Background
Approximately 20% of patients with Crohn’s disease (CD) have perianal fistulizing CD, which are most often complex perianal fistula (PAF). Studies have shown that 4-5% of patients with CD present with PAF without luminal disease, termed isolated perianal CD (iPCD). Little is known about their natural history.

Methods
We performed a retrospective cohort study of patients seen within the Mount Sinai Health System (New York) with an ICD-10 diagnosis code for PAF between 7/22/2008 and 7/22/2022. Medical record review was conducted to identify patients with PAF, considered as possible iPCD, who did not have evidence of luminal disease and had > 6 months of follow-up (Table 1). Demographics, comorbidities, imaging, endoscopy, surgeries, hospitalizations and medication data were collected. These patients were reviewed to determine progression to luminal disease, which was defined as any evidence of luminal ulcers, erosions, or strictures on endoscopy, video capsule exam, or MRE. Descriptive and univariate analyses were performed. Complex PAF was defined by American Gastroenterological Association (AGA) criteria. Complicated disease course was defined as a composite of any major CD-related surgery, PAF/IBD-related hospitalization or biologic use.

Results
Among 887 patients with PAF, 62.2% (n=552) patients underwent luminal evaluation, of whom 80 patients (14.5%) were found to have no luminal disease (Table 1). 57 of these 80 patients had >6 months follow-up. All 57 patients had complex PAF per AGA criteria and were considered as having iPCD. Luminal disease developed in 13 patients (23%) with a mean time to diagnosis of 4.2 +/- 3.8 years after PAF presentation. Patients who developed luminal CD were more likely to be <30 years old at PAF diagnosis (p=0.0225), had >2 fistulas on imaging (p=0.002), and have concomitant autoimmune conditions (p=0.046) (Table 2). During the follow-up period, the patients who developed luminal CD were more likely to receive biological therapy (77% vs 16%, p<0.0001) or undergo CD/PAF-related hospitalization (62% vs 30%, p =0.0356) (Table 2). Overall, patients who developed luminal CD had a more complicated disease course (31% vs 2.3%, p=0.007) than those who did not.

Conclusion
Approximately 1 in 4 patients who presented with possible iPCD developed luminal CD at follow-up. Patients who developed luminal CD were more likely to be younger at diagnosis, had more fistula tracts, and had concomitant autoimmune conditions.

Serre-Yu Wong

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