Interstitial Ectopic Pregnancy

Interstitial ectopic pregnancy is implantation in the part of the fallopian tube that passes through the uterine wall. It accounts for 2 to 4 percent of ectopic pregnancies. Because the surrounding myometrium allows growth to 8 to 12 weeks, rupture causes catastrophic haemorrhage and carries a higher mortality than other tubal sites.

• Risk factors: previous salpingectomy on the affected side, assisted reproduction, previous ectopic, uterine anomaly.
• Presentation is later than tubal ectopic.
• Early ultrasound diagnosis is difficult.

Patients have amenorrhoea of 8 to 12 weeks, lower abdominal pain and sometimes minimal bleeding. Rupture presents with sudden collapse and peritonism.

• Uterine asymmetry with a lateral bulge.
• Massive haemoperitoneum on rupture.
• Differential diagnosis: intrauterine pregnancy in a bicornuate uterus, cornual pregnancy, intramural fibroid, tubal ectopic, molar pregnancy.
• Complications: uterine rupture, haemorrhagic shock, hysterectomy, death, recurrence.

The approach is detailed transvaginal ultrasound with hCG.

• Ultrasound criteria: empty uterine cavity, gestational sac more than 1 cm from the lateral edge of the cavity, thin myometrial layer below 5 mm around the sac, and interstitial line sign.
• Magnetic resonance imaging or three dimensional ultrasound where available if doubt exists.
• hCG baseline; haemoglobin, cross-match 4 units, creatinine, liver enzymes.
• Laparoscopy confirms the diagnosis.

The principle is early diagnosis, then medical treatment if stable and early, or cornual resection if large or unstable.

• Supportive: two large-bore cannulae; blood ready; admit; Ringer's lactate 1 litre if unstable.
• Pharmacological: methotrexate 50 mg/m2 IM single dose for stable women with hCG below 5000 IU/L and no fetal heart, with hCG on days 4 and 7 and weekly until negative; for higher hCG use 4 dose regimen: methotrexate 1 mg/kg IM on days 1, 3, 5 and 7 with folinic acid 0.1 mg/kg IM on days 2, 4, 6 and 8; paracetamol 1 g oral 6-hourly (maximum 4 g in 24 hours).
• Procedures: laparoscopic cornuostomy or cornual resection with suturing of the myometrium; hysteroscopic removal under ultrasound guidance in selected cases; laparotomy and hysterectomy if rupture with uncontrollable bleeding.
• Special groups: Rhesus negative women give anti-D immunoglobulin 250 IU IM within 72 hours; the uterine scar requires caesarean delivery at 36 to 37 weeks in a later pregnancy.
• Red flags: sudden collapse, severe pain, falling haemoglobin, peritonism.
• Follow-up: hCG weekly until negative; scan at 6 weeks; plan caesarean delivery in future pregnancy with early scan.