Oligohydramnios: How to Best Diagnose It and What It Really Means
By John C. Hobbins, MD
Professor, Department of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora
Dr. Hobbins reports no financial relationships relevant to this field of study.
SYNOPSIS: A multicenter randomized, clinical trial involving large numbers of patients has shown that using the maximal vertical pocket instead of the amniotic fluid index to detect oligohydramnios more than halves the amount of inductions for the diagnosis of oligohydramnios without affecting the overall outcome.
SOURCE: Kehl S, Schlekle A, Thomas A, et al. Single deepest vertical pocket or amniotic fluid index as an evaluation test for predicting adverse pregnancy outcome (SAFE Trial): A multicenter, open label, randomized controlled trial. Ultrasound Obstet Gynecol 2016;47:674-679.
Oligohydramnios gets the attention of providers for good reason. It is seen more frequently in patients whose fetuses have intrauterine growth restriction (IUGR) and in those with ruptured membranes (PROM). In the IUGR, it has been thought that the fetus will adapt to intrauterine deprivation by sending more blood to the brain at the expense of renal plasma flow. In PROM, the etiology is obvious and, if dealing with prolonged severe oligohydramnios, there is great concern for fetal pulmonary hypoplasia. Since oligohydramnios has been noted to correlate with outcome in IUGR and, to some extent, in PROM, attempts to detect it have been expanded to include, seemingly, any high-risk patient. In fact, it has become the staple of every fetal surveillance regimen.
The two most common methods to evaluate the amount of amniotic fluid in the uterus are the amniotic fluid index (AFI) and the maximum vertical pocket (MVP). Even though most studies1,2,3 and a Cochrane meta-analysis4 have suggested the MVP to be the superior method, the AFI-leaning providers have not appeared to be swayed by the results of these studies.
In a recent multicenter study from Germany, the authors initiated a randomized trial in low-risk and high-risk patients who had ultrasound exams after 36 weeks and who delivered within one week of their last exams. One group of patients was randomly assigned to have MVPs as the method of choice in their assessments and the other group had AFIs. The patients were all managed according to the desires of the primary providers. However, oligohydramnios was used as a diagnostic trigger for induction. Neonatal outcomes were based on many variables.
Data were available for 498 patients in the AFI group and 504 patients in the MVP group. About 82% of the study group were labeled as low risk. Oligohydramnios was diagnosed more frequently by AFI (9.8% vs. 2.2%; relative risk [RR], 4.51; 95% confidence interval [CI], 4.8-1.50), resulting in more inductions of labor for oligohydramnios (12% vs. 3.6%; RR, 3.50; 95% CI, 1.76-6.96). Interestingly, although there was an increase in “abnormal cardiotocography” in the AFI group (36.3% vs. 26.2%; RR, 1.23; 95% CI, 1.02-1.50), there was no increase in the need for cesarean section for “fetal distress” (RR, 1.26; 95% CI, 0.88-1.79). Most importantly, there were no differences in any of the many neonatal outcomes investigated (newborn special care unit admissions, Apgar scores, blood gases, and meconium-stained amniotic fluid).
Amniotic fluid assessment is part of every ultrasound exam done throughout pregnancy and is an integral part of the biophysical profile (BPP), which is incorporated into surveillance protocols in late pregnancy. The BPP originally was developed by Platt and Manning in 1980.5 The authors decided to use a vertical pocket of ≤ 1 cm to define oligohydramnios. However, it soon was clear that this cutoff was too stringent, so the MVP threshold was liberalized to 2 cm. It was then realized that even when there is an obvious paucity of amniotic fluid, it is possible with some creativity to get a thin slice of fluid in a vertical plane to measure > 2 cm. This led to the addition of a horizontal dimension to the definition of oligohydramnios (i.e., a pocket of < 2 cm × 2 cm, or < 2 cm × 1 cm, as was used in the above study).
Later, based on the concept of “if some is good, more is better,” Phelen adapted the technique to use the sum of four vertical pockets in each of four quadrants of the uterus. The thresholds of concern were AFIs of < 5 cm for oligohydramnios and > 20 cm for polyhydramnios.6
Again, this study shows that by using MVP as a primary tool in fetal assessment, fewer patients are labeled as having oligohydramnios, thereby subjecting fewer patients to inductions. Although this study did not show an increase in cesarean sections with the AFI, other studies have shown an increase.4,7 Of significant note was the fact that this increased intervention did not result in improvement in any of the adverse neonatal outcomes studied.
Although oligohydramnios can be very concerning, true oligohydramnios is not a usual feature of IUGR. The most common form of IUGR is late IUGR in which most surveillance tests, including umbilical artery waveforms, nonstress tests, and BPPs (including amniotic fluid assessment) are normal. Why? Although the supply line is sufficient to sustain these fetuses for many weeks, they simply get to a point, generally after 34 weeks, where they demand more than their partially challenged placentas can provide. To adapt, they begin prioritizing by sparing their brains. This shift in blood flow does not curtail urine production immediately, so by the time they get our attention, oligohydramnios has not yet had a chance to develop. If oligohydramnios does complicate IUGR, it is in the early, severe, forms of placental insufficiency in which there are many other worrisome clues that surface first.
I am amazed at how often providers quibble about the subtle differences in MVPs or AFIs. These methods represent indirect reflections of a dynamic process of amniotic fluid production and absorption that vary not just on a day-to-day basis, but on an hour-to-hour basis. In addition, the methods themselves have wide inter- and intra-observer variabilities. Now that amniotic fluid assessment has been applied to high-risk and low-risk patients, its value, if isolated, should be put into proper perspective. Even in conditions such as IUGR and PROM, oligohydramnios rarely should be a “game changer,” by itself, with regard to intervention.
- Alfirevic Z, Luckas M, Walkinshaw SA, et al. A randomized comparison between amniotic fluid index and maximum pool depth in the monitoring of post-term pregnancy. Br J Obstet Gynaecol 1997;104:207-211.
- Magann EF, Chauhan SP, Bofill JA, Martin JN Jr. Comparability of the amniotic fluid index and single deepest pocket measurements in clinical practice. Aust N Z J Obstet Gynaecol 2003;43:75-77.
- Magann EF, Chauhan SP, Doherty DA, et al. The evidence for abandoning the amniotic fluid index in favor of the single deepest pocket. Am J Perinatol 2007;24:549-555.
- Nabhan AF, Abdelmoula YA. Amniotic fluid index versus single vertical pocket as a screening test for preventing adverse pregnancy outcome. Cochrane Database Syst Rev 2008; CD006593.
- Manning FA, Platt LD, Sipos L. Antepartum fetal evaluation: Development of fetal biophysical profile. Am J Obstet Gynecol 1980;136:787-795.
- Phelan JM, Smith CV, Broussard P, Small M. Amniotic fluid volume assessment with the four-quadrant technique at 36-42 weeks’ gestation. J Reprod Med 1987;32:540-542.
- Magann EF, Doherty DA, Field K, et al. Biophysical profile with amniotic fluid volume assessments. Obstet Gynecol 2004;104:5-10.
A multicenter randomized, clinical trial involving large numbers of patients has shown that using the maximal vertical pocket instead of the amniotic fluid index to detect oligohydramnios more than halves the amount of inductions for the diagnosis of oligohydramnios without affecting the overall outcome.
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