Placenta accreta spectrum disorders

Placenta accreta spectrum (PAS) disorders describe the degree to which there is an invasion by chorionic villi into the myometrium because of a defect in the decidua basalis.

Epidemiology

• placenta accreta:

  • the commonest type of placental invasion (~75% of cases)

  • occurs in ~1 in 7000 pregnancies

  • combination of previous C-section and an anterior placenta previa raises the probability of a placenta accreta

  • maternal mortality of up to 7% depending on location

• placenta increta:

  • ~25% of cases

  • occurs in ~1 in 50,000 pregnancies

• placenta percreta:

  • ~5% of cases

The incidence of all forms of abnormal placental villous adherence is increasing, which is felt to be due to the increased practice of cesarean sections

Risk factors 

• prior cesarean section

• placenta previa

• advanced maternal age

• uterine anomalies

• intrauterine adhesion bands

• previous surgery

• multiparity ⁴

Pathology

A defective uterine wall, usually in scar dehiscence, leads to defective decidua and subsequent abnormal implantation of the trophoblasts (rather than trophoblasts having an invasive nature) ⁵.

Placental villous adherence is classified on the basis of the depth of myometrial invasion:

• placenta accreta:

  • mildest form 

  • villi are attached to the myometrium but do not invade the muscle

• placenta increta:

  • intermediate form

  • villi partially invade the myometrium

• placenta percreta:

  • severest form

  • villi penetrate through the entire myometrium or beyond the serosa 

Radiographic features

Imaging features can vary depending on the extent of invasion.

Ultrasound

• may show placenta previa

• may show placental lacunae

  • variably-sized vascular structures in the placenta creating a "moth-eaten" or "Swiss cheese" appearance.

  • can be seen as parallel linear vascular channels extending from placental parenchyma into the myometrium: they tend to show turbulent flow (cf. placental venous lakes show laminar flow)

• abnormal color Doppler

  • turbulent flow, i.e. disruption of the normal continuous color flow in the myometrium

  • increased vascularity is seen in the myometrium, and even in the urinary bladder in cases of placenta percreta

• loss of retroplacental clear space

• reduced myometrial thickness: anterior myometrial thickness <1 mm

MRI

The joint Society of Abdominal Radiology (SAR) and European Society of Urogenital Radiology (ESUR) established guidelines to standardize MRI use in the diagnosis of placenta accreta spectrum disorders that was published in 2020 ⁴.

Patient preparation

• 1.5-3 T scanner, using a surface coil in the supine position

• optimal time for scanning: between 28-32 weeks gestation

• no contrast agent required

• moderately distended urinary bladder

Image acquisition

• mandatory sequences: obtained in axial, coronal and sagittal planes

  • T2: half Fourier single-shot turbo spin echo (HASTE), single-shot fast spin echo (SSFSE), single-shot turbo spin echo (SSH-TSE), or ultra-fast spin echo (UFSE) according to vendor

  • T1 turbo spin echo (TSE) with fat suppression: axial or sagittal plane, for hemorrhage detection

  • axial images are obtained in the oblique plane perpendicular to the placenta–myometrium interface

  • slice thickness ≤4 mm

• optional sequences:

  • steady-state free precession (SSFP): multiplanar acquisition

  • DWI/ADC: using low and high b values

Interpretation

The presence of placenta previa and the location of the placenta should be documented.

Recommended findings suggestive of placenta accreta spectrum disorders, in order of decreased accuracy:

• placental/uterine bulge

  • hour-glass configuration

  • highly associated with placenta increta and percreta

• bladder wall interruption

  • irregularity or tenting of the bladder wall

  • visualization of placental tissue in the bladder lumen

  • visualization of tortuous flow voids traversing space between uterus and bladder (bladder vessel sign)

• placental bed abnormal vascularization

  • aggressive placentation

  • heterogenous vessels invading the myometrium that may extend to the serosa, bladder, or parametrium

• focal exophytic mass

  • denotes placenta percreta

  • can be seen anteriorly toward the bladder or laterally toward the parametrium

• T2 dark bands

  • most sensitive

  • usually linear, in contact with the outer (maternal) surface of the placenta

• loss of retroplacental T2 hypointense interface

  • has high sensitivity (97%) but poor specificity (36%)

• myometrial thinning <1 mm

Uncertain findings suggestive of placenta accreta spectrum disorders:

• placental heterogeneity

• placental asymmetry in shape or thickness

• placental infarction

  • acute: areas of high T2 and low T1

  • chronic: with associated areas of asymmetric placental thinning

• placental abnormal internal vascularity

  • T2 flow voids within the placenta