An endometrioma, often called a "chocolate cyst," is a type of ovarian cyst that forms when endometrial tissue (the lining of the uterus) grows on or within the ovary. Over time, this tissue bleeds during menstrual cycles, and the trapped blood thickens into a dark, tar-like fluid that gives the cyst its characteristic appearance on MRI.
An endometrioma is never truly an isolated finding — it is a marker of systemic endometriosis, and its presence on imaging should prompt evaluation for additional implants throughout the pelvis. Simple ovarian cysts and ovarian cystadenomas can look similar on ultrasound, making MRI essential for confident characterization. In severe cases, bilateral endometriomas adhere the ovaries together behind the uterus — a pattern called "kissing ovaries" — and deep infiltrating endometriosis involving the rectum, bladder, or uterosacral ligaments is frequently present, making complete preoperative MRI mapping critical for surgical planning. Ovarian cancer must be excluded in any complex ovarian cyst, and MRI plays a central role in that distinction.
Causes
Endometriosis.
Endometriomas are a direct manifestation of endometriosis — a condition in which tissue similar to the uterine lining grows outside the uterus, most commonly in the pelvis. When endometrial implants establish themselves on or within the ovarian cortex, they respond to each monthly hormonal cycle by proliferating and bleeding. Because this blood cannot escape, it accumulates within a progressively enlarging cyst — the characteristic dark, viscous "chocolate" fluid being degraded blood products (hemosiderin) at various stages of breakdown. The cyst wall becomes fibrotic over time and adheres to adjacent structures, making surgical removal technically challenging.
Other contributing factors.
The exact cause of endometriosis — and therefore endometriomas — is not fully understood. The leading theory is retrograde menstruation: menstrual blood carrying viable endometrial cells flows backward through the fallopian tubes into the pelvis, where cells implant and proliferate. However, retrograde menstruation is common while endometriosis affects only 10% of women, suggesting that immune system dysfunction — failure to clear the ectopic implants — plays a central permissive role. Genetic predisposition is significant, with first-degree relatives of affected women having a 7–10 times higher risk. Hormonal factors, including estrogen dependency of the implants, explain why endometriomas grow during the reproductive years and typically regress after menopause. Peak incidence is between ages 25 and 45.
Symptoms
Many women with endometriomas experience chronic pelvic pain, severely painful periods (dysmenorrhea) that worsen over time rather than improving, pain with intercourse (dyspareunia) — particularly with deep penetration reflecting posterior cul-de-sac involvement — and cyclical pain with bowel movements or urination during menstruation when implants involve the rectovaginal septum or bladder. Infertility affects approximately 30–50% of women with endometriosis and is one of the most common presenting concerns. Importantly, cyst size does not reliably predict symptom severity — some women with large endometriomas have minimal pain, while others with small cysts are severely symptomatic due to widespread peritoneal disease. Small endometriomas may be entirely asymptomatic and discovered incidentally on imaging performed for unrelated reasons.
Diagnosis
Pelvic ultrasound is typically the first imaging test and often shows the characteristic "ground glass" appearance — homogeneous low-level internal echoes within a unilocular cyst — that is highly suggestive of an endometrioma. However, ultrasound cannot reliably detect deep infiltrating endometriosis, evaluate the extent of pelvic disease, or confidently distinguish an endometrioma from a hemorrhagic functional cyst or other complex cysts.
An MRI of the pelvis is the most accurate imaging modality for endometrioma characterization and is the study of choice for surgical planning. Endometriomas have a distinctive MRI signature: high T1 signal (bright) reflecting blood products, with persistent T2 hypointensity ("T2 shading") that reflects the chronic nature of the hemorrhage — a finding specific to endometriomas that distinguishes them from hemorrhagic functional cysts, which lose their T1 signal on fat-suppressed sequences. MRI also maps the full extent of pelvic endometriosis — identifying deep infiltrating implants in the uterosacral ligaments, rectovaginal septum, bladder wall, and bowel — information essential for preoperative surgical counseling that ultrasound cannot provide. MRI simultaneously evaluates for features that would raise concern for ovarian malignancy, such as solid enhancing nodules or irregular wall thickening.
Classification
Endometriomas are characterized by size, laterality, and associated findings on imaging and at surgery.
- Simple endometrioma: Single cyst with the characteristic T1-bright, T2-shading MRI appearance and no complex internal features. Low malignant potential.
- Complex endometrioma: Multiple loculations, septations, or solid components. Requires careful MRI evaluation to exclude superimposed malignancy (endometrioid or clear cell ovarian carcinoma arising within an endometrioma is a recognized though uncommon complication).
- Bilateral endometriomas: Cysts on both ovaries, typically indicating more extensive pelvic endometriosis. The "kissing ovaries" pattern — ovaries adhered together in the posterior cul-de-sac — is a specific MRI finding associated with severe disease and difficult surgical dissection.
Treatments
Treatment depends on symptom severity, cyst size, age, fertility goals, and the extent of associated endometriosis. The goal is to relieve pain, preserve ovarian function, and optimize fertility outcomes where relevant.
Observation: Small endometriomas (typically under 3 cm) that are asymptomatic may be monitored with periodic pelvic ultrasound or MRI, particularly in women approaching menopause when spontaneous regression is expected. Serial imaging confirms stability and excludes interval growth or development of complex features.
Medical management.
Hormonal suppression therapies — combined oral contraceptives, progestins (dienogest is particularly effective for endometriosis pain), GnRH agonists (leuprolide) with add-back hormonal therapy, or the levonorgestrel IUD — suppress ovarian cycling, reduce endometrial implant activity, and provide meaningful pain relief. These treatments do not substantially shrink established endometriomas but can prevent growth, reduce recurrence after surgery, and manage symptoms in women who are not pursuing immediate fertility. GnRH antagonists (elagolix, relugolix) offer a newer oral option with faster onset and reversibility.
Surgery.
Laparoscopic cystectomy — excision of the endometrioma cyst wall while preserving the surrounding ovarian tissue — is the standard surgical treatment for symptomatic cysts, large cysts (typically over 4 cm), or cysts with features concerning for malignancy. Cystectomy reduces recurrence and pain more effectively than simple drainage and ablation. During the same procedure, visible peritoneal endometriosis implants are excised or ablated and adhesions are released. Deep infiltrating endometriosis of the bowel or bladder may require multidisciplinary surgical involvement. Recurrence rates after cystectomy are significant — approximately 20–30% at 5 years — and postoperative hormonal suppression is recommended to reduce recurrence risk.
Fertility considerations.
Endometriomas reduce ovarian reserve by damaging the surrounding follicular tissue, and laparoscopic cystectomy can further reduce reserve — particularly with repeat procedures on the same ovary. Women who wish to conceive should discuss timing carefully with a reproductive endocrinologist. Egg or embryo freezing before surgery may be appropriate when ovarian reserve is already compromised. For women undergoing IVF, the decision to operate on an endometrioma before egg retrieval is individualized based on cyst size, symptoms, and remaining ovarian reserve.
Get an MRI to Confirm Your Diagnosis
Before surgical planning or starting treatment, a clear MRI diagnosis ensures the right path forward. First Look MRI offers self-pay Pelvis MRI scans — no doctor's order or insurance required — at our locations in Georgia, Texas, and Colorado.