A 78-year-old man visited our department following classification B in gastric cancer screening by ABC method. Upper gastrointestinal endoscopy revealed multiple black mucous membranes in his stomach. Subsequent hematoxylin and eosin staining of sections revealed small clusters of eosinophilic non-structural substances that also tested positive for amyloids through Congo red staining. Additional testing indicated a diagnosis of AL-type amyloidosis, revealing systemic amyloidosis in the lower gastrointestinal tract, kidneys, and bone marrow. Typically, amyloid deposits are found in the gastrointestinal tract in 90% of systemic amyloidosis cases, with endoscopic findings providing variable results. AL-type deposits are found deeper in the mucosal muscularis and may also present with a submucosal mass-like ridge. Thus, we encountered a rare case of frequent black mucosa where submucosal hemorrhage was presumably formed by amyloid deposition in the mucosal muscularis area causing peripheral circulatory disturbance.
