Watery Diarrhea Caused by AA-Amyloid Deposition Mimicking Collagenous Colitis in Antisynthetase Syndrome-Lupus Overlap

Collagenous colitis-like amyloid deposition in the gastrointestinal tract is an uncommon pattern of gastrointestinal amyloidosis. This pattern is usually associated with AA-amyloid. The recognition of this pattern is difficult for an unaware pathologist. We report collagenous colitis-like deposition of AA amyloid in a 45-year-old woman at the time of autopsy. The woman suffered from antisynthetase syndrome—systemic lupus erythematosus overlap syndrome—was on continued immunosuppression and presented with fever, watery diarrhea, and vomiting. The amyloid showed a band-like congophilic subepithelial deposition throughout the large intestine and terminal ileum. This pattern of amyloid deposition can be distinguished from collagenous colitis by its pale blue-green color on Masson trichrome stain, frayed/fuzzy edges, congophilia with apple-green birefringence, and SAA-positivity.