Vol. 19: Spring 2001
Prenatal Diagnosis
Invasive Prenatal Diagnostic Techniques - Chorionic villus sampling (CVS)
Chorionic villus sampling (CVS) provides the ability to obtain fetal tissue from the developing trophoblast for diagnostic studies in the first trimester. The chorion frondosum, which contains the most mitotically active cells, is the area that is sampled. The indications for CVS are similar to those for amniocentesis as the specimens can be evaluated for fetal chromosome constitution as well as other molecular or biochemical studies. However, neural tube defects and other structural malformations cannot be detected with CVS. CVS is usually performed at 10-13 weeks gestation and can be accomplished using either a transcervical or transabdominal approach. Prior to the procedure, an ultrasound is performed to assess fetal viability, gestational age and placental position. The fetal nuchal translucency may be measured as well.
Transcervical CVS was first described in the late 1960's, prior to the introduction of ultrasound guidance in 1979. The procedure involves passing a polyethylene catheter with a malleable obturator through the cervix to the thickest part of the placenta using ultrasound guidance. Placental trophoblast is then aspirated through the catheter into a 20 cc syringe that contains tissue culture medium.
Transabdominal CVS was first described in 1984. A needle is placed through the long axis of the placenta under ultrasound guidance. The stylet is withdrawn from the needle, a syringe containing tissue culture medium is attached to the hub of the needle and suction is applied as the needle is moved up and down through the placenta until an adequate amount of tissue is obtained. Following the CVS procedure the sample should be inspected to ensure that an adequate sample of chorionic villi have been obtained.
In most cases, physician or patient preference will dictate which method is used, and either procedure is usually successful. The techniques have been shown to be equally safe and efficacious provided that the operator is experienced with both approaches.
In approximately 3-5 percent of cases, clinical circumstances will support one approach over the other. Absolute contraindications to the transcervical approach include active cervical or vaginal infections such as herpes, gonorrhea or chlamydia. Known maternal blood group sensitization is a contraindication for either approach.
Complications/risks of chorionic villus sampling (CVS)
The National Institute of Child Health and Human Development (NICHD) sponsored a seven center non-randomized study that evaluated the safety and efficacy of transcervical CVS. Although it did not reach statistical significance, the loss rate associated with CVS was 0.8% greater than amniocentesis. There was an increased loss rate associated with procedures in which more than one attempt was made, particularly those requiring three or four passes.
There have been a number of concerning reports suggesting that CVS may cause limb reduction defects. Since the initial report, there have been many other publications both supporting and refuting this association. It appears as though there may be an association of transverse limb defects with CVS procedures performed very early in gestation. There also may be an association of limb reduction defects with less experienced operators.
Based on the data available, it appears that there is a slightly higher risk of pregnancy loss associated with CVS compared to second trimester amniocentesis. The data would suggest that CVS performed after 10 weeks by an experienced operator is not associated with an increased incidence of limb defects compared to the general population. It is important that women are educated regarding the potential benefits and risks including a slightly higher loss rate compared to second trimester amniocentesis and the transverse limb defect controversy. CVS procedures should not be performed prior to 10 weeks gestation.
The Genetic Drift Newsletter is not copyrighted. Readers are free to duplicate all or parts of its contents. The Genetic Drift Newsletter is published semiannually by the Mountain States Regional Genetic Services Network for associates & those interested in Human Genetics. In accordance with accepted publication standards, we request acknowledgement in print of any article reproduced in another publication. The views expressed in the newsletter do not necessarily reflect local, state, or federal policy. For additional information, contact Carol Clericuzio, M.D., Editor, Department of Pediatrics, The University of New Mexico, Albuquerque, NM, 87131
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