Fact-sheet: Anal fistula
Updated on 11/04/2024 at 2:13 PM
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Definition
Anal fistula is a complex pathological tract that develops following an infection of digestive origin. The starting point is generally an anal gland located at the dentate line of the anus. The infection then spreads through the anal wall and creates a fistulous tract, a true tunnel that can traverse various muscles and spaces of the Perineum.
Main features:
- Primary opening: Located within the anal lumen, it marks the starting point of the fistula.
- Fistulous tract: Abnormal path that crosses the tissues of the Perineum. It may be simple or complex, with branches, secondary tracts, and diverticula.
- Secondary opening: Exit point of the fistulous tract, which may be cutaneous, opening onto the skin of the anal margin or buttock. There may be several secondary openings for a single primary opening.
- Discharge: The fistula allows drainage of the infection, manifesting as a more or less abundant purulent discharge.
- Abscess: In case of tract obstruction, pus may accumulate and form an abscess, causing pain and inflammation.
Complex anal fistulas:
Certain fistulas have features that make them more difficult to treat, notably:
- Multiple tracts
- Secondary tracts
- Blind diverticula
- Involvement of adjacent organs (vagina, prostate...)
- Distant extensions
Special case of Crohn disease: Crohn disease is a common cause of complex anal fistulas. Fistulas related to Crohn disease are often deeper, more branched, and more difficult to treat.
Clinical features
The clinical presentation of anal fistula varies according to the stage of the disease, alternating between acute and chronic phases.
Acute phase:
- Abscess
- Pain of the anus or an ischioanal fossa, of progressive intensity that can become very severe
- Painful swelling +/- close to the anal margin
- Purulent discharge, allowing evacuation of the pus and relieving the pain
Chronic phase:
- Persistent purulent discharge through one or more anoperineal cutaneous openings
- Pain often present
- Retention and abscess
Other possible symptoms:
- Skin irritation
- Fever
In cases of Crohn disease, anal fistulas are often more complex and symptoms may be more severe.
Ultrasound
Endoanal ultrasound can be used to evaluate anal fistulas. It can be useful for the evaluation of simple anal fistulas. However, MRI is generally preferred for complex cases as it offers better contrast resolution and more comprehensive anatomical analysis.
- Anatomical landmarks: The main landmarks are the muscular structures of the anal sphincter. The internal anal sphincter appears homogeneous and hypoechoic.
- The external anal sphincter is striated and hyperechoic.
- The puborectalis sling is visible at the upper part of the external sphincter.
- Fistulous tract: hypoechoic image. It may contain small hyperechoic air bubbles.
- Abscess: rounded anechoic or hypoechoic image.
- Limitations: requires an experienced operator.
- cannot be performed in patients with anal stenosis.
- limited for assessing the depth extension of fistulas.
- lower diagnostic performance than MRI for complex fistulas.
- may be painful for the patient, sometimes requiring general anesthesia.
- It is difficult to differentiate an active fistula from residual fibrotic lesions.
MRI
Reference imaging technique for the evaluation of anal fistulas, particularly in complex cases. It allows precise anatomical analysis of fistulous tracts, their relationship with adjacent structures, and the presence of complications such as abscesses.
MRI sequences and their value:
- T2 sequences: allow visualization of the overall anatomy of the Perineum and highlight fistulous tracts, which appear hyperintense. They are particularly useful for analyzing the relationship of fistulous tracts with the sphincter muscles and the levator ani muscle.
- T2 sequences with fat suppression (Fat Sat): improve visualization of fistulous tracts. They provide better contrast to identify active fistulas, characterized by hyperintensity.
- T1 sequences with gadolinium injection: allow visualization of inflammation around fistulous tracts and abscesses. Enhancement of the fistulous tract walls indicates lesion activity.
- Diffusion sequences: highly sensitive for detecting inflammatory fistulous tracts, which appear hyperintense.
MRI signs of anal fistulas:
- Fistulous tracts: Appearance: Fistulous tracts typically present as linear T2 hyperintense images, with a peripheral hypointense rim corresponding to the fibrosis surrounding active fistulous tracts.
- Classification: The Parks classification is used (see below)
- Inactive or residual fistulas: the T2 hyperintensity and enhancement disappear.
- Openings: Primary: The primary opening, located in the anal lumen, can be difficult to visualize on MRI.
- Secondary: The secondary opening, usually cutaneous, should be described even if visible on clinical examination.
- Abscess: Appearance: Abscesses present as rounded or ovoid T2 hyperintense images, with a wall enhancing after gadolinium injection.
- Content: They may contain air, visible as hypointensity on all sequences.
- Other semiological findings: Rectal wall thickening: May be observed in the context of Crohn disease.
- Presence of setons: Small linear signal-void images within the fistulous tracts, reflecting the placement of drains.
- Sphincter ring disruption: Discontinuity in the sphincter ring, which may lead to anal incontinence.
The report should include the following elements:
- Type of fistula: simple, complex, horseshoe, anovaginal
- Relationship with the levator ani: infralevator, supralevator
- Parks classification: intersphincteric, transsphincteric, suprasphincteric, extrasphincteric
- Openings: primary, secondary
- Signs of inflammation: T2 hyperintensity, enhancement, abscess
- Presence of secondary tracts or diverticula
- Involvement of adjacent structures
Vascular intervention
Potential applications:
- Percutaneous drainage of deep or difficult-to-access anal abscess.
- Arteriography with embolization in cases of significant rectal bleeding associated with an anal fistula.
Classification
Parks classification, based on the relationship of the fistulous tract to the external anal sphincter:
- Intersphincteric fistula: Tract located between the internal and external sphincters, generally draining at the anal margin.
- Transsphincteric fistula: Tract that crosses the external sphincter and extends into the ischioanal fossa.
- Suprasphincteric fistula: Tract that courses above the external sphincter and passes over the levator ani muscle.
- Extrasphincteric fistula: Tract originating not from the anal canal but from another segment of the digestive tract.
Differential diagnosis
- Anal abscess
- Crohn disease
- Anal cancer
- Hemorrhoids
- Anal fissure