Ectopic Pregnancy
Review of ectopic pregnancy for radiology board review. Check out the free study guide by clicking here. Prepare to succeed!
Show Notes/Study Guide:
What is an approximate beta hCG cutoff below which a normal early pregnancy may be non-visualizable on ultrasound?
It is commonly taught that with beta hCG levels below 2000 mIU/mL, a normal early intrauterine pregnancy may be present but not able to be visualized on ultrasound. For transvaginal ultrasound a beta hCG of 1,500 mIU/mL has also been proposed at a cutoff for with a normal pregnancy should be identifiable.
What is the most common site of implantation in ectopic pregnancy?
The fallopian tube, comprising about 95% of ectopic pregnancies, with the ampullary region being the most common site of implantation, seen in nearly 70% of all ectopic pregnancies. The 2nd most common site of ectopic pregnancy is a cornual ectopic pregnancy, also termed an interstitial ectopic pregnancy, in which implantation occurs within the uterine muscular wall about the proximal portion of the fallopian tube.
What are classic ultrasound features of an ectopic pregnancy?
Let’s start with classic ultrasound findings in the uterus in ectopic pregnancy. The uterus should show no intrauterine pregnancy. In most cases the uterine cavity will be empty. However, in up to 20% of ectopic pregnancies the uterine cavity will not be empty but may show a pseudogestational sac. More on that later. Thickened echogenic endometrium may also be seen, without an intrauterine pregnancy. The most specific ultrasound finding of ectopic pregnancy is visualization of a live extrauterine pregnancy with presence of fetal cardiac activity. However, this is not able to be visualized in most cases, especially with early ectopic pregnancies.
In terms of the adnexa, an extra-adnexal complex appearing cystic or mass-like lesion, with positive beta hCG and no intrauterine pregnancy, nearly confirms a tubal ectopic pregnancy, with greater than 90% certainty by many estimates. A simple appearing adnexal cyst without other adnexal findings denotes some risk of adnexal pregnancy in the setting of elevated beta hCG and no intrauterine pregnancy. This may be all that is seen in the adnexal region in a minority of adnexal ectopic pregnancies. Note that a cystic mass within the ovary itself is more likely to be a corpus luteum rather than an adnexal pregnancy.
Two classic ultrasound signs need to be discussed as well. The tubal ring sign, and the ring of fire sign. More on those later.
In terms of the surrounding pelvis and peritoneal cavity, free pelvic fluid, or more specifically hemoperitoneum in the rectouterine pouch of Douglas (the most dependent portion of the peritoneal cavity), and/or in the right subhepatic space (Morison’s pouch) increase the likelihood of ectopic pregnancy. A large amount of hemoperitoneum, and clinical signs of hemodynamic instability, support a ruptured ectopic pregnancy.
What proportion of ectopic pregnancies may not show a visualizable extra-uterine gestation on ultrasound?
Approximately 15-35% of ectopic pregnancies may fail to demonstrate an extra-uterine gestation on ultrasound. Note that a visualized extra-uterine gestation is commonly considered the most reliable ultrasound finding of ectopic pregnancy.
What is the tubal ring sign?
The tubal ring sign is seen as an echogenic ring around an ectopic pregnancy that confirms with high positive predictive value an ectopic pregnancy, typically unruptured. This can be mimicked by a normal corpus luteum, but seeing normal ovarian tissue surrounding the ring supports corpus luteum.
What is the ring of fire sign?
This is very similar to the tubal ring sign but on color or pulsed Doppler imaging showing a ring of hypervascularity due to increased vascularity of an ectopic pregnancy. However, this sign can be seen with both a corpus luteum and an ectopic pregnancy. Note as with many other of the signs described in this episode, absence of color Doppler ring of fire appearance does not exclude ectopic pregnancy.
Given that both can demonstrate a so-called ring of fire appearance on ultrasound, what are other ultrasound features to help differentiate a corpus luteum from an ectopic pregnancy?
It is important to remember that an ectopic pregnancy is most often located within the fallopian tube whereas a corpus luteum is located within the ovary. A corpus luteum should therefore move with the ovary whereas an ectopic pregnancy will move separate from the ovary, and this can be potentially visualized upon applying focused pressure at the site with the ultrasound probe. With an ectopic pregnancy, the surrounding fallopian tube tissue may appear as a ring that is hyperechoic compared to the echogenicity of typical ovarian parenchyma, and this ring of tissue around the ectopic pregnancy is typically thicker than the relatively thin rim of ovarian tissue that often surrounds a corpus luteum.
What is a so-called pseudogestational sac?
A pseudogestational sac is intra-cavitary fluid within the endometrial canal that can potentially be confused for evidence of an intrauterine gestation in the setting of a positive beta hCG test. However, this was historically more of a transabdominal ultrasound finding on older ultrasound equipment. Modern equipment when scanning with transvaginal technique that shows an intrauterine sac-like fluid collection is reported to be much more likely to be a true gestational sac in comparison to when this finding was originally described using prior generation ultrasound technology. On ultrasound a pseudogestational sac can be seen as a centrally located fluid collection in the endometrial cavity versus an eccentric location which is more typical of a true gestational sac, with no evidence of a yolk sac and no other signs of a true gestational sac. A pseudogestational sac often has irregular-shape or pointed edges.
The intradecidual sac sign is also a potential indicator of early intrauterine pregnancy which is seen when a gestational sac or intrauterine fluid collection or even an echogenic area is seen within one side of the uterine cavity that shows marked adjacent decidual thickening. However, as with many of the signs associated with intrauterine pregnancy, absence of this sign does not exclude a true intrauterine pregnancy.
What is a so-called heterotopic pregnancy?
A heterotopic pregnancy occurs when there is both an intrauterine pregnancy and ectopic pregnancy at the same time. Though rare, there is Increased risk with in vitro fertilization.
Final note:
Serial beta hCG monitoring is a helpful clinical tool used to potentially differentiate between normal and abnormal pregnancies. It can help determine when to intervene and when to expectantly follow a pregnancy by comparing a patient's hCG levels to expected curves. Beta hCG levels increase at a slower rate in most ectopic and nonviable pregnancies than in viable intrauterine pregnancies. A decline in beta hCG levels can help confirm a nonviable pregnancy, especially if there is diagnostic uncertainty on imaging.
