Amniocentesis

CHAPTER 161 Amniocentesis



Dale A. Patterson, John J. Andazola


Amniocentesis is very helpful for evaluating an inaccessible in utero patient. Although genetic studies were first performed on amniotic fluid in the 1950s, before the 1970s the primary indication for amniocentesis was the Rh-immunized patient. Since that time, the indications have expanded to include evaluation of fetal lung maturity, fetal genetics, rupture or infection of the amniotic membranes or fluid, and other factors related to fetal health. In some centers caring for high-risk patients, amniocentesis is a routine procedure in approximately 15% of pregnancies. However, the overall frequency of amniocentesis has decreased every year since 1989—down to 1.7% of pregnancies in 2003. It is important to remember that complications from amniocentesis can be serious and occasionally lethal.



Indications



Prenatal Diagnosis (First and Second Trimester)



• Chromosomal studies. This is the most common reason for amniocentesis in the United States. Amniotic fluid contains fetal and amniotic cells. The fetal cells include desquamated squamous cells and cells from the gastrointestinal tract, respiratory tract, and urinary system. Although it requires 2 to 3 weeks for results, culture of these cells allows accurate fetal chromosome analysis for chromosomal, sex-linked, and metabolic disorders. For chromosomal analysis, amniocentesis is usually performed at 15 to 17 weeks, when there are sufficient numbers of desquamated fetal cells to allow successful culture. Although early amniocentesis (i.e., 10 to 14 weeks gestation) can be performed, chorionic villus sampling can also be performed around this time for similar indications. The risks and benefits of these procedures are similar but vary slightly by location and indication. Local availability and standards may influence the choice of procedure. Indications for early amniocentesis include the following:
• Advanced maternal age (35 years of age or older)

• Parent who is a carrier of a genetic disease that can be diagnosed by amniocentesis

• Mother who is a carrier of an X-linked disorder

• History of a child with a chromosomal disorder, neural tube defect, inherited biochemical disorder, or multiple anomalies

• Mother with a history of three or more spontaneous abortions

NOTE: Some experts recommend that mothers be referred to specialized medical centers that perform 50 or more procedures per year for first-trimester or early second-trimester amniocentesis and genetic studies. Less fluid is available at this stage, and there may be less risk of complications with more operator experience. This is particularly important if the procedure must be performed through the placenta. A cell culture is also very fragile, and proper transport and assurance against loss or mix-up are essential. It may be best to perform the study at the institution where the cells will be cultured.


• Abnormal prenatal testing results. Triple screen, quad screen, and combined first-trimester screening are examples of frequently used prenatal tests for birth defects. Careful sonographic fetal evaluation in patients with abnormal prenatal testing results may diagnose (or exclude) an open neural tube defect or Down syndrome. Occasionally, amniocentesis may be required for clarification.

• Evaluation for amnionitis. In a patient with ruptured membranes and clinical signs of amnionitis, Gram’s stain and culture of amniotic fluid may be performed before initiation of treatment (i.e., antibiotics and prompt delivery of the infant).


Evaluating Fetal Health (Late Second or Third Trimester)


Bilirubin levels, measured spectrophotometrically as ΔOD450, are no longer the standard of care for following the Rh or other blood group isoimmunized pregnancy. This has been replaced by Doppler ultrasonographic measurement of the peak velocity of systolic blood flow in the middle cerebral artery. The sensitivity and specificity of Doppler ultrasonography have been shown to be superior to those of measurement of the amniotic fluid ΔOD450. Doppler ultrasonography is noninvasive and thus does not carry the risk associated with amniocentesis. It is recommended that patients with Rh alloimmunization be referred to a center where Doppler ultrasonography of the middle cerebral artery is performed.



Evaluating Fetal Maturity (Third Trimester)


Another common reason for performing amniocentesis is to assess fetal lung maturity. Lecithin (L), sphingomyelin (S), and phosphatidylglycerol (PG) are phospholipids in the newborn lung that act as surfactants and lower the surface tension in the alveoli. Using amniocentesis to determine the L/S ratio, the presence of PG, or both, may minimize the risk of delivering an infant who will develop respiratory distress syndrome (RDS). Other methods to help determine fetal lung maturity include the foam stability index, fluorescence polarization, and lamellar body counts (Table 161-1). Kits are commercially available to help determine these measurements, or variations on them, and are therefore useful for predicting fetal lung maturity.



TABLE 161-1 Commonly Used Direct Tests of Fetal Lung Maturity

image


Therapeutic Interventions



• Relief of symptomatic polyhydramnios, although this is temporary because the fluid rapidly reaccumulates

• Intrauterine transfusion for Rh-hemolytic disease


Contraindications



Absolute



• Infected lesions of the abdominal wall where amniocentesis must be performed

• Patient refusal

• When the results of tests will not change the clinical course


Relative



• Maternal coagulopathy

• Placental abruption

• Problems not diagnosable by evaluation of the amniotic fluid (e.g., teratogen exposure, radiation exposure, drug use early in pregnancy, history of genetic disorders not diagnosable by amniocentesis)


Equipment and Supplies



• Real-time diagnostic ultrasonography unit

• Commercial amniocentesis tray or sterile tray containing at least three plain sterile specimen tubes (5 to 10 mL each) with caps; standard-length 20- or 22-gauge spinal needle (no larger than 20 gauge should be used); 20-mL syringe; 5-mL syringe; image-inch, 22- or 23-gauge needle; sterile 4 × 4 gauze pads; sterile Band-Aids; and sterile towels for drapes

• Skin antiseptic (e.g., povidone–iodine, chlorhexidine gluconate)

• Fetal heart rate monitor

• Sterile gloves

• Local anesthetic solution (e.g., 1% or 2% lidocaine, without epinephrine) (optional)

NOTE: To obtain disposable amniocentesis trays, the clinician should check with a local surgical supplier.



Preprocedure Patient Education and Forms


The clinician should discuss the procedure with the patient (and the patient’s partner, if available) beforehand. Both people should be informed of the risks and the benefits of having an amniocentesis. In addition, the clinician should describe alternative modes of evaluation or treatment (if any) and obtain signed informed consent. (See the sample patient education and consent forms available online at www.expertconsult.com.)

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May 14, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Amniocentesis

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