1. Context
Endometriosis is a chronic inflammatory condition characterized by the growth of endometrium-like epithelium and/or stroma outside the uterus. It affects approximately 2 - 10% of women in the general population and could be seen in up to 50% of women with fertility problems (1). The most common symptoms of endometriosis include dysmenorrhea, dyspareunia, menorrhagia, and infertility (2). Deep infiltrating endometriosis (DIE) is endometrium-like tissue lesions in the abdomen, extending on or under the peritoneal surface, usually in nodular form, with the ability to invade adjacent structures, and in association with fibrosis and disruption of normal anatomy (3). This is the most severe type of endometriosis, which can involve the intestines and urinary tract, leading to severe symptoms in patients.
Appendiceal endometriosis (AE) is a rare site of DIE. In the literature, AE prevalence is highly variable (from 0.2% to 39%) based on the study population. Among patients undergoing appendectomy for suspected acute appendicitis, the prevalence of AE has been reported as 2.67%. The type and severity of endometriosis may influence AE prevalence; rates of 11.6% in women with superficial endometriosis and 39.0% in those with DIE have been reported (4, 5). The symptoms of AE can mimic those of acute or chronic appendicitis. Preoperative imaging diagnosis is challenging, and AE is often diagnosed after appendectomy on histopathological examination. Bowel obstruction, bowel intussusception, bowel habit disturbance, cyclic acute abdominal symptoms, and positive occult blood test/colonoscopy are other reported symptoms of AE. Timely, accurate imaging assessment is essential as endometriosis has a heterogeneous presentation and a substantial impact on quality of life (6-11).
Noninvasive imaging methods such as transvaginal sonography (TVS) and magnetic resonance imaging (MRI) can help determine the exact location and spread of endometriosis. In addition, magnetic resonance enterography (MRE) can assist particularly in the detection of bowel DIE and surgical planning in cases with multiple lesions. Laparoscopic surgery is the preferred approach for surgical planning and treatment of endometriosis, and appendectomy may be performed when appendiceal involvement is suspected (5, 12-20). Accurate preoperative imaging assessment of symptomatic AE is essential for selecting the most appropriate treatment through precise disease mapping. In this review, we focus on the imaging findings of AE in symptomatic patients to highlight the utility of imaging modalities in timely and accurate preoperative diagnosis.
2. Objectives
The present systematic review aimed to review and collect the imaging abnormalities associated with symptomatic AE confirmed by histopathology, and to describe detection patterns by modality to aid in preoperative planning.
3. Methods
3.1. Study Design
This systematic review adhered to the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. Given the rarity of symptomatic AE, we included case reports to capture detailed imaging findings. Additionally, we aimed to include case series and observational studies to enhance data richness, though no suitable comparative studies were identified during screening.
3.2. Search Strategy
We conducted a comprehensive literature search across five databases: PubMed, Scopus, Web of Science, CINAHL Plus, and the Cochrane Library. The search included combinations of the following terms: "appendix", "endometriosis", "magnetic resonance imaging", "transvaginal sonography", and "transrectal high-intensity focused ultrasound". Boolean operators were used to refine search queries. Filters were applied to include only English-language articles with full text available, published up to 2024. The proposed search strategy is as follows: PubMed (Title/Abstract): (“appendix” AND “endometriosis”) AND (MRI OR “magnetic resonance imaging”) OR (“transvaginal sonography”) OR (“transrectal high-intensity focused ultrasound”)).
3.3. Eligibility Criteria
We included studies that reported imaging findings in symptomatic patients diagnosed with AE and provided histopathological confirmation of diagnosis. We excluded randomized controlled trials, review articles, cohort studies, and systematic reviews, conference abstracts, non-English articles, and studies without full-text availability.
3.4. Study Selection and Data Extraction
Two reviewers independently screened titles/abstracts using Covidence software. Discrepancies were resolved through discussion. Full-text articles of potentially relevant studies were reviewed to determine eligibility. Out of 494 initial records, 128 duplicates were removed. After title and abstract screening, 323 records were excluded. Following full-text review, 39 studies (comprising 40 cases) were included. Data extracted included study characteristics, patient demographics, imaging modalities used, imaging findings, and treatment outcomes. Extracted data were managed using Excel.
3.5. Risk of Bias and Quality Assessment
The quality of included case reports was assessed using the Joanna Briggs Institute (JBI) critical appraisal checklist for case reports. Each case was evaluated for completeness in patient history, diagnostic methods, and outcome reporting. Risk of bias was considered based on clarity of imaging interpretation, potential confounding conditions, and consistency with histopathological findings.
3.6. Data Analysis
Given the nature of case reports, data were analyzed descriptively. Frequencies and percentages were calculated for imaging modalities and findings. No statistical tests were conducted due to the absence of comparative or quantitative data.
3.7. Protocol Registration
This review was registered with PROSPERO (Registration No.: CRD42022335388) and approved by the ethics committee (IR.TUMS.IKHC.REC.1401.094). These steps ensure transparency, credibility, and adherence to ethical standards in the research process, aligning with best practices in systematic reviews and academic research as highlighted in the provided sources.
4. Results
From 39 studies, we identified 40 cases of symptomatic AE with reported imaging findings. The mean patient age was 37.2 years (SD ± 7.07). Of the 40 patients, 8 (20%) were pregnant. An overview of the included studies and patient characteristics is summarized in Table 1.
Table 1.Characteristics of Included Case Reports of Appendiceal Endometriosis a
| No | Article title | Country | First author | Patient age | Clinical characteristics | Reasons for inclusion in this review (case features) | Pregnancy status |
|---|---|---|---|---|---|---|---|
| 1 | A Case of Endometriosis of the Appendix with Adhesion to Right Ovarian Cyst Presenting as Intussusception of a Mucocele of the Appendix | Japan | Akagi (21) | 35 | Intussusception, ovarian cyst adhesion | Symptomatic AE, intussusception | No |
| 2 | A Rare Case of Lower Quadrant Pain | Portugal | Eduardo (22) | 40 | RLQ pain | Symptomatic AE, RLQ pain | No |
| 3 | Acute Appendicitis Secondary to Appendiceal Endometriosis | Brazil | Drumond (23) | 32 | Acute appendicitis | Symptomatic AE, acute appendicitis | No |
| 4 | Acute Small Bowel Obstruction Secondary to Intestinal Endometriosis, an Elusive Condition: A Case Report | United Kingdom | Slesser (24) | 33 | Small bowel obstruction | Symptomatic AE, small bowel obstruction | No |
| 5 | Appendiceal Endometriosis | Saudi Arabia | A. Al-Talib (25) | 31 | Endometriosis of the appendix | Symptomatic AE, abdominal pain | No |
| 6 | Appendiceal Endometriosis in a Pregnant Woman Presenting with Acute Perforated Appendicitis | United States | Lebastchi (26) | 33 | Acute perforated appendicitis | Symptomatic AE, acute perforated appendicitis | Yes |
| 7 | Appendiceal Endometriosis Invading the Sigmoid Colon: A Rare Entity | France | Lainas (27) | 41 | Endometriosis invading sigmoid colon | Symptomatic AE, sigmoid colon involvement | No |
| 8 | Appendiceal Intussusception from Endometriosis | Philippines | Lopez (28) | 39 | Intussusception due to endometriosis | Symptomatic AE, intussusception | No |
| 9 | Appendiceal Intussusception Resulting from Endometriosis Presenting as Acute Appendicitis | Spain | Marin (29) | 29 | Acute appendicitis due to intussusception | Symptomatic AE, acute appendicitis | No |
| 10 | Appendiceal Intussusception Secondary to Endometriosis: A Rare Etiology of Right Lower Quadrant Abdominal Pain | Belgium | Trefois (30) | 30 | RLQ pain | Symptomatic AE, RLQ pain | No |
| 11 | Appendicitis Caused by Endometriosis Within the Bowel Wall | United States | Gupta (31) | 36 | Appendicitis due to bowel wall endometriosis | Symptomatic AE, appendicitis | No |
| 12 | Appendicitis with Submucosal Fecalith Mimicking a Submucosal Tumor: A Case Report | Japan | Bekki (32) | 40 | Submucosal fecalith | Symptomatic AE, appendicitis | No |
| 13 | Appendicular Endometriosis as a Cause of Chronic Abdominal Pain Alone in the Right Iliac Fossa: Case Report and Literature Review | Brazil | Basso (33) | 44 | Chronic abdominal pain | Symptomatic AE, chronic abdominal pain | No |
| 14 | Appendicular Endometriosis: A Case Report and Review of Literature | India | Gupta (34) | 35 | Endometriosis of the appendix | Symptomatic AE, abdominal pain | No |
| 15 | Cecal Endometriosis Presenting as Acute Appendicitis | Iran | Alizadeh Otaghvar (35) | 43 | Acute appendicitis | Symptomatic AE, acute appendicitis | No |
| 16 | Characteristic Findings of Appendicular Endometriosis Treated with Single Incision Laparoscopic Ileocolectomy: Case Report | Japan | Hakoda (36) | 51 | Laparoscopic treatment of appendicular endometriosis | Symptomatic AE, laparoscopic findings | No |
| 17 | Colonic Endometriosis Presenting as a Sigmoid Stricture Requiring Laparoscopic Colonic Surgery for Diagnosis and Treatment | United States | Nojkov (37) | 29 | Sigmoid stricture due to endometriosis | Symptomatic AE, sigmoid stricture | No |
| 18 | Continuous Amenorrhea May Be Insufficient to Stop the Progression of Colorectal Endometriosis | France | Millochau (38) | 26 | Amenorrhea related to endometriosis | Symptomatic AE, amenorrhea | No |
| 19 | Deciduosis of the Appendix During Pregnancy | Japan | Tsunemitsu (39) | 35 | Deciduosis during pregnancy | Symptomatic AE, pregnancy-related symptoms | Yes |
| 20 | Endometriosis Causing Acute Appendicitis Complicated with Hemoperitoneum | Spain | Curbelo (40) | 39 | Acute appendicitis with hemoperitoneum | Symptomatic AE, acute appendicitis | No |
| 21 | Endometriosis of the Appendix Causing Small Bowel Obstruction in a Virgin Abdomen | Australia | Choi (41) | 29 | Small bowel obstruction | Symptomatic AE, bowel obstruction | No |
| 22 | Endometriosis of the Appendix: A Trap for the Unwary | Saudi Arabia | Khairy (42) | 33 | Endometriosis of the appendix | Symptomatic AE, abdominal pain | No |
| 23 | Endometriosis of the Duplex Appendix: A Case Report and Review of the Literature | China | Zhu (43) | 44 | Duplex appendix with endometriosis | Symptomatic AE, duplex appendix | No |
| 24 | Endometriosis of the Terminal Ileum: A Diagnostic Dilemma | Turkey | Karaman (44) | 27 | Diagnostic challenges in terminal ileum | Symptomatic AE, ileal symptoms | No |
| 25 | Endometriosis of the Vermiform Appendix Presenting as a Tumor | Japan | Terada (45) | 41 | Tumor-like presentation of AE | Symptomatic AE, tumor-like symptoms | No |
| 26 | Ileal Endometriosis Presenting as Acute Small Intestinal Obstruction: A Case Report | Nigeria | Alatise (46) | 34 | Small intestinal obstruction | Symptomatic AE, intestinal obstruction | No |
| 27 | Incidental Appendiceal Mass as the Only Manifestation of Endometriosis | Lebanon | Yaghi (47). | 34 | Incidental finding of appendiceal mass | Symptomatic AE, incidental findings | No |
| 28 | Laparoscopic Partial Cecum Resection in Appendiceal Intussusception | Turkey | Zenger (48) | 35 | Laparoscopic treatment of intussusception | Symptomatic AE, intussusception | No |
| 29 | Leiomyomatosis Peritonealis Disseminata Associated with Appendiceal Endometriosis: A Case Report | South Korea | Lee (49) | 31 | Endometriosis with leiomyomatosis | Symptomatic AE, leiomyomatosis | No |
| 30 | Mucocele of the Appendix due to Endometriosis: A Rare Case Report | Japan | Tsuda (50) | 43 | Appendiceal mucocele | Symptomatic AE, mucocele | No |
| 31 | Multifocal Abdominal Endometriosis: A Case Report | United States | Porter (51) | 52 | Multifocal presentation of endometriosis | Symptomatic AE, multifocal symptoms | No |
| 32 | Preoperative Evaluation of an Appendiceal Mucocele in a Woman with Endometriosis | Italy | Morotti (52) | 35 | Preoperative assessment of mucocele | Symptomatic AE, mucocele assessment | No |
| 33 | Preoperative Hormonal Therapy for a Patient With Appendiceal Endometriosis | Japan | Shichiri (53) | 40 | Hormonal therapy prior to surgery | Symptomatic AE, hormonal therapy | No |
| 34 | Lower Quadrant Pain During Pregnancy | United States | How (54) | 26 | RLQ pain during pregnancy | Symptomatic AE, pregnancy-related pain | Yes |
| 35 | Rupture of Appendiceal Mucocele due to Endometriosis: Report of a Case | Japan | Miyakura (55) | 56 | Ruptured appendiceal mucocele | Symptomatic AE, ruptured mucocele | No |
| 36 | Small Bowel Obstruction Caused by Appendiceal and Ileal Endometriosis: A Case Report | Japan | Kobayashi (56) | 37 | Small bowel obstruction due to endometriosis | Symptomatic AE, bowel obstruction | No |
| 37 | Small Bowel Obstruction Caused by Ileal Endometriosis with Appendiceal and Lymph Node Involvement Treated with Single-Incision Laparoscopic Surgery: A Case Report and Review of the Literature | Japan | Koyama (57) | 40 | Small bowel obstruction with lymph node involvement | Symptomatic AE, lymph node involvement | No |
| 38 | Small Bowel Obstruction due to an Endometriotic Ileal Stricture with Associated Appendiceal Endometriosis: A Case Report and Systematic Review of the Literature | India | Sali (58) | 44 | Endometriotic ileal stricture | Symptomatic AE, ileal stricture | No |
| 39 | Two Cases of Endometriosis in the Cecum Detected by Contrast-Enhanced Computed Tomography with Air/Carbon Dioxide Insufflation | Japan | Iwamuro (59) | 40 and 40 | Endometriosis in the cecum | Symptomatic AE, cecal symptoms | No |
Abbreviations: AE, appendiceal endometriosis; RLQ, right lower quadrant.
a Because all included items are case reports, study-level inclusion criteria do not apply. This table lists review-level eligibility (symptomatic AE, histopathologic confirmation, and sufficient clinical/imaging/surgical detail). No exclusion criteria were prespecified; all case reports meeting these features were included.
4.1. Clinical Presentation
The most frequent symptom was abdominal pain, particularly in the RLQ, reported in 77.5% of cases. Other common symptoms included vomiting (42.5%), nausea (30%), abdominal tenderness (55%), and bowel obstruction (20%). Less frequent symptoms included bowel habit disturbance (10%), dysmenorrhea (15%), and fever (5%). The clinical symptoms observed among patients are detailed in Table 2.
Table 2.Summary of Main Clinical Symptoms
| Clinical symptoms | No. (%) |
|---|---|
| Bowel obstruction | 8 (20) |
| Bowel intussusception | 2 (5) |
| Bowel habit disturbance | 4 (10) |
| Cyclic symptoms | 1 (2.5) |
| Abdominal pain | 31 (77.5) |
| Pelvic pain | 1 (2.5) |
| Abdominal tenderness | 22 (55) |
| Abdominal distension | 10 (25) |
| Guarding | 7 (17.5) |
| Fluid accumulation | 5 (12.5) |
| Diarrhea | 6 (15) |
| Constipation | 3 (7.5) |
| Nausea | 12 (30) |
| Vomiting | 17 (42.5) |
| Anorexia | 4 (10) |
| Fever | 2 (5) |
| Leukocytosis | 11 (27.5) |
| Dysmenorrhea | 6 (15) |
| Irregular bleeding | 4 (10) |
4.2. Imaging Modalities
1. Computed tomography (n = 30): Positive findings were seen in 26 patients (86.6%). Common findings included RLQ mass, appendiceal wall thickening, and ascites or free fluid.
2. Magnetic resonance imaging (n = 11): Positive findings were seen in 7 patients (63.6%). Key features included RLQ mass with signal heterogeneity, wall thickening, and nodular lesions with T2 hypointensity.
3. Ultrasound (n = 13): Positive findings were seen in 11 patients (84.6%), identifying features like wall thickening, mass, and signs of intussusception.
Of the 40 patients included in this review, 14 underwent more than one imaging modality, which enabled cross-modality comparison of findings in a subset of cases.
4.3. Comparative Imaging Trends
Among imaging findings:
- Right lower quadrant mass was most commonly detected by MRI (54.54%) and CT (33.33%).
- Appendiceal wall thickening was seen across all modalities but most frequently in CT (26.67%).
- Magnetic resonance imaging identified unique soft tissue characteristics, useful for differentiating endometriosis from other pathologies.
- Sonography remained useful in initial assessment, especially in pregnant patients.
A comparative analysis of imaging findings across modalities is shown in Table 3.
Table 3.Detailed Comparison of Imaging Findings by Sonography, Computed Tomography, and Magnetic Resonance Imaging a
| Imaging findings | Sonography (n = 13) | CT (n = 30) | MRI (n = 11) |
|---|---|---|---|
| Wall thickening | 3 (23.07) | 8 (26.67) | 1 (9.09) |
| Mass | 2 (15.38) | 10 (33.33) | 6 (54.54) |
| Cystic mass | 0 (0.00) | 0 (0.00) | 0 (0.00) |
| Solid lesion in the left ovary | 0 (0.00) | 1 (3.33) | 0 (0.00) |
| Mucocele | 1 (7.69) | 2 (6.66) | 1 (9.09) |
| Intussusception | 2 (15.38) | 3 (10.00 | 2 (18.18) |
| Obstruction | 0 (0.00) | 4 (13.33) | 1 (9.09) |
| Suspected bowel obstruction | 0 (0.00) | 1 (3.33) | 0 (0.00) |
| Appendicitis | 1 (7.69) | 2 (6.66) | 0 (0.00) |
| Fluid | 2 (15.38) | 7 (23.33) | 0 (0.00) |
| Abscess | 0 (0.00) | 2 (6.66) | 1 (9.09) |
Abbreviations: CT, computed tomography; MRI, magnetic resonance imaging.
a Values are expressed as No. (%).
4.4. Interpretation
Across imaging techniques, RLQ mass and wall thickening emerged as the most consistent findings suggestive of AE. The MRI, due to its soft tissue resolution, added value in identifying concurrent pelvic endometriosis lesions. These trends support MRI as the modality of choice in complex or inconclusive cases, while CT remains the workhorse in acute settings. Sonography complements both but is limited in specificity. This synthesis improves our understanding of imaging findings in a rare condition and provides guidance for diagnosis and surgical planning.
5. Discussion
Despite advances in medical and surgical treatment, women with DIE experience significant impairment in quality of life (60). Endometriosis of the appendix presenting with acute appendicitis is rare and accounts for less than 1% of all appendiceal pathologies that can resemble the clinical picture of acute appendicitis (61). Patients with cyclic bowel symptoms, chronic RLQ pain, and severe endometriosis are at a higher risk for developing AE. However, in our study, only 55% of patients were known cases of endometriosis who presented with abdominal tenderness (62).
Despite none of the patients having definitive imaging findings of endometriosis before surgery, retrospective evaluation of MRI in patients suggested findings in favor of AE, including: Concomitant hypo-intense T1 and T2 nodularity along the terminal ileum serosal surface, hypo-intense T1 and T2 mass in the cecal base and appendix orifice, and skipped DIE lesions in the rectum and rectosigmoid. In 2023, Medeiros et al. conducted a systematic review on the accuracy of MRI for DIE and reported that MRI has a high sensitivity and specificity for the detection of intestinal endometriosis [pooled sensitivity of 0.84 (95% CI 0.78 - 0.88) and specificity of 0.97 (95% CI 0.94 - 0.98)] (63). These findings suggest that careful evaluation of pelvic MRI in women of reproductive age with RLQ symptoms could help suggest the preoperative imaging findings of AE and provide patients with benefits from non-surgical treatments. It should be noted that in some conditions, differentiation of AE in nodular form is impossible from a carcinoid tumor, and definitive diagnosis often relies on surgical and histopathological findings.
In 2020, Aas-Eng et al. in Norway reviewed the literature on endometriosis imaging, focusing on TVS and MRI for DIE and adenomyosis. The study suggested that TVS and MRI are reliable methods for diagnosing endometriosis, adenomyosis, and especially DIE. The information obtained from these imaging methods can assist physicians in planning surgery and estimating its risks. Therefore, the use of TVS and MRI should be the first step in the imaging findings and treatment of endometriosis patients (16).
In 2020, Indrielle-Kelly et al. conducted a prospective observational study to investigate the accuracy of TVS and MRI in identifying pelvic DIE. The study included 49 out of 111 patients who underwent imaging with these two methods to plan surgical treatment. Both methods had similar sensitivity and specificity in identifying lesions of the upper rectum and rectosigmoid. The TVS had lower sensitivity and more specificity than MRI in evaluating the bladder, uterosacral ligament, vagina, rectovaginal septum, and pelvis in general. MRI was significantly superior to TVS in identifying lesions in the uterosacral ligament. The study concluded that the use of both methods is useful in identifying pelvic DIE (62).
Bazot et al. conducted a study in 2020 to review the use of MRI in diagnosing DIE involving the small intestine, including its protocols, indications, technical requirements, patient preparation, and criteria. According to the study, MRI should be used as the second-line tool after TVS for evaluating endometriosis in the rectosigmoid colon. It is also recommended to use MRI before surgery to determine the stage of the disease. In addition, MR-enterography should be performed to check for ileocecal and appendicular lesions (63).
The RLQ mass and appendiceal wall thickening were the most common imaging findings in our review. In addition, RLQ mass was the most frequent MRI finding. Although the exact prevalence and accuracy of imaging findings in AE are not defined in the literature, the reported imaging findings include: An enlarged appendix involved by hypodense soft tissue masses, luminal dilation or focal nodules within the appendiceal body in CT, and discrete serosal hyperintense foci on pre-contrast fat-saturated T1 images to nodular lesions that appear hypointense on T2 images, occupying the tip or body of the appendix, luminal obstruction resembling an appendiceal mucocele on MRI (64).
The imaging findings of AE causing acute appendicitis can be challenging, as it is often mistaken for other diseases. In cases of acute appendicitis, the exact cause is not always clear but is often attributed to infection or obstruction. Although endometriosis is a relatively common disease in women of reproductive age, isolated involvement of the appendix is rare. The results of our study suggest that CT and MRI are the preferred modalities for detecting RLQ pathologies in patients with underlying endometriosis, particularly MRI because of the higher soft tissue resolution and ability to detect concomitant endometriotic lesions in both pelvic and extrapelvic locations (65). The RLQ mass, bowel intussusception, mucocele, and bowel wall thickening were the most prevalent reported findings in MRI (66). The bowel wall thickening, obstruction, appendicitis, and free fluid were the most prevalent reported findings in CT.
A major limitation of our review is the absence of eligible observational studies or case series, primarily due to the rarity of symptomatic AE, which reduces the generalizability and strength of the synthesized findings. Our systematic search did not identify any analytical studies containing sufficient cases for inclusion. Even hypothetically, if such studies existed, their descriptive findings would likely focus broadly on clinical outcomes rather than detailed imaging-specific data, potentially introducing heterogeneity and interpretational bias.
In conclusion, RLQ mass and bowel wall thickening are the most commonly reported findings in patients with AE. The MRI appears to be a useful modality in patients suspected of appendicitis and has the added benefit of detecting other foci of pelvic or abdominal endometriosis. We recommend the use of MRI in clinical settings where endometriosis complications are suspected. Further studies are required to retrospectively evaluate the imaging findings of the appendix in pathologically confirmed AE, particularly in patients undergoing pelvic surgery.
