Amnioreduction: Technique

TTTS. Amnioreduction: Technique

Amnioreduction is a technically simple procedure, although appropriate patient selection, patient positioning, and drainage mechanisms are important to minimize procedure-related complications. The procedure may be accomplished without sophisticated technology, although there has been a recent trend in the use of vacuum-assisted devices to withdraw the amniotic fluid rather than manual techniques.

There is wide variation in the amnioreduction technique in terms of needle gauge used, the volume and rate of amniotic fluid withdrawn, and the completion MVP or amniotic fluid index (AFI). Interestingly, despite these procedural differences, the reported success rates of this technique are remarkably similar.27,28 Patient sedation with a benzodiazepine is helpful for maternal comfort and to reduce excessive fetal movement of the recipient fetus during the procedure. 

Positioning of the woman is critical to the successful completion of the procedure (Figure 8.2). Lateral tilt will reduce the potential for aortocaval compression and supporting pillows behind the spine and between the knees will aid maternal comfort. 

The needle insertion site is selected so as to avoid transplacental needle passage or disruption of the dividing membrane. In addition, the needle insertion site should not be too cephalad due to the marked alteration in uterine morphology that follows decompression and the acute angulation which may occur secondarily to this phenomenon. 

Our practice is to use an 18-gauge spinal needle, although a variety of needle gauges have been reported, inserted under continuous ultrasound guidance following infiltration of local anesthetic to the skin and deeper tissues. Amniotic fluid is drained either by a manual system or a vacuum bottle system (Figure 8.3a,b) until the volume of the recipient sac is normal. 

There is a linear relationship between the volume of amniotic fluid removed and the alteration in AFI, with a decrease in the AFI of approximately 1 cm/100 ml fluid removed.31,32 This alteration in AFI appears to be independent of gestation and preprocedure AFI, and can provide a preprocedure approximation of the volume to be drained. Other units use single measures of amniotic fluid as a guide, typically reducing the amniotic fluid volume until an MVP of 5–6 cm is obtained.

COMPLICATIONS 

Amnioreduction has been associated with four main procedure-associated complications: preterm labor and/or amniorrhexis, fetal demise, infectious complications, and placental abruption. It is difficult to dissociate the primary condition being treated from the first two adverse outcomes, as polyhydramnios has a clear association with preterm birth and intrauterine fetal demise.

Inadvertent septostomy has been reported following amnioreduction with umbilical cord entanglement.27 The reported complication rates from large volume amnioreduction are quite variable. In single-center series the complication rate has ranged from 1.5% (3/200 procedures)33 to 4.6% (8/174).34 Multiple-center series have been associated with a tendency to increased complication rates, varying from 3.2% (9/281)27 to 6% (46/760).