Collagenous gastritis: clinical features, histologic correlates and unanswered questions.

Collagenous gastritis (CG) is a rare gastrointestinal disorder characterized by subepithelial collagen deposition and lamina propria inflammation. Despite its first description over four decades ago, the pathogenesis remains unclear, with no standardized pathologic criteria/classification, treatment or established prognosis. A systematic PubMed search identified all English-language case reports, series and observational studies describing CG. Data on demographics, clinical presentation, endoscopic appearance and histologic features were extracted. Of the 133 patients with available demographic data, 101 patients had corresponding histologic information, reflecting the overall rarity and limited characterization of collagenous gastritis in the literature. The most common presenting symptoms were abdominal pain and chronic anaemia. A subset of patients had concurrent collagenous colitis, collagenous sprue, Helicobacter pylori gastritis or celiac disease. The predominant endoscopic finding was gastric mucosal nodularity, typically diffuse or corpus-predominant. Collagen band thickness ranged widely, with a median of 50.5 μm and a maximum of 225 μm. Band distribution was most commonly pan-gastric or corpus-predominant. Among the proposed histologic subtypes of CG, the atrophic pattern was most frequently observed and often correlated with isolated gastric involvement and lack of clinical or histologic remission. This review highlights that histologic classification may help guide differential diagnoses and prognosis. Accordingly, we advocate for explicit reporting of these features in pathology reports. Key gaps in pathogenesis, including the roles of environmental and genetic factors, are also reviewed. This review synthesizes current knowledge of CG and underscores the need for further studies to clarify disease mechanisms. Improved histologic classification and exploration of underlying aetiologies may enhance diagnosis, treatment and research in this underrecognized condition.