Synonyms: An interstitial pregnancy develops in the uterine portion of the falopian tube, a cornual pregnancy develops in the cornu of the uterus, angular pregnancy originates in the interstitial portion of the fallopian tube and grows into the adjacent uterine cavity.

Definition: Pregnancy located in the interstitial portion of the fallopian tube.

Prevalence: 1.4% of all pregnancies are ectopic; 2-4% of ectopic pregnancies are interstitial.

Etiology: Abnormal implantation of a fertilized ovum, in the interstitial portion of the fallopian tube.

Pathogenesis:  Related to abnormal transport of the fertilized ovum within the fallopian tube. This can occur secondary to damage to the fallopian tube following pelvic inflammatory disease or prior conservative surgery for ectopic pregnancy. Hormonal imbalances caused by ovulation induction or exogenous estrogen and progesterone administration can also alter transport mechanisms within the fallopian tube. Cornual pregnancy is associated with bicornuate uterus.

Differential diagnosis: Abdominal pregnancy, other forms of tubal ectopic pregnancy.

Prognosis: The prognosis for the fetus is poor. The maternal mortality rate is 1 per 1000 interstitial pregnancies. This is twice the mortality rate of tubal ectopic pregnancies.

Recurrence risk: Among women who have had an ectopic, the subsequent overall conception rate is approximately 60%. Of these, 10% are repeat ectopic gestations.

Management: Surgical removal of the pregnancy via salpingectomy or salpingostomy. Medical management with methotrexate is used in abdominal pregnancies, but is not employed in ectopic gestations confined to the fallopian tube or uterus.

MESH Pregnancy, -tubal, -ectopic ICD9 633.4 CDC 633.400

Address correspondence to: Robert S. Shapiro, MD, Mount Sinai School of Medicine, Dept. of Radiology, Box 1234, One Gustave L. Levy Place, New York, NY 10029-6574. Ph: 212-241-7401; Fax: 212-427-8137

Introduction

Interstitial pregnancy is an uncommon form of ectopic pregnancy with a disproportionately high incidence of complications. To the best of our knowledge, the case presented in this report is the first description of color Doppler sonography of this entity.

Case report

A 22-year-old G₂P₁ woman presented to her physician for an elective termination of pregnancy. The patient was 12 weeks pregnant by dates and had no prior history of ectopic pregnancy or pelvic inflammatory disease. An ultrasound performed at an outside institution was interpreted as abnormal. A gestational sac was identified outside the normal location in the uterus, but the exact location of the gestation was unclear. The patient was referred to our institution for further diagnostic evaluation and treatment.

An ultrasound exam performed at our institution revealed a gestational sac directly abutting the right side of the uterus..

Color Doppler sonography revealed blood flow associated with the sac, and a spectral tracing revealed a high velocity - low resistance pattern consistent with trophoblastic flow. The crown-rump length was 52 mm, consistent with a gestational age of 12 weeks. Because the gestational sac directly abutted the lateral aspect of the uterus and the pregnancy had progressed to a relatively advanced stage, it was felt that the ectopic gestation was most likely interstitial in location. Because of the advanced stage of the gestation, and since an abdominal pregnancy could not be excluded with certainty, a preoperative angiogram and embolization procedure were performed. The angiogram revealed a vascular extrauterine mass consistent with an ectopic gestation (fig. 1). The embolization procedure was performed to minimize blood loss during the ensuing surgery.

Fig. 1: Angiogram of the pelvis shows the uterus (white arrows) and an extrauterine vascular mass (open arrows), representing an ectopic gestation.

 At surgery, an ectopic pregnancy was identified within an intact fallopian tube. The pregnancy was located in the interstitial portion of the tube, close to the uterine cornu. A right salpingectomy with removal of a cuff of the adjacent uterine cornu was performed. There was minimal blood loss during surgery, and the postoperative course was unremarkable.

Pathological review of the surgical specimen revealed an intact fallopian tube with an attached fragment of myometrial tissue (fig. 2).

Fig. 2: Surgically removed, intact fallopian tube.

 Upon opening the sac, a male fetus was identified. Fragments of myometrium showing an implantation site with chorionic villi were seen. The findings were interpreted as an ectopic pregnancy, interstitial in location, with placental implantation involving the uterine cornu (fig. 3).

Fig. 3: Opened fallopian tube reveals the fetus (cyan arrow), placenta (pink arrow), and a fragment of the uterine cornu (blue arrow).

Discussion

Frequency

An interstitial pregnancy is an uncommon type of ectopic pregnancy, accounting for 2-4% of all ectopic pregnancies^1-3.

Importance

The interstitial portion of the fallopian tube is a highly vascularized, muscular site that offers more support and distensibility to the embryo than any other portion of the fallopian tube. These anatomic features allow the gestation to advance much further into its development than when the embryo implants in other portions of the tube. There are reported cases of rupture at all gestational ages, up to and including full term with surgical delivery⁴. While interstitial pregnancies account for only 2-4% of all ectopic gestations, they cause a disproportionately high incidence of hemoperitoneum and shock, and the mortality rate is approximately twice that of other types of ectopic pregnancies⁴.

Diagnosis

The ultrasound findings of interstitial pregnancy have been described previously^4-6. A gestational sac surrounded by an incomplete or asymmetric uterine myometrial mantle is highly indicative of an interstitial pregnancy. A suggestive but nonspecific sign is an eccentrically located gestational sac. These findings are similar to those in the case described above, in which a gestational sac was identified which directly abutted the uterus. Color Doppler sonography revealed flow associated with the gestational sac. A spectral tracing revealed a high velocity - low resistance pattern, characteristic of trophoblastic blood flow.

The high velocity, low impedance flow around gestations results from the hemodynamics of early placentation⁷. By the third gestational week, the circulation between the placenta and the embryo is probably functional. By six weeks, some maternal spiral arteries open directly into the intervillous spaces. The high pressure gradient between the maternal spiral arteries and the intervillous spaces, combined with the low impedance in the intervillous spaces, produces the high velocity - low resistance flow pattern seen in trophoblastic tissue. This pattern is the same whether the pregnancy is intrauterine or ectopic. Detection of a high velocity - low resistance flow pattern in an adnexal mass has been shown to be of value in diagnosing ectopic pregnancy. In the case presented in this report, with the aid of color Doppler sonography, trophoblastic flow was identified around an interstitial pregnancy.

Classification

The exact classification of the various types of ectopic gestations is somewhat controversial. Gestations implanted within the ampullary or isthmic portions of the fallopian tube are referred to as tubal ectopics (fig. 4).

Fig. 4: Classification of tubal ectopic pregnancies (see text).

 Interstitial pregnancies occur when the gestation implants in the interstitial portion of the fallopian tube.

While some consider the terms interstitial and cornual pregnancies to be synonymous, others reserve the term cornual pregnancy for a gestation in one horn of a bicornuate or septate uterus^1-4.

A seldom used term, angular pregnancy, refers to a gestation in which the pregnancy extends beyond the interstitium into the adjacent uterine cavity^1,8.

In the case presented in this report, the gestation was confined to the interstitial portion of the fallopian tube and therefore was classified as an interstitial pregnancy.

Management

Another unusual aspect of this case is the preoperative angio­gram and embolization procedure. This was performed to minimize blood loss, especially if abdominal pregnancy was encountered at surgery. While the pregnancy was, in fact, interstitial in location, it is interesting to note that there was minimal blood loss at surgery following embolization.

References

1. Bond AL, Grifo JA, Chervenak FA, et al. Term interstitial pregnancy with uterine torsion : sonographic, pathologic, and clinical findings. Obstet Gynecol 73:857-859,1989.

2. Mattingly RF, Thompson JD. TeLind"s operative gynecology 6th edition. Philadelphia: JP Lippincott, 1985:439.

3. Kerr LM, Anderson DF. Angular pregnancy : A clinical entity. Br Med J 1:1113-4,1934.

4. Maliha WE, Gonella P, Degnan EJ. Ruptured interstitial pregnancy presenting as an intrauterine pregnancy by ultrasound. Ann Emerg Med 20:910-912,1991.

5. Coady DJ, Synder JR, Golstein SR, et al. Ultrasound diagnosis of interstitial pregnancy. NY State J Med 85:655-666,1985.

6. Jafri SZ, Loginsky SJ, Bouffard JA, et al. Sonographic detection of interstitial pregnancy. JCU 15:253-257,1987.

7. Taylor KJW, Ramos IM, Feyock AL, et al. Ectopic pregnancy : Duplex Doppler demonstration. Radiology 173:93-97,1989.