Fistula Anorektal

01Pathophysiology

The dominant theory behind anorectal fistula formation is the cryptoglandular hypothesis. The anal canal contains 6 to 10 anal glands (glands of Hermann and Desfosses) that are located at the level of the dentate line and whose ducts empty into the anal crypts. These glands penetrate the internal sphincter and terminate in the intersphincteric space. When a crypt becomes obstructed, stasis of glandular secretions creates a favorable environment for bacterial overgrowth, leading to an intersphincteric abscess. This abscess can then track along tissue planes to present as a perianal, ischiorectal, or supralevator abscess depending on the direction of spread.
Once an abscess is drained, either surgically or through spontaneous rupture, the acute infection resolves but the epithelialized tract that connected the internal opening (at the infected crypt) to the external opening (on the perianal skin) may persist. This persistent communication is the fistula. The reason it does not heal is that the internal opening continues to seed the tract with enteric organisms, and the epithelial lining of the tract prevents it from collapsing and closing. This is why patients classically present with ongoing perianal discharge weeks to months after an abscess was drained, and why recurrent abscess in the same location should always raise suspicion for an underlying fistula.
In patients with Crohn disease, the pathogenesis is different. The transmural inflammation characteristic of Crohn directly penetrates through the bowel wall and perianal tissues, forming complex, often multiple fistulous tracts that do not follow the typical cryptoglandular pattern. This distinction is critical because Crohn-related fistulas require medical optimization before any surgical intervention.

02Classification and Clinical Manifestation

The Parks classification is the standard anatomical system used to describe anorectal fistulas based on the relationship of the tract to the anal sphincter complex. Understanding this classification is essential because it directly determines surgical approach and risk of postoperative incontinence.

A fistula is classified as complex when any of the following features are present: the tract crosses more than 30% of the external sphincter (high transsphincteric, suprasphincteric, or extrasphincteric), there are multiple tracts or secondary extensions, it is anterior in a female patient, it occurs in the setting of Crohn disease, there is preexisting incontinence, or there has been prior radiation. Simple fistulas are low intersphincteric or low transsphincteric tracts without any of these features.

Goodsall rule is a clinical guide used to predict the location of the internal opening based on the position of the external opening:

Note that Goodsall rule is a clinical approximation and is less reliable for anterior fistulas, fistulas with external openings more than 3 cm from the anal verge, and Crohn-related fistulas.

03Diagnostic Workup

The workup begins with a thorough physical examination. Inspect the perianal region for an external opening, which typically appears as a small area of granulation tissue or a dimple with expressible discharge. Digital rectal examination may reveal the internal opening as a nodularity or pit at the dentate line, and a palpable cord of induration between the openings. Assess resting and squeeze sphincter tone, as this baseline is critical for surgical planning.
When the anatomy is straightforward and the fistula appears simple on clinical examination, imaging may not be required before proceeding with fistulotomy. However, MRI of the pelvis is indicated for any complex, recurrent, or Crohn-related fistula. MRI provides the highest sensitivity (approximately 90 to 95%) for mapping the primary tract, identifying horseshoe extensions, and detecting occult abscess collections that would cause surgical failure if left undrained. The St James University Hospital MRI classification is commonly used to grade fistula complexity on imaging.
Endoanal ultrasound is a reasonable alternative when MRI is unavailable or contraindicated. It visualizes the sphincter complex well but is less reliable for suprasphincteric and extrasphincteric tracts compared to MRI.
If there is any clinical suspicion for Crohn disease (young patient, recurrent or multiple fistulas, non-midline posterior internal opening, presence of skin tags or anal stricture), perform colonoscopy to evaluate for underlying inflammatory bowel disease before definitive surgical repair.

04Management and Treatment

Fistulotomy is the treatment of choice for simple, low fistulas. The procedure involves laying open the entire fistula tract from internal to external opening, curetting out granulation tissue, and allowing the wound to heal by secondary intention. The key requirement is that the amount of sphincter muscle divided must be small enough that continence is preserved. For low intersphincteric and low transsphincteric fistulas, fistulotomy achieves healing rates exceeding 90% with minimal risk of incontinence.
For complex fistulas where fistulotomy would require dividing a significant portion of the sphincter, a seton is placed. A seton is a thread or vessel loop passed through the fistula tract. A draining seton (loose seton) is left in place long-term to maintain drainage, prevent recurrent abscess, and promote fibrosis around the tract; it is commonly used in Crohn patients as a bridge to medical therapy. A cutting seton is progressively tightened over weeks, gradually dividing the sphincter muscle while allowing fibrosis to occur behind it, theoretically preserving continence. However, cutting setons carry a higher incontinence risk and are used less frequently today.
The LIFT (Ligation of Intersphincteric Fistula Tract) procedure is a sphincter-sparing technique where the tract is ligated and divided in the intersphincteric space. It preserves the external sphincter entirely and has success rates reported between 60 and 80%.
Endorectal advancement flap involves excising the internal opening and advancing a flap of rectal mucosa, submucosa, and partial internal sphincter to cover the defect. It is indicated for complex transsphincteric fistulas and is often used in female patients with anterior fistulas where sphincter division would risk incontinence.
For Crohn-related fistulas, the cornerstone of treatment is medical optimization with anti-TNF therapy (infliximab or adalimumab), often combined with immunomodulators (azathioprine or 6-mercaptopurine). A draining seton is placed first to control sepsis, and medical therapy is initiated to reduce inflammation. Fistulotomy is contraindicated because the diseased tissue heals poorly and the risk of non-healing wounds and worsening incontinence is unacceptably high. Definitive surgical repair (advancement flap or LIFT) is considered only after inflammation is controlled.
When an abscess coexists with a fistula, the first step is always incision and drainage of the abscess. Attempting fistula repair in the setting of active sepsis leads to failure. The fistula is addressed in a staged fashion once the acute infection has resolved.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students miss questions on anorectal fistula is by failing to recognize that a recurrent perianal abscess in the same location is essentially diagnostic of an underlying fistula. The vignette will describe a patient who had an abscess drained weeks or months ago and now returns with persistent drainage or a new swelling in the same area. The expected answer is not to simply drain again, but to investigate for and treat the fistula. A second trap involves surgical decision-making: students often select fistulotomy as the answer for all fistulas, but fistulotomy is only appropriate for simple, low fistulas. Choosing fistulotomy for a high transsphincteric, suprasphincteric, or Crohn-related fistula is a critical error because it risks fecal incontinence. When the vignette mentions Crohn disease, the answer almost always involves medical therapy first and a sphincter-sparing approach. Another common pitfall is confusing pilonidal disease with a fistula-in-ano; the discriminator is the location (natal cleft versus anal verge) and the absence of a connection to the anal canal in pilonidal disease. Finally, remember that if an abscess is present alongside a suspected fistula, the immediate next step is drainage of the abscess, not definitive fistula repair. The core competency being tested is the ability to classify fistula complexity, match the classification to the correct surgical strategy, and recognize when medical optimization must precede surgical intervention.