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Frequently Asked Questions - Chromosome Disorders
A Certified Genetic Counselor developed the following FAQ guide* to explain chromosome disorders, Trisomy 13 and provide other important information for parents.
Click here for frequent questions and concerns regarding premature infants.
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What
are trisomy 13 and trisomy 18?
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Trisomy 13 and Trisomy 18 are conditions caused by extra genetic material that is present in each
cell of the body. The onset occurs before birth, and babies born with these conditions tend to be
small and have a specific group of abnormalities. Trisomy 18 and Trisomy 13 occur in both boys
and girls, and affect all populations of the world equally.
Trisomy 13 and Trisomy 18 are known as " syndromes." A syndrome describes a particular set of abnormalities that is seen repeatedly in a group of individuals, all of whom have the same
condition. An example of a more commonly recognized syndrome is Trisomy 21, also known as Down syndrome. A particular individual, who is diagnosed with a syndrome, may have many, but will probably not have all of the features or birth defects known to occur in that condition. However, the pattern of the findings seen in that individual, will be consistent with the findings of that syndrome. Consultation with a qualified physician is necessary to diagnose these conditions.
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What are the common findings in trisomy 13?
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Infants born with Trisomy 13 can have structural abnormalities of the brain and face, which often include facial clefting. Problems with the structural formation of the heart, extra fingers and toes (polydactyly), and effects on many other organ systems can be seen. Mental retardation is usually severe, and the average lifespan of these children is severely compromised.
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What are the common findings in trisomy 18?
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Infants born with Trisomy 18 are often small for their gestational age, and have altered gestational timing with approximately one third being premature, and one third of these babies being post mature. These infants often had feeble fetal activity and a weak cry at birth. The back part of the head may be prominent, and many of these babies have a clenched fist with a particular pattern of overlapping fingers Heart defects are also common, and many other organ systems can be affected. Mental retardation is severe, and the average lifespan of these children is severely compromised.
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What causes trisomy 13 and 18?
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Our genetic material is packaged in structures called chromosomes. A complete set of chromosomes is needed in each cell of the body. Forty six (46) chromosomes forms a complete set. Chromosomes are arranged in pairs, so we normally have 23 pairs of chromosomes in each cell. We can identify the different chromosomes by their size and by the special banding patterns that can be seen under a microscope. Chromosome number 1 is the largest, and chromosomes # 21 and #22 are the smallest. The last pair of chromosomes is identified differently, with two X chromosomes determining the female sex, and an X and Y chromosome determining the male sex.
A trisomy occurs, when, instead of having two of each chromosome, there is a third copy of one of the chromosomes.
- Trisomy 13 means the extra chromosome is a number 13 chromosome
- Trisomy 18 means the extra chromosome is a number 18 chromosome
Another example of a trisomy is Down syndrome, which is caused by an extra chromosome # 21. The presence of the extra chromosome is caused by an abnormality in cell division. It is present from the time of conception. Cell division is a molecular process over which we have no control. Therefore, there is nothing that you can do to cause or prevent abnormal cell division from happening.
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What does the extra genetic material do?
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Having extra genetic material affects the normal function and structure of many organ systems in our bodies. The systems affected can include the central nervous system and the brain, circulation and the heart, and the structural development of the growing fetus. Because the genes on chromosomes 13 and 18 appear to be vital for normal development, having this extra material severely affects all of these systems causing the severe degree of mental retardation and alters the normal lifespan of these individuals.
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Who is at risk to have a baby with trisomy?
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Faulty chromosomal distribution can occur in any pregnancy, but is more likely to occur at older maternal ages. However, the average age of mothers who have a baby with Trisomy 18 is 32 years and the average age for mothers of babies with Trisomy 13 is 30.9 years. Screening tests and diagnostic tests are available during pregnancy to help identify those couples at increased risk, however, not every affected baby is diagnosed prenatal.
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Although the cause of a trisomy is due to having extra genetic material, the majority of cases of Trisomy 13, 18 and 21 are NOT inherited. The chance for this to happen again in a subsequent pregnancy is low, usually no greater than one percent ( one chance in one hundred). A discussion with a clinical geneticist and/or genetic counselor can help to clarify the risks
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What tests are available to identify
trisomy 13 or trisomy 18?
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A physical examination by a qualified physician (often with training in genetics) is the first step needed in determining an exact diagnosis.
An analysis of an individuals genetic material can be performed in conjunction with the physicians examination. The genetic material ,or DNA, is usually obtained from a blood sample. The DNA is visualized under a microscope, and a picture is taken. This " picture" is called a karyotype. A normal female has a karyotype of 46,XX and a normal male has a karyotype of 46,XY. The following are karyotypes which identify Trisomy 13 and Trisomy 18:
- Girl with Trisomy 13 47,XX+13
- Boy with Trisomy 13 47,XY+13
- Girl with Trisomy 18 47,XX+18
- Boy with Trisomy 18 47,XY+18
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What tests are available before a baby is born (prenatal testing)?
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Prenatal testing can be divided into two different areas. Diagnostic tests provide a definitive answer and can confirm a suspected diagnosis. Screening tests are tools by which we see if a pregnancy is at increased risk. Screening tests are not diagnostic, but can help to determine if additional testing might be made available to a family.
PRENATAL SCREENING TESTS INCLUDE:
- Ultrasound examination ultrasonographic evaluation which looks at the structural growth and development of the fetus
- Maternal serum screening tests which include multiple marker screening . These tests are performed by obtaining a blood sample from the mother, at a specific gestational age.
Measurements of specific chemicals in the blood, along with the age of the mother as well as other factors can provide a statistical risk calculation for Down syndrome, Trisomy 18 and neural tube defects. Currently, multiple marker screening does not screen for Trisomy 13.
DIAGNOSTIC TESTS INCLUDE:
- Amniocentesis- is usually performed at 16-18 weeks gestation. Amniocentesis involves removing about one ounce (25 cc) of amniotic fluid from the amniotic sac surrounding the fetus. A thin needle is inserted into the uterine cavity, under ultrasound guidance by a trained physician. The fluid contains fetal cells which can be analyzed to obtain a karyotype. The fluid is usually also tested for neural tube defects, as well as some other conditions if indicated by the family or medical history. Amniocentesis has approximately a 1/250 risk for complications that could lead to a miscarriage. This risk may vary at different centers. Amniocentesis should be performed by appropriately trained physicians.
- CVS ( Chronic Villus Sampling) -is usually performed at about 10-12 weeks gestation. In this test, a small sample of chronic villi (which is essentially placental in origin, and is made up of the same cells that determine the genetic make-up of the embryo) is obtained under ultrasound guidance, by aspiration of the tissue through the cervix or abdominal wall. These cells can also be analyzed to obtain a karyotype. CVS carries a risk of up to one percent for miscarriage, and up to a 1/1000 ( 0.1% ) risk for limb reduction defects.
Diagnostic testing is useful in obtaining information regarding a pregnancy with an uncertain prognosis.
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What do I need to know about mosaic, partial or translocation trisomy's?
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Most babies born with Trisomy 13 or 18 have a "full" trisomy, which means they have a whole, separate copy of the extra chromosome. The recurrence risk for full trisomy's is low, and the parents chromosomes are expected to be normal. .
In a partial trisomy ,only a piece of the chromosome is involved, and the physical effects may be different. Sometimes a partial trisomy can be due to a rearrangement (or translocation) in one of the parents chromosomes, which can increase the recurrence risk. Discussions with a clinical geneticist and genetic counselor can help to identify if this is a concern for a particular family.
In mosaic trisomy's, only some, not all, of the cells in the body are abnormal, The effect of fewer abnormal cell lines may mean fewer effects on the baby, but this may not be the case. Recurrence of a mosaic trisomy is quite rare and should be discussed with your genetics professional
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*FAQ reference: Smith's: Recognizable Patterns of Human Malformation
publ.- Saunders, 5th ed. 1997.
Click here for resource center main page
*Disclaimer
All material provided in this website is provided solely
for educational and informational purposes, and is not meant to
supplement or replace medical advice of professionals familiar with your
specific condition. The statements of the professionals on this website
are only their opinions based upon the information you have provided to
them. You should consult with your own physician or other medical
professional regarding the applicability of any of the opinions or
recommendations expressed herein with respect to your own symptoms or
medical conditions.
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