Anorectal Fistula

1. Definition

Let's get the terminology straight first — this trips people up.

• Fistula = an abnormal connection between two epithelial (or epithelialised) surfaces. The word comes from Latin fistula = "pipe" or "tube." So it's a tube where there shouldn't be one.
• Sinus = a blind-ending tract from an epithelial surface into a cavity lined by granulation tissue (i.e., one opening only). Think of it as a dead-end alley.
• Anorectal fistula (fistula-in-ano) = an abnormal tract communicating the rectum or anal canal with the perianal skin ^[1]. It consists of an internal opening (usually in the anal canal near the dentate line) and an external opening (on the perianal skin), connected by a tract lined with granulation tissue ^[2][3].

Crucially, anorectal fistula represents the chronic phase of the perirectal suppurative process. The acute phase is the anorectal abscess. When an abscess ruptures or is surgically drained, the track may persist and epithelialise → fistula ^[2][3].

2. Epidemiology

• Incidence: ~1–2 per 10,000 population per year in Western data. In Hong Kong, anorectal fistulae are a common colorectal outpatient presentation.
• Age: Peak incidence 30–50 years (working-age adults) ^[2].
• Sex: Male predominance (M:F ≈ 2–3:1). This is thought to relate to the greater number and density of anal glands in males and possibly hormonal influences on gland activity.
• Preceding abscess: Approximately 30–50% of patients who have an anorectal abscess will develop a subsequent fistula ^[3]. Conversely, > 90% of fistulae originate from a preceding cryptoglandular abscess ^[2].
• IBD association: In Crohn's disease, perianal fistulae occur in ~20–30% of patients (higher in colonic and anorectal Crohn's). This is a major cause of morbidity ^[4].

3. Anatomy and Function

Understanding fistula anatomy requires mastering the anal canal and its spaces. Without this, classification and surgical planning make no sense.

3.1 Anal Canal

• Surgical anal canal: From anal verge to anorectal ring (~4 cm).
• Dentate (pectinate) line: Located ~2 cm from the anal verge. This is the watershed landmark:
  • Above: columnar epithelium, visceral nerve supply (painless), superior rectal artery, internal haemorrhoidal plexus, lymphatic drainage to internal iliac nodes.
  • Below: squamous epithelium, somatic nerve supply (painful!), inferior rectal artery, external haemorrhoidal plexus, lymphatic drainage to inguinal nodes.

3.2 Anal Glands and Crypts of Morgagni

• At the dentate line sit 6–14 anal columns (columns of Morgagni) with intervening crypts of Morgagni (anal crypts).
• Anal glands (4–10 in number) open into these crypts. Their ducts penetrate through the internal anal sphincter (IAS) and terminate in the intersphincteric plane (between IAS and EAS).
• This is why the intersphincteric plane is "ground zero" for cryptoglandular infection — infection begins in the gland, spreads to the intersphincteric space, and then tracks along paths of least resistance ^[1][2].

3.3 Sphincter Complex

3.4 Perianal and Perirectal Spaces

Infection (and therefore fistula tracts) can spread through these spaces:

4. Etiology

4.1 Cryptoglandular Infection (> 90%) [1][2][3]

This is the dominant cause. The mechanism:

1. Obstruction of an anal gland duct → stasis.
2. Bacterial overgrowth within the obstructed gland (mixed flora: E. coli, Bacteroides, Enterococcus).
3. Abscess formation in the intersphincteric space (this is the primary site because the gland terminates here).
4. Pus tracks along the path of least resistance — this determines the type of abscess and the subsequent fistula tract.
5. If the abscess drains (spontaneously or surgically) but the internal opening persists and the tract epithelialises → fistula.

Why does the internal opening persist? Because the infected crypt continues to seed bacteria into the tract, preventing healing. This is the rationale for identifying and treating the internal opening in fistula surgery.

4.2 Other Causes

5. Pathophysiology

5.1 Cryptoglandular Theory (Parks' Theory) [1][2][3]

This is the cornerstone — understand it step-by-step:

Anal crypt gland obstruction
        ↓
Intersphincteric gland infection
        ↓
Intersphincteric abscess (primary)
        ↓
Pus tracks along path of least resistance
        ↓
Secondary abscess formation
(perianal / ischiorectal / supralevator)
        ↓
Spontaneous rupture or surgical drainage
        ↓
Persistent internal opening + granulation-lined tract
        ↓
FISTULA-IN-ANO

Why does it become chronic?

• The internal opening (at the infected crypt) continuously seeds bacteria.
• The tract becomes lined with granulation tissue and eventually partly epithelialised, preventing spontaneous closure.
• The ongoing faecal contamination from the anal canal keeps the inflammatory cycle going.

5.2 Crohn's Disease Pathophysiology

• Transmural inflammation is the hallmark of Crohn's — inflammation penetrates the full thickness of the bowel wall.
• This leads to microperforations and abscess formation, which then track to form fistulae.
• Crohn's fistulae are often complex (multiple tracts, horseshoe configurations, rectovaginal).
• Fistulising disease accounts for ~20% of Crohn's disease ^[3][4].

5.3 Why Some Abscesses Form Fistulae and Others Don't

• If the abscess is adequately drained AND the internal opening heals → no fistula.
• Pus culture showing gut flora (e.g., E. coli, Bacteroides) suggests communication with the anal canal lumen → higher likelihood of fistula formation ^[3].
• Pus culture showing skin flora (e.g., Staphylococcus) → less likely to form a fistula (infection was superficial) ^[3].

6. Classification

6.1 Parks Classification [1][2][3]

This is THE classification. It describes the fistula tract's relationship to the external anal sphincter (EAS).

Not included in Parks: Superficial fistula — does not involve the sphincter at all (subcutaneous/submucosal tract). Easy to treat ^[3].

6.2 Simple vs Complex Classification [2][3]

This classification determines surgical strategy — simple fistulae can usually be laid open safely; complex ones cannot.

Simple fistula:

• Superficial fistula
• Intersphincteric (Type I)
• Low transsphincteric (Type II) involving < 30% of EAS ^[2][3]

Complex fistula:

• High transsphincteric (Type II) involving ≥ 30% of EAS ^[2][3]
• Suprasphincteric (Type III)
• Extrasphincteric (Type IV)
• Horseshoe fistulas
• Multiple tracts
• Recurrent fistula
• Fistula associated with Crohn's disease ^[2][3]
• Fistula related to TB, HIV, radiation ^[2]
• Women with anterior fistulas / rectovaginal fistulas (thinner anterior sphincter in females → higher incontinence risk) ^[3]
• Patients with pre-existing anal incontinence ^[2]

6.3 High vs Low Classification

• Low fistula: Below the dentate line / involving minimal sphincter → can usually fistulotomy safely.
• High fistula: Above the sphincter complex or involving significant sphincter → need sphincter-sparing techniques.

7. Clinical Features

7.1 Symptoms

7.2 Signs

7.3 Goodsall's Rule [3]

This is a clinical "rule of thumb" to predict the location of the internal opening based on the position of the external opening.

• Draw an imaginary transverse line across the anus (through the ischial spines/tuberosities).
• External opening ANTERIOR to the transverse line → fistula tracks in a straight (radial) line to the nearest crypt at the dentate line.
• External opening POSTERIOR to the transverse line → fistula follows a curved track to open at the posterior midline of the anal canal.

Goodsall's rule has higher reliability when the external opening is within 3 cm of the anal verge ^[3]. Beyond 3 cm, the rule is less reliable (the tract is more likely to be complex).

Why does this pattern occur? The posterior midline is where the anal glands are most concentrated. Additionally, the posterior anal canal has a relatively avascular area (the "posterior commissure"), which provides a path of least resistance for infection to track through.

7.4 Associated Features to Assess

When evaluating a patient with a fistula, always look for features suggesting an underlying cause:

• Crohn's disease: multiple fistulae, skin tags ("elephant ear" tags), fissures away from the midline, oral ulcers, abdominal pain, diarrhoea, weight loss, perianal disease preceding intestinal disease.
• TB: chronic non-healing fistula, constitutional symptoms (night sweats, weight loss), granulomata on histology (caseating).
• Malignancy: hard, irregular edges of fistula, bloody discharge, weight loss, change in bowel habit.
• HIV/immunosuppression: unusual organisms, poor healing.

8. Pathophysiology–Clinical Feature Connections (Summary Table)

References

^[1] Lecture slides: GC 179. Anal pain perianal lesions and sepsis.pdf (p49, p54) ^[2] Senior notes: felixlai.md (Anorectal fistulas section, Anorectal abscess section) ^[3] Senior notes: maxim.md (Anal fistula, Anorectal abscess sections) ^[4] Lecture slides: Inflammatory bowel disease.pdf; Senior notes: felixlai.md (IBD section), maxim.md (Surgical procedures for CD)

Differential Diagnosis of Anorectal Fistula

The differential diagnosis of anorectal fistula essentially boils down to one question: what else can cause a perianal opening with discharge, pain, or swelling? Let's work through this systematically from first principles.

A patient presenting with perianal discharge, pain, and/or a visible external opening on the perianal skin could have several conditions. The key is to distinguish a true fistula (communication between two epithelial surfaces) from conditions that mimic it. Diagnosis can usually be made by careful history and examination ^[1], but you must actively consider — and exclude — the differential diagnoses below.

Approach to Differential Diagnosis

The differentials can be organised by the dominant presenting feature:

Detailed Differential Diagnoses

1. Anorectal Abscess

2. Anal Fissure

3. Pilonidal Sinus

4. Perianal Crohn's Disease

5. Anal Carcinoma

6. Hidradenitis Suppurativa

7. Presacral Epidermoid / Dermoid Cyst

8. Anal Ulcers or Sores [2]

9. Rectal Prolapse [9]

10. Proctalgia Fugax [7]

Summary Differential Diagnosis Table

Clinical Decision-Making Flow

When you see a patient with a perianal external opening and discharge, your systematic approach should be:

1. Is it a fistula or something else? — Examine for a tract, look for internal opening, check location relative to anal canal.
2. If it's a fistula, is it simple or complex? — Parks classification, assess sphincter involvement.
3. Is there an underlying cause? — Always consider Crohn's disease (colonoscopy), TB (especially in HK), and malignancy (biopsy).
4. Common anorectal conditions are mostly benign but colorectal neoplasm needs to be excluded ^[1].

References

^[1] Lecture slides: GC 179. Anal pain perianal lesions and sepsis.pdf (p46, p56, p57, p77) ^[2] Senior notes: felixlai.md (Anorectal fistulas — Clinical manifestation, Differential diagnosis) ^[3] Senior notes: maxim.md (Anal fistula — Assessment section) ^[4] Senior notes: maxim.md (Anorectal abscess — Clinical diagnosis, Management) ^[5] Senior notes: felixlai.md (Anal fissures — Clinical manifestation) ^[6] Senior notes: maxim.md (Anal fissure section) ^[7] Senior notes: maxim.md (Pilonidal sinus, Proctalgia fugax, Anal carcinoma sections) ^[8] Lecture slides: Inflammatory bowel disease.pdf (p45, p48) ^[9] Senior notes: felixlai.md (Rectal prolapse section)

Diagnosis of Anorectal Fistula

Diagnostic Principles

Before diving into individual investigations, understand the three goals of diagnosing a fistula — every investigation you order serves one of these purposes ^[1]:

1. Define the anatomy of the pathological process — where is the internal opening, external opening, primary tract, and any secondary extensions? What is the relationship to the sphincter complex? (This determines Parks classification → determines surgery.)
2. Help in the diagnosis and assess the severity of disease — is this simple or complex? Is there active sepsis?
3. Exclude diseases in the proximal bowel and associated bowel problems (e.g., inflammatory bowel disease) — is there an underlying cause like Crohn's, TB, or malignancy? ^[1]

Diagnostic Algorithm

Investigation Modalities

1. Clinical Examination (The Foundation)

Anorectal fistula is primarily a clinical diagnosis ^[1]. Clinical examination is the first and most important step ^[1].

a. History

Approach to anorectal diseases requires a focused history ^[1]:

• Present illness: anorectal symptoms — pain, bleeding, mass, prolapse ^[1]. For fistula specifically: intermittent perianal discharge, cyclical pain (worse when blocked, better when drains), recurrent abscess at the same site.
• Past health: medical diseases, previous surgery ^[1] — prior anorectal abscess drainage? Previous fistula surgery? IBD? TB? Radiation?
• Family history ^[1] — IBD.
• Sexual history ^[1] — LGV, HIV, HPV (anal carcinoma).

b. External Inspection

• External opening visible on perianal skin ^[1][2]:
  • Typically a small dimpled area of granulation tissue that drains pus or blood on manual compression ^[2].
  • Pustule-like lesion in perianal or buttock area ^[2].
  • Perianal skin is excoriated and inflamed from chronic discharge ^[2].
• Count the number of external openings — multiple openings suggest complex fistula or Crohn's disease.
• Note the position of the external opening relative to the anus → apply Goodsall's rule ^[3]:
  • Anterior to transverse anal line → straight radial tract.
  • Posterior to transverse anal line → curved tract to posterior midline.
  • Higher reliability if external opening within 3 cm of anal verge ^[3].

c. Digital Rectal Examination (DRE)

• PR: induration with cord-like structure ^[1] — the fistula tract is palpable as a firm, cord-like subcutaneous structure running from the external opening towards the anal canal.
• May feel a pit or area of induration at the dentate line corresponding to the internal opening.
• Assess sphincter tone (resting and squeeze) — baseline continence assessment before surgery is critical.
• Feel for any abscess (fluctuance, tenderness) or mass (hard, irregular = malignancy).

d. Proctoscopy / Sigmoidoscopy

• Internal opening in the anus can be viewed with anoscopy ^[2].
• Internal opening in the rectum can be viewed with sigmoidoscopy ^[2].
• May see pus exuding from the internal opening.
• Also assesses for concurrent pathology (haemorrhoids, fissure, rectal mucosal changes suggesting Crohn's or malignancy).

2. Imaging Studies

Imaging studies are usually not necessary for straightforward simple fistulae ^[1]. However, they are essential in specific scenarios.

a. MRI Pelvis (Gold Standard Imaging)

MRI is the accurate technique for evaluation of the primary track of fistula and any extensions ^[1].

When to order MRI ^[1]:

• Complicated high fistula
• Recurrent fistula
• When anatomy is not obvious

MRI and EAUS are necessary to document before definitive treatment in perianal Crohn's disease ^[8].

Key MRI findings and interpretation:

• Primary fistula tract: linear hyperintense tract on T2W connecting internal and external openings. Classify its relationship to EAS → Parks type.
• Secondary extensions/branches: additional tracts branching from the primary tract (e.g., horseshoe extension in the ischiorectal fossa or supralevator space).
• Abscess: well-defined fluid collection with enhancing rim post-gadolinium. Must be drained before definitive fistula surgery.
• Internal opening: identified as a defect in the IAS at the level of the dentate line, often with surrounding inflammation.
• Sphincter integrity: assess IAS and EAS for defects (from prior surgery or obstetric injury) — affects surgical planning.

St James's University Hospital MRI Classification (widely used):

Higher grade = more complex = higher recurrence risk, needs sphincter-sparing surgery.

b. Endoanal Ultrasound (EAUS / Transrectal Ultrasound)

Transrectal ultrasound can be used to assess complex fistula ^[1] and assess the sphincter muscles in patients with faecal incontinence ^[1].

• Technique: High-frequency (7–16 MHz) rotating endoprobe inserted into the anal canal.
• Instilling H₂O₂ (hydrogen peroxide) via the external opening improves visualisation of the fistula tract — the microbubbles appear hyperechoic (bright) on ultrasound ^[3].

Advantages: No radiation, real-time imaging, good sphincter assessment, bedside availability. Limitations: Operator-dependent, limited field of view (cannot see supralevator disease as well as MRI), uncomfortable without anaesthesia.

MRI vs EAUS: Both have similar accuracy for classifying primary tracts (~85–90%). MRI is superior for detecting secondary extensions and supralevator disease. In practice, MRI is preferred for complex fistulae; EAUS is a good alternative if MRI is unavailable or for sphincter assessment.

c. Fistulography [1][2]

Fistulogram ^[1] — injection of water-soluble contrast through the external opening followed by radiographs.

• Largely superseded by MRI and EAUS in modern practice.
• Can demonstrate the tract, but has poor sensitivity for secondary extensions and does not show the relationship to sphincter muscles well.
• May have a role when MRI/EAUS are unavailable.

d. CT Scan

• Less useful than MRI for fistula tract delineation (poorer soft-tissue contrast).
• Main role: CT/MRI scan indicated in a non-palpable abscess ^[5] — particularly useful for detecting deep abscesses (supralevator, ischiorectal) that are not clinically apparent.
• CT abdomen/pelvis may also be used to assess for Crohn's disease complications (strictures, other fistulae).

3. Examination Under Anaesthesia (EUA) — The Definitive Assessment

Examination under anesthesia (EUA) is required before surgery to trace the fistula tract ^[3]. This is the most important investigation because it combines diagnostic confirmation with therapeutic planning — and is often performed immediately before definitive surgery.

What happens during EUA:

1. Visual inspection of perianal area (external opening, scarring, Crohn's features).
2. DRE with relaxed sphincter — assess internal opening, induration, sphincter integrity.
3. Proctoscopy — identify internal opening (may see pus draining from a crypt at the dentate line).
4. Fistula probing ^[2]:
   • A fistula probe (malleable metal probe) is gently inserted into the external opening and passed along the tract towards the internal opening.
   • Determine whether the fistula is simple or complex based on the extent of anal sphincter muscle involvement ^[2].
   • Performed with caution to avoid creating a false passage by penetrating the fistula wall ^[2].
5. Injection of hydrogen peroxide or methylene blue through external opening — helps visualise the internal opening (dye emerges at the crypt).
6. Assessment of sphincter bulk around the tract — the surgeon palpates how much muscle lies between the probe and the skin to decide if fistulotomy is safe.

4. Investigations to Exclude Underlying Causes

These are not for diagnosing the fistula per se but for answering "Why does this patient have a fistula?" — essential in atypical cases.

a. Colonoscopy [3][8]

• Indication: Colonoscopy if suspect Crohn's disease ^[3].
• Any patient with complex, recurrent, multiple, or atypical fistulae should have a colonoscopy.
• Look for: skip lesions, cobblestoning, aphthous ulcers, non-caseating granulomata on biopsy.
• 24.5% of patients with Crohn's disease have perianal involvement ^[8]; sometimes perianal disease is the first manifestation.

b. Histology / Biopsy

• Tissue from the fistula tract (curettings from EUA) should be sent for histology.
• Granulomata: if found, must differentiate Crohn's (non-caseating) from TB (caseating). Must exclude TB — especially in Hong Kong ^[2].
• Dysplasia/malignancy: biopsy any hard, irregular, or suspicious tissue.
• Actinomycosis: "sulphur granules" on histology.

c. TB Workup

• CXR, sputum AFB (if pulmonary involvement suspected).
• Tissue AFB staining and TB culture from fistula curettings.
• Interferon-gamma release assay (IGRA) / Mantoux test.
• Important in Hong Kong (TB is endemic).

d. Blood Tests (Supportive)

e. Pus Culture and Sensitivity [4]

• After abscess drainage: pus for C/ST (culture and sensitivity testing).
• Gut flora (E. coli, Bacteroides) → communication with anal canal lumen → likely fistula → arrange follow-up ^[4].
• Skin flora (Staphylococcus) → superficial infection → unlikely fistula → reassurance ^[4].

Summary: Which Investigation and When?

References

^[1] Lecture slides: GC 179. Anal pain perianal lesions and sepsis.pdf (p4, p13, p17, p18, p19, p57) ^[2] Senior notes: felixlai.md (Anorectal fistulas — Diagnosis section) ^[3] Senior notes: maxim.md (Anal fistula — Assessment section) ^[4] Senior notes: maxim.md (Anorectal abscess — Management, Pus C/ST) ^[5] Senior notes: felixlai.md (Anorectal abscess — Diagnosis section) ^[8] Lecture slides: Inflammatory bowel disease.pdf (p45)

Management of Anorectal Fistula

Overarching Principles

Before we discuss specific procedures, let's establish the fundamental management philosophy. Every decision in fistula surgery revolves around one tension:

Goal of surgery is to eradicate the fistula while preserving fecal continence ^[2].

This is the central trade-off. The more aggressively you divide sphincter muscle, the better the cure rate — but the higher the risk of incontinence. The more you try to preserve sphincter, the lower the cure rate — but incontinence is avoided. Every treatment modality exists on this spectrum.

Three pre-operative requirements must be met ^[2][3]:

1. Probing of the fistula is required before surgery to determine its anatomy ^[2] — this is done at EUA, not in clinic.
2. Active sepsis must be controlled first — if there is an undrained abscess, drain it before definitive fistula surgery. Operating on an inflamed, septic field increases recurrence and complication rates.
3. Underlying disease must be addressed — Crohn's fistulae are managed very differently from cryptoglandular ones (medical-first approach).

Management Algorithm

Treatment Modalities

A. Conservative / Supportive Measures

These are adjunctive — no fistula will heal with conservative measures alone (except some Crohn's fistulae with biologics), but they support surgical healing.

Lifestyle modification ^[2]:

• Avoid constipation — hard stools traumatise the healing wound and internal opening. Use fibre supplementation, adequate fluid intake, stool softeners.
• Maintain proper anal hygiene — reduce bacterial load around the wound. Sitz baths (warm water soaks) after defecation promote wound cleanliness and comfort.

B. Surgical Treatment — Cryptoglandular Fistulae

1. Fistulotomy (Laying Open)

Fistulotomy/fistulectomy for simple low fistula ^[1].

The principle: If you can safely divide the tissue overlying the fistula tract without causing incontinence, you lay the tract open to heal from the base up (secondary intention healing). This is called "fistulotomy" — fistula + Greek -tomē = "cutting."

Technique ^[2][3]:

1. A fistula probe is inserted into the external opening and gently passed along the tract to the internal opening.
2. Most critical step is to identify and curette the internal opening to reduce the risk of recurrence ^[2] — remember, the internal opening is the source of ongoing cryptoglandular infection. If you don't eliminate it, the fistula recurs.
3. An incision is made over the entire length of the fistula using the probe as a guide → all overlying tissue (skin, subcutaneous tissue, and the involved portion of sphincter) is divided.
4. The tract is gently curetted to remove granulation tissue.
5. Marsupialization of tract is preferred as it may speed up wound healing ^[2] — the edges of the laid-open wound are sutured to the base with absorbable sutures, reducing the wound surface area and accelerating healing.
6. The wound is packed with gauze and covered with dressing → heals by secondary intention.

2. Seton Techniques

A seton (from Latin seta = "bristle") is a thread, suture, or loop of material passed through the fistula tract. Setons serve different purposes depending on the type used.

a. Loose (Draining / Non-cutting) Seton

Loose Seton for 6 weeks as a first-stage procedure + secondary sphincter-sparing procedure ^[3].

Why use it?

• Establish drainage and eradicate the septic focus ^[3] — the seton keeps the tract open, allowing continuous drainage of pus. This prevents re-accumulation of abscess.
• Preserve sphincter mechanism ^[3] — no muscle is divided.
• Acts as a bridge to definitive surgery once inflammation has settled.

Technique:

• Loose seton is usually inserted at EUA ^[3].
• Material: silastic vessel loop ^[3] — a soft, elastic tube that sits comfortably in the tract.
• The seton is threaded through the tract from external to internal opening and loosely tied. It is NOT tightened.
• Left in place for ~6 weeks to allow the tract to mature and sepsis to resolve.

Indications:

• High risk of incontinence / complex fistula ^[3].
• Active sepsis with complex fistula — drain first, definitive surgery later.
• Crohn's-associated fistula (often left as a long-term draining seton if the patient is asymptomatic with the seton in place).

b. Cutting (Tight / Snug) Seton

Principle: The seton is progressively tightened at regular intervals → it slowly cuts through the sphincter muscle while fibrosis occurs behind it → the sphincter is divided gradually with simultaneous scarring, preserving some degree of continence ^[2][3].

• Cutting seton = elastic tightened regularly → slowly cut through the tract to cause scarring ^[3].
• Think of it like a cheese wire slowly cutting through a block — as it cuts forward, scar tissue forms behind it, maintaining structural support.

Advantage: Lower incontinence rate than single-stage fistulotomy for complex fistulae. Disadvantage: Painful (each tightening causes discomfort), multiple clinic visits, still a risk of incontinence (~10–20%), largely falling out of favour in modern practice compared to sphincter-sparing techniques.

3. Sphincter-Sparing Procedures

These are for complicated high fistula or transsphincteric fistula with significant amount of muscle involvement ^[1].

a. Endorectal Advancement Flap

Endorectal advancement flap ^[1].

Principle: Close the internal opening by a mucosal flap → allow the fistula tract to heal and close ^[3]. By obliterating the internal opening (the source of infection), you remove the driver of the fistula without dividing any sphincter muscle.

Technique:

1. A flap of rectal mucosa, submucosa, and sometimes a small cuff of internal sphincter is raised above the internal opening.
2. The internal opening is curetted and closed.
3. The flap is advanced downward to cover the closed internal opening and sutured in place.
4. The external opening is left open to drain.

b. LIFT — Ligation of Intersphincteric Fistula Tract

Ligation of intersphincteric fistula tract (LIFT) ^[1].

Principle: The fistula tract passes through the intersphincteric space (between IAS and EAS) in transsphincteric fistulae. LIFT involves making an incision in the intersphincteric groove, identifying the tract as it crosses this space, ligating (tying off) the tract on both sides, and dividing it. This seals the internal opening side and allows the external tract to drain and collapse.

Technique:

1. Incision in the intersphincteric groove.
2. Identify the fistula tract in the intersphincteric plane.
3. Ligate the tract close to the internal opening.
4. Ligate the tract close to the external sphincter side.
5. Divide the tract between the two ligatures.
6. Curette the external component and leave it open for drainage.

c. Anal Fistula Plug

Anal fistula plug ^[1].

Principle: A bioabsorbable plug (made of porcine small intestinal submucosa or synthetic material) is inserted into the fistula tract from the internal opening. It acts as a scaffold for tissue ingrowth, occluding the internal opening and allowing the tract to close.

d. Fibrin Sealant

Fibrin sealant ^[3].

Principle: Fibrin glue is injected into the fistula tract to obliterate it. The glue solidifies and acts as a matrix for tissue ingrowth.

e. Modified Hanley Procedure

Modified Hanley procedure: horseshoe fistulas ^[3].

Principle: Horseshoe fistulae have a primary tract (usually posterior midline) with extensions through the deep postanal space into both ischiorectal fossae. The Modified Hanley procedure involves:

1. Laying open the deep postanal space (posterior midline) to address the primary tract and internal opening.
2. Counter-incisions over each ischiorectal fossa extension for drainage.
3. The posterior midline component is laid open; the lateral extensions are drained and allowed to heal by secondary intention.

4. Newer / Emerging Treatment Modalities

Other treatment modalities from the lecture slides ^[1]:

C. Management by Parks Classification — Summary Table

D. Management of Perianal Crohn's Disease Fistulae

Crohn's perianal fistulae are a special category — the approach is fundamentally medical-first, with surgery playing a supportive/drainage role ^[5][8][9].

Perianal CD management ^[8][9]:

Step 1: Control Infection

• Metronidazole/ciprofloxacin for infection control ^[9]
  • Why antibiotics? Reduce the bacterial load driving the inflammatory process. Metronidazole and ciprofloxacin penetrate well into pelvic/perianal tissue. Used as bridge therapy until immunomodulators/biologics take effect.
  • From CD medical notes: Antibiotics are indicated in perianal diseases such as fistula ^[10].

Step 2: Drain Any Abscess

• Abscess — simple drainage ^[9]
  • I&D under GA. Do NOT perform definitive fistula surgery at this stage — tissue is inflamed, anatomy distorted.

Step 3: Manage the Fistula Based on Complexity

• Simple fistula — fistulotomy ^[9]
• Complex fistula ^[9]:
  • Seton insertion for drainage — loose seton to control sepsis and maintain drainage.
  • Azathioprine — immunomodulator to reduce Crohn's inflammation ^[8][9].
  • Biologics — Anti-TNF +/- AZA and/or seton drainage ^[8]. Give biologics after drainage of pus to close the fistula tract ^[5].
    • Why biologics after drainage? If you give biologics (which suppress immune response) while there is an undrained abscess, you risk uncontrolled sepsis. Drain first → control sepsis → then introduce biologics to promote fistula closure.
    • Must screen for TB with CXR/QuantiFERON-TB Gold + HBV with HBsAg before starting anti-TNF ^[10].
  • Colostomy or proctectomy — if all else fails ^[9].
    • If failed, consider surgery (defunctioning stoma ± proctectomy) ^[5].

Enterocutaneous Fistula in Crohn's — uses the SNAP principle ^[2][10]:

• Sepsis control: Antibiotics + abscess drainage ^[11]
• Nutrition support: High vs low output assessment, TPN aids closure ^[11]
• Anatomy: CTE or MRE to delineate anatomy ^[11]
• Procedure: Medication adjustment, closure with biological agents, en-bloc resection of involved bowel and fistula ^[11]

E. Management of Anorectal Abscess (Acute Phase)

Since abscess and fistula are the same disease at different time points, managing the abscess correctly can prevent or set up proper management of the subsequent fistula.

Treatment of anorectal abscess ^[1][4]:

• Incision and drainage of abscess ^[1]
• Little role for antibiotics except in patients with severe cellulitis, valvular heart disease, prosthetic heart valves, and immunosuppression ^[1]
• ? Primary fistulotomy ^[1] — if a fistula is clearly identified at the time of abscess drainage (e.g., probe passes easily from abscess to internal opening), some surgeons will perform a primary fistulotomy at the same sitting. This is controversial and typically only done for simple, low fistulae with clearly identified anatomy.
• Drainage approach by site ^[4]:
  • Perianal abscess: skin incision close to the anal verge.
  • Ischiorectal abscess: buttock skin incision → drain as close to sphincter complex as possible to minimise the length of a potential fistula tract.
  • Intersphincteric abscess: rectal incision (internal sphincterotomy) → drain transanally to avoid creating an extrasphincteric fistula ^[4].

Comprehensive Summary: Treatment Selection Decision Table

References

^[1] Lecture slides: GC 179. Anal pain perianal lesions and sepsis.pdf (p53, p58, p60, p61) ^[2] Senior notes: felixlai.md (Anorectal fistulas — Treatment section) ^[3] Senior notes: maxim.md (Anal fistula — Definitive treatment section) ^[4] Senior notes: maxim.md (Anorectal abscess — Management section) ^[5] Senior notes: maxim.md (Surgical procedures for CD — Perianal CD section) ^[8] Lecture slides: Inflammatory bowel disease.pdf (p45) ^[9] Lecture slides: Inflammatory bowel disease.pdf (p48) ^[10] Senior notes: felixlai.md (Crohn's disease — Treatment section, Antibiotics, Biologics) ^[11] Lecture slides: Inflammatory bowel disease.pdf (p33, p34)

Complications of Anorectal Fistula

Complications can be divided into two broad categories: complications of the disease itself (untreated or inadequately treated fistula) and complications of surgical treatment. Understanding both is essential because the management of fistula is essentially a balancing act — you're always weighing the risks of leaving the disease untreated against the risks of the surgery intended to fix it.

A. Complications of the Disease Itself

1. Recurrent Anorectal Abscess

Mechanism: The fistula maintains a persistent communication between the anal canal lumen (via the internal opening at the infected crypt) and the perianal tissues. Faecal bacteria continuously seed the tract → intermittent accumulation of pus → cyclical abscess formation ^[2][3].

• This is the most common complication of an untreated fistula.
• Each recurrence can extend the septic process into new tissue planes → the fistula becomes progressively more complex (new secondary tracts, horseshoe extensions).
• Can expand into adjacent tissues including ischiorectal and supralevator space or even progress into generalised systemic infection ^[2].

Why does this matter? Each cycle of abscess → drainage → recurrence creates more scarring and fibrosis around the sphincter complex, making eventual definitive surgery more difficult and increasing the risk of incontinence from the surgery itself.

2. Complex Fistula Formation / Branching

Mechanism: Undrained or inadequately drained sepsis tracks along paths of least resistance through perianal/perirectal tissue planes. Over time:

• A simple intersphincteric fistula can extend through the EAS → becomes transsphincteric.
• Pus can track through the deep postanal space (behind the anal canal) from one ischiorectal fossa to the other → horseshoe fistula.
• Multiple secondary tracts develop → complex fistula with multiple external openings.

The more complex the fistula becomes, the harder it is to treat surgically without damaging the sphincter, and the higher the recurrence rate.

3. Perianal Sepsis and Systemic Infection

Mechanism: If the external opening blocks and there is no adequate drainage path, pus accumulates under pressure. This can progress to:

• Cellulitis of the perianal skin and perineum.
• Necrotising fasciitis of the perineum (Fournier's gangrene) — a life-threatening surgical emergency. Polymicrobial infection spreads rapidly through subcutaneous tissues and fascial planes. Particularly dangerous in immunocompromised patients (diabetes, HIV, on immunosuppressants) and those with perianal Crohn's disease.
• Bacteraemia / sepsis — the perianal region is richly vascular; uncontrolled infection can seed the bloodstream.

4. Chronic Perianal Skin Changes

Mechanism: Persistent discharge from the external opening causes:

• Chronic dermatitis and maceration of the perianal skin — from continuous moisture and irritant contact with pus/faecal material.
• Pruritus ani — secondary to chronic skin inflammation and excoriation ^[2].
• Scarring and fibrosis — repeated cycles of inflammation and healing lead to thickened, indurated perianal skin.

5. Malignant Transformation (Rare but Important)

Mechanism: Chronic inflammation over many years (typically > 10 years of untreated fistula) can lead to squamous cell carcinoma or mucinous adenocarcinoma arising in the fistula tract. This is analogous to other chronic inflammatory conditions that predispose to malignancy (e.g., Marjolin's ulcer in chronic wounds, cholangiocarcinoma in PSC).

• Extremely rare but well-documented in case reports.
• Should be suspected in any long-standing fistula that suddenly changes character — increased pain, harder tissue, bloody rather than purulent discharge, rapid growth.
• Always send fistula tract curettings for histology at the time of surgery to exclude occult malignancy.

6. Complications Specific to Crohn's-Associated Fistulae

Crohn's perianal fistulae carry additional disease-specific complications due to the transmural inflammatory nature of the underlying disease ^[12][8]:

• Fistula formation (additional types beyond perianal) ^[12]:
  • Enteroenteric fistula → palpable mass ^[12]
  • Enterovesical fistula → urinary symptoms due to UTI (pneumaturia = air in urine, faecaluria = faeces in urine) ^[12][13]
  • Enterovaginal fistula → passage of gas or faeces through vagina ^[12][13]
  • Enterocutaneous fistula → drainage of bowel contents to surface of skin ^[12][13]
  • Retroperitoneal fistula → psoas abscess or ureteral obstruction with hydronephrosis ^[12]
• Abscess formation — sinus tracts may lead to abscess → acute localised peritonitis with fever, abdominal pain, and tenderness → perforation can lead to diffuse peritonitis ^[12].
• Perianal disease — formation of skin tags, anal fissures, perirectal abscess, and anorectal fistulas ^[12]. 83% of perianal Crohn's patients required surgery ^[8].
• Anal stenosis / stricture — chronic fibrosis from repeated inflammation narrows the anal canal.

B. Complications of Surgical Treatment

This is where the exam loves to test you — the complications of each surgical procedure and why they occur.

1. Faecal Incontinence

This is the most feared complication of fistula surgery and the primary reason complex fistulae require sphincter-sparing approaches rather than fistulotomy.

Risk of incontinence should be informed before any fistula surgery ^[1].

Mechanism: Division of the anal sphincter muscle during surgery → loss of the sphincter's ability to maintain closure → involuntary leakage of gas, liquid stool, or solid stool.

Risk factors for incontinence after fistula surgery:

• Pre-existing sphincter damage (previous obstetric injury, prior fistula surgery, prior lateral internal sphincterotomy for fissure).
• Female sex — thinner, shorter anterior sphincter.
• Anterior fistula in women — even a small division of the anterior EAS can cause incontinence.
• Recurrent fistula surgery — each operation removes more sphincter tissue; cumulative damage.
• Older age — reduced baseline sphincter tone due to age-related muscle atrophy and pudendal neuropathy.

2. Recurrence

This is the second most common complication. Recurrence rates vary significantly by procedure:

Common reasons for recurrence:

1. Failure to identify the internal opening — if the cryptoglandular source is not addressed, the infection cycle restarts. The most critical step is to identify and curette the internal opening ^[2].
2. Missed secondary tracts — undrained extensions continue to harbour infection. This is why MRI is essential for complex fistulae.
3. Inadequate drainage of associated abscess — residual sepsis prevents healing.
4. Underlying Crohn's disease not treated medically — ongoing transmural inflammation drives recurrence regardless of surgical technique. More complicated courses of fistula tract in Crohn's ^[8].
5. Creation of false passage during probing → new iatrogenic tract that is not addressed.

3. Sphincter Injury (Without Overt Incontinence)

Complications: sphincter injury ^[3].

Even when patients do not report overt incontinence, fistula surgery can cause subclinical sphincter damage detectable on endoanal ultrasound or anorectal manometry. This may only become clinically significant later in life when age-related sphincter degeneration further reduces continence reserves.

4. Wound-Related Complications

Fistulotomy wounds heal by secondary intention (from the base up, without primary closure). This creates specific wound-related issues:

• Delayed wound healing: Fistulotomy wounds in the perianal region take 6–12 weeks to heal fully. The perineum is a moist, contaminated environment (faecal bacteria, constant movement from sitting/walking), which slows healing.
• Wound infection: Despite the wound being intentionally left open, secondary bacterial overgrowth can cause wound cellulitis or a new abscess.
• Bleeding: Minor postoperative bleeding from the granulating wound bed is common. Significant bleeding (reactionary or secondary) is uncommon but may require return to theatre for haemostasis.
• Anal stenosis / stricture: Excessive fibrosis during wound healing can narrow the anal canal → difficulty with defecation, thin stools, straining. More common with extensive fistulotomy (large wound) or circumferential wounds. May require finger or endoscopic balloon dilatation ^[14].

5. False Passage Creation

Mechanism: During EUA, if the fistula probe is forced through the tract wall rather than following the true tract, a new iatrogenic passage is created through tissue that was previously uninvolved ^[2].

• Performed with caution to avoid creating a false passage by penetrating the fistula wall ^[2].
• This can convert a simple fistula into a complex one, or create a new fistula in addition to the original one.
• Avoided by: gentle probing, using a malleable (not rigid) probe, never forcing the probe.

6. Complications Specific to Certain Procedures

7. Stoma-Related Complications (When Faecal Diversion Is Required)

For extrasphincteric fistulae (Type IV), refractory Crohn's perianal disease, or salvage procedures, a defunctioning stoma may be created. This carries its own set of complications ^[2][14]:

Early ( < 30 days):

• Stomal bleeding
• Stomal necrosis (ischaemia of the bowel stoma — appears dark/dusky)
• Stomal retraction (stoma sinks below skin level → appliance leakage)
• Mucocutaneous separation (dehiscence of the stoma-skin junction)
• Skin irritation and dermatitis (especially ileostomy — high-output alkaline enzymatic effluent is highly corrosive to skin)

Late ( > 30 days):

• Parastomal hernia (bowel herniates through the fascial defect around the stoma)
• Stomal prolapse
• Stomal stenosis

C. Summary: Complications at a Glance

References

^[1] Lecture slides: GC 179. Anal pain perianal lesions and sepsis.pdf (p58) ^[2] Senior notes: felixlai.md (Anorectal fistulas — Treatment section; Anorectal abscess — Overview) ^[3] Senior notes: maxim.md (Anal fistula — Definitive treatment, Complications) ^[8] Lecture slides: Inflammatory bowel disease.pdf (p45) ^[12] Senior notes: felixlai.md (Crohn's disease — Signs and symptoms of complications) ^[13] Lecture slides: Inflammatory bowel disease.pdf (p33) ^[14] Senior notes: felixlai.md (Colorectal surgery — Complications section); Senior notes: maxim.md (Post-operative complications section)