 46 chromosomes, one of them is altered. In such cases, one of the parents is a carrier, even if they show no symptoms.
• Mosaicism: This is the least common form of Down syndrome. It occurs when embryonic cells begin dividing and, at some point, the pair of chromosome 21 fails to separate. From that moment on, all cells that descend from the affected one will carry the same alteration. The severity of the symptoms depends on when the anomaly occurred: the later it happens, the fewer cells are affected.

In addition to slower-than-usual intellectual development, Down syndrome causes specific physical traits and abnormalities in the function of certain organs.

Symptoms

Each person presents different characteristics, as some have severe health problems associated with trisomy, while others show few symptoms. Despite this variability, the most common traits of Down syndrome generally include:

• Delayed speech and language development
• Smaller-than-average head and ears
• Small mouth and large tongue
• Flattened facial features
• Small hands and short fingers
• Palpebral fissures (upward-slanting eyes)
• Short stature
• Low muscle tone
• Heart or gastrointestinal defects

Causes

Since chromosomes are made up of many genes, the exact causes of trisomy and the specific characteristics seen in people with Down syndrome remain unknown. However, research agrees that the excess genetic material affects the formation of organs, as protein production and genetic regulation are altered.

Risk Factors

The main risk factors for Down syndrome include:

• Advanced maternal age (over 35 years): The primary reason is that older eggs have a higher likelihood of chromosomal division errors.
• Heredity: If one of the parents has a genetic translocation—in other words, a portion of chromosome 21 attached to another chromosome—they may pass the condition on to their children. Nevertheless, Down syndrome is rarely hereditary.
• Having another child with the syndrome: In these cases, it is advisable for the parents to undergo genetic testing to determine whether they are carriers.

Complications

People with Down syndrome are more likely to develop certain conditions, such as leukemia, sleep apnea, obesity, dementia, and immune system disorders.

When someone is born with trisomy 21, their life expectancy is generally shorter than average, as the syndrome itself causes premature aging. Depending on the condition’s specific characteristics and any related diseases, individuals typically do not live beyond 60 years of age.

Prevention

There is no way to prevent Down syndrome. However, individuals who may be at risk of passing it on can seek genetic counseling to understand the risks before planning a pregnancy.

What kind of doctor treats Down syndrome?

Geneticists are responsible for studying the characteristics of Down syndrome. In addition, pediatricians, gynecologists, general practitioners, and preventive medicine specialists are involved in its diagnosis. Patients with this condition are treated by specialists in physical medicine and rehabilitation, speech therapists, and psychologists.