Deep infiltrating endometriosis (DIE) of the bowel remains an under-recognized challenge in endoscopic practice. Lesions frequently mimic subepithelial tumors or infiltrative neoplasia, leading to diagnostic errors reported in up to 60–70% of reviews and resulting in delayed diagnosis. Rectosigmoid involvement predominates, whereas small-bowel and duodenal disease are exceptionally rare. The absence of reproducible endoscopic criteria and the infrequent use of EUS further complicate preoperative recognition, contributing to misclassification as malignancy.

Aims: To describe reproducible endoscopic and EUS markers of bowel DIE with histological confirmation and to evaluate the role of EUS-FNB in diagnosing atypical subepithelial lesions.

Two patients with histologically confirmed intestinal DIE complicated by gastrointestinal bleeding were analyzed. A comprehensive correlation of endoscopic, EUS, and histopathological findings was performed, including EUS-guided fine-needle biopsy (FNB).

A structured literature search and analysis were conducted using PubMed, Embase, and Scopus databases (2019-2025), focusing on bowel endometriosis and its endoscopic visualization.

1. Sigmoid involvement

Colonoscopy: segmental wall rigidity, concentric luminal narrowing, mucosal deformation with preserved epithelium.

EUS: hypoechoic thickening of the muscularis propria with mesenteric involvement.

Histology: confirmed DIE.

2. Duodenal involvement (exceptionally rare)

EGD: nodular, bleeding protuberant lesions distal to the major papilla.

EUS: deep hypoechoic infiltrate with loss of wall stratification and extension into paraduodenal tissue.

EUS-FNB: morphological confirmation of endometriosis (PAX8+, ER+).

Morphologically verified cases of duodenal DIE diagnosed by EUS-FNB are exceedingly rare or absent in the published literature.

3. Literature data

Bowel DIE occurs in ~5-12% of patients with endometriosis, predominantly affecting the rectosigmoid segment.

EUS demonstrates high sensitivity for assessing depth of infiltration (~82–94% in clinical series), whereas standard endoscopy remains limited in detecting submucosal disease.

Practical implications / Reproducible markers

Based on clinical observations and literature analysis, the following reproducible endoscopic and EUS markers of bowel DIE were identified:

    1.    Wall rigidity and luminal narrowing without ulceration.

    2.    Mucosal deformation with preserved epithelial surface.

    3.    Atypical bleeding nodular lesions in the duodenum.

    4.    Hypoechoic infiltrative contour on EUS with involvement of adjacent tissues.

These markers emphasize the diagnostic value of combined endoscopy and EUS, supporting accurate preoperative recognition of bowel DIE and reducing the risk of misinterpretation as neoplasia.

These observations enhance clinical awareness of endoscopic and EUS features of bowel DIE and demonstrate the diagnostic value of EUS-FNB even in extremely rare duodenal involvement. The findings may serve as a foundation for developing standardized imaging criteria and for future multicenter prospective validation under the auspices of ESGE.