Spondyloarthritis Associated With Collagenous Colitis-Case Report and Literature Review.

Collagenous colitis (CC) is a subtype of microscopic colitis characterized by chronic watery diarrhea with normal endoscopic findings. Although often linked to autoimmune diseases, its association with spondyloarthropathy (SpA) is rare and poorly documented. A 33-year-old man with longstanding diarrhea and polyarthralgia was initially misdiagnosed with irritable bowel syndrome and fibromyalgia. After developing palmoplantar pustulosis and worsening musculoskeletal pain, repeat colonoscopy with biopsies confirmed CC. Imaging revealed sternal erosions and metatarsophalangeal effusions, supporting a diagnosis of SpA. Treatment with budesonide, mesalazine, and methotrexate provided partial improvement. A TNF-α inhibitor is being considered to target both intestinal and joint involvement. This case underscores the diagnostic complexity of CC and its potential association with SpA. Shared immune mechanisms suggest a common pathogenesis, highlighting the importance of multidisciplinary management for optimal care.