Clinical characteristics of isolated perianal Crohn’s disease and validation of the TOpClass criteria: a retrospective cohort pilot study

Background: Isolated perianal Crohn’s disease (ipCD) is characterized by recurrent or refractory complex perianal fistulas without luminal inflammation. This study aimed to compare clinical characteristics among ipCD, perianal Crohn’s disease (pCD) with luminal involvement, and refractory/recurrent cryptoglandular disease (CGD), assess the diagnostic performance of the TOpClass criteria, and evaluate treatment outcomes in ipCD. Methods: This retrospective cohort pilot study included patients with complex perianal fistula confirmed by pelvic magnetic resonance imaging. ipCD was defined as recurrent or refractory complex perianal fistula persisting beyond 6 months post-surgery, absence of luminal inflammation, and at least a clinical response to biologics. Results: Among 33 patients (ipCD = 9, pCD = 9, CGD = 15), ipCD patients were older than pCD (38 vs 27 years, P = .020), had lower fecal calprotectin (268 vs 2498 mg/kg, P = .014), and more complex fistula types (P = .029). Compared to CGD, ipCD patients were younger (38 vs 47 years, P = .032), more likely to have anal stenosis (55.6% vs 0%, P = .003), branched fistula (100.0% vs 46.7%, P = .009), multiple internal openings (44.4% vs 6.7%, P = .047), horseshoe extensions (77.8% vs 20.0%, P = .010), and deeper internal opening (3.8 vs 2.7 cm, P = .006). The TOpClass criteria demonstrated 75.0% sensitivity and 100% specificity for diagnosing ipCD. Fistula remission at 6 months was significantly lower in ipCD than pCD (11.1% vs 100%, P = .007). Conclusion: ipCD represents a distinct phenotype with advanced fistula complexity and limited response to biologics. Younger age, anal stenosis, and more advanced fistula characteristics favor ipCD over CGD. Applying TOpClass criteria showed high specificity but moderate sensitivity; incorporating additional fistula features may improve sensitivity.