A number of ultrasound findings have been described in the setting of an ectopic pregnancy. These have been discussed in a previous case (click here to view this case in a new window). Vascular flow around an ectopic is variable and depends on the amount of trophoblastic tissue. The case shown here demonstrates the classical "ring of fire" sign seen on color Doppler studies in cases of ectopic gestation. Trophoblastic flow is characterized by high velocity systolic flow, and low resistance diastolic flow when seen in an extrauterine location. However, most studies now conclude that color Doppler does not significantly increase the diagnostic accuracy in cases of ectopic gestation, but can be used as a supplemental test to detect ectopic trophoblastic flow.
The closest differential of an ectopic is a corpus luteum cyst. Identification of a tubal ring that displays an echogenic chorionic rim with low resistance flow surrounding a hypoechoic structure, favors an ectopic pregnancy over a corpus luteum cyst. However, sometimes the corpus luteum can have highly vascular walls, which may simulate the ring of fire sign. According to a study by Rottem, et al, three criteria can aid in distinguishing between an ectopic gestation and corpus luteum cyst:
1.The echogenicity of a suspected finding relative to the ovary
2.The presence or absence of flow
3.The relationship of the sign to an intentionally displaced ovary
According to published reports, the tubal ring of an ectopic is usually more echogenic than the ovarian parenchyma, and may be more echogenic than the endometrium. In contrast, the walls of the corpus luteum usually appear less echogenic or about the same as the ovarian parenchyma. This can be used as an ancillary sonographic finding.
Also demonstrated in this case is the presence of a small anechoic area with barely perceptible walls, adjacent to the endometrial cavity. This may represent a simple endometrial cyst or a decidual cyst. The presence of decidual cysts is now known to be associated with ectopic pregnancy. Since no appreciable decidual reaction is seen, decidual cyst is less likely and it may be a simple endometrial cyst.