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Placenta accreta

Dr Ayush Goel and Radswiki et al.

Placenta accreta (PA) is both the general term applied to the abnormal placental adherence, and also the condition seen at the milder end of the spectrum of abnormal placental adherence. This article focuses on the second, more specific definition.

In a placenta accreta, the placental villi extend beyond the confines of the endometrium and attach to the superficial aspect of the myometrium but without deep invasion.

Epidemiology

It is the most common form of placental invasion (76% of cases). It is thought to occur in approximately 1 in 7,000 pregnancies. The incidence may be increasing due to increased practice of Caesarean sections.

Pathology

The abnormal implantation is thought to result from a deficiency in the decidua basalis, in which the decidua is partially or completely replaced by loose connective tissue. In a placenta accreta, chorionic villi and/or cytotrophoblasts directly attach to the myometrium with little or no intervening decidua. 

Risk factors

Recognised primary risk factors for placenta accreta include: 

Radiographic features

Accurate prenatal diagnosis of placenta accreta is vital because this abnormality is an important cause of significant hemorrhage in the immediate post-delivery period with resultant maternal and fetal morbidity and mortality. However, the diagnosis is not often made prospectively. 

Ultrasound

According to one study 9, ultrasound has sensitivity of 89.5%, positive predictive value of 68% and negative predictive value of 98% for diagnosis of placenta accreta.

Several sonographic criteria for the diagnosis of placenta accreta have been reported. They include: 

  • marked thinning or loss of the retroplacental hypoechoic zone
  • interruption of the hyperechoic border between the uterine serosa and bladder
  • presence of mass-like tissue with echogenicity similar to that of the placenta, and
  • visualization of prominent vessels or lakes within the placenta or myometrium.

When a placenta accreta occurs on the posterior or lateral walls of the uterus, it may be difficult to detect by ultrasound.

MRI 

Magnetic resonance imaging has also been used to diagnose placenta accreta. Specific fast acquisition sequences (i.e HASTE, true FISP) may minimize fetal and maternal motion artifacts.

The demonstration of uterine bulging and loss of normal uterine contour.

  • T2
    • on T2 weighted MR images, the mass is hyperintense and may be heterogeneous.
    • in addition, T2 weighted MR images are useful in the assessment of focal thinning of the myometrium and interruption of the junctional zone.

Complications

Treatment and prognosis

A definitive treatment for placenta accreta consists of a hysterectomy with possible resection of adjacent organs if percreta is present. A placenta accreta is reported to be the most common indication for emergency peripartum hysterectomy.

In certain instances, however, conservative treatment may be used, especially if uterine preservation is desired. Conservative measures include curettage, oversewing of the placental bed, and ligation of the uterine arteries or the anterior divisions of the internal iliac arteries. 


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