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What is Anal and Perianal Fistula?
A fistula is an abnormal, pathological communication (essentially a small tunnel or tract) that forms between two different body structures or between an internal structure and the skin surface. In the case of an anal fistula (also known as a fistula-in-ano), this tunnel connects the anal canal to the perianal skin (the skin surrounding the anus).
Epidemiology
Anal fistulas are a relatively common proctological condition:
- They are up to five times more frequent in men than in women
- They most commonly affect individuals between the ages of 30 and 40
- Their incidence is estimated at approximately 1–2 per 10,000 population per year in Western countries
How Does an Anal Fistula Form?
Anal fistulas typically develop as a consequence of incompletely healed anal abscesses. The process unfolds as follows:
- The anal glands are small glands located between the internal and external anal sphincters that drain into the anal canal and they become obstructed or infected
- Blocked secretions and fecal bacteria accumulate, leading to the formation of an anal abscess (a localized collection of pus)
- In approximately 50% of cases, the abscess finds no drainage route through the anal canal and instead burrows outward through the intersphincteric space to the perianal skin surface, creating a persistent fistulous tract
What Does a Fistula Feel Like?
The main clinical manifestation is a dull but persistent pain in the anal region, which characteristically worsens during defecation. The associated abscess can also serve as a focus for systemic infection, manifesting as fever and general malaise.
Other common symptoms include:
- Perianal swelling
- Discharge of pus, serous fluid, blood, or feces from the fistula opening
- Maceration (softening and irritation) of the perianal skin
- Constipation
- Tenesmus (a persistent urge to defecate with incomplete or absent evacuation)
Treatment Overview
Treatment is always surgical and consists of:
- Complete drainage of the associated abscess
- Definitive closure of the fistulous tract
Causes
The Pathway from Abscess to Fistula
The formation of an anal fistula is a direct consequence of an anal abscess that fails to drain naturally into the anal canal and instead creates an alternative pathway to the skin surface, forming the anomalous communication known as a fistula.
To understand anal fistula formation, it is essential to understand what drives anal abscess formation in the first place.
Primary Cause: Anal Gland Infection (90% of Cases)
In approximately 90% of cases, the anal abscess originates from inflammation of the anal glands (cryptoglandular theory). These glands, situated in the anal canal between the internal and external sphincters, can become infected through two main mechanisms:
1. Obstruction of Normal Glandular Drainage. The most common mechanism. The outflow of the anal gland into the anal canal becomes blocked by:
- Prolonged stasis of large amounts of fecal material in the rectal ampulla
- Mucus infiltrating the glandular ducts
- Foreign bodies
When glandular contents stagnate, fecal bacteria colonize the retained material, triggering infection and eventually abscess formation.
2. Direct Bacterial Infection of the Gland. It is Less common. infection can occur when bacteria directly invade the glandular tissue. This is more likely in people with weakened immune defenses, such as those who are immunosuppressed or have poorly controlled diabetes.
Secondary Causes (10% of Cases)
In a minority of cases, the anal abscess, and subsequently the fistula, develops from:
-
Inflammatory bowel diseases (IBD):
- Crohn’s disease: The most clinically significant association. Perianal fistulas occur in 20–40% of patients with Crohn’s disease and often represent a particularly complex and refractory form of fistula
- Diverticulitis: Complicated sigmoid diverticulitis can rarely cause a colovaginal or colovesical fistula
- Ulcerative colitis
-
Colorectal cancer: Tumor necrosis or local invasion can lead to fistula formation
-
Anorectal trauma: Including surgical trauma (post-operative complication of rectal surgery)
-
Hidradenitis suppurativa: A chronic skin condition causing recurrent abscesses in the perianal and inguinal regions that can lead to complex fistula networks
-
Tuberculosis: Rare in high-income countries but an important cause globally
-
Sexually transmitted infections (STIs): Particularly syphilis, lymphogranuloma venereum (LGV, caused by Chlamydia trachomatis), and herpes simplex
Symptoms
The clinical presentation of an anal fistula encompasses both the symptoms of the underlying anal abscess and the specific manifestations of the fistulous tract itself.
Key Signs and Symptoms
Pain:
- Dull, persistent perianal pain, present throughout the day and significantly worsened during bowel movements
- In the acute phase of abscess formation, pain may be severe, throbbing, and constant
Discharge and Drainage:
- Spontaneous discharge of pus, serous fluid, blood, or (less commonly) feces from the external fistula opening
- Discharge may be intermittent, temporarily relieving pain when the tract drains, or continuous
- A characteristic unpleasant odor often accompanies fistula discharge
Perianal Skin Changes:
- Anal itching (pruritus ani) sometimes accompanied by excoriation from scratching
- Perianal swelling from the underlying abscess or inflammatory reaction
- Maceration and irritation of the perianal skin from chronic moisture exposure caused by fistula discharge
- Possible skin induration (hardening) along the fistula tract
Systemic Symptoms (indicating abscess or systemic infection):
- Fever, particularly in the acute abscess phase
- General malaise and fatigue
Bowel Symptoms:
- Constipation: The anticipation of pain during defecation often leads patients to voluntarily defer bowel movements, worsening constipation
- Tenesmus: A persistent and uncomfortable urge to defecate without real evacuation, or a sensation of incomplete emptying
- Blood and/or pus in the stool
Important: Any persistent perianal pain, discharge, or swelling should prompt medical evaluation. These symptoms should never be self-managed or ignored, particularly given the association between perianal disease and colorectal malignancy.
Diagnosis
The diagnosis of anal fistula is primarily clinical, based on medical history and proctological examination. However, imaging plays a critical role in mapping complex fistula anatomy before surgical planning.
Medical History
The patient typically reports a characteristic history of persistent perianal pain worsening with defecation, often with a prior episode of perianal swelling (consistent with abscess formation) that either spontaneously drained or was surgically incised. Chronic fistula symptoms, recurrent discharge, itching, and perianal skin irritation, are highly suggestive of the diagnosis.
Proctological Examination
A specialized proctological (anorectal) examination is indispensable and consists of two components:
1. External Inspection of the Perianal Region
- Identifies the external fistula opening (often visible as a small granulation tissue nodule or dimple on the perianal skin)
- Detects perianal swelling, skin maceration, scarring, or signs of active infection
- The Goodsall’s rule is a clinical guide to predicting the internal fistula opening based on the position of the external opening
2. Digital Rectal Examination (DRE)
- Allows palpation of the fistula tract as a firm, indurated cord within the anal canal and perianal tissues
- Identifies the internal fistula opening in the anal canal
- Assesses sphincter integrity, which is critical for surgical planning
Transanal Ultrasound (Endoanal Ultrasound)
For complex or deep fistulas where the tract anatomy cannot be fully defined clinically, 360° rotating transanal ultrasound is the preferred first-line imaging investigation:
- Performed using a specialized rotating probe inserted into the anus and advanced through the rectum
- Provides detailed images of the anal sphincters, fistula tract, associated abscesses, and adjacent musculo-ligamentous structures
- Diagnostic accuracy: 80–90%
- Non-invasive, well-tolerated, and relatively inexpensive compared to MRI
Pelvic MRI (Magnetic Resonance Imaging)
Reserved for the most anatomically complex cases, particularly Crohn’s-associated fistulas or recurrent fistulas after prior surgery, where ultrasound findings are inconclusive:
- Provides superior soft tissue resolution and can precisely map multi-branched or supralevator fistula tracts
- Identifies occult abscesses not visible on ultrasound
- Considered the gold standard for complex perianal fistula mapping before major surgical intervention
- Higher cost and longer acquisition time compared to ultrasound limit its routine use
Additional Investigations
In selected patients, further workup may include:
- Colonoscopy: To exclude Crohn’s disease or colorectal cancer as an underlying cause
- Blood tests: White cell count, CRP, and ESR if systemic infection is suspected; glucose to exclude uncontrolled diabetes
- STI screening: If clinically indicated
Treatment
The definitive treatment of anal fistula is always surgical. No pharmacological treatment alone can close an established fistulous tract. When an associated perianal abscess is present, its management must be addressed simultaneously or immediately prior to definitive fistula repair.
Management of the Associated Abscess
Superficial abscesses can be managed as an outpatient procedure:
- Local anesthesia is administered
- The skin overlying the abscess is incised and the purulent contents are drained externally
Deep abscesses require hospitalization and formal surgical drainage:
- Performed under:
- Sedation with local anesthesia, or
- Regional (loco-regional) anesthesia, spinal or epidural
- The abscess cavity is thoroughly evacuated and irrigated
- A drain may be left in place to ensure continued drainage
Surgical Options for the Fistula
1. Fistulotomy
The most commonly performed procedure for simple, superficial fistulas:
- The entire length of the fistula tract is laid open (surgically cut along its full length)
- The wound is left open to heal by secondary intention, granulating from the inside out into a flat, linear scar
- Highly effective for superficial intersphincteric and low transsphincteric fistulas
- Not suitable for complex or deep fistulas crossing significant amounts of sphincter muscle, due to the risk of fecal incontinence
2. Seton Technique
Reserved for deep, complex, or high transsphincteric fistulas where division of the sphincter muscle in a single procedure would risk fecal incontinence:
- A flexible thread or soft rubber loop (seton) is passed through the fistula tract and loosely or tightly tied around the sphincter muscle
- The seton is progressively tightened at regular outpatient visits, gradually cutting through the sphincter muscle over several months while simultaneously allowing fibrosis and healing of the already divided portion
- Treatment duration: several months, during which the patient can maintain normal daily activities
- The staged nature of the procedure allows gradual sphincter division with simultaneous fibrotic reinforcement, significantly reducing incontinence risk
3. Fistulectomy
A more extensive procedure involving:
- Complete en-bloc excision of the fistula tract
- Repair of the resulting defect
- Reconstruction of the anorectal mucosa using mucosal and submucosal advancement flaps
- Provides thorough pathological tissue assessment, important when malignancy must be excluded
- Higher tissue disruption than fistulotomy, reserved for selected cases
4. Fibrin Glue Injection
A minimally invasive, sphincter-preserving technique:
- Fibrin glue (a biological sealant) is injected directly into the fistula tract to seal it
- Extremely simple, safe, and causes no sphincter damage
- Major limitation: High recurrence rate of 40–50%, making it unsuitable as a standalone definitive treatment for most patients
- May be used as a temporizing measure or in combination with other techniques
Innovative Minimally Invasive Techniques
Modern technology has enabled the development of advanced sphincter-preserving procedures that offer the benefits of reduced hospital stay, faster recovery, and lower complication rates compared to traditional open surgery:
LIFT (Ligation of Inter-Sphincteric Fistula Tract):
- A sphincter-preserving procedure in which the intersphincteric portion of the fistula tract is identified, ligated (tied off), and divided through a small incision in the intersphincteric groove
- Preserves sphincter integrity entirely
- Success rates of 40–85% in published series, with variable outcomes depending on fistula complexity
- Can be combined with fistula tract curettage to improve results
VAAFT (Video-Assisted Anal Fistula Treatment):
- A cutting-edge endoscopic procedure using a miniature fistuloscope (a thin endoscopic camera) inserted into the fistula tract to visualize its entire course under direct vision
- The internal opening is identified and the fistula tract is destroyed from within using electrocautery or laser
- The internal opening is then closed using a stapler, advancement flap, or other closure technique
- Advantages: precise identification of complex tract anatomy, minimal tissue trauma, sphincter preservation
- Particularly useful for complex multi-branched fistulas that are difficult to map with conventional imaging
Post-Operative Care and Follow-Up
After any anal fistula surgery:
- Sitz baths (warm water soaks) 2–3 times daily to promote healing and comfort
- High-fiber diet and adequate hydration to ensure soft, regular stools and minimize defecation-related trauma
- Prescribed topical treatments (antiseptic creams or healing ointments) as directed
- Avoid prolonged sitting on hard surfaces during recovery
- Regular outpatient follow-up to monitor wound healing and detect early signs of recurrence
- Recurrence rates vary by technique and fistula complexity, and they remain a challenge, particularly in Crohn’s-associated fistulas
Sources and Bibliography
- Harrison TR, et al. Harrison’s Principles of Internal Medicine, 18th ed. CEA Casa Editrice Ambrosiana, Milan, 2012.
- Dionigi R. Chirurgia, 2002.
- Parks AG, Gordon PH, Hardcastle JD. A classification of fistula-in-ano. Br J Surg. 1976;63(1):1–12.
- Steele SR, et al. Practice Parameters for the Management of Perianal Abscess and Fistula-in-ano. Dis Colon Rectum. 2011;54(12):1465–1474.
- Garg P. Is fistulotomy still the gold standard in present era and is it highly underutilized? Int J Surg. 2020;82:1–5.
- World Journal of Gastroenterology: Perianal fistula: pathogenesis and treatment
- American Society of Colon and Rectal Surgeons (ASCRS): Perianal Abscess and Fistula-in-ano
- Limura E, Giordano P. Modern management of anal fistula. World J Gastroenterol. 2015;21(1):12–20.
