AA Amyloidosis in a Patient with Rheumatoid Arthritis and Systemic Lupus Erythematous

Introduction: Amyloid A (AA) amyloidosis, one of the most severe complications of rheumatoid arhritis, is a serious, potentially life-threatening disorder caused by deposition of AA amyloid fibrils in multiple organs. These AA amyloid fibrils derive from the circulatory acute-phase reactant serum amyloid A protein. New biologics may permit AA amyloidosis secondary to RA to become a treatable, manageable disease. Here, we present a clinical case of AA amyloidosis in a patient wih rheumatoid arhritis and systemic lupus erythematous. Case Description: A 49-year-old woman presented with subnephrotic proteinuria since December 2024 and creatinine elevation. She had history of rheumatoid arthritis since 2016, initially treated with methotrexate for four years, however, she stopped taking it by own decision and replaced with homeopathy and steroids, she developed Cushing syndrome in 2023, currently only taking opioids, and systemic lupus erythematous recently diagnosed by rheumatology in November 2024 in the presence of anti-double-stranded DNA antibody positive, hypocomplementemia, and arthralgias. On the 24-hour urine collection, she had 1078 mg of proteinuria, 343 mg of albuminuria. A renal biopsy was performed, expecting a lupus nephritis class III, but revealed amyloid deposits within tubulointerstitial and vascular components, Congo red stain was positive, and AA amyloidosis was diagnosed. Discussion: Among patients with AA amyloidosis, rheumatoid arthritis is the most common associated etiology within rheumatic diseases. However, this clinical case is complex, with the recent diagnosis of systemic lupus erythematous, we expected a lupus nephritis class III in the biopsy, remarking the importance of the tubulointerstitial component in the renal biopsy.