An interstitial ectopic pregnancy can be confused with an intrauterine pregnancy on both point of care ultrasound and formal radiology department ultrasound. Misdiagnosis as an IUP is dangerous.
From Essentials of Point-Of-Care Ultrasound:
Fortunately, they [interstitial ectopic pregnancies] will betray themselves by having at some point, a very thin layer of myometrium around the gestational sac. This is referred to as the “myometrial mantle.” Your last step before calling something an IUP is therefore to measure the myometrium at its thinnest point. Radiologists and gynecologists generally accept 5 mm as being adequate. If you have any doubts, err on the side of caution and only accept 8 mm. This is invariably safe.
What follows are two resources to help avoid this potentially dangerous confusion.
Basically, if there is any uncertainty whether the pregnancy is a normal intrauterine pregnancy or an interstitial ectopic pregnancy, get a three dimensional ultrasound. If the diagnosis remains unclear, then request an MRI evaluation.
The following is from Challenges in the diagnosis and management of interstitial and cornual ectopic pregnancies from The Middle East Fertility Society Journal via ScienceDirect:
Approximately 2–4% of all ectopic pregnancies are interstitial. Because of the danger of rupture, interstitial ectopic pregnancies can be life-threatening. An interstitial pregnancy may rupture between 8 and 16 weeks. Interstitial ectopic pregnancies can be distinguished from eccentrically-located intrauterine pregnancies with the help of three-dimensional ultrasonography.
The difference between an interstitial ectopic pregnancy and an eccentrically located intrauterine pregnancy can be ambiguous (14). Myometrium around the gestational sac is likely the most useful ultrasonographic feature in addressing the distinction (15). That said, in a subseptate uterus, the eccentric location of the embryo may create difficulty in distinguishing an interstitial pregnancy from a cornual pregnancy (15). In the event that an ultrasound is inconclusive, then magnetic resonance imaging (MRI) can be used. The same criteria should be used for an MRI diagnosis as with an ultrasonographic diagnosis.
While all ectopic pregnancies are associated with a risk of hemorrhage, interstitial pregnancies are associated with the highest risk of massive, uncontrollable bleeding. In contrast to the common clinical notion that rupture occurs only between 12 and 16 weeks, in interstitial pregnancies rupture could happen at any time in early pregnancy. Hence, conservative management of interstitial pregnancies should depend on close ultrasonographic follow-up and clinical acumen. Otherwise, rupture could happen suddenly, as will be discussed in this manuscript.
Significant progress has been made in the diagnosis and management of interstitial and cornual ectopic pregnancies 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. This progress has had three pillars: ultrasonographic diagnosis, successful laparoscopic management, and laparoscopic suturing of the uterine cornua. We present in this manuscript interstitial pregnancies and cornual pregnancies at different stages with their challenges in management and outcome.
Interstitial ectopic pregnancy occurs when the embryo implants in the interstitial or intramural portion of the Fallopian tube (Figure 1.1, Figure 1.2, Figure 1.3, Figure 1.4, Figure 1.5, Figure 1.6, Figure 1.7, Figure 1.8 and Figure 1.9). This portion of the Fallopian tube is tortuous and measures 0.7 mm in diameter and 1–2 cm in length (11). Two to 4% of ectopic pregnancies are interstitial 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 and 24. An interstitial pregnancy is suspected when ultrasonography demonstrates an eccentric implantation of the gestational sac at the superior fundal level of the uterus (Figure 1.6, Figure 1.7, Figure 1.8 and Figure 1.9). Three sonographic criteria can be used for the diagnosis of interstitial pregnancy: (a) Empty uterine cavity; (b) Chorionic sac separated 1 cm from the most lateral edge of the uterine cavity; and (c) Thin myometrial layer surrounding the chorionic sac (13). The interstitial line sign (the echogenic line extending into the upper part of the uterine horn bordering the margins of the intrauterine gestational sac) is also helpful in diagnosing an interstitial pregnancy(13).
Managing an interstitial pregnancy is dependent upon whether the ectopic pregnancy has ruptured and the stability of the patient. If the ectopic pregnancy has ruptured, diagnosis requires laparotomy, and often, hysterectomy (15). Ultrasonography and a high index of suspicion have allowed for early diagnosis and increased success of conservative management for the interstitial ectopic pregnancy.
The following is from the post Cornual (interstitial) Ectopic Pregnancy from Sonoworld:
The overwhelming majority of extrauterine gestations occur in the ampullary portion of the fallopian tube. Ectopic pregnancies that implant in the interstitial (cornual) portion of the fallopian tube represent only 2 to 4.7% of all ectopic gestations. Risk factors include previous salpingectomy and assisted reproduction. Despite their rarity, they pose a significant diagnostic and therapeutic challenge and carry a greater maternal mortality risk (up to 2.2%) then ampullary ectopic pregnancies. Because of myometrial distensibility, cornual pregnancy tend to present relatively late, at 7 to 12 weeks gestation. Significant maternal hemorrhage leading to hypovolemia and shock can result from cornual rupture.
Evaluation with endovaginal sonography allows accurate diagnosis which is critical for management. One study has shown that early diagnosis of cornual pregnancy with EVS allows for first trimester conservative management with methotrexate. If the diagnosis is made later in gestation, however, surgical treatment with cornual resection or even hysterectomy may be required.
The sonographic findings of cornual pregnancy include:
· Absence of a normally positioned intrauterine pregnancy
· Heterogeneous mass in the cornua
· Eccentrically placed gestational sac with myometrial mantle surrounding only portion of the gestational sac. The gestational sac is usually in the lateral portion of the uterus early in gestation but in advanced cornual pregnancies, the gestational sac may be located above the uterine fundus and may be confused with an eccentric intrauterine pregnancy.
· “Interstitial line” sign
