Appendiceal endometriosis is implantation of ectopic endometrial tissue into the serosal, subserosal, or less commonly muscularis and mucosal layers of the vermiform appendix. Occurs in 3-18% of women with pelvic endometriosis and is usually part of the deep infiltrative endometriosis (DIE) spectrum. T1 hyperintense focus on MRI (blood products — methemoglobin) is pathognomonic. Cyclic right lower quadrant pain (worsening with menstruation), dyschezia, and rarely cyclic hematochezia are clinical clues. May be nonspecific on CT; MRI is the gold standard for diagnosis. Can mimic appendicitis leading to unnecessary emergency surgery, or may be detected histologically in incidental appendectomy specimens.
Age Range
18-50
Peak Age
32
Gender
Female predominant
Prevalence
Rare
Appendiceal endometriosis is extension of pelvic endometriosis pathology to the appendix. Through retrograde menstruation (Sampson theory), endometrial cells shed from the fallopian tubes into the pelvic cavity and implant on peritoneal surfaces. Since the appendix is located in the right lower quadrant, endometrial cells reach the appendiceal serosa via peritoneal fluid circulation. The implanted ectopic endometrial tissue undergoes cyclic changes under estrogen and progesterone influence like normal endometrium — proliferation, secretory transformation, and desquamation (bleeding). This cyclic bleeding triggers chronic inflammation in surrounding tissue: hemosiderin deposition, fibrosis, and adhesion develop. T1 hyperintensity on MRI reflects the presence of methemoglobin (subacute blood) — this paramagnetic substance shortens T1 relaxation producing bright signal. Low T2 signal is due to fibrosis and hemosiderin deposition — hemosiderin (iron) strongly reduces T2 signal through magnetic susceptibility effect. Serosal implants begin as small nodules, and over time may infiltrate subserosal and muscularis layers — muscularis infiltration can lead to luminal narrowing and obstructive symptoms, mimicking acute appendicitis.
A nodule with high T1 signal and low T2 signal (shading) in the appendiceal serosal/subserosal layer is pathognomonic for endometrial implant. T1 hyperintensity reflects methemoglobin (subacute blood), T2 hypointensity reflects hemosiderin and fibrosis. Preservation of T1 signal on fat-sat confirms blood rather than fat. This MRI signal pattern — T1 bright + T2 dark + preservation on fat-sat — is the endometriosis triad and has >95% specificity for endometrial implants anywhere in the body including the appendix.
High signal nodule on T1-weighted images in the appendiceal serosal/subserosal layer. Source is methemoglobin (subacute blood product) within the hemorrhagic endometrial implant. Signal persistence on fat-sat sequences (confirms blood, not fat) increases diagnostic specificity.
Report Sentence
A T1 hyperintense nodule measuring approximately ___ mm is seen in the appendiceal serosal/subserosal layer, consistent with endometrial implant.
Endometrial implant shows low signal on T2-weighted images — 'shading' effect. Results from fibrotic tissue and hemosiderin deposition. More pronounced in chronic endometriosis. May be accompanied by T2 signal decrease in surrounding tissue (fibrotic adhesions).
Report Sentence
Low T2 signal and shading effect are observed in the appendiceal nodule, consistent with chronic endometrial implant.
Focal wall thickening or small soft tissue nodule in the appendiceal wall. May be nonspecific on CT — differentiation from appendicitis, carcinoid tumor, or adenoma is difficult. Shows mild enhancement. May be accompanied by minimal periapendiceal fat stranding.
Report Sentence
Focal appendiceal wall thickening is observed; in the appropriate clinical context (reproductive-age woman, cyclic symptoms), endometrial implant should be considered in the differential; further evaluation with MRI is recommended.
Hypoechoic nodule in the appendiceal wall or serosa. Transvaginal US is the primary modality for pelvic endometriosis evaluation and may also detect appendiceal endometriosis. The nodule is usually small (<15 mm), has irregular borders, and shows minimal vascularity.
Report Sentence
A hypoechoic nodule measuring approximately ___ mm is seen in the appendiceal wall; endometrial implant should be considered in the appropriate clinical context.
Preservation of hyperintense nodule signal on T1 fat-sat (fat-suppressed) sequence. This finding confirms that the high signal in the nodule is blood (methemoglobin), not fat. Critical for differentiation from fat-containing lesions (dermoid cyst, lipoma, fatty metaplasia).
Report Sentence
The hyperintense signal in the appendiceal nodule on T1 is preserved on fat-sat sequence, consistent with blood product (methemoglobin), supporting the diagnosis of endometrial implant.
Diffusion restriction is variable — may be present in acute hemorrhagic phase but generally not prominent in chronic fibrotic lesions. DWI hyperintensity and low ADC may be seen during acute inflammatory flare-up. T1 and T2 characteristics are more reliable for differentiation from malignant lesions.
Report Sentence
Diffusion restriction in the appendiceal lesion is ___; together with T1 and T2 signal characteristics, consistent with endometrial implant.
Criteria
Endometrial implant confined to appendiceal serosal surface. Most common subtype. Usually small nodules (<10 mm). May be symptomatic or incidentally detected.
Distinct Features
Conservative management or hormonal therapy. Surgery usually not needed. May be incidentally resected during pelvic endometriosis surgery.
Criteria
Endometrial tissue infiltrates beyond subserosal layer into muscularis propria. May cause luminal narrowing and obstructive symptoms. Larger and more prominent nodule on MRI. Can mimic appendicitis.
Distinct Features
Surgical resection (appendectomy) may be needed. Luminal obstruction can create acute appendicitis presentation. Interluler endoscopic evaluation may be beneficial. Should be planned combined with pelvic endometriosis surgery.
Criteria
Rarest subtype — endometrial tissue extends to mucosal layer. May present with cyclic hematochezia (GI bleeding with menstruation) and recurrent appendicitis episodes. Requires pathologic diagnosis.
Distinct Features
Requires appendectomy. Even if suspected preoperatively with MRI, definitive diagnosis is histological. Malignant transformation risk is minimal but exists (endometrioid adenocarcinoma). Endometrial glands and stroma in pathologic examination are diagnostic.
Distinguishing Feature
In acute appendicitis, the appendix is dilated (>6 mm), periappendiceal fat stranding and acute clinical picture (fever, leukocytosis, McBurney tenderness) predominate — T2 hyperintense on MRI, no specific T1 signal. In appendiceal endometriosis, T1 hyperintensity (methemoglobin) is pathognomonic, clinical course is cyclic and chronic, appendix is usually not dilated.
Distinguishing Feature
Carcinoid tumor is a solid enhancing nodule — shows prominent arterial phase enhancement. On MRI, T1 low/intermediate signal, T2 intermediate/high signal; does not show T1 hyperintensity. Endometriosis is T1 hyperintense (methemoglobin) and T2 hypointense (hemosiderin/fibrosis); enhancement is mild.
Distinguishing Feature
Colorectal (rectosigmoid) endometriosis shows the same MRI signal characteristics (T1 hyper, T2 hypo) but localizes to the rectosigmoid region — appendiceal endometriosis localizes to the appendix in the right lower quadrant. Both conditions may coexist with pelvic endometriosis. Rectosigmoid involvement is more common and extensive.
Distinguishing Feature
Mucinous neoplasm shows low-density (mucinous) cystic dilation of the appendix — wall calcification, peritoneal pseudomyxoma spread may occur. Endometriosis is solid/mixed nodular, does not show cystic dilation. On MRI, mucinous content is T2 hyperintense and T1 variable — endometriosis shows T1 hyper T2 hypo pattern.
Urgency
routineManagement
medicalBiopsy
Not NeededFollow-up
6-monthAppendiceal endometriosis is generally managed with medical (hormonal) treatment — GnRH agonists, oral contraceptives, or progestins reduce cyclic symptoms. Surgery (appendectomy) is performed in symptomatic patients or cases mimicking acute appendicitis — especially the deep infiltrative form may cause luminal obstruction. The appendix should be routinely evaluated during pelvic endometriosis surgery and resected if necessary (3-18% coexistence rate). Malignant transformation is extremely rare (<1%) but endometrioid adenocarcinoma development has been reported in documented cases. Critical message for radiologists: in reproductive-age women with right lower quadrant pain and appendiceal wall thickening, especially with cyclic symptoms, endometriosis should be considered alongside appendicitis and further evaluation with MRI should be recommended. Presence of pelvic endometriosis increases diagnostic confidence.
Medical treatment (hormonal) or surgical resection (appendectomy) is applied. Appendix should also be evaluated during pelvic endometriosis surgery. In rare cases, luminal obstruction may mimic acute appendicitis. Malignant transformation is extremely rare.