Systematic Review
Accreta placentation: a systematic review of prenatal ultrasound imaging and grading of villous invasiveness

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Background

Determining the depth of villous invasiveness before delivery is pivotal in planning individual management of placenta accreta. We have evaluated the value of various ultrasound signs proposed in the international literature for the prenatal diagnosis of accreta placentation and assessment of the depth of villous invasiveness.

Objective

We undertook a PubMed and MEDLINE search of the relevant studies published from the first prenatal ultrasound description of placenta accreta in 1982 through March 30, 2016, using key words “placenta accreta,” “placenta increta,” “placenta percreta,” “abnormally invasive placenta,” “morbidly adherent placenta,” and “placenta adhesive disorder” as related to “sonography,” “ultrasound diagnosis,” “prenatal diagnosis,” “gray-scale imaging,” “3-dimensional ultrasound”, and “color Doppler imaging.”

Study Design

The primary eligibility criteria were articles that correlated prenatal ultrasound imaging with pregnancy outcome. A total of 84 studies, including 31 case reports describing 38 cases of placenta accreta and 53 series describing 1078 cases were analyzed. Placenta accreta was subdivided into placenta creta to describe superficially adherent placentation and placenta increta and placenta percreta to describe invasive placentation.

Results

Of the 53 study series, 23 did not provide data on the depth of villous myometrial invasion on ultrasound imaging or at delivery. Detailed correlations between ultrasound findings and placenta accreta grading were found in 72 cases. A loss of clear zone (62.1%) and the presence of bridging vessels (71.4%) were the most common ultrasound signs in cases of placenta creta. In placenta increta, a loss of clear zone (84.6%) and subplacental hypervascularity (60%) were the most common ultrasound signs, whereas placental lacunae (82.4%) and subplacental hypervascularity (54.5%) were the most common ultrasound signs in placenta percreta. No ultrasound sign or a combination of ultrasound signs were specific of the depth of accreta placentation.

Conclusion

The wide heterogeneity in terminology used to describe the grades of accreta placentation and differences in study design limits the evaluation of the accuracy of ultrasound imaging in the screening and diagnosis of placenta accreta. This review emphasizes the need for further prospective studies using a standardized evidence-based approach including a systematic correlation between ultrasound signs of placenta accreta and detailed clinical and pathologic examinations at delivery.

Introduction

Placenta accreta (PA) is an iatrogenic 20th century disorder of human placentation, characterized by the abnormal attachment or invasion of placental tissue to the underlying uterine musculature.1, 2, 3, 4, 5 PA may have been observed before the 20th century but all epidemiologic studies have shown a direct association between the increase in cesarean delivery (CD) and the increased incidence of PA in subsequent pregnancies.1, 6, 7, 8 PA is not exclusively a consequence of CD and much smaller surgical damage to the integrity of the uterine lining, such as those following “uterine curettage, manual delivery of the placenta, postpartum endometritis and previous hysteroscopic surgery, endometrial resection, and uterine artery embolization (UAE)”, has been associated with PA in subsequent pregnancies.1, 4, 5, 9 The development of PA was also reported in women with no surgical history but presenting with uterine pathology such as bicornuate uterus, adenomyosis, submucous fibroids, and myotonic dystrophy.1, 4, 5 These individual case reports suggest that intramyometrial implantation of villous tissue is not always secondary to uterine surgery and may explain the few rare cases of PA observed before the 20th century.

PA was first defined in 1937 by Irving and Hertig,10 as the “abnormal adherence of the afterbirth in whole or in parts to the underlying uterine wall.” The failure of the placenta to separate normally from the uterus after delivery is typically accompanied by severe postpartum hemorrhage, and attempts to remove a PA typically provoke further major hemorrhage, which is associated with increased maternal morbidity and mortality. Modern pathologists have graded PA into placenta creta (PC) or placenta vera, placenta increta (PI), and placenta percreta (PP) according to the depth of villous invasiveness.4, 5 In PC, the villi adhere to the myometrium with no intermediate decidual layers between the tip of the anchoring villi and the muscular cells but do not invade the myometrium. In PI, the villi penetrate deeply into the myometrium up to the external layer whereas in PP, the invasive villous tissue reaches and/or penetrates through the uterine serosa. The PA spectrum can therefore be subdivided into PC for abnormally adherent placentation and PI and PP for abnormally invasive placentation. Cases of PA are also often subdivided into total, partial, or focal according the amount of placental issue involved. More recently, it has been suggested that cesarean scar pregnancy represents a precursor of one of the different grades of PA.11, 12, 13

Several concepts have been proposed to explain the pathophysiology of PA. The oldest is based on a theoretical primary defect of the biological functions of the trophoblast, leading to excessive adherence or invasion of the myometrium. The other prevailing hypothesis is that of a secondary defect of the endometrial-myometrial interface leading to a failure of normal decidualization in the area of the uterine scar allowing trophoblastic infiltration beyond the superficial myometrium and villous development inside the myometrium.1, 4, 5 Although, the pathogenic mechanisms of the different types of accreta placentation, including scar pregnancies, are similar, the anatomical and clinical consequences vary widely. In placenta vera, the villi simply adhere to the superficial layer of the myometrium whereas in PI and PP the villous tissue invades into and may penetrate through the entire uterine wall thickness and reach the surrounding pelvic tissues and organs.

The worst clinical outcome arises when PA and, in particular the invasive forms of PA i.e. PI or PP are unsuspected at the time of delivery and the surgeon attempts to remove the invasive part of the placenta leading immediately to major hemorrhage and an increasing need for emergency hysterectomy.14, 15, 16 Prenatal diagnosis of PA has therefore become essential for the safe management of this increasingly common obstetric complication.17, 18, 19 However, recent population studies from the United Kingdom and the United States have shown that PA remains undiagnosed before delivery in between half20 and a third21 of cases. Determining the depth of placental invasion is essential for planning of individual management of women diagnosed with PA.

The objective of this review is to evaluate the value of the various ultrasound signs described in the international literature for the prenatal diagnosis of PA in general and for the assessment of the depth of villous invasiveness in the uterine wall in particular.

Section snippets

Information sources and search strategy

We conducted a systematic review of the literature and selected relevant studies published from the first prenatal ultrasound description of PA by Tabsh et al22 in 1982 through March 30, 2016. We undertook a PubMed and MEDLINE search using combinations of key words “placenta accreta,” “placenta creta,” “placenta increta,” “placenta percreta,” “abnormally invasive placenta,” “morbidly adherent placenta,” and “placenta adhesive disorder” as related to “sonography,” “ultrasound diagnosis,”

Case reports characteristics

The case reports included 38 individual cases with prenatal ultrasound findings. PA grading was confirmed clinically or histopathologically as PC in 13 cases, PI in 16 cases, and PP in 9 cases.

A planned CD hysterectomy with or without UAE or UAL was performed in 2 PC, 9 PI, and 7 PP.32, 35, 36, 37, 38, 39, 41, 46, 48, 52, 53 In the other 20 cases conservative management was attempted including focal myometrial resection with or without suture or MTX,29, 31, 34, 43, 46, 50 uterine curettage,55

Main findings

Although some ultrasound signs are more often associated with PA, no ultrasound sign or combination of ultrasound signs are specific of the depth of accreta placentation. The wide heterogeneity in terminology and study design in the published series on the prenatal ultrasound diagnosis of PA could explain the low detection rate during routine ultrasound examination.

Comparison with existing literature

The origin and first use of the terminology “placenta accreta” is unknown. Langhans108 and Hart,109 who first described the

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    The authors report no conflict of interest.

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