Polyhydramnios Imaging

Updated: Oct 14, 2015
  • Author: Prabhakar Rajiah, MD, MBBS, FRCR; Chief Editor: Eugene C Lin, MD  more...
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Overview

Overview

Polyhydramnios is the presence of excess amniotic fluid in the uterus. By definition, polyhydramnios is diagnosed if the deepest vertical pool is more than 8 cm or amniotic fluid index (AFI) is more than 95th percentile for the corresponding gestational age. With a deep pocket of 8 cm as criteria of polyhydramnios, the incidence is 1-3% of all pregnancies. About 20% are associated with fetal anomalies.

Polyhydramnios has a variety of causes affecting the mother or the fetus. The presence of polyhydramnios should prompt a search for other fetal anomalies. Some of the anomalies can be diagnosed with sonography, while others require karyotyping.

Preferred examination

The diagnostic approach to polyhydramnios consists of (1) physical examination of the mother with an investigation for diabetes mellitus, diabetes insipidus, and Rh isoimmunization; (2) sonographic confirmation of polyhydramnios and assessment of the fetus; (3) fetal karyotyping; and (4) maternal serologic testing for syphilis. [1, 2]

Limitations of techniques

Clinical examination can reveal a uterine size larger than that expected for the corresponding gestational age of the fetus.

Ultrasonography is the most reliable method for diagnosing and quantifying polyhydramnios. Experienced operators make the diagnosis based on subjective assessment. The height of the deepest pocket and the AFI are objective semiquantitative measurements of the amniotic fluid.

MRI is not necessary for the diagnosis of polyhydramnios, but polyhydramnios can be detected during MRI for other indications. Three limitations of MRI are notable. First, MRI is not cost effective. Second, it is time consuming. For example, the time required for magnetic resonance volumetry can be as long as 6 hours, which is not ideal when sonography can be performed in only a few minutes. Third, MRI requires knowledge of computers and postprocessing.

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Computed Tomography

CT scanning is generally avoided during pregnancy.

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Magnetic Resonance Imaging

MRI is not essential in the imaging protocol for polyhydramnios, but if it is performed for fetal or maternal imaging, it can also be used for diagnosing polyhydramnios.

A customized medical image postprocessing software package can be used for segmentation and 3-dimensional (3D) modeling. Once the structures of interest in a 3D image volume are segmented, the postprocessing software creates a corresponding 3D surface model and automatically calculates the volume of each 3D reconstruction

Although allowances must be made for a smaller fluid volume (except in polyhydramnios) and for segmentation of the amniotic fluid in vivo being slightly more difficult owing to fetal motion, volumetric measurements are likely to represent the real values.

Degree of confidence

MRI is good for assessing the volume of amniotic fluid and for diagnosing polyhydramnios. However, MRI is not mandatory for the diagnosis of polyhydramnios.

Kubik et al found that MRI is accurate in measuring the amniotic fluid volume, as it is in measuring placental volume and fetal weight. [3]

A good correlation was obtained between MR volumetry studies and the actual amniotic fluid volume. Although this would not be a cost effective method of diagnosing polyhydramnios, it would be of greater help in monitoring therapeutic response to polyhydramnios treatment. The common sequences used are T2-weighted single-shot fast spin-echo and high-spatial-resolution T1-weighted fast spin-echo images subsequent to a spoiled gradient-echo localizer.

False positives/negatives

Measuring the amniotic fluid volume is difficult when the quantity is low because no difference in signal intensity can be noted between a thin rim of fluid and the placenta and uterine wall; this similarity makes postprocessing and automatic segmentation difficult.

Although 3D reconstruction of a fetus is better with a large amount of amniotic fluid present, fetal motion adversely affects image processing and reconstruction.

Use of an automatic threshold for excluding amniotic fluid excludes other tissues containing the same signal intensity, such as fetal brain and fluid-filled fetal organs (eg, the urinary bladder).

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Ultrasonography

Ultrasonography is the main modality for the diagnosis of polyhydramnios and evaluation of the fetus. Features that are assessed in polyhydramnios include amniotic fluid, possibly of multiple pregnancy, chorionicity in multiple pregnancy, fetal macrosomia, fetal thorax, fetal central nervous system, fetal gastrointestinal tract, cervical length, and posttreatment follow-up results. [4, 1, 2]

Amniotic fluid

There are at least 3 methods for measuring amniotic fluid: (1) depth of the deepest vertical pool, (2) the 2-diameter pocket (depth X width of the longest pocket), and (3) the AFI. [5, 6, 7, 8, 9]

With the AFI method, the uterus is divided into 4 quadrants. The depths of the deepest vertical pool in the 4 quadrants are measured and added to give the index. Occasionally, at less than 20 weeks, only the right and left lower quadrants are used. The normal index is 5-24. In polyhydramnios, it is more than 24. AFI of a normal population (ie, normative values) corresponding to the gestational age can be noted, and the percentile value of the particular patient can be calculated by using the mean and standard deviation.

The graph below shows the normal limits of AFI based on gestational age. The mean AFI for normal pregnancies is 11-16 cm. Polyhydramnios is diagnosed when the AFI is more than the 95th percentile value. Normative values are not available before 16 weeks of gestation.

Graph illustrating amniotic fluid index in a norma Graph illustrating amniotic fluid index in a normal singleton pregnancy. The solid line is the mean AFI, the lower dotted line is the 5th percentile value, and the upper dotted line is the 95th percentile value (data adapted from Moore, 1990). Image courtesy of Christopher L. Sistrom, MD.

The incidence of polyhydramnios can vary with the technique used. A single deep pocket more than 8 cm is diagnostic of polyhydramnios. With the single-pocket technique, the incidence is 0.7% (1.1% for oligohydramnios). With the 2-diameter pocket, the rate is 3% (30% for oligohydramnios), and with the AFI method, the rate is 0% (8% for oligohydramnios). Therefore, the single-deep-pocket method is the best technique because it classifies the least number of cases as being abnormal.

A simple rule of thumb is that in the first trimester, the fluid is more than the embryo/fetus; in the second trimester, the fluid is equal to the fetus; and in the third trimester, the fluid is less than the fetus.

Chorionicity in multiple pregnancy

Twins can be monochorionic or dichorionic. The difference can be assessed by careful observation. In dichorionic twins, the intermembrane septum is thick, with 3 or 4 membrane layers, and the membrane is more than 2 mm. The triangular sign is present and very specific. In dichorionic twins, the cause of polyhydramnios is the same as that in a singleton pregnancy.

In monochorionic twins, the intermembrane septum is thin, and the junction of membranes forms a T shape. In monochorionic twins, the most common cause of polyhydramnios is twin-to-twin transfusion syndrome

Fetal growth

Sonography may be useful in screening for growth retardation.

Fetal gastrointestinal tract

Ultrasonography may be helpful in evaluating the mouth, stomach, small bowel, and abdominal wall.

Fetal bladder dynamics

One technique involves the change in bladder dimension observed over 20-minute intervals. These changes can differentiate fetal polyuria from other causes of polyhydramnios. However, this technique has its limitations. It underestimates the degree of fetal urine production by at least 50%, and it is not useful in severe hydramnios because the bladder is already filled with urine, and any further increase in the bladder size is minimal. [10]

Cervical length

Cervical length is essential for assessing the risk of preterm labor. If the fetus is less than 24 weeks and if after amniotic drainage the cervical length is less than 25 mm, a cervical suture is required to prevent preterm labor. [11]

Posttreatment follow-up results

The AFI should be monitored twice a week when the patient is being treated with indomethacin. The treatment is stopped when the AFI is less than normal. The response is seen usually between 4 and 20 days.

Doppler imaging of the ductus arteriosus is also done within 24 hours of starting treatment and once weekly thereafter. Indomethacin is known to cause premature closure of ductus arteriosus, and if this happens, indomethacin is stopped.

Degree of confidence

The values for amniotic fluid index, single deepest pocket, and 2 diameter pockets are not normally distributed throughout pregnancy. Therefore, a logarithmic transformation is required for gestational age–specific ranges. Normative values also vary within a population.

The incidence of detection of polyhydramnios varies with the technique used, as discussed in Amniotic fluid above.

Sonographic assessment of amniotic fluid is a poor indicator of amniotic volume. The 95% confidence limit is wide compared with the dye-dilution technique for the measurement of amniotic fluid volume.

Magann et al showed that, while sonography and the single-deep-pocket method are good for measuring normal amniotic fluid volume (83-94%), they are not accurate in diagnosing polyhydramnios (33-46%) and oligohydramnios (11-27%). [6, 7]

If color Doppler imaging is used along with normal scanning, the AFI is less than that obtained without Doppler techniques. This difference increases the diagnosis of oligohydramnios.

False positives/negatives

Chorioangiomas larger than 5 cm can produce complications including polyhydramnios, preeclampsia, preterm delivery, congenital malformation, congestive cardiac failure, antepartum hemorrhage, intrauterine growth retardation, and microangiopathic hemolytic anemia.

Sonograms show a placental mass with anechoic spaces, which demonstrate flow on color Doppler studies and pulsatile flow on spectral Doppler trace studies.

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Nuclear Imaging

Nuclear medicine studies have no role in the evaluation of polyhydramnios.

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