Association between intestinal functional disorders and anal fistula: evidence from a retrospective case–control study

Background Clinical observations indicate that some individuals experience bowel dysfunction, such as diarrhea or constipation, prior to the diagnosis of anal fistula (AF). These observations suggest a potential association between pre-existing intestinal dysfunction and the occurrence of AF. However, systematic clinical evidence remains limited. This study aimed to evaluate the association between pre-diagnostic bowel dysfunction and AF and to explore whether such associations differ across AF anatomical subtypes. Methods We conducted a retrospective hospital-based case-control study including 251 patients with newly diagnosed AF between July 2024 and July 2025. During the same period, 251 age- and sex-matched individuals without AF were selected as controls (1:1 matching; age ± 3 years). Demographic characteristics, lifestyle factors, and bowel function within 1 year before the index date were collected from medical records and supplementary interviews where applicable. Conditional logistic regression, accounting for the matched-pair design, was used to assess the association between bowel dysfunction and AF. Both crude (unadjusted) and adjusted analyses were performed. Stratified analyses were performed to explore heterogeneity across AF subtypes. Results Pre-existing bowel dysfunction was associated with a higher likelihood of developing AF ( p < 0.05). Diarrhea showed a significant association with AF (OR ≈ 2.2), whereas constipation was not significantly associated. Smoking, alcohol consumption, prolonged sedentary or standing occupations, and higher body mass index (BMI) were also associated with AF. In exploratory analyses of AF subtypes, diarrhea showed modest heterogeneity across anatomical classifications, whereas constipation showed no significant variation across subtypes. Conclusions This retrospective case-control study indicates that pre-existing intestinal functional disorders, particularly chronic diarrhea, are associated with a higher likelihood of AF, with the association appearing more pronounced among low anal fistulas. No consistent subtype-specific pattern was observed overall, although diarrhea showed modest heterogeneity across anatomical classifications in exploratory analyses. Given the retrospective observational design, these findings support association rather than causation and warrant confirmation in multicenter prospective studies. Clinically, these results suggest that earlier identification and risk assessment of patients with chronic diarrhea and other intestinal functional disturbances may provide valuable insights, but further research is needed to confirm these findings.