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Abstract & Commentary
Synopsis: Does oligohydramnios with intact membranes in a high-risk pregnancy represent a reason to deliver?
Source: Magann EF, et al. Obstet Gynecol. 2000;96:640-642.
Ultrasound assessment of amniotic fluid "adequacy" has been a component of fetal assessment ever since the introduction of the biophysical profile. The amniotic fluid index (AFI) represented an attempt to further refine the quantification of amniotic fluid. The "modified biophysical profile" that involved only 2 variables, nonstress test and assessment of amniotic fluid, emerged as a substitute for those clinicians not keen on spending the time and effort to observe fetal behavior (breathing, movement, and tone).
The obsession over the amount of amniotic fluid stems from the fact that conditions in which oligohydramnios occurs, such as intrauterine growth restriction (IUGR) and postmaturity, are associated with higher perinatal mortality and morbidity.
Over the last few years, a few reports have surfaced that question the concept that oligohydramnios, as such, is a reason to deliver. In one of these studies, Magann and colleagues evaluated 79 high-risk patients at 34 weeks or greater who had AFIs of less than 5 cm. These patients were empirically induced as part of a management protocol. Each patient’s perinatal outcome was then compared with that of the next patient who had a similar high-risk condition but had an AFI of 6 cm or more.
There was no significant difference in any outcome variable they analyzed, which included meconium- stained amniotic fluid, variable fetal heart rate decelerations, emergency cesarean section, Apgar scores, cord gases, and admissions to neonatal intensive care. Magann et al concluded that "high risk pregnancies with AFIs less than 5 cm appear to carry intrapartum complication rates similar to those of high risk pregnancies with an AFI greater than 5 cm."
COMMENT BY JOHN C. HOBBINS, MD
Oligohydramnios seems to have gotten a "bum rap." In many cases, its presence, in the absence of ruptured membranes, simply represents an adaptive maneuver of the fetus to shift blood away from organs such as the kidneys in favor of the brain, heart, and adrenals. Although this mechanism is put into action when the fetus is demanding more than the placenta can deliver, oligohydramnios is an early, rather than late, sign of fetal compromise, and by itself certainly is only a symptom rather than the culprit in a "supply line" problem.
In IUGR, oligohydramnios will precede a nonreassuring fetal heart rate tracing and/or metabolic acidemia (the best correlate of neurological morbidity) by many weeks.
Researchers have found a strong correlation between oligohydramnios and increased resistance (and decreased blood flow) in the fetal kidneys. We have noticed that oligohydramnios associated with IUGR is accompanied in virtually all cases with increased end diastolic flow in the middle cerebral arteries (MCA), suggesting simply an adaptive maneuver of the fetus to "spare" his/her brain in the face of adversity. However, the message that the fetus is sending us, especially in the premature fetus, is "don’t deliver me unless there is other evidence of fetal compromise such as worrisome umbilical artery dopplers (or more recently, ductus venosus wave form abnormalities), or a nonreassuring fetal heart rate pattern." These are far more precise indicators of fetal condition than an ultrasound finding that is difficult to quantify, has a wide inter- and intra-patient day-to-day variation, and is so nonspecific.
Severe oligohydramnios should represent a reason to deliver a post-term patient, but isolated oligohydramnios is an undergrown fetus with otherwise normal testing may well be subjecting many fetuses to the unnecessary complications of prematurity and their mothers to injudicious inductions and unnecessary cesarean sections.
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