Functional cyst (corpus luteum)

The corpus luteum (CL) is a transient structure formed as a result of ovulation, due to the midcycle luteinizing hormone surge from the pituitary gland. The CL is responsible for the production of progesterone, hence the term “functional cyst.” It is necessary for regulating menses and for maintaining a pregnancy until it develops the ability to make its own progesterone. If a pregnancy does not occur, the CL breaks down. It can, on occasion, undergo internal hemorrhage and develop into a hemorrhagic cyst. When such a cyst continues producing progesterone, it is a hemorrhagic corpus luteum. If progesterone synthesis ceases, but the cyst persists, then it is considered a hemorrhagic cyst. Hemorrhagic cysts can enlarge up to 5 cm or more, causing pain, and may occasionally rupture, resulting in a hemoperitoneum. The pain typically resolves within a few days, whereas the cyst may take 1 to 3 months to regress. Patients with symptomatic hemorrhagic cysts typically present with acute unilateral pelvic pain and have a complex-appearing lesion on ultrasound evaluation. Often they are asymptomatic and can be an incidental finding. The nonspecific and confusing sonographic appearance of the hemorrhagic corpus luteum and hemorrhagic cyst often results in misdiagnosis and unnecessary surgery.

The CL is an ovarian cystic structure, typically 2 to 3 cm in size. Gray scale ultrasound characteristics include an irregular thick wall, unilocular cyst, often with internal debris or echogenic material. The most constant and specific feature of the CL is the “ring of fire” pattern of color Doppler, showing intense and abundant circumferential blood flow. The hemorrhagic CL often has a fine reticular or fishnet-like internal pattern and/or a solid area consistent with a retracting clot. Color Doppler reveals circumferential flow but no internal blood flow. The specific diagnosis is often possible sonographically, but because the CL is a mimicker of adnexal pathologies, a follow-up scan may be helpful when uncertain of the diagnosis, as discussed in clinical recommendations. A hemorrhagic cyst will have the same appearance as a hemorrhagic corpus luteum but without color flow. If internal hemorrhage occurs with cyst rupture or partial rupture, then complex fluid may be seen in the cul-de-sac or higher, or surrounding the ovary.

The correct diagnosis is often challenging because of variations in size, irregularity of the cyst wall, and internal solid areas (clot), all of which are nonspecific sonographic findings mimicking pathology. Knowing the menstrual cycle day is very helpful, although not always possible, in patients with irregular bleeding or menses.