Introduction

Endometriosis is a chronic inflammatory gynecologic disorder characterized by the presence of endometrial-like tissue outside the uterine cavity. These ectopic implants respond to hormonal stimulation and undergo cyclic bleeding, which triggers inflammation, fibrosis, and anatomical distortion of pelvic organs. One of the most characteristic manifestations of the disease is the ovarian endometrioma, a cystic lesion formed within the ovary that contains altered hemolyzed blood. Because of its dark brown appearance during surgery, the lesion is often referred to as a “chocolate cyst.”

Endometriomas represent more than a simple cystic ovarian lesion. They are considered a marker of disease progression, frequently associated with moderate or severe endometriosis, pelvic adhesions, deep infiltrating endometriosis, infertility, and chronic pelvic pain. Understanding their development, biological evolution, and imaging characteristics is essential for accurate diagnosis and appropriate clinical management. Among available imaging techniques, transvaginal ultrasound has become the principal diagnostic tool for identifying endometriomas and evaluating the broader spectrum of pelvic endometriosis.

Pathogenesis and Development of Endometriomas

The formation of an ovarian endometrioma is generally explained by the implantation of ectopic endometrial tissue on the ovarian cortex followed by repeated cyclic hemorrhage.

The most widely accepted mechanism is the cortical invagination theory, originally described by Hughesdon in 1957 after histologic examination of surgical specimens.

According to this mechanism, endometriotic implants first develop on the surface of the ovary. These implants undergo cyclical bleeding during menstruation. Repeated bleeding leads to localized inflammation and fibrosis, which progressively pulls the ovarian cortex inward. Over time, the ovarian tissue folds inward and creates a cavity within the ovarian cortex. This cavity gradually enlarges and accumulates degraded blood products, forming the cystic lesion known as an endometrioma. Histological studies demonstrate that the wall of the cyst contains fibrotic ovarian cortex with endometrial glands and stroma, supporting this mechanism.

An alternative hypothesis proposes that functional ovarian cysts may become secondarily colonized by endometriotic implants. However, pathological evidence supporting this mechanism is limited, and most contemporary studies support cortical invagination as the primary pathway.

Source

Hughesdon PE. The structure of endometriotic ovarian cysts. Journal of Obstetrics and Gynaecology of the British Commonwealth. 1957.

Natural Evolution of Endometriomas

Endometriomas usually develop gradually rather than appearing suddenly. In early stages of ovarian endometriosis, small superficial implants are present on the ovarian cortex. These lesions may measure only a few millimeters and may not yet form a cystic cavity.

With repeated menstrual cycles, the implants bleed repeatedly and inflammation progressively damages the surrounding ovarian tissue. The ovarian cortex begins to invaginate and a small cavity forms within the ovary. As the process continues, the cavity enlarges and progressively fills with hemolyzed blood products. Over time, the cyst wall becomes thicker and more fibrotic due to chronic inflammation.

As the lesion evolves further, adhesions frequently develop between the ovary and surrounding pelvic structures such as the uterus, fallopian tube, bowel, or pelvic sidewall. The cyst fluid becomes increasingly dense because it contains high concentrations of hemoglobin degradation products, hemosiderin, inflammatory proteins, and cellular debris. This biochemical composition explains the characteristic ultrasound appearance of homogeneous low-level echoes, often described as ground-glass echogenicity.

Relationship Between Endometriomas and Disease Severity

The presence of an endometrioma is strongly associated with advanced stages of endometriosis according to the classification developed by the American Society for Reproductive Medicine.

The ASRM classification divides endometriosis into four progressive stages. The earliest stages of the disease, referred to as minimal and mild disease, are characterized mainly by superficial implants and small peritoneal lesions with minimal anatomical distortion. In contrast, the more advanced stages—moderate and severe disease—are defined by deeper lesions, pelvic adhesions, and ovarian involvement. Endometriomas are typically observed in these advanced stages, particularly when the disease has progressed enough to involve the ovarian cortex and produce adhesions.

Epidemiologic studies suggest that approximately 30–50% of women diagnosed with endometriosis eventually develop ovarian endometriomas, highlighting the close association between ovarian involvement and disease progression.

Sources:

Giudice LC. Endometriosis. New England Journal of Medicine. 2010.
Vercellini P et al. Human Reproduction Update. 2014.

Clinical Significance. Impact on Fertility

Endometriomas have significant implications for reproductive function. Several mechanisms may contribute to infertility in affected patients. The cyst itself can damage the ovarian cortex, leading to loss of functional follicles and reduction of ovarian reserve. In addition, the inflammatory microenvironment associated with endometriosis can impair folliculogenesis, oocyte quality, and embryo implantation.

Endometriomas are also frequently associated with adhesions that distort the normal anatomical relationship between the ovary and the fallopian tube, interfering with ovulation and oocyte capture. Multiple studies have demonstrated that women with endometriomas often exhibit lower anti-Müllerian hormone (AMH) levels, suggesting diminished ovarian reserve.

Source: Raffi F et al. Human Reproduction. 2012.

Chronic Pelvic Pain

Endometriomas are commonly associated with chronic pelvic pain symptoms, including dysmenorrhea, dyspareunia, and non-cyclic pelvic pain. However, the cyst itself is not always the main source of pain. Instead, the pain frequently arises from the inflammatory reaction, nerve infiltration, and adhesions associated with surrounding endometriotic lesions, particularly deep infiltrating endometriosis.

Surgical Considerations

Surgical removal of endometriomas is often considered in patients with significant symptoms or infertility. The most common procedure is ovarian cystectomy, which involves removal of the cyst capsule.

Although surgery may relieve symptoms and improve fertility in selected cases, it also carries risks. During cystectomy, healthy ovarian cortex may be inadvertently removed, which can further reduce ovarian reserve. For this reason, careful imaging evaluation is essential before surgical planning.

Role of Ultrasound in Diagnosis

Transvaginal ultrasound is widely regarded as the first-line imaging modality for diagnosing ovarian endometriomas. Clinical guidelines from organizations such as the European Society of Human Reproduction and Embryology and the American Institute of Ultrasound in Medicine recommend ultrasound as the initial diagnostic examination in women suspected of having endometriosis.

Ultrasound offers several advantages: it provides high spatial resolution of pelvic organs, allows real-time dynamic evaluation of pelvic mobility, is widely available, and does not involve ionizing radiation. When performed by experienced operators, ultrasound can accurately identify the typical imaging characteristics of endometriomas and evaluate associated pelvic disease.

Characteristic Ultrasound Appearance

Endometriomas exhibit distinctive sonographic features that help differentiate them from other ovarian cysts. The most typical finding is the presence of homogeneous low-level internal echoes, which give the cyst a characteristic ground-glass appearance. The cyst wall is usually well defined and may appear slightly thickened. Unlike malignant ovarian tumors, endometriomas typically lack papillary projections and internal solid components.

Doppler evaluation usually shows minimal or absent vascular flow within the cyst content. Posterior acoustic enhancement is often present because the cyst contains fluid material. These imaging characteristics have been validated in studies conducted by the International Ovarian Tumor Analysis (IOTA) group.

Source: Timmerman D et al. Ultrasound Obstetrics & Gynecology. 2016.

Differential Diagnosis

Despite their typical appearance, endometriomas can sometimes resemble other ovarian lesions. Hemorrhagic ovarian cysts may produce internal echoes due to blood products and therefore represent an important differential diagnosis. Dermoid cysts, cystadenomas, and certain borderline tumors may also mimic some sonographic features of endometriomas.

In cases where imaging features are atypical, structured diagnostic systems such as the IOTA classification can assist clinicians in distinguishing benign from potentially malignant ovarian lesions.

Ultrasound Mapping of Endometriosis

Modern ultrasound protocols emphasize comprehensive evaluation of the pelvis rather than simple detection of ovarian cysts. The systematic ultrasound approach proposed by the International Deep Endometriosis Analysis (IDEA)group recommends a structured evaluation that includes assessment of the uterus, ovaries, pelvic mobility, and the presence of deep infiltrating lesions.

During this evaluation, the examiner assesses ovarian mobility, the sliding sign between pelvic organs, and the anterior and posterior pelvic compartments. This comprehensive mapping allows ultrasound to detect not only ovarian endometriomas but also deep infiltrating endometriosis involving the bowel, uterosacral ligaments, rectovaginal septum, and bladder.

Source: Guerriero S et al. Ultrasound Obstetrics & Gynecology. 2016.

Role of Ultrasound in Follow-Up

Ultrasound is also essential for monitoring the progression of endometriomas over time. Serial examinations allow clinicians to evaluate cyst growth, detect changes in morphology, and assess response to medical therapy. Ultrasound can also identify recurrence after surgical treatment, which has been reported in approximately 20–30% of cases within five years after cystectomy.

In addition, ultrasound can evaluate ovarian reserve by measuring antral follicle count and ovarian volume, which is particularly important when managing patients seeking fertility treatment.

Source: Vercellini P et al. Human Reproduction Update. 2014.

Emerging Ultrasound Technologies

Recent technological developments in ultrasound may improve the characterization of ovarian endometriomas. Three-dimensional ultrasound allows better visualization of the spatial relationships between pelvic structures. Elastography may provide information about tissue stiffness, potentially helping to distinguish fibrotic lesions from other cystic structures. Contrast-enhanced ultrasound has also been investigated for evaluating vascular patterns in ovarian lesions.

However, current major clinical guidelines do not yet recommend these techniques as standard practice for diagnosing endometriomas. Their clinical role remains under investigation.

Endometriomas represent one of the most characteristic manifestations of endometriosis and frequently indicate advanced disease. Their formation is primarily explained by cortical invagination of the ovary resulting from repeated bleeding of endometriotic implants. Over time, these lesions evolve into cystic structures filled with hemolyzed blood and surrounded by fibrotic ovarian tissue.

Beyond their structural presence, endometriomas carry important clinical implications. They are associated with infertility, chronic pelvic pain, pelvic adhesions, and potential surgical challenges. Transvaginal ultrasound plays a central role in the diagnosis, characterization, and monitoring of these lesions. When performed using systematic pelvic mapping protocols, ultrasound can provide comprehensive evaluation of both ovarian disease and associated deep endometriosis.

As imaging techniques continue to evolve and standardized protocols become more widely adopted, ultrasound is increasingly recognized as a powerful tool not only for detecting endometriomas but also for improving early diagnosis and guiding individualized management of patients with endometriosis.

March 2026.