There is often a delay in the diagnosis of an interstitial pregnancy, as this segment has increased distensibility, leading to less pain. In our patient, the diagnosis of an interstitial ectopic pregnancy was missed on ultrasonography and pelvic CT scan. It was a surprise intra operative finding. It poses a significant diagnostic and s therapeutic challenge and it carries a greater maternal mortality risk than an ampullary ectopic pregnancy. After 2-3 months of amenorrhoea, the vaginal spotting commonly begins. The developing chorionic villi may eventually erode into the blood vessels of the cornu, causing a severe haemorrhage. An interstitial pregnancy occurs at the most richly vascularized site of the female pelvis, the junction of the branches of the uterine and the ovarian arteries. The significant maternal haemorrhage which leads to hypovolaemia and shock, can rapidly result from an interstitial pregnancy rupture [2
]. The Maternal And Child Health Report for 2000-02 in Black pool, Victoria Hospital, UK, stated that 4 out of 11 deaths from ruptured ectopic pregnancies were due to interstitial pregnancies [3
]. The haemorrhage can be severe because an interstitial tubal pregnancy is often more developed than an extra uterine tubal pregnancy. It presents late and the blood supply is large. The clinicians should be aware of the difficulties which are faced in both the clinical and the radiological diagnoses.
The aetiological factors of an interstitial pregnancy are pelvic in- flammatory diseases, fibroids, a previous pelvic surgery and the use of ART procedures [2
]. A high number of transferred embroyos, a transfer near the uterine horn, excessive pressure on the syringe during the transfer or the difficulties which are encountered during the embryo transfer procedure increase the risk [4
]. Bilateral salpingectomy is likely to be another risk factor [5
]. For the nonsalpingectomized patients, the peri and the intra tubal adhesions which are related or not related to endometriosis, are an additional risk factor. Certain authors also consider the quality of the embryos and the hormonal milieu at the moment of the transfer as a possible cause [6
]. With the increasing use of ART procedures, the clinicians should be equipped with skills to diagnose this evasive form of ectopic pregnancies as early as possible. The present maternal mortality rate is 2-2.5 % for all the interstitial ectopic pregnancies.
The diagnosis of an interstitial pregnancy is made by a critical evaluation of all the criteria which are used for other types of tubal pregnancies. The symptoms are acute abdominal pain, intraperitoneal bleeding, a low hematocrit value and a positive serum or urine pregnancy test. The diagnostic tests include the sensitive beta-HCG radioimmunoassay, culdocentesis and ultrasonography. An interstitial pregnancy is diagnosed with the ultrasonographic criteria in the presence of a positive HCG level, which indicates a pregnancy [7
Timor Trisch and colleagues established the transvaginal ultrasound criteria for interstitial pregnancies [8
]. These criteria include a) an empty uterine cavity b) a chorionic sac which is seen separately and which is less than 1 cm from the most lateral edge of the uterine cavity and c) a thick myometrial layer which surrounds the chorionic sac. All these parameters are relatively specific (88%- 93%), but they lack the sensitivity (only about 40%) which is required for the diagnosis of an interstitial pregnancy [2
]. Ackerman et al have described the “interstitial line sign” [9
]. A thin echogenic line extends directly upto the centre of the interstitial gestational sac. This represents either the endometrial cavity or the interstitial portion of the fallopian tube, depending upon the size of the interstitial chorionic sac. A transvaginal, three dimensional ultrasound scanning can reproduce the coronal plane of the uterus and it can facilitate the exact localization of the gestational sac which is relative to the uterine cornu.
The features that are helped with the use of the 3-dimensional Trans Vaginal Ultrasonography (TVS) include a live embryo in a gestational sac which is surrounded by the myometrium. It lies below the cornu and outside the endometrium [10
]. Another diagnostic aid is laparoscopy, which has the advantage of allowing both the diagnosis and the treatment. One study has shown that an early diagnosis of an interstitial pregnancy with TVS allows a first trimester conservative management with methotrexate. However, if the diagnosis is made later in the gestation, as in our patient, a surgical treatment with cornual repair, resection or even hysterectomy may be required [11
The Royal College of Obstetricians and Gynaecologists recommends that the women with tubal pregnancies who are most suitable for the methotrexate therapy are those who have serum beta HCG levels of 3000IU/ml and minimum symptoms. There is some evidence which suggests that the women who present with interstitial pregnancies and beta HCG levels of <5000IU/mL can be treated successfully with a single dose of methotrexate [12