In a healthy adult with a single low trans‑sphincteric or inter‑sphincteric anal fistula and no Crohn’s disease, abscess, prior anorectal surgery, or active proctitis, how do I distinguish a simple from a complex fistula and what is the appropriate management for each?

Distinguishing Simple vs. Complex Anal Fistula and Management

Classification Criteria

A simple fistula is defined by: low anatomical origin (intersphincteric or low transsphincteric), single external opening, absence of pain or fluctuation suggesting abscess, no rectovaginal involvement, and no anorectal stricture. 1

A complex fistula has one or more of: high anatomical origin (high transsphincteric, suprasphincteric, or extrasphincteric), multiple external openings, associated pain or fluctuation suggesting abscess, rectovaginal involvement, or active rectal disease on endoscopy. 1

Anatomical Definitions

• Intersphincteric fistulas (54% of all anal fistulas) have tracts running between the internal and external anal sphincters—these are classified as simple when low-lying. 1

• Low transsphincteric fistulas (subset of the 21% transsphincteric category) cross through the lower portion of the external sphincter into the ischioanal fossa—these are classified as simple when involving minimal sphincter muscle. 1

• High transsphincteric, suprasphincteric, and extrasphincteric fistulas are always classified as complex due to significant sphincter involvement and risk of incontinence. 1

Diagnostic Evaluation

Examination under anesthesia (EUA) by an experienced colorectal surgeon is approximately 90% accurate in detecting and classifying perianal fistulas, but preoperative imaging with MRI or endoanal ultrasound is recommended for surgical planning. 1

• MRI with gadolinium contrast demonstrates sensitivity of 92-97% and specificity of 85-96% for discriminating complex from simple perianal fistula, superior to endoanal ultrasound (75% sensitivity, 64% specificity). 2

• The American College of Radiology recommends pelvic MRI with multichannel phased array body coil as the standard for imaging perianal fistula, particularly for complex cases with clinically occult tracts. 2

• Gadolinium-based IV contrast is preferred because active inflammation in fistulous tracts enhances avidly, abscesses show rim-like enhancement, and contrast enables differentiation of inactive tracts (diffuse enhancement) from active tracts (ring enhancement). 2

Critical Clinical Pearls

• Digital rectal examination should identify the internal opening at the dentate line (pathognomonic for cryptoglandular fistulas), palpable cord-like structures, and assess for fluctuation or abscess. 3

• Do not probe for occult fistulas during examination, as this risks creating iatrogenic fistula tracts. 3

• Mandatory exclusion of Crohn's disease is required in any patient with recurrent perianal fistulas through focused history for inflammatory bowel disease symptoms (diarrhea, weight loss, abdominal pain), as Crohn's disease occurs in approximately one-third of patients with anorectal abscess and markedly reduces surgical success rates. 3

Management Approach

Simple Fistulas (Intersphincteric or Low Transsphincteric)

Fistulotomy is the gold standard surgical treatment for simple fistulas, with healing rates exceeding 95% and low recurrence rates. 4

• Fistulotomy (laying open the fistula tract) is relatively safe for simple, distal fistulas where minimal sphincter muscle is involved. 5

• Patient selection is crucial for low transsphincteric fistulas to determine whether safe fistulotomy can be performed versus requiring a sphincter-saving technique. 4

• Fistulectomy (excision of the entire tract) is an alternative to fistulotomy for simple fistulas, though fistulotomy remains more commonly performed. 6

Complex Fistulas (High Transsphincteric, Suprasphincteric, Extrasphincteric)

Fistulotomy is contraindicated in complex fistulas due to risk of anal sphincter damage with subsequent fecal incontinence; only sphincter-preserving techniques should be used. 7, 4

The ligation of intersphincteric fistula tract (LIFT) procedure and rectal advancement flaps provide optimal outcomes with healing rates of 60-90% in complex fistulas. 4

Sphincter-Preserving Options

• LIFT procedure appears safe and effective and may be routinely considered for complex anal fistula, with success rates of 60-95%. 8, 5

• Rectal advancement flaps achieve healing rates of 60-90% while preserving sphincter function. 4

• Seton placement (62% of procedures in one large series) is commonly used as first-line treatment for complex fistulas and Crohn's patients, providing drainage and allowing staged definitive repair. 6

• Novel techniques including transanal opening of intersphincteric space (TROPIS), fistula laser closure (FiLaC), and video-assisted anal fistula treatment (VAAFT) show healing rates of 65-90% but require further evaluation. 8, 4

• Anal fistula plug is safe but modestly effective with variable success rates (24-88%), with failures often due to plug extrusion. 5

• Fibrin glue shows poor and variable healing rates (14-74%). 5

Common Pitfalls

• Never perform fistulotomy on complex fistulas involving significant sphincter muscle, as this inevitably causes incontinence. 7

• Preoperative imaging should not be omitted for complex cases, as clinical examination alone may be insufficient for accurate classification. 1

• Adjuvant medical therapy is required for both palliative and definitive procedures in complex fistulas. 7

• Incontinence risk exists even with sphincter-preserving techniques; reported major incontinence rates are 1.3% with careful technique. 6