placenta accreta
description
placenta accreta refers to the abnormal adherence of the placenta directly to the uterine wall. the decidua basalis that characteristically underlies the placenta is focally or diffusely absent. pathologic studies suggest that there is a progressive loss of decidual elements rather than a primary deficiency. placenta accreta has been documented as early as 12 weeks, menstrual age. placenta increta occurs when the placental villi invade the myometrium. a further extension of the placenta to the serosal surface of the uterus is defined as placenta percreta. the extension of the placenta into the bladder is a classic finding of placenta percreta. placenta accreta occurs more commonly (60%); the remaining cases are evenly divided between placenta increta and placenta percreta. the incidence of placenta accreta, increta, and percreta combined is approximately 0.3 per 1,000 births. predisposing factors for placenta accreta include: prior placenta previa; prior cesarean section, with the prevalence increasing with the number of operative deliveries; prior dilatation and curettage; uterine synechia; submucosal leiomyomas; cornual resection; advanced maternal age; and multiparity. the presence of placenta accreta with placenta previa is between 3% and 9.3%. miller and co-workers have reported that placenta accreta occurred in only 7 of 155,080 (0.005%) women without placenta previa. among women with a placenta previa and a prior cesarean section, the incidence of placenta accreta was 22%, in contrast to a rate of 1 in 68,000 women with neither a placenta previa or a history of a prior cesarean section. in this study 80% of the confirmed cases of placenta accreta occurred in the 0.4% of the population with placenta previa. the likelihood of placenta accreta increases from 0.65% after one cesarean section to 10% after 4 cesarean sections. placenta accreta has been associated with an elevated second trimester maternal serum alphafetoprotein level. the most common complication of placenta accreta is severe and persistent vaginal bleeding. hysterectomy is frequently the only treatment that will prevent maternal exsanguination. cox and colleagues have suggested four possible conservative approaches for the management of placenta percreta: 1) leaving the placenta in-situ; 2) localized resection and uterine repair; 3) over-sewing the uterine defect ; and 4) curettage of the uterine cavity. when the placenta is left in situ, it will gradually resorb. adjuvantive chemotherapy with methotrexate is recommended by some authors to arrest placental development. methotrexate effectively reduces viable trophoblast tissue and is associated with a significant reduction in the b-hcg level. if the placental mass does not decrease in size, re-exploration and total abdominal hysterectomy is best performed 4 weeks postpartum, in order to avoid complications related to possible low-grade disseminated intravascular coagulation. because of the possibility of uncontrollable vaginal hemorrhage, subsequent transcervical removal of the placenta by suction curettage should not be attempted. successful pregnancies have been reported after a placenta accreta has been left in-situ and managed expectantly. the primary neonatal complication associated with placenta accreta is prematurity. however, it is the presence of an associated placenta previa that results in the increased prematurity rate, not the degree of placental invasion into, or through, the myometrium.
diagnosis
the sensitivity of ultrasound in the detection of abnormal placenta adherence varies with the extent of involvement of the uterine wall. the sonologist/sonographer should only realistically expect to diagnosis a diffuse placental accreta; focal areas of placenta accreta may not be appreciated. since the placenta-myometrial interface must be assessed, the sonologist/sonographer is limited to diagnosing anterior placenta accretas or posterior placenta previa/accretas with extensive intervillous flow. an additional sonographic sign of placenta accreta is the presence of extensive lacunar pulsatile and laminar flow in the lower uterine segment. these areas represent areas of intervillous space attenuation; their prominence is attributed to the extension of the histologic changes characteristic of the spiral arteries in normal pregnancy to the deeper myometrial radial arteries. it has been suggested that this finding represents an adaptation to a deficient placental blood supply.
differential diagnosis
placenta accreta is an abnormally adherent placenta. occasionally, a retroplacental hemorrhage will compress the hypoechoic border between the placenta and myometrium. however, as the retroplacental clot ‘matures’, it will become more sonolucent, making the diagnosis of placental abruption easier.
sonographic features
the sonologist/sonographer must first be aware of the predisposing factors that would make the diagnosis more likely.
with respect to placenta accreta, the characteristic hypoechoic boundary between the placenta and myometrium is lost.
this is the most common sonographic sign associated with placenta accreta.
when there is an extension of placental tissue beyond the serosa, a diagnosis of placenta percreta is assured.
secondary hemorrhage may occur into the peritoneal cavity.
color doppler transvaginal sonography may be helpful in diagnosing placenta accreta by identifying the extension of placental lacunae with turbulent flow into the surrounding myometrium.