Index
It is the presence of a single extra chromosome, producing three chromosomes of that particular type rather than a pair.
Trisomy 16 is a chromosomal deformity in which there are three copies of chromosome 16 instead of the usual two.
Severely affected children with partial or complete trisomy affecting only the short arm (p arm) or long arm (q arm) have been described.
Trisomies are the most common chromosome-related cause found in post- miscarriage tests.
Of all trisomies, trisomy 16 appears to be the most common in humans, occurring in 1% of all pregnancies.
There are three main trisomies 16, depending on whether a complete or partial extra chromosome is present: complete trisomy 16, partial trisomy 16, and mosaic trisomy 16.
This chromosomal disorder occurs when an extra copy of chromosome 16 is found after fertilization.
Trisomy 16 is one of the most common chromosomal abnormalities responsible for more than 100,000 pregnancy losses per year.
Experiencing a pregnancy with trisomy 16 does not affect the likelihood of trisomy 16 in subsequent pregnancies.
Types of trisomy
There are three types of trisomy 16: complete, mosaic, and partial.
Complete trisomy 16
Complete trisomy 16 means that all cells in the embryo contain the extra chromosome and is the most severe form of trisomy 16.
Complete trisomy 16 is not compatible with life, and all affected babies are often spontaneously aborted in the first trimester or die in early childhood.
Mosaic trisomy 16
Mosaic trisomy 16 means that some of the embryo’s cells are affected, while others have the average number of chromosomes.
Mosaic trisomy 16 generally results from trisomy present at “corrective” conception during early cell division in fetal development, leaving some cells affected but not others.
Mosaic trisomy 16 is less severe when only some of the cells in the body have an extra copy of chromosome 16.
Mosaic trisomy 16 can cause complications during pregnancy, including intrauterine growth restriction and pre-eclampsia. Still, it is possible to carry a pregnancy to term in the case of mosaic trisomy 16.
A baby with mosaic trisomy 16 may have some congenital disabilities, depending on the number of cells affected by the trisomy.
If few cells are affected, the baby can develop normally. As more cells are affected, the probability of defects increases.
Defects can include heart defects, facial deformities, developmental delay, respiratory abnormalities, and premature delivery.
Partial trisomy 16
Partial trisomy 16 refers to the presence of an extra copy or a missing part of a segment of a chromosome. The cells of the placenta are affected by trisomy, but the developing fetus is not.
Because the fetus contains only normal cells, it usually develops without any indication of trisomy 16. This is the rarest form of trisomy 16.
Consequences and symptoms
Patients may experience complete or mosaic trisomy 16.
If the patient experiences a total trisomy 16, they are likely to die at birth or shortly after that.
If the patient experiences mosaic trisomy 16, the symptoms will vary greatly depending on the number of affected cells compared to the number of unaffected cells.
Also, those with partial trisomy 16 will have various symptoms depending on the part of the chromosome affected.
In other words, sometimes babies are born without evidence of abnormalities, but others may have a distinctive set of health problems, depending on the degree of mosaicism.
16p minus (16p-)
In this disorder, part of the short (p) arm of chromosome 16 is missing. A disorder associated with 16p- is Rubinstein-Taybi syndrome.
16p more (16p +)
Duplication of some or all of the short arms (p) of chromosome 16 can cause:
- Poor growth of the fetus during pregnancy and the baby after birth.
- Small round skull.
- Few lashes and eyebrows.
- Round and flat face.
- Prominent upper jaw with a minor lower jaw.
- Rounded, low-set ears with deformities.
- Thumb abnormalities.
- Severe mental disorder.
16 q minus (16q-)
In this disorder, part of the long arm (q) of chromosome 16 is missing.
Some people with 16q- may have severe growth and development disorders and abnormalities of the face, head, internal organs, and musculoskeletal system.
16q more (16q +)
Duplication of part or all of the long arm (q) of chromosome 16 can cause the following symptoms:
- Poor growth
- Mental handicap
- Asymmetric head.
- High forehead with prominent short or pointed nose and thin upper lip.
- Common anomalies.
- Genitourinary abnormalities.
Causes
Researchers have not determined what causes these cell division errors, but they appear to occur randomly.
Complete trisomy 16 is usually an error in cell division that affects the sperm or egg, which means that the abnormality is already present at conception.
After the sperm fertilizes the egg, the embryo should have 46 chromosomes in each cell. When a trisomy occurs, there is an extra copy of one or more chromosomes.
In trisomy 16, the extra copy is on chromosome 16. This extra chromosome can have a range of effects depending on how many cells in the embryo are affected.
Diagnosis
The breakdown of fetal cells in the maternal circulation makes it possible for laboratories to detect fetal DNA during pregnancy.
When fetal cells break down, cell-free DNA is released.
It is possible to duplicate many copies of this DNA so that tests are possible. This non-invasive prenatal test can be performed as early as the 10th week of pregnancy.
Trisomy 16 can be found during chorionic villus sampling or amniocentesis, although if there is a total trisomy 16, the pregnancy can be aborted before these tests are done.
Genetic testing can be done on fetal tissue after an abortion to determine if chromosomal abnormalities contributed to the miscarriage.
Treatment
Doctors and parents must work as a team for a multidisciplinary and appropriate treatment for each child.
