Risk Factors for Fistula-in-Ano
The majority of anal fistulas arise from preexisting anorectal abscesses due to infection of the intersphincteric anal glands, with male sex, age 20-40 years, and Crohn's disease being the most significant risk factors. 1
Primary Risk Factors
Demographic Factors
- Male sex: Men are affected twice as commonly as women (2:1 ratio), with males having a 15.8% incidence compared to 11.6% in females 1
- Age: Peak incidence occurs in young adults aged 20-40 years, with a mean age at presentation of 40 years 1
- Second age peak: Elderly patients aged 76-90 years show increased incidence 1
Pathophysiologic Mechanisms
- Preexisting anorectal abscess: The majority of anal fistulas develop from prior abscesses, representing a spectrum of the same disease process 1
- Cryptoglandular infection: Obstruction of the draining duct from infected intersphincteric anal glands is the most commonly accepted mechanism 1
- Anorectal abscess association: Approximately one-third of patients with anorectal abscesses will develop fistulas 1
Disease-Associated Risk Factors
Crohn's Disease (Most Important Secondary Cause)
- High prevalence: Perianal fistulas occur in 13-27% of all Crohn's disease patients 1
- Colonic disease with rectal involvement: Prevalence reaches as high as 92% when rectum is involved 1
- Initial manifestation: Perianal fistulas may be the first sign of Crohn's disease in up to 81% of patients who develop perianal disease 1
- Complex fistulas: Crohn's disease patients more commonly develop complex and multifocal fistulae 1
- Gender difference in Crohn's: Men with Crohn's disease are more likely to develop perianal disease than women 1
Other Medical Conditions
- Diabetes mellitus: Undetected diabetes increases risk of anorectal sepsis and subsequent fistula formation 1
- HIV infection: Immunocompromised states predispose to infectious complications 1
- Tuberculosis and actinomycosis: Chronic infections can cause fistula formation 1
Iatrogenic and Trauma-Related Risk Factors
Surgical and Medical Interventions
- Prior anal surgery: Previous anorectal procedures increase fistula risk 1
- Radiation proctitis: Pelvic radiation therapy can lead to fistula development 1
- Foreign body: Retained foreign material in the anorectum 1
Obstetric Trauma
- Childbirth injury: Obstetric trauma is the most common cause of anovaginal fistulas 2
Malignancy-Associated Risk Factors
Cancer Considerations
- Anal carcinoma: Carcinomas may rarely arise in chronic fistulas in the anorectum 1
- Crohn's-associated malignancy: Carcinoma has been reported in association with anal fistulae in Crohn's disease 1
- Colovesical/colovaginal fistulas: Approximately 11% are caused by underlying malignancy 1, 3
Clinical Pitfalls to Avoid
- Recurrent fistulas: Always exclude undiagnosed Crohn's disease in patients with recurrent anorectal abscesses or fistulas, especially if previous surgical repair has failed 1, 2
- Proctosigmoidoscopy: Should be performed routinely in initial evaluation since concomitant rectosigmoid inflammation has prognostic and therapeutic relevance 1
- Complete examination: Inspect perineum for surgical scars, anorectal deformities, and other signs of perianal Crohn's disease in all patients 1