Gastrointestinal amyloidosis: an often unexpected finding with systemic implications.

The gastrointestinal (GI) tract is a common site of amyloidosis, but the incidence, clinicopathologic features, and systemic implications of different types of GI amyloidosis are not well understood. GI amyloid specimens (N = 2511) typed using a proteomics-based method between 2008 and 2021 were identified. Clinical and morphologic features were reviewed in a subset of cases. Twelve amyloid types were identified, including AL (77.9%), ATTR (11.3%), AA (6.6%), AH (1.1%), AApoAIV (1.1%), AEFEMP1 (0.7%), ALys (0.4%), AApoAI (0.4%), ALECT2 (0.2%), Aβ2M (0.1%), AGel (0.1%), and AFib (<0.1%). Amino acid abnormalities indicative of known amyloidogenic mutations were detected in 24.4% ATTR cases. AL, ATTR, and AA types all commonly involved submucosal vessels. They also showed some characteristic patterns of involvement of more superficial anatomic compartments, although there was significant overlap. Common indications for biopsy were diarrhea, GI bleed, abdominal pain, or weight loss. Amyloidosis was usually an unexpected finding, but most AL and ATTR patients were ultimately found to have cardiac involvement (83.5% of AL; 100% of ATTR). Although most GI amyloid is of AL type, over 10% are ATTR, over 5% are AA, and twelve different types were identified in total. GI amyloid is often unexpected but usually signals systemic amyloidosis, thus there should be a low threshold to perform biopsy with Congo red stain in patients with unexplained GI symptoms. Clinical and histologic features are nonspecific, and typing should be performed via a robust method such as proteomics as treatment hinges on correctly identifying the amyloid type.