Managing placenta accreta
In the past, surgery was the only option for women with abnormally adherent placentae, but conservative medical management may be an alternative for select patients. Here, the authors review recent trends and describe medical and surgical options.
- Placenta accreta occurs in approximately 1 in 2,500 deliveries.
- Risk factors include placenta previa, Asherman’s syndrome, the existence of a prior hysterotomy scar, and advanced maternal age or parity.
- Almost 50% of all cases of placenta accreta are diagnosed antepartum.
- MRI combined with ultrasound has a sensitivity of 100% in identifying placenta accreta.
- Medical management should be considered only when the patient wishes to preserve her fertility and when no active uterine bleeding is present.
- Gravid hysterectomy has been associated with a mortality rate of 7.4%, with a 90% incidence of transfusion, a 28% incidence of postoperative infection, and a 5% incidence of ureteral injuries or fistula formation.
Placenta accreta is an uncommon but potentially lethal complication of pregnancy. It occurs when the placenta is abnormally adherent to the uterine myometrium as a result of partial or complete absence of the decidua basalis and Nitabuch’s layer. The depth of invasion determines the histologic classification: Placenta accreta indicates direct attachment of the placenta to the myometrium; placenta increta describes placental invasion into the myometrium; and placenta percreta indicates full-thickness compromise of the myometrial layer. Deeper invasion is associated with more serious complications.
Incidence and pathophysiology
The incidence of placenta accreta has increased threefold over the past 20 years. Breen and colleagues reported a rate of 1 in 7,000 deliveries in 1977,1 while a later review suggests an incidence closer to 1 in 2,500 deliveries for the period from January 1985 through December 1994.2
Placenta accreta can develop in any setting in which there is an abnormally thin or denuded decidual layer, allowing easy access to the underlying myometrium by the invading trophoblastic tissue. Risk factors include placenta previa, Asherman’s syndrome, the existence of a prior hysterotomy scar, and advanced maternal age or parity. The major contributor to the rise in the incidence of placenta accreta appears to be a concurrent increase in the rate of cesarean section, which is associated with an increased risk for placenta previa. 3,4
When placenta accreta occurs in the setting of a prior hysterotomy, the placenta is implanted over the uterine scar, where the decidual layer is already thinned. Clark et al reported the association between placenta accreta and prior cesarean section in a retrospective review of over 97,000 deliveries. They discovered a 5% risk of clinically diagnosed placenta accreta with placenta previa alone, but found this risk increased to 24% with a single prior hysterotomy, to 47% with 2 prior hysterotomies, and to 67% with 3 or more (TABLE 1).3 Miller and colleagues recently demonstrated that women with placenta previa have a 9.3% incidence of placenta accreta, compared with a 0.005% incidence in women with normally located placentae.2
Incidence of placenta accreta in women with placenta previa and prior hysterotomy
NUMBER OF HYSTEROTOMIES
3 or more
In the past, diagnosis was typically made clinically, suggested by significant postpartum hemorrhage or a placenta that did not separate easily from its uterine attachment. The result was treatment in an emergent setting at the time of delivery. Today, thanks to a better understanding of risk factors and improved diagnostic testing, nearly half of all cases of placenta accreta are diagnosed antepartum.5 Earlier diagnosis makes it possible for the clinician to prepare in advance for delivery and its potential complications, thus improving the ultimate outcome.
Prenatal diagnosis. The assessment of placental morphology and location is a standard part of the obstetric ultrasound examination, allowing many cases of abnormal placentation to be diagnosed antenatally. Ultrasonographic diagnostic criteria (TABLE 2) for placenta accreta include the following:
- thinning or loss of the hypoechoic retroplacental myometrial zone to less than 2 mm6,7;
- absence of the hypoechoic myometrium in the lower uterine segment between the placenta and bladder6;
- thinning or disruption of the hyperechoic uterine serosa-to-bladder interface6;
- focal exophytic masses or extension of the placenta beyond the myometrial boundaries6,7;and
- • lacunar flow within the placenta with prominent venous lakes.8
While these findings are not definitive, they are highly suggestive of the diagnosis. Most authors agree that ultrasound has a sensitivity and specificity exceeding 85% in the detection of this condition.6,9 Transvaginal studies may be preferable to transabdominal ultrasound for improved resolution. In addition, Doppler velocimetry may allow for better identification of venous lakes and areas of increased vascularity within the myometrium. The sonographic detection rate is reduced when the placenta is located posteriorly.
In cases where ultrasound is equivocal, magnetic resonance imaging (MRI) is a useful adjunct. MRI provides better delineation of tissue planes, including the placenta, myometrium, and vasculature. Kay reported 3 cases where MRI was used to identify placenta previa when ultrasonic findings were equivocal.10 Similarly, Levine et al demonstrated a sensitivity of 100% for the identification of placenta accreta using MRI with ultrasound,9 and Thorp and colleagues demonstrated the efficacy of MRI in delineating bladder involvement in a case of placenta percreta.11 As would be expected, MRI has proved most useful when the placenta is located posteriorly. Besides being safe for both mother and fetus, MRI requires little in the way of preparation. Unfortunately, it lacks portability and is more expensive to perform than ultrasound.
Consider medical management only when no active uterine bleeding is present.
Some established biochemical markers have been applied in novel ways in diagnosing placenta accreta. For example, Zelop et al retrospectively reviewed the cases of 11 women who had undergone cesarean hysterectomy for placenta previa with accreta and compared them to 14 women with placenta previa alone. In 5 of 11 cases, women with accreta had alpha-fetoprotein (AFP) levels greater than 2 multiples of the median (MOM), compared to none in the previa-only group.12 This suggests that abnormal placental attachment results in myometrial invasion with increased diffusion of fetal AFP into the maternal circulation.
Hung and colleagues reviewed over 9,000 deliveries in the Taiwan Down Syndrome Screening Group.13 After other causes of elevated maternal AFP were excluded, regression analysis showed a relative risk of 8.3 for the presence of accreta when AFP levels exceeded 2.5 MOM in the second trimester. Ophir et al reported 2 cases of women with elevated creatine kinase levels as early as 22 weeks’ gestation who subsequently were diagnosed with placenta accreta.14 The investigators theorized that trophoblastic invasion of the myometrium results in muscular damage and elevated serum creatine kinase levels. While more studies are needed, serum markers may exist for the presence of accreta, providing another asset for earlier diagnosis and preparation.
Ultrasound criteria for diagnosis of placenta accreta
Thinning of the hypoechoic retroplacental myometrium to <2 mm
Absence of the hypoechoic myometrium in the lower uterine segment between placenta and bladder
Disruption of the hyperechoic uterine serosa-to-bladder interface
Extension of the placenta beyond the myometrial boundary
Lacunar flow and venous lakes within the placenta
In recent years, reports of select patients undergoing medical management for placenta accreta have begun to appear. Although the number of these patients has been small, with some women ultimately requiring surgical intervention, the vast majority have done well. Even so, medical management should be considered only when the patient wishes to preserve her fertility and when no active uterine bleeding is present. Adequate discussion of the potential risks and benefits also is crucial.
Methotrexate (MTX) is the cornerstone of medical management, although case reports also have described the use of antibiotics, uterotonics, surveillance with ultrasound, and the monitoring of human chorionic gonadotropin (hCG) levels. There is no agreed-upon regimen for the use of MTX or adjunctive therapies such as antibiotics and oxytocin. However, after reviewing the relevant literature, we can suggest some general guidelines.
At the time of delivery, the cord and membranes should be ligated as high as possible. Broad-spectrum antibiotics, for prophylaxis, and oxytocin should be administered during the initial 72 hours. In addition, ultrasound should be performed daily to monitor involution and placental vascularity, which should decrease over time.
If hCG levels plateau, placental vascularity persists, or placental involution stalls after this initial 72-hour period, MTX should be administered (1 mg/kg) on alternate days for a total of 4 to 6 doses. Medical management should be stopped if liver function tests are 2 or more times the normal value or there is evidence of thrombocytopenia (platelet levels below 100,000), neutropenia (white blood cell count below 2,000), or renal dysfunction (creatinine levels greater than 1.5 mg/dL). If the patient becomes clinically unstable or placental tissue fails to resolve following MTX therapy, hysterectomy should be considered.
Expectant management is another valid approach in select cases (TABLE 3). It is more likely to be successful when vascularity is no longer present on ultrasound examination of the placenta. Panoskaltsis and colleagues reported 2 cases of expectant management.15 In 1 case, the placental mass and vascularity regressed spontaneously with time following vaginal delivery, and normal menses resumed at 9 months postpartum. In the second case, MTX was given when the placental mass maintained vascularity on ultrasound exam at postpartum day 12. Ultimately, this mass involuted to a 5-cm mass without vascularity at 1 year. Normal menses resumed, and hCG levels returned to zero. Follow-up in these patients has been short. Fertility has yet to be documented in either patient, although the resumption of menses is an encouraging sign.
When it is successful, medical management has many potential benefits. A woman retains her future fertility and avoids the morbidity and mortality of gravid hysterectomy. Even with antenatal diagnosis of placenta accreta, gravid hysterectomy can result in high-volume blood loss and coagulopathy due to the difficult nature of the procedure.5 Proponents of medical management would further argue that there are few disadvantages to attempting medical management in clinically stable patients, provided follow-up is close. Even when a placental mass fails to resolve or vascularity or vaginal bleeding occurs, an interval of even a few days after delivery may simplify hysterectomy due to uterine involution and a concurrent decrease in vascularity.
Opponents of medical management suggest that it increases the risk of sudden hemorrhage, infection, and/or emergent surgery. While there have been reports of infection, all cases were confined to endometritis and were well controlled with an oral antibiotic regimen. One case report describes a patient given MTX for 6 weeks (50 mg per week). Human chorionic gonadotropin levels decreased, the placental mass was resolving, and there was no evidence of vascularity on ultrasound. However, when a suction dilatation and curettage (D&C) was performed for mild bleeding at 8 weeks postpartum, a massive hemorrhage occurred. Ultimately, the patient required a transfusion of 18 units of packed red blood cells and emergent hysterectomy.16
Opponents of medical management suggest it increases the risk of sudden hemorrhage.
Surgical options for the management of placenta accreta are dictated by the patient’s clinical status, comorbidities, age, and parity, as well as the desire to preserve future fertility. Practitioners should be prepared to manage placenta accreta when suspicious radiologic findings or significant risk factors are present. However, radiologic studies are subject to interpretive errors and definitive diagnosis can be made only at the time of delivery. The physician should lay the groundwork for surgery by counseling the patient extensively regarding possible complications and outcomes.
If hemorrhage occurs, follow a stepwise approach to ensure hemostasis.
Preoperative considerations. The best way to decrease surgical complications is through adequate preparation. To that end, the following steps should be considered when planning an operative delivery for a patient with suspected placenta accreta17:
- Notify anesthesia staff of the potential for a prolonged procedure with significant blood loss.
- Assemble an adequate surgical team, including backup by an experienced gynecologist, gynecologic oncologist, general surgeon, or urologist.
- Notify the blood bank of the potential need for significant blood products in the form of packed cells, clotting factors, and platelets. (Blood should be present in the room at the start of the procedure.)
- Ensure that items such as compression boots, a warming blanket, and a 3-way Foley are available. (The 3-way catheter allows the bladder to be back-filled to check for incidental cystotomy.)
- Consider ureteral stent placement to aid in the identification and protection of ureters if significant dissection is indicated.5
- Consider preoperative placement of angiocatheters for intraoperative embolization of the hypogastric arteries to control operative bleeding.18,19
- If bladder involvement is suspected, preoperative cystoscopy can confirm the diagnosis, allowing mobilization of the urology team.