Types of Down Syndrome: Causes and Symptoms
Trisomy occurs when three copies of a chromosome are present instead of two (all chromosomes normally come in pairs). While most expectant parents are aware of Down syndrome and will undergo prenatal screening to detect it, other potentially more serious trisomies can occur, including Edwards syndrome, Patau syndrome and others.
Some may cause few or no symptoms. Others can lead to serious malformations that make life, or even pregnancy, unbearable.
What are trisomies?
Genes that contain all of the DNA-encoded information related to physiological makeup and metabolic function are found on chromosomes. Each human cell nucleus typically contains 46 chromosomes, 23 of which we inherit from each genetic parent.
Of these, 22 pairs are autosomes, which determine our unique biological and physiological characteristics. The 23rd pair is that of the sex chromosomes (X or Y), which largely determine biological sex.
In rare cases, a coding error can occur when a cell divides during fetal development. Instead of dividing cleanly into two identical chromosomes, the newly divided chromosome will have extra genetic material.
This can lead to complete trisomy (in which a complete third chromosome is created) or partial trisomy (in which only part of the chromosome is copied). From then on, the error will repeat and repeat as the cell continues to divide.
Down syndrome, the most common genetic disease in humans, is called trisomy 21 because there is an extra copy of chromosome 21 in the nucleus of every cell. Other genetic diseases have the same name.
Causes and consequences
Trisomies affecting the sex chromosomes – in which genetic females usually have two X chromosomes (XX) and genetic males have one X and Y chromosome (XY) – tend to be less severe. Autosomal trisomies often result in severe physical and intellectual disabilities, especially autosomal complete trisomies, where early death is common.
In addition to birth defects, trisomies can affect the viability of a pregnancy. In fact, more than half of all miscarriages are believed to be directly associated with a chromosomal abnormality. Of these, many are due to trisomies.
Nobody knows for sure why chromosome 21 is so vulnerable to trisomy. Of all the trisomies identified by researchers, Down syndrome is known to affect nearly one in every 800 births worldwide. These other trisomies are much less common but worth knowing.
Types of trisomy
There are several types of trisomy. Some can give birth to a live baby, while others can cause miscarriage in the early months of pregnancy.
Edwards syndrome (trisomy 18)
Edwards syndrome (trisomy 18) is usually caused by an extra chromosome 18. About 5% of cases are due to an error known as a translocation in which the building blocks of one chromosome are inserted into another.
Edwards syndrome is rare, affecting only one in 5,000 births.
Edwards syndrome is characterized by low birth weight, an abnormally small head, and abnormalities of the heart, kidneys, lungs, and other organs. While a few children with Edwards syndrome survive into adolescence, the majority die within the first year (and often days) of life.
Patau syndrome (trisomy 13)
Most cases of Patau syndrome (trisomy 13) are related to complete trisomy; a very small proportion is caused by translocation or a similar condition known as mosaicism in which the chromosomal building blocks are rearranged.
Patau syndrome is the third most common autosomal disease in newborns after Down syndrome and Edwards syndrome.
Children with Patau syndrome will often have cleft lips and palates, extra fingers or toes, heart defects, severe brain abnormalities, and malformed or rotated internal organs. The severity of the symptoms is such that a baby with Patau syndrome rarely lives beyond the first month.
Warkany syndrome (trisomy 8)
Warkany syndrome (trisomy 8) is a common cause of miscarriage and usually results in the death of the newborn within the first few months. Babies born with Warkany syndrome typically have a cleft palate, distinctive facial features, heart defects, joint defects, abnormal or missing kneecaps, and an abnormally curved spine (scoliosis).
Babies born with trisomy mosaic 8 (T8MS) can survive, but the condition is rare. Worldwide, T8MS occurs in approximately one in every 25,000 to 50,000 live births.
Complete trisomy 16 is incompatible with life. While most fetuses with this anomaly are spontaneously aborted by the 12th week of gestation, a few have survived until the second trimester.
Trisomy 16 is the most common autosomal trisomy seen in miscarriages, accounting for at least 15% of first trimester pregnancy losses.
In contrast, the chances of survival for children with mosaic trisomy 16 were once thought to be low, with most deaths occurring in early childhood.
Advances in genetic research have since shown that some previously unidentified children with mosaic trisomy 16 have no abnormalities of any kind and that the risk of miscarriage and birth defects is directly related to the number of cells carrying the mosaic. chromosomal mutation.
That being said, more than half of babies with mosaic trisomy 16 will have fetal abnormalities. Symptoms of trisomy 16 can include musculoskeletal abnormalities, distinctive facial features, undersized lungs, and atrial septal defect (a hole between the upper chambers of the heart). The life expectancy of children with mosaic trisomy 16 depends on the extent of these types of problems.
Males will often have hypospadias in which the opening of the urethra grows over the shaft of the penis rather than at the end. Developmental delays can occur but are less common than other trisomies.
Most fetuses with complete trisomy 22 miscarry before the first trimester. The severity of physical and organic malformations is such that full-term babies are unable to survive for more than a few hours or days.
Trisomy 22 is the second most common chromosomal cause of miscarriage.
Some babies with mosaic trisomy 22 survive. The severity of birth defects is determined by the number of cells with the mutated chromosomal copy. Characteristic findings include heart abnormalities, kidney problems, intellectual disability, muscle weakness, and cognitive and developmental delays.
Trisomy 9 is a rare disease in which complete trisomy is usually fatal within the first 21 days of life. Newborns with trisomy 9 will have a smaller head, distinctive facial features (including a bulbous nose and slanted forehead), a deformed heart, kidney problems, and often severe muscle and skeletal deformities.
Babies born with partial or mosaic trisomy 9 have a much higher chance of survival. This is especially true with mosaic trisomy 9 in which organ defects tend to be less severe and intellectual disabilities do not necessarily impede basic language, communication, or social-emotional development.
Since the disorder was first identified in 1973, few cases of trisomy mosaic 9 have been positively identified in the medical literature.
Klinefelter syndrome (XXY syndrome)
Klinefelter syndrome, also known as XXY syndrome, is a condition affecting males caused by an extra X chromosome. People with Klinefelter syndrome typically produce low testosterone, which leads to reduced muscle mass, facial hair, and body hair.
Characteristic symptoms include small testicles, developmental delay, breast enlargement (gynecomastia) and reduced fertility. The severity of symptoms can vary greatly.
Klinefelter syndrome is common, affecting 0.1% to 0.2% of newborn boys.
Some people with Klinefelter syndrome may also have learning disabilities, which are usually focused on language, although intelligence is usually normal. Testosterone replacement therapy is often used to treat the disorder alongside assisted fertility treatments for those wishing to father children.
Triple X Syndrome (Trisomy X)
Some women are born with triple X syndrome, involving an extra X chromosome. Triple X syndrome, also known as XXX syndrome, is not associated with physical characteristics and often causes no medical symptoms.
XXX syndrome is reported in approximately 1 in 1,000 newborn female babies. Since XXX syndrome may not cause obvious symptoms, it is thought to be underdiagnosed.
A small proportion of those affected may have menstrual irregularities as well as learning disabilities, speech delay and compromised language skills. When Down syndrome causes symptoms, treatment usually involves physical and speech therapy and classroom support. However, most people with trisomy X will grow normally and unhindered.
Most males born with an extra Y chromosome do not have any distinguishing physical characteristics or medical issues. On the contrary, people with XYY syndrome can sometimes be taller than average and may be at increased risk for learning disabilities, as well as speech and language delays.
XYY syndrome is reported to affect 1 in 1,000 newborn males. Because there are few symptoms, however, a diagnosis is thought to only be made in about 15% of cases.
Impairment, if any, tends to be mild. Most adults with XYY syndrome have normal sexual development and are able to conceive children.
Frequently Asked Questions
What are the 3 most common trisomic anomalies?
The three most common trisomies are Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13). Children with Down syndrome often have a good prognosis, with a life expectancy of up to 60 years. However, most children with Edwards syndrome and Patau syndrome die in infancy.
What is the life expectancy of a person with trisomy X?
Trisomy X does not affect life expectancy. Most people born with Down syndrome lead normal lives, although there is an increased risk of learning disabilities.
How common are trisomy 16 miscarriages?
Some sources estimate that trisomy 16 causes up to 15% of all miscarriages that occur in the first trimester.