Clinical Review

Diagnosing placenta accreta spectrum with prenatal ultrasound

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Uterovesical interface

Studies also have reported that abnormalities of the uterovesical interface are predictive of PAS. The uterovesical interface is best evaluated in a sagittal plane containing the lower uterine segment and a partially full bladder in gray-scale and color Doppler US.15 The normal uterovesical interface appears as a smooth line, without irregularities or increased vascularity on sagittal imaging.

Abnormalities include focal interruption of the hyperechoic bladder wall, bulging of the bladder wall, and increased vascularity, such as varicosities (FIGURES 5, 6, and 7).15 These findings may be seen as early as the first trimester but are more commonly noted in the second and third trimesters.7 The authors of a recent meta-analysis concluded that irregularity of the uterovesical interface is the most specific marker for invasive placentation (99.75% confidence interval; range, 99.5% to 99.9%).13

Other US markers and modalities

Other proposed US markers of PAS include placental bulge or focal exophytic mass (FIGURE 8). More concerning is disruption of the uterine serosa with placental extension, suggesting an exophytic mass, most commonly into the bladder.21

Three-dimensional US. Studies have evaluated the role of 3-dimensional (3D) US for predicting PAS. Application of 3D US in vascular mode has shown promise because it allows for semiquantitative assessment of placental vasculature.22 Using 3D US to screen for PAS presents drawbacks, however: The technology is not well-standardized and requires significant operator expertise for volume acquisition and manipulation. Prospective studies are needed before 3D US can be applied routinely to screen for and diagnose PAS.

Color Doppler US. As an adjunct to gray-scale US, color Doppler US can be used for making a diagnosis of PAS. Color Doppler US helps differentiate a normal subplacental venous complex with nonpulsatile, low-velocity venous blood flow waveforms from markedly dilated peripheral subplacental vascular channels with pulsatile venous-type flow, which suggests PAS. These vascular channels are often located directly over the cervix. In addition, the observation of bridging vessels linking the placenta and bladder with high diastolic arterial blood flow also suggests invasion.21 In a meta-analysis, overall sensitivity of color Doppler US for the diagnosis of PAS was 91%, with specificity of 87%.13

AT OUR INSTITUTION……we evaluate the uterovesical interface on transvaginal grayscale and color Doppler US in a midline sagittal view, in which the bladder wall is seen as a hyperechoic band between the uterine serosa and bladder lumen. We subjectively define irregularity of the posterior bladder wall as disruption of the normally smooth bladder wall. We measure the thinnest portion of the myometrium at the uterovesical interface, perpendicular to the long axis of the wall of the uterus with an empty maternal bladder.

The value of utilizing multiple markers

The accuracy of US diagnosis of PAS is likely improved by using more than 1 sonographic marker. Pilloni and colleagues,20 in a prospective analysis, found that 81% of cases of confirmed PAS had ≥2 markers and 51% of cases had ≥3 markers.

Several scoring systems have been proposed for making the diagnosis of PAS using combinations of sonographic markers, placental location, and clinical history.19,24,25 In 2016, Tovbin and colleagues,25 in a prospective study, evaluated a scoring system that included:

  • number of previous CDs
  • number of, maximum dimension of, and presence of blood flow in lacunae
  • loss of uteroplacental clear zone
  • placental location
  • hypervascularity of the uterovesical or uteroplacental interface.

Tovbin assigned 1 or 2 points to each criterion. Each sonographic marker was found to be significantly associated with PAS when compared to a high-risk control group. A score of ≥8 was considered “at high risk” and predicted 69% of PAS cases.

Regrettably, no combination of US markers reliably predicts the depth of invasion of the placenta.26

ON OUR US UNIT……we apply color Doppler US to the retroplacental–myometrial interface and the uterovesical interface to evaluate for abnormal subplacental and uterovesical hypervascularity, defined subjectively by the presence of striking amount of color Doppler US signals in the placental bed, with numerous, closely packed, tortuous vessels demonstrating multidirectional flow and aliasing artifact.23

Continue to: A standardized approach is needed

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