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MRI in the differential diagnosis of angular pregnancy and interstitial pregnancy during the first trimester1Beijing Friendship Hospital Affiliated to Capital Medical University, Beijing, China, 2School of Clinical Medicine, Jining Medical University, Jining, China, 3Jining No. 1 Peoples’ Hospital, Jining, China
Synopsis
The study evaluated the accuracy of MRI in the diagnosis of 22 angular pregnancy cases and 37 interstitial pregnancy cases retrospectively during the first trimester. Two senior obstetrics radiologists reviewed the MR images and analyzed several features. The study found two key features were useful to diagnose angular pregnancy, namely, “medial free edge” and “medial free edge plus above-cutoff endometrial thickness”. One key feature to diagnose interstitial pregnancy was “intact lateral junctional zone”. MRI is a novel imaging approach to make the precise differential diagnosis of angular pregnancy and interstitial pregnancy.
INTRODUCTION
It is challenging to differentiate angular pregnancy from other ectopic pregnancies in the uterotubal junction, among which interstitial pregnancy is most common. 1, 2 It is of great importance to make a precise diagnosis of angular pregnancy and interstitial pregnancy during the first trimester, since a misdiagnosis of these two entities may result in either the termination of a viable pregnancy 2, 3 or life-threatening gynecologic emergencies 4-6.First-trimester ultrasonography is a routine imaging approach for assessing patients with suspected ectopic pregnancies. However, the unusual implantation sites of angular and interstitial pregnancies, which may also coexist with heavy hemorrhage, bowel gas, and ovarian masses, may interfere with the diagnostic accuracy of this operator-dependent imaging modality.7-9 MRI may have unique advantages for the differential diagnosis of angular pregnancy and interstitial pregnancy, which are easily misdiagnosed and confused with each other during the first trimester.3, 4
Using operative and pathological reports as the reference standard, we established a retrospective cohort of patients preoperatively diagnosed with uterotubal junctional pregnancies. We hypothesized that angular pregnancy and interstitial pregnancy can be precisely differentiated by MRI.
METHODS
This retrospective study involved 59 patients (age: 23 - 44 years; gestational age: 6 - 10 weeks) with a preoperative imaging diagnosis of uterotubal junctional pregnancy. Using operative and pathological reports as the reference standard, 22 patients were diagnosed with angular pregnancy and 37 patients were diagnosed with interstitial pregnancy (Fig 1). Two senior obstetrics radiologists, who were blinded to the patients’ information, reviewed the MR images. Any disagreement was resolved by discussion to achieve a consensus. The sensitivity and specificity of each MRI finding were calculated in accordance with the reference standard.RESULTS
The endometrial thickness in the angular pregnancy groups was significantly larger than in the interstitial group (p = 0.001). The cutoff value of the endometrial thickness was 11.5 mm with a sensitivity, specificity, Youden’s index, and area under the curve that were 77.3%, 64.9%, 42.4%, and 0.743, respectively (Fig 2). Two key features were useful to diagnose angular pregnancy (Fig 3), namely, “medial free edge” and “medial free edge plus above-cutoff endometrial thickness”. The sensitivity and specificity of the medial free edge were 100% and 94.9%, respectively. The sensitivity and specificity of the medial free edge plus above-cutoff endometrial thickness were 77.3% and 100%, respectively. The key feature to diagnose interstitial pregnancy (Fig 4) was an “intact lateral junctional zone”, of which the sensitivity and specificity were 94.6% and 100%, respectively. Two patients with interstitial pregnancy were false-negative for this sign because the gestational sac in both patients protruded into the uterine cavity, leading to an interruption of the junctional zone ( Fig. 5).DISCUSSION
The medial free edge was a novel MRI feature to diagnose angular pregnancy. Similar to the medial free edge we demonstrated, Bradly et.al. 10 and Grant et.al. 3 identified “double sac” and “surrounding endometrium” by ultrasonography. The MRI and hysteroscopic results in our study, together with the previous ultrasonographic findings 3,10, indicate that the medial free edge is a highly sensitive feature for the diagnosis of angular pregnancy.Endometrial thickness is an independent risk factor that has been used to predict the occurrence of ectopic pregnancy.11-14 Similar to findings in previous publications 11-13,15, the endometrial thickness in the angular pregnancy group was significantly higher than in the interstitial pregnancy group, in our study. After combining the above-cutoff (11.5 mm) endometrial thickness and medial free edge, the specificity was increased to 100% since the two patients who were false positive for the medial free edge sign had an endometrial thickness that was less than 11.5mm. Given that the endometrial thickness is proportional to the pregnancy viability 12, 16 and a medial free edge indicates the eccentric, angular implantation of a gestational sac, the extremely high specificity of the combination of these two signs suggests the high concordance between MRI features and the biological characteristics of angular pregnancy, which is a potentially viable intracavitary pregnancy implanting in the uterine cornua.
While the sign of “ intact lateral junctional zone” has been previously reported 17,18,19, we further confirmed this sign not only by introducing a solid reference standard and using a larger sample size, but also by emphasizing the anatomical location that is between the uterine cavity and the external tangent gestational sac. The false-negative rate for this sign was 5.4% (2/37) since the two false-negative patients showed gestational sacs that protruded into the uterine cavity, resulting in an interrupted junctional zone. Not surprisingly, the same patients were also false positive for the sign of medial free edge.
The presence of flow-void in MRI suggests enhanced blood flow through dilated blood vessels 20,. Thus, the sign of enhanced ipsilateral flow-void in the patients with interstitial pregnancy might be owing to the ectopically implanted gestational sac that facilitates the proliferation of ipsilateral blood vessels.
CONCLUSION
MRI is a novel imaging approach that can be used to make the precise differential diagnosis of angular pregnancy and interstitial pregnancy during the first trimester.Acknowledgements
We thank the obstetrics and gynecology team of Jining No.1 People’s Hospital for collecting and providing the hysteroscopic images; and Jane Charbonneau, DVM, from Liwen Bianji (Edanz) (www.liwenbianji.cn/) for editing the English text of a draft of this manuscript.
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