Chorionic villus sampling
The chorionic villus sampling , also under the synonyms CVS , chorion test , placenta puncture or placental puncture , is aware of the removal and examination of cells of the chorion . It is an invasive examination method in the context of prenatal diagnostics that can be used at an earlier stage of pregnancy than amniocentesis . Both methods examine prenatally whether the unborn child is affected by chromosomally- related peculiarities or metabolic diseases .
Principle of investigation
During the first trimester of pregnancy, extrafetal tissue wraps the outside of the amniotic cavity . Although the tissue does not belong to the fetus , it is usually genetically identical to it. In the area of the umbilical cord attachment it thickens to form the “chorion frondosum”, which is covered with tree-like branching structures (the chorionic villi ) and later develops into the placenta .
This area can be demarcated from about the 10th to the 12th week of pregnancy using ultrasound . Components of the placenta (= placenta), the chorionic villi, are removed and examined by means of a biopsy . The growing child and the placenta that supplies it are formed from a common cell of origin. Chromosomal deviations between the cells of the mother cake and the cells of the child (placenta mosaic ) are therefore rare, although possible in principle: A placenta mosaic is a different chromosome configuration of either cells within the placenta (i.e. there are two different cell lines next to each other) or a different chromosome configuration of placenta and growing baby. In about 1 to 2 out of 100 chorionic villus sampling, a placenta mosaic created by mutations after the zygote has formed is found.
The chorionic villus sampling developed in the USA is not a routine examination and is only used if the pregnant woman or the parents expressly request it. The attending physician is obliged to advise the pregnant woman in detail before the examination and to discuss the advantages and disadvantages in an understandable manner. This also includes the reference to the fact that there is no therapy for a causal cure for most of the identifiable peculiarities , and thus, in the event of a positive examination finding , ultimately only the acceptance of the child with his peculiarity, the postnatal release of the child for adoption or the postnatal delivery the child to a foster family / home or abortion as alternatives.
Optimal time for examination and weighing up Chorionic villus sampling or amniocentesis
Since the introduction of the risk assessment of age-dependent chromosomal disorders according to the standards of the "Fetalmedicine Foundation UK" fetalmedicine.com through the assessment of neck transparency and biochemical methods in the time window of pregnancy week 11 + 0 to 13 + 6, the number of chorionic villus sampling has decreased significantly. The examination was previously used from around the 9th to 11th week of pregnancy. Today, a chorionic villus sampling is usually not recommended before the 11th completed week of pregnancy. In prenatal diagnostic centers, for some years now, early fine ultrasound has been increasingly performed, taking into account the risk assessment for trisomy 21 , trisomy 13 and trisomy 18, preferably in the time window of the 13th week of pregnancy. Depending on the findings and risk assessment for the age-dependent chromosomal disorders, the question of a chorionic villus sampling often arises from a new perspective.
If there is an increase in risk due to thickened neck transparency or a severe malformation (major anomaly) e.g. B. Heart defects with normal neck transparency, a chorionic villus sampling, if possible only performed after prior genetic counseling, can very quickly help to clarify the karyotype . In the case of normal neck transparency, possibly in combination with normal biochemical markers, the so-called "age risk" for trisomy 21, trisomy 13 or trisomy 18 will be reassessed and will be lower. This leaves time to rethink an invasive procedure and for further advice. So it makes sense to take advantage of genetic and / or psychosocial counseling. With the knowledge about the nature, risks and objectives of the methods and the consequences of a conspicuous finding, a decision for or against an invasive diagnosis can then be considered more carefully.
However, if an invasive examination to assess the karyotype to rule out a chromosomal disorder is desired, starting around the 16th – 17th century. An amniocentesis should be performed at the 1st week of pregnancy. The advantage here is the lower mosaic rate (see below), which is approx. 2% in chorionic villus sampling and the assessment of the chromosomes from a cell culture of the amniotic fluid analysis is described under the microscope as being qualitatively better than the cell cultures of a chorionic villus sampling.
Procedure of the investigation
There are two ways to collect cells during a chorionic villus sampling:
- 1. by inserting a pin through the abdominal wall (= transabdominal):
- Under ultrasound guidance, a needle is inserted into the abdominal wall of the pregnant woman and advanced into the placenta. This is called a " puncture ". Tissue is removed and examined in the laboratory. The main difference to amniocentesis (removal of amniotic fluid) is that the needle does not penetrate the amniotic sac, but is guided past it to reach the placenta. If the placenta is very unfavorable, then a transabdominal chorionic villus sampling is not possible.
- 2. via the cervix (= transcervical):
- In early pregnancy, a thin tube ( catheter ) is pushed through the vagina and the cervix into the placenta to collect cells . However, since there are comparatively more risks with this method, cells are now more often carried out transabdominally, i.e. by means of a puncture (= insertion of a needle) through the abdominal wall of the pregnant woman.
A total of approx. 20 to 30 mg villi are removed and a karyotyping (creation of a chromosome image / karyogram ) is carried out in the laboratory . A DNA analysis for the detection of molecular genetic diseases in the unborn is only carried out in special cases and usually after prior genetic advice.
result of the investigation
The examination serves to prove some genetically determined characteristics of the child (including Down syndrome (trisomy 21), Patau syndrome (trisomy 13), Edwards syndrome (trisomy 18), and trisomy 8 ) as well as some metabolic diseases . Neural tube malformations (e.g. forms of spina bifida aperta and anencephaly ) and abdominal wall malformations cannot be diagnosed. Also occur trisomies (= triplicates of chromosomes) sometimes referred to as so-called " mosaic " on. This means that the respective chromosome is not present in triplicate in all cells of the child, but there are also cells with an inconspicuous set of chromosomes. It is therefore possible that a mosaic trisomy will not be detected in the chromosome examination.
As already mentioned, it is seldom, but in principle also possible, that there is a chromosomal discrepancy between the cells of the mother cake and the cells of the child (placenta mosaic), an amniocentesis can be recommended if the result is positive. In 1997 Hahneman and von Vejerslev reported three cases of trisomy 14 in chorionic villus sampling: two had no cell mosaic, one did. However, all three findings only concerned the placenta and could not be detected when examining the cells of the three children. Especially in the case of conflicting results, e.g. B. with sonographically completely normal fetus and abnormal short-term culture or with unclear evaluation of the chromosome picture under the microscope, the result of the long-term culture should always be awaited, which is available after about 10 to 20 days and has a diagnostic reliability of approx. 99.8%. However, this probability value is so high that it already implies a relative “security” which is sufficient as an indication for an abortion in the event of an abnormal finding. In some cases, e.g. B. If there is evidence of a mosaic or other somewhat unclear findings, an additional amniocentesis should be performed if necessary. In such diagnostic gray areas, all further examinations should only be carried out after genetic counseling, so that the parents-to-be can be informed in detail about the problems of the diagnosis and the possible consequences. In particular, it should be known that cytogenetic diagnoses are also not possible with 100 percent certainty.
Invasive examinations such as the chorionic villus sampling are usually carried out due to the risks associated with the procedure, especially when the risk of miscarriage as a result of the procedure is lower than the statistically expected probability of the presence of a chromosomal peculiarity or a hereditary disease . Expectant parents are advised to take advantage of counseling services before using prenatal diagnostics in general and chorionic villus sampling in particular. It is advisable to go to a genetic counseling center, especially if you have a family history .
Before the procedure, the doctor is legally obliged to explain all existing risks - also in relation to the expected benefit - in a generally understandable manner, to name current risks and, if requested, their sources (studies) and to give the patient sufficient reasons for concern Allow information time. The patient can only legally consent to the intervention if she is in a position to assess the benefit and / or disadvantage of the intervention.
After the procedure, the pregnant woman should consistently take it easy for a while. Even days to weeks later, the risk for z. B. Bleeding and miscarriages increased.
The following risks of varying degrees of risk should be weighed up by the pregnant woman or the parents before agreeing to a chorionic villus sampling:
The numbers for miscarriage after the procedure are given as 8.8: 100 and slightly below.
Jauniaux and Rodeck (1995) give the increase in the risk of a spontaneous abortion after a chorionic villus sampling compared to a late amniocentesis with 0.5% to 4%. (Compare the risks of late amniocentesis ). This range results from the considerable differences in the design of the comparative studies and especially the different number and experience of the diagnosticians involved. The weighted mean of the data given by Jauviaux and Rodeck (1995) results in a risk of 8.8% for a spontaneous abortion after a chorionic villus sampling compared to 5.7% after an amniocentesis. It must be noted, however, that chorionic villus sampling is usually performed between the 9th and 12th week of pregnancy, whereas amniocentesis is usually not performed until after the 15th week. Some studies indicate that the risk of spontaneous abortion after a chorionic villus sampling is lower than after an early amniocentesis (Jauvinaux & Rodeck, 1995).
Antibody formation in case of rhesus incompatibility
Since the maternal tissue is injured during the puncture of the maternal abdominal wall and the child’s tissue is then removed, the child’s blood enters the mat’s body. Even when the needle is withdrawn, the maternal tissue is contaminated by blood components from the child . If there is a blood group incompatibility ( Rhesus incompatibility ) to the Rhesus factor - antigen "D", i. H. between Rh-negative (Rh−, rh, genotype dd) mother and Rh-positive (Rh +, Rh, genotype Dd, dD, DD) child, antibody formation by the mother (called "anti-D") against the child's blood would be likely if it is through the above Process of introducing child's blood or its components ( erythrocytes or their components) into the maternal bloodstream . This process can also be initiated by other invasive interventions on the child or during pregnancy in the uterus (e.g. induced abortion or termination of pregnancy ; umbilical cord puncture ; amniocentesis ). The risk increases depending on the extent of the invasiveness or the risk of injury and bleeding.
"Anti-D" is an irregular erythrocyte antibody that Rhesus negative people can produce if they are immunized by Rhesus positive erythrocytes . The drug ("anti-D prophylaxis", such as RHOPHYLAC from CSL Behring and RHESONATIV from Octapharma), which is usually administered prophylactically against this process and its consequences in children (see Rhesus incompatibility # pathogenesis ) is also called this.
Deformation of the extremities
Deformations of the child's extremities ( limb reduction defects ) are mentioned as long-term consequences (Jauniaux & Rodeck, 1995). In the general population, this malformation occurs with a frequency of 0.03–0.06%. Studies aimed at quantifying the risk of leg deformity after a chorionic villus sampling have shown incidences of 0.06–2%. The weighted mean is 0.178% and thus represents - at a low level - a 3 to 5-fold increase in risk.
- Vascular injuries and bleeding after the procedure occasionally occur.
- Infections are comparatively rare.
Development of the investigation method
Initial attempts at prenatal diagnosis in the first trimester of pregnancy were made when amniocentesis was still in development. The main motivation was to avoid the high medical risk and massive psychological stress caused by a late termination of pregnancy, for example in the 21st week, in which the pregnant woman often feels child movements. For these reasons, diagnosticians have high hopes for the method of analyzing cells from the chorion , which was first described by Ward et al. (1983) and Rodeck et al. (1983) , which enables karyotyping from the 8th week of pregnancy. The actual breakthrough of the technology on the way to the safest possible method for tissue extraction came with the use of ultrasound diagnostics in the early 1980s.
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- Network of prenatal diagnostics
- Prenatal Diagnostics Working Group
- Familienplanung.de - Chorionic villus sampling : The information portal of the Federal Center for Health Education (BZgA)
- Interdisciplinary Forum Prenatal Diagnostics Berlin e. V.
- Information on prenatal diagnostics