What are the risk factors for developing a fistula in ano, particularly in adults with a history of anal abscesses or anorectal diseases such as Crohn's disease or ulcerative colitis?

Risk Factors for Fistula in Ano

The primary risk factor for developing a fistula in ano is a preceding anorectal abscess, which leads to fistula formation in approximately 33-50% of cases, with Crohn's disease being the most important underlying disease predisposition. 1, 2

Major Risk Factors

Anorectal Abscess (Most Common)

• The majority of anal fistulae arise from preexisting abscesses and represent a spectrum of the same cryptoglandular disease process. 3, 1
• Anorectal abscesses result from infection of the intersphincteric anal glands, with obstruction of the draining duct producing infection that can rupture through the external sphincter. 3
• The recurrence rate after simple abscess drainage can be as high as 44%, with inadequate drainage, loculations, and time from disease onset to incision being key risk factors for fistula development. 1
• Approximately 40% of anorectal abscesses progress to fistula formation. 2

Crohn's Disease (Most Important Underlying Condition)

• Perianal fistulae occur in 13-27% of patients with Crohn's disease, making it the most significant disease-associated risk factor. 3
• Perianal fistulae may be the initial manifestation of Crohn's disease in up to 81% of patients who develop perianal disease. 3
• In a small subset of patients, perianal fistula is the only manifestation of their Crohn's disease. 3
• Complex and multifocal fistulae are more common in Crohn's disease compared to cryptoglandular fistulae. 3
• Crohn's colitis is associated with anal lesions much more frequently than small bowel Crohn's disease (52% vs. 14%). 4
• It is mandatory to exclude underlying Crohn's disease, especially with recurrent presentations, by assessing for inflammatory bowel disease symptoms including diarrhea, weight loss, and abdominal pain. 1

Additional Risk Factors

Infectious and Inflammatory Conditions

• Radiation proctitis can predispose to fistula formation. 3
• Specific infections including HIV, tuberculosis, and actinomycosis are associated with anal fistula development. 3

Iatrogenic and Traumatic Factors

• Prior anal surgery increases fistula risk. 3
• Foreign body penetration can lead to fistula formation. 3

Malignancy-Associated Risk

• Approximately 11% of colovesical and colovaginal fistulae are caused by malignancy. 3
• Carcinoma has been reported in association with anal fistulae in Crohn's disease, and carcinomas may rarely arise in chronic fistulae. 3

Demographic Risk Factors

• Male sex (2:1 male-to-female ratio). 3
• Mean age at presentation is 40 years. 3

Clinical Pitfalls to Avoid

• When an anal lesion is the manifesting sign, Crohn's disease will soon develop elsewhere in the intestine, so physicians must always consider inflammatory bowel disease when dealing with suspicious anal lesions. 4
• The absence of acute infection (no swelling, fever, or leukocytosis) suggests a chronic fistula rather than an active abscess. 1
• Concern for malignancy should be raised when imaging shows a soft mass, mass-like thickening of the anorectal wall, or malignant-appearing lymphadenopathy during evaluation for suspected benign inflammatory disease. 3