An interstitial pregnancy is a uterine but ectopic pregnancy; the pregnancy is located outside the uterine cavity in that part of the fallopian tube that penetrates the muscular layer of the uterus. The term cornual pregnancy is sometimes used as a synonym, but remains ambiguous as it is also applied to indicate the presence of a pregnancy located within the cavity in one of the two upper "horns" of a bicornuate uterus. Interstitial pregnancies have a higher mortality than ectopics in general. Anatomy
The part of the Fallopian tube that is located in the uterine wall and connects the remainder of the tube to the endometrial cavity is called its "interstitial" part, hence the term "interstitial pregnancy"; it has a length of 1–2 cm and a width of 0.7 cm. Its borders are the opening (ostium) of the tube to the endometrial cavity within the uterus and, laterally, the visible narrow segment of the tube. The area is well supplied by the Sampson artery which is connected to both the uterine and the ovarian arteries. Surrounded by uterine muscle (myometrium) it can expand significantly when it hosts a pregnancy. Interstitial pregnancies can be confused with angular pregnancies; the latter, however, are located within the endometrial cavity in the corner where the tube connects; typically those pregnancies are viable although a high rate of miscarriage has been reported. A pregnancy located next to the interstitial section laterally is an isthmic tubal pregnancy.
In a 2007 series, 22% of patients presented with rupture and hemorrhagic shock, while a third of the patients were asymptomatic; the remainder had abdominal pain and/or vaginal bleeding. Cases that are not diagnosed until surgery show an asymmetrical bulge in the upper corner of the uterus. Treatment
Choice of treatment is largely dictated by the clinical situation. A ruptured interstitial pregnancy is a medical emergency that requires an immediate surgical intervention either by laparoscopy or laparotomy to stop the bleeding and remove the pregnancy.Surgical methods to remove the pregnancy include cornual evacuation, incision of the cornua with removal of the pregnancy (cornuostomy), resection of the cornual area or a cornual wedge resection, typically combined with an ipsilateral salpingectomy, and hysterectomy. Because of the vascularity of the interstitial region particularly during pregnancy, blood loss during surgery may be substantial. Postoperatively, patients with conservative surgical therapy are at risk for development of a persistent ectopic pregnancy due to the presence of deeply embedded surviving trophoblastic tissue; thus, monitoring of hCG levels is indicated until they become undetectable. In patients with an asymptomatic interstitial pregnancy methotrexate has been successfully used, however, this approach may fail and result in cornual rupture of the pregnancy. Selective uterine artery embolization has been successfully performed to treat interstitial pregnancies. Subsequent pregnancies
Patients with an ectopic pregnancy are generally at higher risk for a recurrence, however, there are no specific data for patients with an interstitial pregnancy.