Endometriosis

01Pathophysiology

Endometriosis is defined by the implantation and growth of endometrial-type tissue (glands and stroma) outside the uterine cavity. These ectopic implants are most commonly found on the ovaries, uterosacral ligaments, posterior cul-de-sac (pouch of Douglas), and the pelvic peritoneum. Less commonly, they can appear on the bowel, bladder, and rarely at distant sites such as the lung (catamenial pneumothorax).

The most widely accepted explanation is the Sampson theory of retrograde menstruation: during menses, endometrial tissue refluxes through the fallopian tubes into the peritoneal cavity. Once there, these cells implant, proliferate, and respond to cyclic hormonal stimulation just like native endometrium. This is why the disease is fundamentally estrogen-dependent. Each menstrual cycle triggers proliferation and subsequent bleeding within these implants, causing a local inflammatory response, fibrosis, and adhesion formation.

The cyclical inflammation explains the hallmark symptom of progressive secondary dysmenorrhea. Pain worsens with each cycle because repeated bleeding leads to cumulative scarring and adhesion formation. Deep dyspareunia results from implants on the uterosacral ligaments and the cul-de-sac, which are mechanically stretched during intercourse. Dyschezia (painful defecation) occurs when implants involve the rectovaginal septum or bowel serosa, and these symptoms also follow a cyclic pattern, worsening perimenstrually.

Infertility in endometriosis is multifactorial. The inflammatory peritoneal environment is toxic to sperm and oocytes. Adhesions can distort tubal anatomy, impairing ovum pickup. Endometriomas within the ovary destroy normal follicular tissue. Additionally, there is evidence of impaired endometrial receptivity even when the eutopic endometrium appears structurally normal.

When an endometrioma forms within the ovary, repeated cyclic bleeding creates a cyst filled with old, hemolyzed blood, giving it a thick, dark brown appearance. This is the classic "chocolate cyst." Over time, these cysts can become quite large and are a frequent finding on imaging.

Other proposed theories include coelomic metaplasia (peritoneal cells transforming into endometrial tissue, which may explain rare cases in men on estrogen therapy or in pre-menarchal girls) and lymphatic/hematogenous spread (explaining distant implants in the lung or brain).

02Classification and Clinical Manifestation

Revised American Society for Reproductive Medicine (rASRM) Staging

The stage does not correlate well with the severity of pain. A patient with Stage I disease can have debilitating pain, while a patient with Stage IV may be asymptomatic and discovered only during an infertility workup.

Clinical Manifestations By Location

03Diagnostic Workup

The diagnosis of endometriosis begins with clinical suspicion based on the triad of dysmenorrhea, dyspareunia, and infertility in a reproductive-age woman. A thorough history focusing on the cyclic nature of symptoms is the single most important clue. On pelvic examination, the classic finding is tender nodularity along the uterosacral ligaments, best appreciated on rectovaginal exam. A fixed, retroverted uterus suggests dense posterior adhesions.

Transvaginal ultrasound is the best initial imaging study. It is most useful for identifying ovarian endometriomas, which appear as well-defined cysts with homogeneous low-level internal echoes (the "ground-glass" pattern). However, ultrasound has poor sensitivity for superficial peritoneal implants and small deep lesions. A normal ultrasound does not rule out endometriosis.

MRI provides superior soft-tissue contrast and is particularly valuable when deep infiltrating endometriosis is suspected (bowel, bladder, ureteral involvement). It helps map disease extent before surgery. However, MRI is not routinely ordered as a first-line study.

CA-125 is sometimes measured but is not reliable for diagnosis. It lacks both sensitivity and specificity. It may be mildly elevated (typically 35-100 U/mL), but this finding can also be seen in ovarian malignancy, pelvic inflammatory disease, and other conditions. It is not recommended as a screening or diagnostic tool.

The gold standard remains laparoscopy with histologic confirmation. Visual inspection during laparoscopy reveals characteristic lesions: classic "powder-burn" or "gunshot" lesions (dark blue-black puckered nodules), red flame-like lesions (early active implants), or white scarred lesions (burned-out disease). Biopsy demonstrating endometrial glands and stroma confirms the diagnosis. In current practice, many clinicians initiate empiric medical therapy based on clinical suspicion without laparoscopy, reserving surgery for patients who fail medical management or who require a definitive diagnosis (e.g., infertility patients).

04Management and Treatment

Medical Management

The first step in management for a patient with suspected endometriosis and pelvic pain is empiric therapy with NSAIDs combined with continuous combined oral contraceptives. The COC is taken without the placebo week to achieve continuous ovarian suppression and prevent the cyclic hormonal fluctuations that drive implant activity. This approach eliminates withdrawal bleeding and reduces the inflammatory stimulus at ectopic sites.

If COCs fail after 3-6 months, the next step is to switch to progestin-only therapy. Options include oral norethindrone acetate (5 mg daily), depot medroxyprogesterone acetate (150 mg IM every 3 months), or a levonorgestrel-releasing IUD. Progestins work by inducing decidualization and subsequent atrophy of endometrial implants. Depot medroxyprogesterone and the levonorgestrel IUD are particularly useful because they often achieve amenorrhea.

GnRH agonists (leuprolide) represent third-line therapy. They work by initially stimulating, then downregulating the hypothalamic-pituitary-gonadal axis, creating a medical menopause state. This profoundly suppresses estrogen production, starving implants of their hormonal fuel. The key limitation is bone mineral density loss. Without add-back therapy, GnRH agonists should not be used for more than 6 months. Add-back therapy with norethindrone acetate (5 mg/day) allows extended use by providing just enough hormonal support to protect bones and reduce vasomotor symptoms while still keeping estrogen levels low enough to suppress endometriosis.

Oral GnRH antagonists (elagolix) are a newer option that offers dose-dependent estrogen suppression without the initial flare effect seen with GnRH agonists. At the lower dose (150 mg daily), partial estrogen suppression allows use for up to 24 months. At the higher dose (200 mg twice daily), more complete suppression limits use to 6 months without add-back.

Danazol, a synthetic androgen, is rarely used today due to androgenic side effects (hirsutism, acne, voice deepening, weight gain, hepatotoxicity). It is essentially historical but may still appear as a distractor.

Surgical Management

Conservative surgery via laparoscopy is indicated when medical therapy fails, when a tissue diagnosis is needed, or when infertility is the primary concern. Surgical goals include excision or ablation of all visible implants, lysis of adhesions, and restoration of normal pelvic anatomy. For endometriomas larger than 3-4 cm, cystectomy (excision of the cyst wall) is preferred over simple drainage and ablation because recurrence rates are lower and subsequent fertility outcomes are better.

Definitive surgery (TAH-BSO) is reserved for patients with severe, debilitating, treatment-refractory disease who have completed childbearing. If the ovaries are removed, estrogen replacement therapy can be considered, but there is a small risk of reactivating residual disease.

Infertility-Focused Management

For patients whose primary concern is infertility, the approach differs. Medical suppression of endometriosis (COCs, GnRH agonists) does not improve fertility and should not be used for this purpose. Instead, options include laparoscopic surgical treatment of implants and adhesions (improves spontaneous conception rates in mild-moderate disease) or proceeding directly to assisted reproductive technology (IVF), especially in moderate-to-severe disease or when other factors contribute.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students lose points on endometriosis questions is by failing to recognize the cyclic nature of symptoms. The vignette will describe pelvic pain, dyspareunia, or even GI or urinary symptoms, but the distinguishing feature is that these symptoms worsen perimenstrually and follow a cyclic pattern. If the pain is constant and unrelated to the menstrual cycle, think of other diagnoses.

A second frequent trap involves confusing endometriosis with adenomyosis. Both cause dysmenorrhea in reproductive-age women, but adenomyosis presents with a diffusely enlarged, boggy, tender uterus and heavy menstrual bleeding (menorrhagia), whereas endometriosis presents with a normal-sized or fixed retroverted uterus, nodularity on rectovaginal exam, and adnexal endometriomas.

Students also commonly forget that the stage of disease does not predict pain severity. A vignette showing minimal laparoscopic findings does not exclude the diagnosis in a patient with classic symptoms.

On treatment questions, remember that hormonal suppression does not treat endometriosis-related infertility. If the vignette presents a patient whose chief complaint is inability to conceive, the answer is never "start OCPs" or "start leuprolide." The correct next step is either laparoscopic surgery or referral for assisted reproduction.

For the rare but testable scenario of cyclic hemoptysis or recurrent unilateral pneumothorax in a young menstruating woman, think of thoracic endometriosis (catamenial pneumothorax). This is a classic "if you know it, you get it" question.

Finally, when a vignette describes a painful nodule at a prior surgical scar (especially a cesarean scar) that enlarges and becomes tender during menstruation, the diagnosis is scar endometriosis. This is an increasingly tested topic and should be recognized immediately from the cyclic pain-at-scar pattern.