How often do untreated anal fissures in Crohn's progress to abscess or fistula, and what risk factors predict progression?

1. How often: what the data show about progression rates

Small cohort and surgical series focused on fissures report that roughly one quarter of unhealed fissures evolved to an abscess or fistula — Fleshner et al. reported 13 of 50 patients (26%) with unhealed fissures developing an anal abscess or fistula ^[1]; larger literature on perianal manifestations of Crohn’s places the cumulative incidence of perianal fistulas in the disease population at about 23–38% over time, with an approximate 25% figure commonly cited as cumulative risk for fistulizing perianal disease ^{[2] [4]}. These two types of estimates are complementary: the fissure‑to‑abscess/fistula conversion in treated cohorts (~25%) and population‑level cumulative incidence of perianal fistulas in Crohn’s patients (≈23–38%) both indicate that progression is a common and clinically important event ^{[1] [2]}.

2. Who is at higher risk: consistent clinical predictors

Several consistent predictors emerge across reviews and cohort studies: rectal or colonic disease location (especially rectal involvement) is the strongest anatomical risk factor for perianal fistula formation ^{[3] [2]}; male sex appears repeatedly as a risk factor for perianal sepsis and fistula in multiple series ^{[5] [6]}; active rectal inflammation or refractory proctitis, presence of anorectal strictures, and prior or concurrent perianal abscess increase the probability that a fissure will progress ^{[6] [7] [3]}. Smoking has been identified as a modifiable risk factor associated with anal abscess and fistula ^[6]. Acute versus chronic and painless versus painful fissures also predicted response to medical therapy in older series (male gender, painless fissure, and acute fissure predicted medical success), implying chronic, painful fissures are likelier to persist and complicate ^{[1] [8]}.

3. Biological plausibility and mechanism linking fissure → abscess/fistula

Anal fissures or deep anorectal ulcers provide a portal for infection or penetrating inflammation; a fissure may seed an anal gland or elongate into surrounding tissues, producing local sepsis and then a draining tract — classic pathophysiologic explanations in the literature propose both cryptoglandular infection and deep penetrating ulceration as origins of Crohn’s fistulae ^{[7] [2]}. Crohn’s‑related impaired wound healing, transmural inflammation, and immunosuppression (from disease or therapy) further favor progression and chronicity ^[7].

4. What this means for management and outcomes

Because unhealed fissures have a documented propensity to develop into abscess or fistula, many authoritative reviews urge prompt medical management of fissures in Crohn’s rather than expectant observation; untreated or refractory perianal disease may ultimately require more invasive interventions including setons, biologic therapy, or even proctectomy in extreme cases ^{[9] [7] [2]}. Older surgical series report higher healing rates after targeted anorectal procedures and warn that proximal intestinal resection does not reliably prevent local perianal complications ^[1].

5. Limits, heterogeneity, and what the literature does not allow

Estimates vary because studies differ by design (small retrospective surgical cohorts vs population‑based series), by how “untreated” or “unhealed” is defined, and by follow‑up completeness; the 26% fissure‑to‑abscess/fistula figure comes from a 50‑patient follow‑up group in a single study and may not generalize to all settings ^[1]. Population cumulative incidence (23–38%) reflects all perianal fistula causes and timing and cannot be translated directly into a per‑fissure short‑term risk without caution ^{[2] [4]}. Non‑fistulizing perianal disease is less well studied than fistulizing disease, adding uncertainty about exact conversion rates across different patient subgroups ^[10].

6. Bottom line

Untreated or unhealed anal fissures in Crohn’s disease carry a meaningful risk of progressing to perianal abscess or fistula — cohort data indicate about one in four such fissures may complicate, while population studies place cumulative fistula risk in Crohn’s patients in the mid‑20% range — and patients with rectal/colonic disease, active rectal inflammation, anorectal stricture, male sex, smoking, and prior perianal sepsis are at higher risk ^{[1] [2] [6] [3]}. Heterogeneity in study designs limits precise individual prognostication, but the balance of evidence supports early, aggressive medical assessment and targeted management of fissures in Crohn’s disease to reduce progression risk ^{[9] [7]}.