Keywords
placenta previa - placenta accreta - ultrasound - magnetic resonance imaging
Palavras-chave
placenta prévia - acretismo placentário - ultrassom - ressonância magnética
Introduction
Placenta accreta is characterized by an anomalous adherence of the placenta to the
uterine wall. Based on the degree of adhesion, placental invasion can be classified
into accreta, increta, or percreta.[1] The global incidence of placenta accreta has been increasing over the years; this
seems parallel to the increased rate of cesarean sections (C-sections). Wu et al[2] reported the global incidence of placenta accreta as 1:533 pregnancies between 1982
and 2002, which is much higher than the incidences of 1:4,027 and 1:2,510 reported
in the 1970s and 1980s.
Ultrasound (US) criteria are used to diagnose placental invasion, namely loss of the
hypoechoic retroplacental myometrial zone, thinning or disruption of the uterine serosa–bladder
interface, presence of exophytic zones and large sonolucent areas in the placenta,
myometrial thickness < 1 mm, and, in Doppler US, turbulent flow of placental lacunae
and bladder–uterine serosa interface hypervascularity.[3]
[4]
[5]
Magnetic resonance imaging (MRI) has been incorporated in the obstetric practice for
some cases of fetal, maternal, and placental assessment in which soft tissues are
clearly visible, thus enabling the assessment of the retroperitoneal space. The MRI
does not seem to increase the possibility of diagnosing anterior placenta accreta;
therefore, it is indicated for cases of posterior placenta, for which visualization
by US is difficult owing to fetal parts.[6]
Few studies compared the accuracy of the MRI and US in the antenatal diagnosis of
placental accretism, and similar prediction results have been found for both methods.[7]
[8]
[9]
[10] Satija et al[8] concluded that the US remains the primary modality for the evaluation of placental
accretism, and the MRI should be reserved for inconclusive cases. Rezk and Shawky[9] compared Doppler US and MRI in patients with placenta previa and uterine scarring.
Even though these authors did not make separate analyses for the placement of the
placenta, they concluded that the MRI should be reserved to exclude accretion in cases
of posterior or lateral placental location.[9]
The objective of the present study was to assess and compare the sensitivity and specificity
of the US and MRI in the diagnosis of placenta accreta in patients with placenta previa.
Methods
The present study is a retrospective cohort study of diagnostic assessment that was
documented using data obtained from the records of patients hospitalized at the Department
of Fetal Medicine, Maternidade Escola Assis Chateaubriand, Universidade Federal do
Ceará, Fortaleza, Brazil, between January 2013 and October 2015. During this period,
there was a total of 13,226 births; 7,009 (53.0%) of these were C-sections, and 6,217
(47%) were live births. The study protocol was analyzed and approved by the Ethics
in Research Committee of Universidade Federal do Ceará (#1.471.764), with the need
for informed consent waived owing to the study's retrospective nature.
The initial sample was composed of all 41 patients admitted to the institution during
the study period with ultrasonographic diagnoses of placenta previa who had been examined
due to complaints of transvaginal bleeding. All of those patients were submitted to
at least one US examination, and, if possible, a single MRI examination. Four patients
were excluded due to incomplete information. Data from the last US and MRI examinations
performed on the last week before delivery were considered for the statistical analysis.
The US diagnosis of placenta previa was considered when the placental mass reached
the internal cervical os. Cases of low placental insertion were not included in this
series. There were 21 cases (56.8%) of total placenta previa, 12 (32.4%) of partial
placenta previa, and 4 (10.8%) of marginal placenta previa. For the diagnosis of placenta
accreta using Doppler US, we used at least 1 of the following criteria: 1) loss of
the hypoechoic retroplacental myometrial zone; 2) disruption of the uterine serosa–bladder
interface; 3) presence of exophytic zones; 4) large sonolucent areas in the placenta;
5) myometrial thickness < 1 mm; 6) turbulent blood flow in the placental lacuna; and
7) hypervascularity of theuterine serosa–bladder interface.[3]
[4]
[5] For the diagnosis of placenta accreta by MRI, we used the following criteria: 1)
presence of uterine bulging; 2) heterogeneous signal intensity within the placenta;
3) dark intraplacental bands on T2-weighted sequences,; 4) abnormal placental vascularity;
5) focal interruptions in the myometrial wall; 6) tenting of the bladder; and 7) direct
visualization of the invasion of adjacent organs.[6]
The variables analyzed were: age; parity; number of C-sections; number of abortions;
number of curettage procedures; type of delivery; and the need for hysterectomy. The
placenta-related variables analyzed were: location of the placenta; and extent of
placenta adherence. The newborn-related variables analyzed were: gestational age;
birth weight; 1- and 5-minute Apgar scores; neonatal complications; and deaths. Histopathology
was considered the gold standard for the diagnosis of placenta accreta; the description
of the intraoperative findings was used in its absence.
Continuous variables were expressed as means and standard deviations, whereas nominal
variables were expressed as absolute frequencies and percentages. The associations
among the variables were investigated using the Pearson chi-squared test and the Mann-Whitney
U-test. The Statistical Package for the Social Sciences (SPSS, IBM Corp., Armonk, NY,
US) software, version 22.0, was used for all statistical analyses. Values of p < 0.05 were considered statistically significant.
Results
Out of the 37 patients included in the study, 16 (43.2%) had placenta accreta, whereas
21 (56.8%) did not. One case (3%) progressed to vaginal birth, and 36 (97%), to C-section.
Out of the 36 patients who had a C-section, 16 (44.4%) underwent concurrent hysterectomy,
and 14 (87.5%) of these had placenta accreta confirmed by histopathology. Out of the
20 (55.6%) patients who had C-sections but no hysterectomy, 2 (10%) had placenta accreta
confirmed by histopathology of the uterus segment that was resected or of the curettage
material in the adhered segment.
Out of the 16 women with placenta accreta, 11 (68.8%) had anterior placentas, and
5 (31.2%) had posterior placentas. Regarding the degree of placental invasion, 9 (56.3%)
women had placenta accreta, 6 (37.5%) had placenta increta, and 1 (6.2%) had placenta
percreta. The mean age was 31.8 ± 7.3 years, the mean number of pregnancies was 2.8 ± 1.1,
and the mean number of deliveries was 1.4 ± 0.7. The mean number of abortions was
0.3 ± 0.6, the mean number of curettage procedures was 0.4 ± 0.7, and the mean number
of previous C-sections was 1.2 ± 0.8.
As for women without a diagnosis of placenta accreta, the mean age was 31 ± 6.8 years,
the mean number of pregnancies was 3.5 ± 2.2, and the mean number of deliveries was
2 ± 2. The mean number of abortions was 0.5 ± 0.7, the mean number of curettage procedures
was 0.5 ± 0.7, and the mean number of previous C-sections was 0.5 ± 0.7 ([Table 1]).
Table 1
Clinical and obstetric characteristics of the patients diagnosed with placenta previa
with and without placenta accreta
|
Variable
|
With placenta accreta
n = 16
|
Without placenta accreta
n = 21
|
p-value*
|
|
Age (mean ± SD), years
|
31.8 ± 7.3
|
31 ± 6.8
|
0.774*
|
|
Pregnancies (mean ± SD)
|
2.8 ± 1.1
|
3.5 ± 2.2
|
0.387*
|
|
Deliveries (mean ± SD)
|
1.4 ± 0.7
|
2 ± 2
|
0.715*
|
|
Abortions (mean ± SD)
|
0.3 ± 0.6
|
0.5 ± 0.7
|
0.617*
|
|
Curettage procedures (mean ± SD)
|
0.3 ± 0.6
|
0.5 ± 0.7
|
0.617*
|
|
Previous cesarean sections (mean ± SD)
|
1.2 ± 0.8
|
0.5 ± 0.7
|
0.008*
|
|
No
|
2 (13.3)
|
13 (86.7)
|
< 0.001#
|
|
Yes
|
14 (63.6)
|
8 (36.4)
|
|
|
1
|
10 (62.5)
|
6 (37.5)
|
|
|
2
|
3 (60)
|
2 (40)
|
|
|
3
|
1 (100)
|
0
|
|
Abbreviation: SD, standard deviation.
Notes: *Mann-Whitney U-test; #chi-squared test.
The mean gestational age at birth for women with placenta accreta was 35.4 ± 1.1 weeks.
There was no need for maternal blood transfusion in any of the cases without placenta
accreta; however, blood transfusion only occurred in one case of placenta accreta.
This patient showed bladder invasion with resection of a bladder wall segment. There
was no pelvic organ injury in any other case. The mean weight of the newborns was
2,635.9 ± 374.1 g, and the mean 1- and 5-minute Apgar scores were 7.9 ± 0.6 and 8.4 ± 0.6
respectively. There were four complications in the newborns in this group. Among the
patients without placenta accreta, the mean gestational age was 35.6 ± 2.2 weeks,
the mean birth weight was 2,486.9 ± 559.6 g, and the mean 1- and 5-minute Apgar scores
were 6.7 ± 2.6 and 8.4 ± 0.9 respectively. There was one death and complications in
nine newborns in this group ([Table 2]). All neonatal complications were infection or jaundice. A total of two neonates
in the accretism group, as well as two neonates in the group without accretism, needed
blood transfusions.
Table 2
Maternal and perinatal results of patients diagnosed with placenta previa with and
without placenta accreta
|
Variable
|
With placenta accreta
n = 16
|
Without placenta accreta
n = 21
|
p-value
|
|
Gestational age at birth (mean ± SD), weeks
|
35.4 ± 1.1
|
35.6 ± 2.2
|
0.751*
|
|
Hysterectomy
|
|
No
|
2 (9.5)
|
19 (90.5)
|
< 0.001#
|
|
Yes
|
14 (87.5)
|
2 (12.5)
|
|
Maternal hospital stay after delivery (days)
|
3.4 ± 1.9
|
2.9 ± 2.2
|
0.735*
|
|
Birth weight (mean ± SD), grams
|
2,635.9 ± 374.1
|
2,486.9 ± 559.6
|
0.596*
|
|
1-minute Apgar score
|
7.9 ± 0.6
|
6.7 ± 2.6
|
0.280*
|
|
5-minute Apgar score
|
8.4 ± 0.6
|
8.4 ± 0.9
|
0.961*
|
|
Neonatal complications
|
4
|
9
|
0.123#
|
|
Neonatal hospital stay (days)
|
7.8 ± 3.1
|
8.2 ± 3.9
|
0.639*
|
|
Perinatal death
|
0
|
1
|
0.245#
|
Abbreviations: SD, standard deviation.
Notes: *Mann-Whitney U-test; #chi-squared test.
Among the 37 (100%) women who underwent the US, the prevalence of placenta accreta
was 20 (54.2%) (95% confidence interval [95%CI]: 40.8–67.3), and among the 21 (56.7%)
who underwent the MRI, it was 66.7% (95%CI: 43.0–85.4). The measurements of the accuracy
of the US and MRI examinations in the diagnosis of placenta accreta are presented
in [Table 3].
Table 3
Measurements of the accuracy of ultrasound and magnetic resonance imaging in the diagnosis
of placenta accreta in pregnancies with placenta previa
|
Measurement
|
Ultrasound
n = 37
|
Magnetic resonance imaging
n = 21
|
|
Sensitivity (95%CI)
|
87.5 (71–96.5)
|
92.9 (66–99.8)
|
|
Specificity (95%CI)
|
44.4 (25.5–64.7)
|
42.9 (9.9–81.6)
|
|
Positive predictive value (95%CI)
|
65.1 (49.1–78.9)
|
76.5 (50.1–93.2)
|
|
Negative predictive value (95%CI)
|
75 (47.6–92.7)
|
75 (19.4–99.4)
|
|
Positive likelihood ratio (95%CI)
|
1.57 (1.10–2.26)
|
1.63 (0.84–3.10)
|
|
Negative likelihood ratio (95%CI)
|
0.28 (0.10–0.77)
|
0.17 (0.02–1.33)
|
Abbreviation: 95%CI, 95% confidence interval.
There was a prevalence of accretism in anterior versus posterior placentas according
to the US exams (41.7%; 95% CI: 22.11–63.36 versus 46.2%; 95%CI: 19.22–74.87). The
measurements of the diagnostic accuracy of the US and MRI at insertions of placenta
are presented in [Tables 4] and [5].
Table 4
Measurements of the accuracy of ultrasound and magnetic resonance imaging in the diagnosis
of anterior placenta accreta in pregnancies with placenta previa
|
Measurement
|
Ultrasound
n = 24
|
Magnetic resonance imaging
n = 13
|
|
Sensitivity (95%CI)
|
100 (69.2–100)
|
100 (66.4–100)
|
|
Specificity (95%CI)
|
71.4 (41.9–91.6)
|
33.3 (4.3–77.7)
|
|
Positive predictive value (95%CI)
|
71.4 (41.9–91.6)
|
69.2 (38.5–90.9)
|
|
Negative predictive value (95%CI)
|
100 (69.2–100)
|
100 (15.8–100)
|
|
Positive likelihood ratio (95%CI)
|
3.5 (1.53–8.0)
|
1.5 (0.85–2.64)
|
|
Negative likelihood ratio (95%CI)
|
−
|
−
|
Abbreviation: 95%CI, 95% confidence interval.
Table 5
Measurements of the accuracy of ultrasound and magnetic resonance imaging in the diagnosis
of posterior placenta accreta in pregnancies with placenta previa
|
Measurement
|
Ultrasound
n = 13
|
Magnetic resonance imaging
n = 8
|
|
Sensitivity (95%CI)
|
66.67 (22.3–95.7)
|
80 (28.4–99.5)
|
|
Specificity (95%CI)
|
100 (59.0–100)
|
100 (2.5–100)
|
|
Positive predictive value (95%CI)
|
100 (39.7–100)
|
100 (39.7–100)
|
|
Negative predictive value (95%CI)
|
77.8 (39.9–97.2)
|
50 (1.3–98.7)
|
|
Positive likelihood ratio (95%CI)
|
−
|
−
|
|
Negative likelihood ratio (95%CI)
|
0.33 (0.11–1.03)
|
0.20 (0.03–1.15)
|
Abbreviation: 95%CI, 95% confidence interval.
Discussion
Placenta accreta is an obstetric condition; it can be fatal, and often requires a
multidisciplinary approach.[11] At our department, 100% of the patients with placenta accreta underwent C-sections,
whereas 14 (87.5%) underwent total abdominal hysterectomy. In 2 (10%) cases, a point
of cleavage was observed, and the placenta could be removed without the need for hysterectomy.
This approach contributed to a very favorable outcome, that is, the absence of maternal
mortality during the study period, and there was only one case of neonatal death.
A total of 11 (68.8%) out of the 16 patients with placenta accreta had anterior placenta,
and 5 (31.2%) had posterior placenta. This finding corroborates with those of other
studies, which reported a higher incidence of anterior placenta in confirmed cases
of placenta accreta.[12]
The mean maternal age of the patients with placenta previa accreta was 31.8 years;
this finding corroborates those found in the literature,[13]
[14] which show means of 31.6 and 32 years. Maternal age and number of children are also
known to be associated with a high risk of placenta previa.[15] The number of previous C-sections was higher in patients with placenta accreta,
thus suggesting a strong relationship between prior uterine scars and the risk of
invasion, a finding confirmed by other authors as well.[15] However, this was not observed in relation to the mean number of pregnancies, deliveries,
and abortions between the two groups. Our results do not corroborate the risk association
reported in another study.[12] Thus, because there was no significant statistical difference between the two groups,
our study contradicts aspects that have already been reported in the reviewed literature.
No statistically significant differences were observed in the comparison of newborn
parameters between the two groups (with and without placenta accreta); According to
our current institutional protocol, when placenta accreta is diagnosed by imaging
during antenatal care, the proposed surgical procedure is the classic uterine incision
with fetal extraction followed by hysterectomy without attempting to remove the placenta
from the uterine wall. Previously, each surgical approach was decided by the obstetrical
team at the time of the C-section. In most cases, Pfannenstiel incisions in the skin,
transverse incisions in the uterus, and attempts to remove the placenta were made.
After this change in protocol, in addition to better neonatal well-being assessed
by higher Apgar scores, we observed a marked decrease in the need for blood products
for both the mother and the newborn.[16]
The consequences of a late diagnosis of placenta accreta can be severe; this emphasizes
the importance of prior detection during antenatal care. The first and most crucial
step must be early investigation, by asking women about their previous uterine surgeries
and other possible factors related to placenta accreta, such as endometrial ablation
or the use of assisted reproduction techniques.[17] Women with myometrial damage primarily caused by previous C-sections are at an increased
risk of developing placenta accreta.[11]
Antenatal diagnosis of placenta previa, placenta accreta, and its variants may reduce
maternal and fetal morbidity and mortality, thus enabling the resolution of the pregnancy
to be scheduled at tertiary institutions with multidisciplinary teams, neonatal intensive
care units, and blood banks, among other resources available at the time of surgery.
These measures can prove effective only if these conditions are created in advance,
and if the correct diagnosis is made.[12]
The US remains the main imaging modality to screen for abnormal placental implantation;
however, the MRI is also a very useful complementary imaging resource in cases of
inconclusive US or posterior placenta.[18] The MRI is clearly indicated when the US results in ambiguous conclusions.[19] The accuracy of the US in the diagnosis of placenta accreta may be biased, as stated
in a study[20] in which a significant increase in the diagnostic accuracy was observed once the
risk factors became known during the examination. In the same study,[20] when a diverse group of patients with unknown obstetric history was assessed, the
diagnostic performance characteristics of the US were shown to be less accurate.
A meta-analysis[21] failed to show any significant difference between the US and the MRI for the diagnosis
of placenta accreta. Both methods are highly specific and sensitive in diagnosing
or ruling out the presence of placenta accreta, with the US being the first choice
in patients with limited time and lower income.[21] Another study[22] also affirmed the good sensitivity of the US, but it showed that, although the MRI
was not as useful as initially expected, it still provided additional information
for women at risk. Another study[23] has indicated the MRI as an excellent method for placenta assessment, particularly
to investigate findings related to placental diseases, thus contributing to the adequate
and timely care of pregnant women.[23]
Several examiners performed the US and MRI in the present study. The radiologists
performing the MRI were blinded to the US results, except for the fact that the patients
were diagnosed with placenta previa, and that the objective was to assess the presence
and degree of placental invasion, which was one of the strengths of the study. However,
during the US examinations, which were usually serial, the radiologists often became
aware of the MRI results; this can be considered a limitation of the US assessment.
This fact may not be relevant because almost 50% of the patients did not undergo an
MRI; therefore, the examiners were blinded to the results.
The patient sampling started at the moment the MRI device was installed. During this
period, the radiologists wished to evaluate all cases of placenta previa without prior
knowledge of the US result in the presence or absence of placenta accreta, in an attempt
to evaluate the previous experience of the service in identifying this finding. There
were requests for MRI examinations; however, they were not performed in all cases
due to the occasional technical unavailability of this examination in the institution.
The calculation of positive and negative predictive values depends on the prevalence
of the disease; therefore, the prevalence for each diagnostic tool in the calculation
of these measurements was taken into account. The prevalence of placenta accreta was
identified separately for each diagnostic tool. The prevalence was higher using the
MRI, but not due to the selection of cases. There were attempts to perform MRIs in
all cases, but some patients were unable to undergo this examination.
The comparison of sensitivity and specificity between the US and the MRI in the diagnosis
of placenta accreta indicated that the US had a good sensitivity, thus confirming
the data from the literature. This eliminates the need for MRI in most cases, a factor
that is particularly important in low-income countries with limited access to this
examination.[18]
Conclusion
In summary, regardless of the location of the placenta, the US and the MRI had similar
sensitivity and specificity for the diagnosis of placenta accreta.
