This 31-year-old woman is due for an elective cesarean section shortly and has had three prior normal pregnancies, also by cesarean section. She has a diagnosis that can sometimes be challenging to make. Can you identify what is indicated by each of the arrows, then put them all together for that diagnosis?
The orange arrow in image 1 demonstrates the placenta covering the cervical os, which is so-called complete placenta previa. Triple red arrows (images 2 and 2A) have been placed, and the intimate inseparable appearance of the placenta from the anterior uterine wall is at least compatible with placenta accreta. The nodular area highlighted by a green arrow (images 2 and 2A, enlarged) is consistent with trans-uterine extension of placental material into the roof of the bladder wall. A differential diagnosis would lie between placenta increta and placenta percreta, with the latter favored.
The red arrows represent high signal with fluid in the collapsed bladder. Placental accreta occurs when all or part of a placenta attaches to the myometrium. The grades of placental attachment are based on depth: Accreta-chorionic villi attach to the myometrium but spare the decidua basalis. Increta-chorionic villi invade into the myometrium. Percreta-chorionic villi invade into and/or through the myometrium or uterine serosa. The only locus where the serosa appears transgressed is along the bladder roof (green arrow, images 2 and 2A).
The major risk for patients with these placental variations is bleeding at the time of attempted vaginal delivery. In this case, placenta previa alone precludes vaginal delivery. This is important to recognize because these patients may require transfusion, and hysterectomy is not infrequently required. An important risk factor for placenta accreta is placenta previa in the presence of a uterine scar. This patient has placenta previa and has had multiple prior C-sections with a uterine scar from those prior C-sections. The approximate locus of that scar is indicated by the pink arrow (image 2).
By itself, placenta previa is an independent risk factor for placental accreta. Other factors include multiparity (this patient has it), advanced maternal age (31 is an intermediate age), prior uterine surgery (this patient has had three C-sections), prior uterine curettage, uterine radiation, endometrial ablation, uterine fibroids, uterine anomalies, scar from prior currettage, Asherman’s syndrome, eclampsia/preeclampsia and smoking.
It is said that people who are pregnant past 35 years of age who have had a cesarean section and now have placenta previa overlying a uterine scar have a 40 percent chance of placental accreta. The diagnosis of placental accreta can be challenging. But when the myometrial thickness is 1 mm, overlying the placenta especially with the scenario described above, one must assume that placental accreta or one of its variations such as increta are present.
Besides bleeding, damage to regional organs such as bowel, bladder or ureters along the pelvic sidewall is a major concern with placenta percreta. Other complications include amniotic fluid embolism, dilutional coagulopathy, consumptive coagulopathy, transfusion-associated complications, ARDS, and electrolyte abnormalities from transfusion. For more case review, visit MRI Online.