Ultrasound is sufficient for prenatal diagnosis of placenta previa accreta: a systematic review and meta-analysis

Ultrasound is highly sensitive and specific in diagnosing placenta previa accreta, after previous cesarean section when performed by skilled sonologist reports the results of a systematic review and meta-analysis by Jauniaux and Bhide published in July issue of Obstetrics and Gynecology.

Placenta accreta is a potential life-threatening condition and requires multidiscipline involvement for successful management and reducing maternal and neonatal morbidity and mortality. Because of increasing incidence of cesarean sections, the incidence of morbidly adherent placenta is on rise.

The incidence of placenta previa accreta was 4.1% in women with 1 prior cesarean and 13.3% in women with ≥2 previous cesarean deliveries.

Prenatal diagnosis of placenta accreta allows for sufficient time and planning by involving the concerned discipline to be prepared in advance.

The researchers included data from 14 cohort studies with 3889 pregnancies presenting with placenta previa or low-lying placenta and 1 or more prior cesarean deliveries in the quantitative analysis.

There were 328 (8.4%) cases of placenta previa accreta, 90% (298 cases) of which were diagnosed with ultrasound.

Two hundred and eight patients underwent cesarean hysterectomy out of 232 cases (89.7%)

The odds of diagnosing placenta accreta were higher in prospective studies (odds ratios = 228.5) as compared to retrospective studies (odds ratio=80.8). Sonography was more accurate in diagnosing placenta accreta as the depth of villous invasion increased.

The authors stressed the need of developing screening protocols for better management of this increasingly common and life threatening obstetric complication.

Placental Accreta Index (PAI) was recently proposed to predict individual risk for morbidly adherent placenta using 2-D and color Doppler sonographic exam. It includes 5 parameters: 2 or > cesarean delivery, lacunae, myometrial thickness, anterior placenta previa and bridging vessels.

Each parameter was weighted to create a 9-point scale in which a score of 0-9 provided a probability of invasion that ranged from 2–96%, respectively. The probability of invasion increases with increasing PAI score, such that a score of 9 confers a 96% chance of histologic placental invasion.

But, the score needs further validation before it can be used as a standardized method in prediction of placenta accreta, but it definitely improved the  antenatal detection.

Differentiating between intrauterine pregnancy and cesarean scar pregnancy in the early first trimester by Ultrasongraphy.

Cesarean scar pregnancy (CSP) consists of implantation of the gestational sac in the previous hysterotomy scar, with some serious complications as the pregnancy progresses. 

CSP was first described by Larsen and Salomon in 1978. The incidence currently is 1/1800 to 1/2200 pregnancies with 0.15% of women with previous Caesarean sections ending in CSP. But, as the rates of cesearean section rates are increasing world wide, with some countries reporting as high as 50%, the incidence of CSPs is bound to increase!

Risk factors for  CSPs are those  with a history of multiple Caesarean sections and Caesarean breech delivery but scar pregnancies have also been reported after dilation and curettage, myomectomy, metroplasty, hysteroscopy, and manual removal of the placenta.

In fact up to 72% of cesarean scar pregnancies occur in women who have had 2 or more cesarean deliveries.

Transvaginal Sonography and color Doppler are the best diagnostic tools at present, but the clinician should have a high index of suspicion to use them.

According to a paper by Osborn D.A et al the following sonographic criteria have been put forward for early diagnosis of cesarean scar pregnancies in the first trimester.

1. Empty uterus with a clearly visualized endometrium;
2. Empty cervical canal;
3. Gestational sac within the anterior portion of the lower uterine segment at the presumed site of the cesarean scar; and
4. Thinned or absent myometrium between the gestational sac and bladder (<5 mm in two-thirds of cases).
5. Other findings include marked peritrophoblastic color Doppler flow around the sac with low-impedance (pulsatility <1), high-velocity flow (>20 cm/s), a resistive index of less than 0.5, and a peak systolic to diastolic blood flow ratio of less than 3.

MRI being useful as a troubleshooting tool when sonography is equivocal or inconclusive before intervention or therapy.

This paper by  Timor-Tritsch I.E et al in the forthcoming issue of American Journal of Obstetrics & Gynecology describe a sonographic method for the differential diagnosis of CSP vs IUP in early gestation.

The study tests the hypothesis that in a first trimester sonography between 5-10 weeks of pregnancy the relative location of the center of gestational sac to the midpoint of the uterus along a longitudinal line between the external cervical os and the fundus can be used for early detection of CSPs.

This is a retrospective study reviewing the electronic record of 242 ultrasound images from 20004- 2015 were analyzed. A total of 57 cases of CSP diagnosed. The following measurements were made for each case:

1. distance from the external cervical os to the uterine fundus,
2. the midpoint axis of the uterus,
3. the distance from the external cervical os to the center of gestational sacs, and
4. the distance from the external cervical os to the most distant edge of the gestational sacs from the cervix.

It was seen that the location of the center of gestational sac relative to the midpoint axis of the uterus between 5-10 weeks of gestation differentiated between IUP and CSP (mean 17.8 vs –10.6 mm, respectively, P = .0001).

Most CSPs are located proximally to the midpoint axis of the uterus and most IUPs are located distally to the midpoint axis.

The diagnostic accuracy of the method:  accuracy: sensitivity 93.0% and specificity 98.9%.

The authors strongly believe that the mounting anecdotal evidence supports instituting a simple, relatively inexpensive very early transvaginal screening ultrasound of every pregnant patient with a previous history of cesarean delivery.

References:

https://med.stanford.edu/news/all-news/2015/12/optimal-c-section-rate-may-be-as-high-as-19-percent-to-save-lives.html

http://www.jultrasoundmed.org/content/31/9/1449.full

http://www.ajog.org/article/S0002-9378%2816%2900316-1/abstract