Diagnosis of Tetrasomy 18p

There are several different reasons that your family’s physicians might suspect that there is an underlying chromosome change present. Some of the more common reasons include:

• •Child missing developmental milestones
• •Birth defects
• •Health problems
• •Minor differences in facial features
• •A family history of a chromosome condition

Although Tetrasomy 18p can be diagnosed prenatally by chorionic villus sampling (CVS) or amniocentesis, it is most frequently diagnosed during infancy or early childhood. There are two tests that are commonly performed to identify a deletion on Tetrasomy 18p. Both of these tests can be performed on a blood sample.

Routine Chromosome Analysis
In this test, white blood cells are grown in the lab. The chromosomes are stained and examined under a microscope by a qualified cytogeneticist. This individual is trained in recognizing extra and missing pieces of chromosomes, as well as other rearrangements, such as translocations and inversions. Results from a chromosome analysis will indicate the number of chromosomes, whether the person is a male or a female, and which part of the chromosome is changed. The chromosome result of a person with Tetrasomy 18p might look something like this:

47,XX,+i(18)(p10)
This result tells us that there are 47 chromosomes instead of the expected 46. This person is also a female, as indicated by the two copies of the X chromosome. The “i” stands for isochromosome, which is composed of material from chromosome 18. The “p10” tells us that the isochromosome is made of chromosome material that starts at the centromere (p10) and extends to the tip of p arm.

Microarray Analysis
A microarray analysis is similar to a routine chromosome analysis in that it determines if there are extra or missing pieces of a chromosome. It can detect chromosome deletions and duplications that are not visible under a microscope. A microarray result of a person with Tetrasomy 18p might look something like this:

arr 18pterp11.21(136,226-15,198,990)x4
This result tells us that there is a change in the region extending from the tip of chromosome 18p to base pair 15,198,990, which is essentially the centromere. The “x4” at the end tells us that there are actually four copies of that region, which is consistent with a diagnosis of Tetrasomy 18p.

Although microarrays are very useful in determining exactly what is missing and what is extra, they cannot determine some other types of chromosome changes. Microarrays cannot detect chromosome changes that do not involve extra or missing chromosome material. For example, they cannot detect certain chromosome rearrangements, such as balanced translocations or inversions. Therefore, both tests may be ordered to fully describe the underlying genetic change.