You are in: eMedicine Specialties > Radiology > OBSTETRICS/GYNECOLOGY OligohydramniosArticle Last Updated: Oct 3, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 11
  Author: Jason K Baxter, MD, Fellow in Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Thomas Jefferson University Hospital Jason K Baxter is a member of the following medical societies: American College of Obstetricians and Gynecologists, American Medical Association, Association of Professors of Gynecology and Obstetrics, Sigma Xi, and Society for Maternal-Fetal Medicine Coauthor(s): Harish M Sehdev, MD, Assistant Professor of Clinical Obstetrics and Gynecology, Department of Obstetrics and Gynecology, University of Pennsylvania; Consulting Staff, Pennsylvania Hospital, University of Pennsylvania Health System; John W Breckenridge, MD, FACR, Clinical Assistant Professor, Department of Radiology, Division of Ultrasound, Thomas Jefferson University Hospital; Chair, Department of Radiology, Abington Memorial Hospital Editors: Christopher L Sistrom, MD, Associate Chair for Research, Assistant Professor, Department of Radiology, University of Florida School of Medicine; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Karen L Reuter, MD, FACR, Professor, Department of Radiology, Lahey Clinic Medical Center; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Eugene C Lin, MD, Consulting Staff, Department of Radiology, Virginia Mason Medical Center Author and Editor Disclosure Synonyms and related keywords: decreased amniotic fluid, amniotic fluid volume, AFV, amniotic fluid index, AFI, maximum vertical pocket, MVP, single deepest pocket, SDP BACKGROUND AND DEFINITIONSSection 2 of 11
  Multiple definitions of oligohydramnios are used because no ideal cutoff level for intervention exists.1, 2, 3, 4, 5, 6, 7 Oligohydramnios is characterized by the following features:
Amniotic fluid volume depends on the gestational age; therefore, the best definition may be AFI less than the fifth percentile. NORMAL PHYSIOLOGYSection 3 of 11
AFV increases steadily throughout most of pregnancy, with a volume of about 30 mL at 10 weeks' gestation and a peak of about 1 L at 34-36 weeks' gestation. AFV decreases in the late third trimester, with a mean AFV of 800 mL at 40 weeks. The rate of decline may be as high as 150 mL/wk at 38-43 weeks' gestation. Mechanisms that produce changes in the AFV are not completely understood; however, they are related to the inflow and outflow of amniotic fluid in an active process. Amniotic fluid constantly circulates, with an estimated exchange rate as high as 3600 mL/h. The 3 major determinants of AFV are (1) the physiologic regulation of flow rates by the fetus, (2) the movements of water and solutes within and across the membranes, and (3) the maternal effects on transplacental fluid movement. AFV is determined by a steady state between inflow and outflow; therefore, the excretion of urine by the fetus is the major source of amniotic fluid production in the second half of the pregnancy. Fluid secreted by the fetal respiratory tract, including that from the lungs and that from the oral-nasal cavity, also contributes to AVF. Fetal swallowing is the major pathway of amniotic fluid clearance in the last half of gestation. Fetal skin is highly permeable in the first half of pregnancy before it becomes keratinized at 22-25 weeks' gestation. Amniotic fluid is probably not transferred across the skin in a significant amount in the third trimester. Water and solutes rapidly move between the amniotic fluid and the fetal blood. They pass across the large surface area of the fetal surface of the placenta and the membranes, probably affecting the AFV. The osmolality and composition of amniotic fluid, as well as membrane permeability, probably play important roles in this movement. The mother's fluid status and, consequently, the fluid status of the fetus have a major effect on the AFV. The ease with which fluid moves across the placenta between the fetal blood and the maternal blood may affect this process. Lastly, increased maternal fluid ingestion has been shown to increase the AFV in women with oligohydramnios. PATHOPHYSIOLOGYSection 4 of 11
Oligohydramnios is secondary to either an excess loss of fluid or a decrease in fetal urine production or excretion. In general, oligohydramnios is related to one of the following 4 conditions:
FREQUENCYSection 5 of 11
In the US, oligohydramnios is a complication in 0.5-5.5% of all pregnancies, and severe oligohydramnios is a complication in 0.7% of pregnancies. Oligohydramnios is more common in pregnancies beyond term because the AFV normally decreases at term. It complicates as many as 12% of pregnancies that last 41 weeks and longer. CLINICAL DETAILS: WORKUPSection 6 of 11
Ultrasonography The diagnosis is confirmed by means of ultrasonography. Oligohydramnios may be discovered incidentally during routine ultrasonography and noted during antepartum surveillance for other conditions. The diagnosis may be prompted by a lag in sequential fundal height measurements (size less than that expected for the dates) or by fetal parts that are easily palpated through the maternal abdomen.8, 9, 10, 11, 12 During ultrasonography of the fetal anatomy, normal-appearing fetal kidneys and fluid-filled bladder may be observed to rule out renal agenesis (see Images 1-2), cystic dysplasia, and ureteral obstruction. Check fetal growth to rule out intrauterine growth restriction (IUGR) leading to oliguria. Sterile speculum examination Sterile speculum examination may be performed to check for ROM. Amniotic fluid may pool in the vagina, and an arborization or ferning pattern may be observed when dried posterior vault fluid is examined microscopically. Cervical mucous may cause false-positive results, as can semen and blood. Nitrazine paper turns blue. The amniotic fluid is more basic (pH 6.5-7.0) than normal vaginal discharge (pH 4.5). Measurement of AFV Initial studies to objectively measure AFV involved dye dilution techniques. The techniques were accurate, although they required amniocentesis, an invasive procedure that increased the risk of perinatal morbidity. The routine use of ultrasonography has created a safe, reliable, and repeatable method of measuring AFV. Early methods of assessing AFV with ultrasonography involved nonquantitative assessments, including sonographers' subjective impression of AFV. Subjective oligohydramnios criteria have included the following: the absence of fluid pockets throughout the uterine cavity, crowding of the fetal limbs, the absence of pockets surrounding the fetal legs, and overlapping of the fetal ribs (in severe cases). The 2 most commonly used objective methods of determining AFV include measurement of the SDP and the summation of the SDPs in each quadrant, or the AFI. These tests are routinely performed with the patient in the supine or semi-Fowler position, although studies have demonstrated accuracy in the lateral decubitus position as well. The ultrasound transducer is held along the maternal longitudinal axis and maintained perpendicular to the floor while the SDP of the amniotic fluid is measured. Pockets should be free of fetal limbs and the umbilical cord, although some authors allow for a single loop of cord to be within the fluid pocket. AFV may be artificially increased if the transducer is not maintained perpendicular to the floor. Excessive pressure on the maternal abdomen with the transducer may lead to an artificially reduced measurement (see Image 4). Phelan et al described the AFI as a quantitative measurement to predict a poor pregnancy outcome and the success of external cephalic versions.1 The pregnant abdomen is divided into 4 quadrants by using the umbilicus as a reference point to divide the uterus into upper and lower halves and by using the linea nigra to divide the uterus into left and right halves. The 4 measurements are summed to obtain the AFI in centimeters. In gestations earlier than 20 weeks, measurements from the 2 halves are divided by the linea nigra to obtain the AFI. Tables of the normal limits for AFI, based on the gestational age, have been published for singleton and multiple pregnancies (see Image 3). The mean AFI for normal pregnancies is 11-16 cm. The test is reproducible, with interobserver and intraobserver variations of about 10-15% or 1-2 cm in pregnancies with normal AFVs. The margin of error is less in patients with decreased amounts of amniotic fluid. Oligohydramnios has been defined as an AFI less than 5 cm, although 8 cm has occasionally been used as a cut-off threshold. Because the AFV depends on the gestational age, oligohydramnios has been defined as an AFI less than the fifth percentile (corresponding to an AFI of <6.8 cm at term). Oligohydramnios has been defined as an SDP less than 2 cm. Perinatal morbidity rates have been shown to increase sharply with SDPs below this value. Some have suggested that an SDP of 2.5-3.0 cm is a better lower limit for separating normal SDPs from those consistent with oligohydramnios. Many studies have shown that the SDP and the AFI methods have equal diagnostic accuracies. The SDP technique may be a better means of assessing the AFV in twin gestations and in pregnancies at an early gestational age. Some study results have shown that the AFI has greater sensitivity and a higher predictive value than the SDP in diagnosing abnormally high and low AFVs. Most obstetricians prefer to assess a broader area of the uterine cavity by using the AFI because the single measurement of the SDP does not allow for an asymmetric fetal position in the uterus. Other examinations MRI and 3-dimensional (3D) ultrasonography are newer (and more expensive) modalities for accurately assessing the AFV. CAUSESSection 7 of 11
Fetal causes include chromosomal factors, congenital factors, intrauterine growth restriction, postterm pregnancy, premature ROM (PROM), and fetal demise. 13 Placental causes include abruption and twin-to-twin transfusion syndrome (monochorionic twins). (Also see the eMedicine articles Placenta, Abruption and Twin-Twin Transfusion Syndrome.) Maternal causes include maternal dehydration, uteroplacental insufficiency, hypertension, preeclampsia, diabetes, and chronic hypoxia. Drug-induced causes include those due to indomethacin and ACE inhibitors. Idiopathic causes are also possible. PROGNOSIS AND SIGNIFICANCESection 8 of 11
The earlier in pregnancy that oligohydramnios occurs, the poorer the prognosis. Fetal mortality rates as high as 80-90% have been reported with oligohydramnios diagnosed in the second trimester. Most of this mortality is a result of major congenital malformations and pulmonary hypoplasia secondary to PROM before 22 weeks' gestation. Midtrimester PROM often leads to pulmonary hypoplasia, fetal compression syndrome, and amniotic band syndrome. The inspiration of amniotic fluid at regular intervals is probably needed for terminal alveolar development. The assessment of AFV is important in pregnancies complicated by abnormal fetal growth or IUGR. AFV has been shown to be predictive in discriminating normal from decreased growth. Oligohydramnios is a frequent finding in pregnancies involving IUGR and is most likely secondary to decreased fetal blood volume, renal blood flow, and, subsequently, fetal urine output. Pregnancies complicated by severe oligohydramnios have been shown to be at increased risk for fetal morbidity. AFV is an important predictor of fetal well-being in pregnancies beyond 40 weeks' gestation. AFV must be closely monitored, with measurements obtained at least once per week. The diagnosis of oligohydramnios may help in identifying postterm fetuses in jeopardy. AFV is a predictor of the fetal tolerance of labor, and it is associated with an increased risk of abnormal heart rate, meconium-stained amniotic fluid, and cesarean delivery. An increased incidence of cord compression is associated with oligohydramnios; this can lead to variable decelerations, with cord occlusion as the proximate cause of fetal distress. TREATMENTSection 9 of 11
Treatment for oligohydramnios is based on gestational age. Before term, expectant management is often the most appropriate course of action, depending on maternal and fetal condition. Ongoing antepartum surveillance, including assessment of fetal growth and follow-up monitoring of AFV, is necessary. Some advocate continuous fetal heart rate monitoring in all pregnancies complicated by oligohydramnios. At term, delivery is often the most appropriate management. With reassuring fetal testing, delivery may be safely delayed on the basis of the parity, the gestational age, the inducibility of the mother's cervix, and the severity of the oligohydramnios. After term, oligohydramnios in the postterm patient is associated with more fetal decelerations, a higher incidence of meconium-stained fluid, and an increased risk for cesarean delivery. Oligohydramnios is considered an indication for delivery in a postterm pregnancy. However, studies have not shown a difference in perinatal outcomes. Increasing the amount of fluid within the amniotic cavity can be accomplished during delivery with the use of amnioinfusion. Warm or room-temperature sodium chloride solution is transcervically infused through an intrauterine catheter. This procedure increases the amount of fluid to provide more padding around the umbilical cord, which has been shown to decrease the frequency and severity of variable decelerations secondary to decreased cord compression. Amnioinfusion lowers the rate of surgical delivery. Some have advocated the injection of fluid before delivery via amniocentesis to improve the intrauterine environment for the fetus prior to labor. The duration of the effect is unknown. Oligohydramnios is usually observed to return within 1 week. Amnioinfusion via amniocentesis has been used in the second trimester to better visualize the fetal anatomy and confirm diagnosis (see Images 1-2). Vesicoamniotic shunts may be used to divert fetal urine to the amniotic fluid cavity in patients in whom a fetal obstructive uropathy is determined to be the cause of oligohydramnios. Findings from human and animal studies suggest that the shunts can prevent or at least ameliorate pulmonary hypoplasia. Unfortunately, such shunts are associated with ROM in about 12% of the cases at the time of shunt placement. The treatment of maternal dehydration with oral or intravenous rehydration has been shown to increase the AFV. MULTIMEDIASection 10 of 11
REFERENCESSection 11 of 11
Article Last Updated: Oct 3, 2007 | |||||||||||||||||||||||||||||||||
