DISCUSSION

A cesarean scar ectopic pregnancy is rare, occurring in approximately 1 in 1,800 to 1 in 2,000 pregnancies, and accounting for 6.1% of ectopic pregnancies in patients with previous cesarean deliveries.^{Reference Seow, Huang and Lin1,Reference Rotas, Haberman and Levgur2} Its incidence is increasing, both due to increased recognition and increased rates of cesarean deliveries being performed worldwide.^{Reference Rotas, Haberman and Levgur2} A cesarean scar ectopic pregnancy is thought to occur by embryo migration through a microscopic scar defect, enabling the embryo to become completely surrounded by myometrium and scar tissue.^{Reference Rotas, Haberman and Levgur2} Patients with cesarean scar ectopic pregnancies typically present for assessment in the first trimester with nonspecific symptoms, including lower abdominal or pelvic cramping and vaginal bleeding.^{Reference Rotas, Haberman and Levgur2} Failure to diagnose a cesarean scar ectopic pregnancy in a timely manner can result in serious morbidity, including uterine rupture, life-threatening hemorrhage, and a hysterectomy.^{Reference Seow, Huang and Lin1,Reference Rotas, Haberman and Levgur2}

Diagnosis is usually made by a combination of consultative transabdominal and transvaginal ultrasonography. Diagnostic criteria vary, however, generally include an empty uterus and cervical canal, gestational sac within the anterior portion of the lower uterus, and thinned or absent myometrium between the bladder and gestational sac.^{Reference Seow, Huang and Lin1–Reference Osborn, Williams and Craig3}

In this report, we present the first case of cesarean scar ectopic pregnancy in the literature diagnosed by a transabdominal point-of-care ultrasound. Point-of-care ultrasound is a common adjunct used in the emergency department for the evaluation of first-trimester, pregnancy-related complaints, including abdominal pain and vaginal bleeding.^{Reference Murray, Baakdah, Bardell and Tulandi4} Of critical importance in patients with these complaints is to rule out ectopic pregnancy, as it is a potential source of morbidity and mortality. In patients without risk factors for a heterotopic pregnancy, such as the use of fertility treatments, point-of-care ultrasound identification of an intrauterine pregnancy is considered to effectively rule out ectopic pregnancy, as the likelihood of a heterotopic pregnancy in the general population is considered to be extremely low.^{Reference Murray, Baakdah, Bardell and Tulandi4,Reference Socransky and Wiss5}

Criteria for the diagnosis of an intrauterine pregnancy by point-of-care ultrasound include identification of the adjacency of the uterus to the bladder (a typical ectopic pregnancy may appear to be surrounded by uterine-like tissue; however, it will usually not be found immediately adjacent to the bladder); identification of a gestational sac; decidual reaction centred within the endometrial stripe; yolk sac and/or fetal pole; and myometrial thickness > 5–8 mm⁵. Failure to meet any of these criteria warrants the physician to declare no definitive intrauterine pregnancy and to seek consultative ultrasound.^{Reference Socransky and Wiss5} In spite of these criteria, a visible ectopic pregnancy on point-of-care ultrasound may mislead the physician into declaring an intrauterine pregnancy. The cesarean scar ectopic pregnancy may be predisposed to this error due to its close approximation to the uterus and bladder. Knowledge of the appearance of a cesarean scar ectopic pregnancy is therefore of particular importance to the emergency physician.

In our case, 4 of 5 criteria for intrauterine pregnancy diagnosis by point-of-care ultrasound diagnosis were met (gestational sac, decidual reaction, yolk sac/heart rate, bladder-uterine adjacency). However, the images reveal a lack of visible myometrium surrounding the gestational sac, particularly on the aspect in contact with the bladder. Further scrutiny also reveals that, unlike a normal intrauterine pregnancy, the cesarean scar ectopic is eccentrically located, being positioned lower in the uterus than the expected site of implantation. In our case, this is the most obviously abnormal feature.