There are a number of ultrasound findings that have been thought to predict placenta accreta over the years. The most sensitive sign is the presence of linear irregular placental spaces with turbulent flow, usually known as lacunae. The nature of these is not known—sometimes the rate per minute suggests fetal origin, but more often the rate suggests maternal origin.
In a prospective analysis of all patients with previous uterine surgery presenting for obstetrical ultrasound over a 12-year period, most patients who developed placenta accreta had ultrasound findings by the time of their screening or booking exam at 18 weeks.11 The most sensitive marker consisted of vascular lacunae, which are long and thin (Figure 8-5A) echolucent spaces, rather than round (Figure 8-5B), and show blood flow within (Figures 8-6 and 8-7). Placental lacunae or vascular spaces as a finding in placenta accreta have been described in several publications.29,30, and 31 Finburg and Williams graded placentas by the number of abnormal vascular spaces and found that the more lacunae there were, the more likely the patient had placenta accreta. This was recently corroborated by Yang et al who graded 51 at–risk cases using a method similar to Finberg and William's.32 Sensitivity for placenta increta or percreta for grade 2 (4 to 6 irregular lacunae) or above was 100%, specificity 97.2%, positive predictive value (PPV) 93.8%, and negative predictive value 100%. Of their cases requiring cesarean hysterectomy all had lacunae, with 2 at grade 1 and all the rest grades 2 to 4. The more lacunae present, the more frequent and the larger were transfusions. If lacunae were present, 23% developed disseminated intravascular coagulation (DIC) and 46% needed intensive care unit (ICU) admission. Finberg and Williams also showed that placenta accreta can occasionally be present with no lacunae, but when there were 4 or more lacunae, placenta accreta was present in 100% of cases.
A: Vascular lacunae are long and narrow echolucent spaces (arrows). B: Normal placental lakes are round or oval. (arrow).
Placenta percreta. A: Vascular lacunae are seen in the anterior part of the placenta. B: Uterus showing growth of the placenta to the edge of the uterine wall (arrow).
Vascular lacunae have blood flow within them.
Another frequently mentioned sign is thinning or absence of the clear zone between the placenta and myometrium. This clear or echolucent zone between the placenta and uterine wall is thought to represent the decidua parietalis (Figure 8-8). However, it is frequently absent anteriorly in the normal patient,33 although rarely in the fundal or posterior areas (Figure 8-9). Thus, absence of the clear zone in an anterior placenta previa can be normal. Actually, the absence of this clear zone as the only finding is the most frequent cause of false–positive diagnoses.11,29 In one large study the PPV of the clear zone was 6% and its sensitivity was 7%.11
The "clear zone" (arrows) is an echolucent space between the placenta and uterine wall.
A: Absence of the clear zone in placenta accreta. B: Absence of the clear zone in a normal anterior placenta.
The clear zone has not been evaluated in posterior placenta accreta. Since McGahan et al found that the clear space was always seen posteriorly, its absence may be much more significant in a posterior placenta than in an anterior one.
Disruption of the usual white line between the bladder and uterus, termed uterine-bladder serosa, has been cited as a very reliable sign of placenta accreta (Figure 8-10). Its disruption does not automatically mean that the placenta has invaded this wall, but is due to enlarged vessels in these spaces. It is an infrequent sign but has been quite specific for a PAD in 2 large studies.11,29
Bladder wall in placenta accreta (arrows). Note that it is not a continuous white line as was seen in Figure 8-8.
Diagnosis in the Patient Without Uterine Scarring
Placenta percreta can occur rarely in patients who have had no previous surgery. A spontaneous rupture through the fundus due to placenta percreta has been reported.27 All the major papers have evaluated patients who have had cesarean sections, most of whom also have placenta previa. Does the ultrasound appearance of placentas not implanted over a lower uterine segment or a surgical scar differ from that described in most cases of placenta accreta with a placenta previa? There is as yet no available data.
Diagnosis of Placenta Percreta
It appears that the more lacunae that are present the more likely there is placenta increta or percreta. Yang et al found that all patients with a grade 2 or above had a PAD, and all but 1 of these had placenta increta or percreta. Invasion can occur into the cervix and broad ligaments as well. Ultrasound detection of this has not been reported.