Isolated perianal Crohn's disease: a systematic review and expert consensus proposing novel diagnostic criteria and management advice.

Perianal fistulae can present a diagnostic challenge when distinguishing perianal Crohn's disease from idiopathic perianal fistulae. This distinction is key, as perianal Crohn's disease requires inflammatory bowel disease (IBD) therapy, whereas idiopathic perianal fistulae are primarily managed surgically. When luminal inflammation is absent, but Crohn's disease is suspected, the term isolated perianal Crohn's disease is sometimes applied. However, no formal guidance exists for diagnosing or managing isolated perianal Crohn's disease. Furthermore, whereas fistula granuloma might provide specific evidence of Crohn's disease, they are rarely detected in perianal fistulae, so are unlikely to sensitively delineate aetiology. This project aimed to develop an opinion-based framework for isolated perianal Crohn's disease. A systematic review evaluated clinical features suggestive of isolated perianal Crohn's disease and evidence for IBD therapies in patients with perianal fistulae without luminal IBD. The findings of this systematic review informed a multidisciplinary consensus process with IBD specialists, resulting in the formulation of diagnostic criteria and management recommendations for isolated perianal Crohn's disease. We then tested this diagnostic approach in a prospective cohort of 50 patients treated as having idiopathic perianal fistulae at a proctology referral centre, identifying six (12%) individuals as meeting the diagnostic criteria. Our findings underscore the scarcity of evidence guiding isolated perianal Crohn's disease diagnosis and the need for a composite risk-based assessment. The proposed framework provides a tool for clinical practice and research but requires validation and refinement. Clear communication with patients is essential, given the diagnostic and therapeutic uncertainties. Future studies should refine these criteria, explore biological markers for isolated perianal Crohn's disease, and establish evidence-based methods to differentiate isolated perianal Crohn's disease within the perianal fistulae spectrum.