Amniocentesis

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Chapter 227 Amniocentesis

DESCRIPTION

Amniocentesis is the sampling of fluid from around the growing fetus for prenatal biochemical or genetic diagnosis. Rarely, amniocentesis may be used to reduce the amount of amniotic fluid present in cases of polyhydramnios.

INDICATIONS

The assessment of fetal genetics or metabolic disorders, lung maturity, fetal infection, or isoimmunization status. Therapeutic amniocentesis may be performed for the reduction of fluid volume or the instillation of agents for fetal therapy or other purposes such as fetal imaging or the diagnosis of rupture of the fetal membranes. Amniocentesis is also a necessary step in other diagnostic and therapeutic procedures such as cordocentesis or fetal transfusion.

CONTRAINDICATIONS

Active skin infections near the site of needle placement. Relative; maternal fever of unknown origin, known or suspected allergies to materials used (e.g., latex, skin preparation materials, local anesthetics). Amniocentesis may be technically difficult to accomplish in patients with multiple gestations.

TECHNIQUE

The indications, contraindications, risks, benefits, and complications should be reviewed and discussed with the patient, and informed consent should be obtained. The patient should be placed in the supine position with the head elevated 20 to 30 degrees. If the pregnancy is advanced, the patient may empty her bladder and be placed in a slightly left decubitus position. Ultrasonography is used to assess fetal well-being, fetal lie, and placental position.

A suitable pocket of amniotic fluid should be identified by use of ultrasound. Ideally, this pocket should be located away from the fetal face and placenta, but it should be accessible with a standard spinal needle. Areas around the fetal extremities are often best. The location of this pocket relative to the skin surface should be noted as a guide to needle insertion.

The skin of the abdominal wall over the chosen pocket of amniotic fluid should be disinfected with a suitable skin preparation solution and technique of the examiner’s choice. If a local anesthetic is to be used, it is established at this juncture using sterile technique: A small skin weal of local anesthetic is placed, and the proposed needle track is infiltrated with a total of less than 4 to 5 mL of anesthetic agent.

With the stylet in place, a 20- or 22-gauge spinal needle is passed perpendicularly through the skin, abdominal, and uterine walls, into the amniotic sac. A slight pop or loss of resistance may be felt as the needle traverses the fascia. After the pocket of fluid has been entered, the stylet is removed from the needle. Free flow of fluid should be demonstrated. If free flow is not found, the needle should be rotated or tipped and rechecked prior to being advanced farther (with the stylet in place). Using ultrasonography to guide the needle’s advancement may facilitate placement of the needle into the amniotic fluid pocket. This can be especially helpful when amniocentesis is performed early in pregnancy.

Once free flow of fluid has been demonstrated, a small syringe should be attached to the needle and 2 to 3 mL of fluid should be withdrawn. This fluid is discarded. Appropriate samples are now taken and placed in sterile specimen tubes. Determination of the amount of material needed and any special handling required for these specimens is dictated by the studies to be performed. If there is any doubt, consultation with the laboratory before the procedure may help to identify any special handling that must be used.

After samples have been obtained, the needle is withdrawn and a self-adhesive bandage is applied to the site of needle puncture. The fetus should be monitored for a short period after the procedure. If bloody fluid was obtained, this monitoring period should be extended by 1 to 2 additional hours or longer, depending on other considerations. An appropriate procedure note should be entered into the patient’s record.

COMPLICATIONS

Early amniocentesis is associated with a fetal loss rate of approximately 2.5% (versus 0.7% for later procedures). Amniotic fluid leakage, frank rupture of the fetal membranes, amnionitis (infection), bleeding, and possible isoimmunization of mothers who are Rh negative are all possible. The risk of direct fetal injury is small when the procedure is carefully performed. When amniocentesis is carried out in the presence of preterm premature rupture of membranes, failure rates are higher. Bleeding and infection are always possible with any invasive procedure.

FOLLOW-UP

When amniocentesis is performed after fetal viability (24 weeks), a period of electronic fetal heart rate monitoring (30 minutes) is usual. If bloody fluid is obtained or the fetus or umbilical cord is perforated, this monitoring is generally increased (1 to 2 hours or longer, as clinically indicated). Patients should report persistent uterine cramping, vaginal bleeding or leakage of fluid, or fever.

CPT CODE(S)

59000 Amniocentesis, any method

76946 Ultrasonic guidance for amniocentesis, physician supervision and interpretation

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