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Placenta accreta.

Placenta accreta is an abnormally invasive implantation of the placenta into the uterine wall. It occurs when the chorionic villi invade into the myometrium. This condition affects 1 in 2500 pregnancies. (1) It is associated with a number of risk factors, including advanced maternal age and previous cesarean deliveries. (2) There has been an increase in incidence of placenta accreta, most likely due to recent increases in cesarean deliveries. The deep adherence of the placenta makes it difficult to detach, which puts the mother at high risk for hemorrhage during delivery. For this reason, placenta accreta can result in a hysterectomy. Sonography is the first modality used to image the fetus and placenta; however, MR often is used when placenta accreta is suspected because it can help differentiate the specific type of invasive placental implantation. Prenatal diagnosis can help health care professionals prepare for the expected complications during delivery, thereby reducing morbidity and mortality of the mother.

Discussion

A normal placenta has 2 portions. The maternal portion develops from the decidua basalis, and the fetal portion develops from the chorion frondosum. The fetal tissue combines with the material decidua to form the circulation. The major functioning unit of the placental circulation is the chorionic villi, which is located in the intervillous spaces. Maternal blood enters the intervillous spaces through the spiral arterioles and bathes the chorionic villi. From there the blood enters the umbilical arteries and moves toward the fetus.

Invasive placental implantation occurs when there is a defect or absence of the decidua basalis, which is replaced by loose connective tissue. This allows the chorionic villi of the placenta to directly attach to the myometrium. There are 3 forms of abnormal placental invasion. The mildest and most common form is placenta accreta. (10) Although this type involves contact with the myometrium, it does not penetrate into the muscle itself. This form accounts for 75% to 78% of all cases. (7) Placenta increta is a variant that involves the extension of the chorionic villi into the muscle of the uterine wall and occurs in 17% of cases. (7) The most severe variant, known as placenta percreta, occurs when the placenta penetrates through the entire uterine wall and may attach to other organs, such as the bladder or rectum. It occurs in 5% to 7% of all cases. (7)

The precise etiology of all 3 forms is unknown. However, there are known factors that increase the risk for developing this condition. The greatest risk factors include anything that may cause scarring of the endometrial cavity, such as grand multiparity, previous cesarean delivery, uterine curettage, myomectomy and Asherman syndrome (a disease that causes intrauterine adhesions). (3) Accreta commonly occurs in conjunction with other conditions (ie, placenta previa). Accreta also can occur if the placenta implants over a submucosal fibroid located in the lower uterine segment, near the rudimentary uterine horn or near the uterine cornu. (2) Five percent to 10% of patients with placenta invasion have previa. (5,7) Placenta previa occurs when the placenta implants low in the uterus, partially or completely covering the internal cervical os. Additional risk factors include hypertensive disorders, smoking and a maternal age older than 35 years. (4) Placenta accreta presents with few signs and symptoms prior to delivery. There may be some vaginal bleeding during the third trimester. In addition, abnormal elevations in [beta]-hCG and maternal alpha-fetoprotein may be measured in the second trimester. (6,7)

When placenta accreta is diagnosed, the pregnancy is monitored closely to prepare for the possible complications at the time of delivery. The primary complication is hemorrhage during delivery because the placenta has difficulty separating from the uterine wall. Additional complications include retained waste products and uterine rupture. (2) Uterine rupture is especially common in cases of placenta increta and percreta. Patients may have a hysterectomy after delivery to control bleeding. Some conservative treatments have been reported, including medical management with methotrexate, angiographic-directed embolization and surgical wedge resection. (8) Another conservative approach is a newer operative technique comprised of a vertical incision in the uterus, eversion of the uterus and manual removal of the placenta. (8)

In the past, the maternal mortality rate was 10% to 25% in cases of placenta accreta. (2) It is lower now that there are better methods of detection, including maternal serum alpha-fetoprotein levels, sonography and MR imaging. Adverse effects to the mother are usually limited to a hysterectomy, but death can occur if hemorrhaging is not stopped in a timely manner. Adverse effects to the fetus include a risk of preterm delivery and low birth weight. (8)

Case study

A 31-year-old pregnant woman, third pregnancy with 2 full-term births prior, with a previous diagnosis of complete placenta previa was admitted to the hospital for vaginal bleeding at 30 weeks gestation. She had 2 previous cesarean sections, a sonogram was ordered to assess the placenta. The equipment used was an Acuson Sequoia (Siemans Medical Solutions, Malvern, Penn) and a curvilinear 2-5 MHz transducer. The sonogram documented a 30-week, 2-day pregnancy with placenta previa. (See Fig. 1.) The images also demonstrated placental lacunae, an echo-free space representing venous lakes (See Fig. 2) and turbulent flow at the placental-uterine border. (See Fig. 3.) All of these signs indicated placenta accreta. An MR exam was ordered to differentiate the grade of placenta accreta. An absence of a welldefined placental-myometrial interface was docmented. (See Figs. 4 and 5.) The placenta and/or myometrium were seen abutting the bladder, and left hydronephrosis was present. (See Fig. 6.) Placenta percreta was ruled out, but accreta or increta could not be differentiated and were both noted as differential diagnoses.

[FIGURES 1-6 OMITTED]

The patient remained stable in the hospital for 3 weeks, at which time she began to hemorrhage and was transferred to labor and delivery for a cesarean section. During the removal of the placenta, she developed a massive uterine hemorrhage and a hysterectomy was performed. The left ureter was transected, and a ureteral neocystostomy was performed. Massive blood and fluid resuscitation were performed, and the patient was taken to surgical intensive care. Postoperatively, she remained hemodynamically stable and was released 1 week later.

Imaging Methods and Appearance

The sonographic appearance of placenta accreta is a subtle finding. Close attention should be paid to the area where the placenta attaches to the myometrium. A thinning or absence of retroplacental myometrium is the most common sonographic finding. (2) Obliteration of the echolucent area located between the uterus and placenta occurs. There may be an uneven posterior bladder wall where it is adjacent to the uterus. This appears as a series of dashed or curved lines instead of a continuous echolucent line. (9) With the severe variants, like placenta percreta, placental villous tissue can be seen invading the bladder.

The bladder wall can appear nodular and thickened. (2) Placental lacunae or venous "lakes," which appear as multiple irregular anechoic spaces within the placenta, are often present. This is also referred to as a "swiss cheese" appearance. Color Doppler imaging shows marked turbulent flow between the placenta and myometrium. It can also show irregular blood flow underlying the bladder. (2) The sensitivity rates for visualizing 2 or more criteria have been documented at 80%, with a positive predictive value of 86%. (9)

When placenta previa is seen, placenta accreta should be assessed because they often are associated with each other. Translabial or transvaginal scanning can be helpful in evaluating a patient with suspected placenta previa. (10)

MR is an accurate way to detect and assess placenta accreta when the results of the sonogram are inconclusive. (11) MR shows soft tissue clearly and is especially helpful in distinguishing cases of placenta increta and percreta. (12) It does remain the modality of choice for posterior uterine lesions that can be difficult to see by sonography, particularly in the third trimester. T2-weighted images have been documented as being most helpful because of the tissue contrast between the placental tissue and normal myometrium. (13) On MR images, thinning and irregularity of the myometrium may be seen at the site of placental invasion. The hallmark of placental invasion would show homogenous tissue with a signal different from that of normal myometrium.

Conclusion

Placenta accreta is a rare but serious obstetrical condition. The only well-documented symptom is vaginal bleeding during the third trimester. Imaging modalities used to diagnose placenta accreta include sonography and MR. It is important to diagnose prenatally to prepare for delivery complications such as uterine rupture and severe hemorrhage. Clinicians should be aware of the risk factors and imaging features associated with this condition so they can provide the best care for the patient.

References

(1.) American College of Obstetrics and Gynecologists. Placenta accreta. ACOG Committee Opinion No. 266. Obstet Gynecol. 2002;99:167-170.

(2.) De Lange M, Rouse GA. Ob/Gyn Sonography: An Illustrated Review. Pasadena, Calif: Davies Publishing Inc; 2004.

(3.) Moon MA. OB/GYN News: Ultrasound helps predict placenta accreta. 2004. Available at: www.findarticles. com/p/articles/mi_m0CYD/is_23_39/ai_n8581175/print. Accessed October 10, 2006.

(4.) Usta I, Hobeika E, Musa A, Gabriel G, Nassar A. Placenta previa-accreta: risk factors and complications. American Journal of Obstetrics and Gynecology. 2005;193(3):1045-1049.

(5.) Gielchinsky Y, Mankuta D, Rojansky N, Laufer N, Gielchinsky I, Ezra Y. Perinatal outcome of pregnancies complicated by placenta accreta. Obstetrics and Gynecology. 2004;104(3):527-530.

(6.) Hung T, Shau W, Hsieh C, Chiu T, Hsu J, Hsieh T. Risk factors for placenta accreta. Obstetrics and Gynecology. 1999 ;93(4):545-550.

(7.) Placenta Accreta. Available at: www.en.wikipedia.org/wiki/ Placenta_accreta. Accessed October 10, 2006.

(8.) Nishijima K, Shukunami K, Arikura S, Kotsuji F. An operative technique for conservative management of placenta accreta. Obstetrics and Gynecology. 2005;105(5):1201-1203.

(9.) Comstock C, Love J, Bronsteen R, et al. Sonographic detection of placenta accreta in the second and third trimesters of pregnancy. American Journal of Obstetrics and Gynecology. 2004;190(4):1135-1140.

(10.) Cooney M, Benson C. Placenta Previa and Placenta Accreta. Brigham Radiology. Available at: http://brighamrad.harvard.edu/Cases/bwh/hcache/139/full.html. Accessed October 10, 2006.

(11.) Maldjian C, Adam R, Pelosi M, Pelosi M, Rudelli RD, Maldjian J. MRI appearance of placenta percreta and placenta previa. Obstetrical & Gynecological Survey. 2000;55(3):131-132.

(12.) Taipale P, Orden MR, Berg M, Manninen H, Alafuzoff I. Prenatal diagnosis of placenta accreta and percreta with ultrasonography, color Doppler, and magnetic resonance imaging. Obstet Gynecol. 2004;104(3):537-540.

(13.) Sonin A. Nonoperative treatment of placenta percreta. American Journal of Roentgenology. 2001;177:1301-1303.

Dora DiGiacinto, M.Ed., RDMS, RDCS, is an assistant professor and a clinical coordinator in the radiologic technology department at the University of Oklahoma Health Science Center in Oklahoma City.

Ashley Hildebrand, B.S.R.T., is a diagnostic medical sonographer at St. John Medical Center in Tulsa, Okla.
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Title Annotation:CASE STUDY
Author:DiGiacinto, Dora; Hildebrand, Ashley
Publication:Radiologic Technology
Geographic Code:1USA
Date:Nov 1, 2006
Words:1812
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