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.