What are the causes, types, and treatment options for perianal fistula?

Perianal Fistula: Causes, Types, and Treatment

Perianal fistulas require a combined surgical and medical approach, with treatment strategy determined by fistula classification as either simple or complex, and the presence of underlying conditions such as Crohn's disease. 1, 2

Causes of Perianal Fistulas

• Most perianal fistulas arise from infection of intersphincteric anal glands, which may obstruct and rupture through the external sphincter 1
• Crohn's disease is a significant cause, with perianal fistulizing disease occurring in 13.7-37% of Crohn's patients 1
• Other causes include radiation proctitis, foreign bodies, prior anal surgery, infections (HIV, tuberculosis, actinomycosis), and malignancy 1
• Men are more likely to develop perianal disease than women (15.8% vs 11.6%), with highest incidence in young adults (16-30 years) and a second peak in the elderly (76-90 years) 1

Types of Perianal Fistulas

• Perianal fistulas are classified as either "simple" or "complex" 1, 3
• Simple fistulas are low (superficial, low intersphincteric, or low transsphincteric), have a single external opening, no pain or fluctuation suggesting abscess, no rectovaginal fistula, and no anorectal stricture 1
• Complex fistulas are high (high intersphincteric, high transsphincteric, extrasphincteric, or suprasphincteric), may have multiple external openings, may be associated with perianal abscess, rectovaginal fistula, anorectal stricture, or active rectal disease 1, 3
• Parks classification uses the external sphincter as reference to describe 5 types: superficial, intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric 1

Diagnostic Approach

• Contrast-enhanced pelvic MRI is the initial imaging procedure of choice for assessment of perianal fistulas 1, 2
• Endoscopic anorectal ultrasound (EUS) is a good alternative if rectal stenosis is excluded 1, 2
• Proctosigmoidoscopy should be routinely performed to assess for concomitant rectosigmoid inflammation 1, 2
• Examination under anesthesia (EUA) is the gold standard for definitive diagnosis and classification when performed by an experienced surgeon 1, 2
• Fistulography is not recommended 1

Treatment of Simple Perianal Fistulas

• For uncomplicated low anal fistulas, simple fistulotomy may be considered 1
• Seton placement in combination with antibiotics (metronidazole and/or ciprofloxacin) is the preferred initial strategy 1, 2
• For recurrent refractory simple fistulizing disease not responding to antibiotics, thiopurines (azathioprine or mercaptopurine) or anti-TNF agents can be used as second-line therapy 1, 2
• Fistulotomy has the highest success rate for simple fistulas 3

Treatment of Complex Perianal Fistulas

• Imaging before surgical drainage is recommended, and EUA for surgical drainage of sepsis is mandatory 1
• Abscess drainage and loose seton placement should be performed as the initial step 1
• Anti-TNF therapy (particularly infliximab) combined with immunomodulators is considered first-line medical treatment after surgical drainage and seton placement 2, 3
• For complex fistulas, non-cutting setons, advancement flaps, and ligation of intersphincteric fistula tract (LIFT) are considered, depending on fistula characteristics 3, 4
• The height of the internal fistula opening (≥15 mm) is associated with recurrence after LIFT procedure 4

Special Considerations for Crohn's Disease

• Active luminal Crohn's disease should be treated if present, in conjunction with appropriate surgical management of fistulas 1, 3
• More than two-thirds of Crohn's disease patients have an abscess associated with their fistula, which must be drained before considering operative intervention 1
• Crohn's patients are more likely to have setons placed and less likely to undergo fistulotomy 1
• Maintenance therapy with thiopurines, infliximab, adalimumab, seton drainage, or a combination is recommended 1

Monitoring Response to Treatment

• Clinical assessment (decreased drainage) is usually sufficient to evaluate response to treatment 1
• MRI or anal endosonography in combination with clinical assessment is recommended to evaluate improvement of fistula track inflammation 1, 2
• Clinical closure does not always equal MRI closure, with high risk of recurrence without complete fibrotic tract on MRI 2

Emerging Treatments

• Platelet-rich fibrin (PRF) sealant has shown promise as an outpatient treatment option, with complete healing in 52.86% of patients 5
• Video-assisted anal fistula treatment (VAAFT) combined with advancement flaps has shown good success rates 3
• For refractory cases, expanded allogeneic adipose-derived stem cells have shown promise 3

Prognosis and Complications

• Despite combined medical-surgical management, active symptomatic complex perianal fistulas may persist in up to 58% of patients 6
• Symptom remission can be achieved in about 21% of cases, with fistula closure in another 21% 6
• Long-term monitoring for development of malignancy in chronic perianal fistula tracts is recommended 3