Gastric involvement in multiple myeloma-associated AL amyloidosis: a case with serial endoscopy and a pooled analysis of 30 published cases.

Objective: To describe serial endoscopic findings in a patient with relapsed/refractory multiple myeloma (MM) and gastric amyloidosis and to summarize 30 published cases, with emphasis on their clinical, endoscopic, and histopathologic features and common diagnostic challenges. Methods: This study comprised a case report and an individual-patient pooled analysis of published cases. We reviewed the clinical course and serial endoscopic assessments of the index patient and searched PubMed, Web of Science, Cochrane Library, and Embase from inception through June 1, 2026. Reports of MM-associated gastric amyloidosis with extractable clinical information were included. Demographic characteristics, symptoms, sites of involvement, endoscopic findings, diagnostic methods, clonal characteristics, treatment, and outcomes were analyzed descriptively. Results: During myeloma relapse, the index patient developed refractory nausea and vomiting. Routine histopathologic examination of a biopsy from a proliferative lesion in the gastric fundus showed a fundic gland polyp, while Congo red staining was weakly positive. Symptoms improved substantially after anti-multiple myeloma treatment was resumed, and repeat endoscopy after two cycles of anti-multiple myeloma treatment showed fewer and smaller polypoid elevations. As the disease subsequently progressed, nausea and vomiting recurred intermittently and the polypoid elevations gradually increased, raising the possibility that the gastric manifestations tracked plasma-cell clonal activity. Thirty previously published patients were included. Bleeding-related manifestations were most common (20/30, 66.7%), followed by nausea and vomiting (10/30, 33.3%). The antrum was the most frequently involved site (16/26, 61.5%). Endoscopic findings were available for 28 patients; nodules, polyps, masses, or other nonhematomatous elevated lesions predominated (16/28, 57.1%). Among 25 patients with a clear diagnostic sequence, 14/25 (56.0%) were diagnosed with MM or smoldering multiple myeloma after gastrointestinal amyloid deposition was identified, whereas 11/25 (44.0%) developed gastric involvement in the setting of previously diagnosed MM. Conclusions: MM-associated gastric amyloidosis lacks specific symptoms and endoscopic features. Bleeding, refractory vomiting or obstruction, and tumor-like lesions are important presentations. In patients with MM who have unexplained persistent gastrointestinal symptoms, or discordance among the clinical presentation, endoscopic phenotype, and histopathologic findings, multiple adequate biopsies should be obtained, with deeper sampling when necessary, followed by Congo red staining, confirmation under polarized light, and amyloid protein typing.