Placenta Accreta

Etiology

Placenta accreta spectrum (PAS) disorders are most commonly associated with a history of a previous cesarean section. This is likely due to the abnormal placentation secondary to the loss of decidua in the cesarean section scar. However, other risk factors are associated with placenta accreta, including advanced maternal age and multiparity. Placenta previa is present in approximately 80% of placenta accreta cases. Placenta accreta has also been linked to other types of uterine surgery, such as myomectomy, uterine curettage, hysteroscopic surgery, prior endometrial ablation, uterine embolization, and pelvic irradiation.[1]

Epidemiology

The incidence of placenta accreta has increased from 1 in 30,000 pregnancies in the 1960s to 1 in 533 pregnancies in the 2000s.[4] One study quotes the current incidence as high as 1 in 272.[5] As previously noted, prior cesarean delivery is a risk factor for placenta accreta, so the rise in placenta accreta incidence over the past decades reflects the rise in cesarean deliveries. Furthermore, it has been established that an increased number of prior cesarean sections increases the risk of placenta accreta. Approximately 6.7% of patients with 5 prior c-sections were noted to have placenta accreta, compared to 0.3% of patients with 1 prior c-section.[6]

Pathophysiology

In typical placentation, trophoblast invasion stops at the spongiosus layer of the decidua. There are many theories as to why placenta accreta may occur. One leading theory is that in patients with prior uterine surgeries, the spongiosus layer of the decidualized endometrium may not be present. Therefore, the typical stop signal is absent. Furthermore, cytotrophoblasts must also reach the spiral arterioles before differentiation into placenta tissue may occur. However, uterine scars have a relative lack of vasculature. It is important to note that, although rare, placenta accreta can occur in nulliparous women and women without prior uterine surgery.[2]

Histopathology

The histology of PAS disorders reveals placental invasion into the uterine myometrium. In the case of placenta percreta, it may also invade the serosa or other organs. There is also an increased rate of trophoblastic inclusions compared to normal placentation. These inclusions are characterized by an inner layer of syncytiotrophoblasts contained within an outer layer of cytotrophoblasts. They are most commonly associated with molar pregnancies and chromosomal aneuploidies. However, 1 group documented their presence in 40% of placenta accreta specimens, compared to 2.4% of controls.[7]

History and Physical

At the initial obstetric visit, it is important to identify risk factors for placenta accreta, including parity and prior uterine surgeries. Rarely do patients have urinary and bowel symptoms in placental percreta involving those organs. Typically, diagnosis is reliant on imaging.

Evaluation

Antenatal diagnosis of placenta accreta is typically made by ultrasound. Ultrasound reveals placenta previa. Furthermore, it is possible to visualize other abnormalities, including loss of hypoechoic division between the placenta and myometrium, increased vasculature, myometrial thinning, and extension of the placenta into serosa or bladder.[2] 

Color Doppler may reveal lacunae with turbulent blood flow. Notably, the sensitivity and specificity of ultrasonography appear to be highly variable. Individual studies of ultrasound diagnosis of accreta report a large range of sensitivities and specificities.[8][9][10] However, a systemic review and meta-analysis of ultrasound demonstrated a sensitivity of 90.8% and a specificity of 96.9%. Important factors in the diagnosis are the presence of lacunae and the loss of hypoechoic retroplacental space.[11] Additionally, the presence of previa increases the diagnosis rate from 6.9% to 72.3%.[12] The experience of the ultrasonographer and clinician must also be taken into account.[2]

Magnetic resonance imaging (MRI) has also been investigated to diagnose placenta accreta. A systemic review of the MRI-based diagnosis of placenta accreta demonstrated a specificity of 84.0% and a sensitivity of 94.4%.[13] While this may interest some providers at first glance, it is worth noting that selection bias is difficult to avoid in these studies. Patients are typically chosen for MRI when their ultrasound findings are inconclusive. Furthermore, it is important to consider the cost and the lack of availability of MRI. At this time, ultrasound remains the modality of choice for diagnosis.[2]

Treatment / Management

Placenta accreta spectrum is best managed when it has been diagnosed antenatally. Many steps can be undertaken to minimize risks. The American College of Obstetricians and Gynecologists (ACOG) has recommended delivery between 34 0/7 and 35 6/7 weeks of gestation via cesarean hysterectomy to optimize neonatal maturity and minimize the risk of maternal bleeding.[2] Before delivery, there should be a consideration for transfer to a Placenta Accreta Center of Excellence (PACE) or a level 3 or 4 center for delivery. Outcomes have been improved by delivery at these facilities due to the availability of a large, interprofessional team. These teams should include perinatologists, pelvic surgeons, intensivists, general surgeons, urologists, and neonatologists. In patients with bleeding, consideration should be made for an early transfer near an appropriate facility. Furthermore, the patient’s hemoglobin level should be optimized before delivery, and coordination should be made with the blood bank to ensure supplies are ready if a massive transfusion is needed.[2](B3)

Cesarean sections are typically performed in a manner that allows for easy conversion to hysterectomy. This includes dorsal lithotomy positioning and a vertical skin incision. The uterine incision should be made in a manner that would avoid the placenta. After the delivery of the neonate, if the placenta does not deliver spontaneously, the current recommendation is to leave the placenta, close the hysterotomy, and perform a hysterectomy. This approach minimizes the risk of hemorrhage. Often, supracervical hysterectomies are not possible due to bleeding. The surgeon may also need to consider cystotomy to separate the placental tissue.

Close monitoring of hemodynamic status and blood loss should be performed. ACOG recommends monitoring blood loss, hemoglobin, electrolytes, blood gas, and coagulation factors to objectively determine the need for transfusion. Massive transfusion protocols often involve a 1-to-1-to-1 ratio of packed red blood cells, fresh frozen plasma, and platelets. However, patients may also consider autologous donation before the procedure.[2] 1g IV tranexamic acid, an antifibrinolytic therapy, can be considered within the first 3 hours of delivery. It has been shown to reduce maternal death due to hemorrhage without an increase in adverse effects.[14] (A1)

After the procedure, ACOG recommends admission to the intensive care unit to closely monitor for signs of bleeding, hypoperfusion, and fluid overload from resuscitation.[2] Of note, some providers also offer delayed hysterectomy. The placenta is left in situ in this practice, and the hysterectomy is performed later. In the limited number of reported cases, it has been shown to decrease blood loss and the need for transfusion. However, this is still considered to be investigational.[15](B3)

In patients with unknown placenta accreta, there are typically two management routes. First, suppose the accreta is discovered before hysterotomy. In that case, the procedure should be paused until the appropriate help has arrived, blood products have been ordered, and the anesthesia team has been made aware. In the second route, the accreta is discovered when the placenta does not deliver with typical maneuvers. In this scenario, the surgeon should proceed with a hysterectomy, as outlined previously. A transfer should be considered if a hysterectomy is impossible at that facility. The surgeon needs to pack the abdomen and consider transfusion and tranexamic acid.[2](B3)

For those wishing to preserve future fertility, conservative management or expectant management can also be considered. In conservative management, the placenta or uteroplacental tissue is removed without the removal of the uterus. This route is more successful in patients with small defects, which can easily be surgically removed or over-sewn.[2] There are also reports involving the successful use of Bakri balloons.[16] (B2)

Expectant management is an area of investigation for women wishing to preserve fertility but has an extensive accreta. This route involves leaving the placenta in situ, with or without adjunctive methotrexate use. In 1 study, 78% of patients were able to avoid hysterectomy.[17] In some cases, the placenta has been removed using hysteroscopic resection. Regardless of the route of fertility conservation, it is important to counsel the patient extensively on potential risks, including the risk of hemorrhage, hysterectomy, and accreta recurrence (15%-30%) in future pregnancies.[2](B2)

Differential Diagnosis

While they are all on the placenta accreta spectrum, it is essential to distinguish between placenta accreta, increta, and percreta. The involvement of other organs is especially important to ascertain ahead of the planned surgery. Additionally, differentiating between placenta previa and placenta previa with accreta allows for the correct preparation and counseling. As previously mentioned, ultrasound is the imaging modality of choice that is currently recommended. However, MRI may be considered to determine the degree of invasion.[2]

Prognosis

The prognosis is better for patients who have placenta accreta without placenta previa. Placenta accreta with previa has a higher risk of hemorrhage and is more likely to undergo a hysterectomy, both of which contribute to morbidity.[12] Patients who have placenta percreta are at an increased risk of complications compared to placenta accreta and increta. These patients have a statistically significant higher rate of renal tract injuries, intensive care unit (ICU) admissions, and the need for additional blood products.[18]

Complications

The most common maternal complication associated with the placenta accreta spectrum is postpartum hemorrhage. This can be associated with intraoperative hypoperfusion, transfusion, post-resuscitation fluid overload, and disseminated intravascular coagulopathy (DIC). Transfusion was required in 80% of cases, and DIC occurred in 28% of cases in one study.[19] Hemorrhage is best reduced if the placenta is left in situ after fetal delivery, as noted previously; however, it is often not avoidable. As mentioned, patients should be monitored in the ICU following surgery to observe for hemorrhage-related complications carefully.

Another major complication is damage to nearby structures. Inadvertent or intentional cystotomy can occur during the procedure. Most commonly, the placenta is anterior and may invade the bladder. In this case, cystotomy may be necessary to separate the placental tissue. Ureteral injury may also occur due to the technical difficulty of a cesarean hysterectomy. Patients should be adequately counseled on these complications.

As with most obstetrical pathologies, the neonate is also affected. Neonatal morbidity and mortality result from preterm birth. Furthermore, maternal hemorrhage can result in decreased fetal oxygenation.[1]