Perianal Abscess and Fistula-in-Ano
Summary
Perianal abscess and Fistula-in-Ano are two stages of the same disease process: Cryptoglandular Infection. An abscess is the acute phase (pus collection), while a fistula is the chronic phase (epithelialised track forming between the anal canal and perineal skin). Approximately 30-50% of abscesses will develop a fistula. The cardinal rule of management is that abscesses require surgical drainage, not antibiotics alone. Fistula management balances the dual (and often conflicting) goals of healing the track and preserving continence (sphincter function). [1,2]
Key Facts
- Cryptoglandular Theory (Eisenhammer): Infection starts in the anal glands (located in the intersphincteric space at the dentate line) which then burrows outwards to the skin.
- Goodsall's Rule (for Fistula):
- Anterior Opening (Top half): Track goes Straight to the anal canal.
- Posterior Opening (Bottom half): Track is Curved, entering the canal at the 6 o'clock midline position.
- Exception: Anterior openings >3cm from the anus may curve to the posterior midline ("Long Anterior Fistula").
- Park's Classification:
- Intersphincteric (Most common, 70%): Between internal/external sphincters.
- Transsphincteric (25%): Crosses external sphincter.
- Suprasphincteric: Goes over the top of the sphincter (levator).
- Extrasphincteric: From rectum to skin (rare, Traumatic/Crohn's).
Clinical Pearls
Antibiotics are Useless: Treating a fluctuating abscess with antibiotics alone is negligence. It delays effective treatment, allows the abscess to grow, and promotes complex fistula formation. "Do not let the sun set on a perianal abscess".
Fournier's Gangrene: If you see black skin, crepitus (gas), or the patient is septic with a "painful scrotum/perineum", this is Necrotising Fasciitis. Immediate radical debridement is life-saving.
Crohn's Disease: Always suspect Crohn's in recurrent, complex, or multiple fistulas. Avoid aggressive cutting (fistulotomy) in Crohn's as healing is poor and incontinence risk is high.
Incidence
- Common surgical emergency.
- Gender: Men > Women (2:1).
- Age: Peak 30-50 years.
Risk Factors
- Diabetes.
- Crohn's Disease.
- Smoking.
- TB / HIV (in endemic areas/high risk).
The "Lifecycle"
- Obstruction of anal gland duct.
- Stasis and Infection (E. coli, Bacteroides).
- Abscess Formation in intersphincteric space.
- Expansion:
- Downwards -> Perianal Abscess (Simple).
- Outwards -> Ischiorectal Abscess (Large, deep fossa).
- Upwards -> Supralevator Abscess.
- Drainage (Spontaneous or Surgical).
- Fistula Formation: persistent track fed by the internal opening (the gland).
Abscess
Fistula
External Inspection
- Abscess: Red, tender, hot, fluctuating lump near anus.
- Fistula: Small "granulation tissue" opening. Pressing it may express pus. Note position (Clock face).
Digital Rectal Examination (DRE)
- In acute abscess, DRE is often too painful and should be deferred to Examination Under Anaesthesia (EUA).
- Induration: Can feel the "track" as a cord-like structure.
- Internal Opening: A pit or depression (usually dentate line).
Imaging
- MRI Pelvis: Gold Standard for Fistula. Defines anatomy of track relative to sphincter muscle. Essential for recurrent/complex cases.
- Endoanal Ultrasound (EAUS): Alternative to MRI.
- CT: Only useful for deep supralevator abscesses or if diagnosis unclear (diverticulitis?). Poor for sphincter anatomy.
Screening
- HbA1c: Rule out Diabetes.
- Faecal Calprotectin: Rule out Crohn's (if suggestive history).
Management Algorithm
PERIANAL COMPLAINT
↓
┌─────────────┴─────────────┐
ACUTE ABSCESS CHRONIC FISTULA
(Pain, Fever, Lump) (Discharge, Track)
↓ ↓
EMERGENCY SURGERY MRI PELVIS
(I & D under GA) ↓
↓ ┌─────────┴─────────┐
Drain Pus LOW / SIMPLE HIGH / COMPLEX
Search for Fistula (No muscle) (Involves muscle)
↓ (e.g., Intersphincteric) (e.g., Transsphincteric)
Leave open ↓ ↓
(+ Abx only if Septic) FISTULOTOMY SETON SUTURE
(Lay Open) (Loose rubber loop)
1. Management of Abscess
- Incision and Drainage (I&D):
- Cruciate incision or excision of skin ("deroofing") to prevent premature closure.
- Cavity packed with dissolvable (Sorbsan) or ribbon gauze.
- Healing by secondary intention (granulating from bottom up).
- Antibiotics: Only indicated for Sepsis, extensive Cellulitis, or Immunosuppression/Diabetes. Metronidazole + Ciprofloxacin (or Augmentin).
2. Management of Fistula
Goal: Healing without Incontinence.
- Fistulotomy (Lay Open): Cutting through the skin and muscle over the track.
- Indication: Low fistulas involving minimal sphincter muscle.
- Cure rate: >90%.
- Seton: Threading a silicone/suture loop through the track and tying it loosely.
- Indication: High fistulas, Crohn's, or acute sepsis.
- Function: Keeps track open (draining), prevents abscess, allows track to mature/fibrose. Does not cure, but controls.
- Sphincter-Sparing Procedures (for high fistulas):
- LIFT (Ligation of Intersphincteric Fistula Tract).
- VAAFT (Video-Assisted Anal Fistula Treatment).
- Advancement Flap: Moving rectal mucosa to cover proper internal opening.
- Fibrin Glue / Plug: Rarely works long term.
- Recurrence: High (30-50%). Commonest cause is a missed internal opening or undiagnosed Crohn's.
- Incontinence: Damage to Internal (sampling reflex) or External (squeeze) sphincter.
- Anal Stenosis: Scarring.
- Simple Abscess: Good outcome with drainage. 50% heal, 50% form fistula.
- Complex Fistula: Often requires multiple planned procedures.
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Fistula-in-Ano | ASCRS / ACPGBI | MRI is imaging of choice. Use Setons for sphincter preservation. |
| Sepsis | NICE | I&D source control is priority. Antbiotics adjunct. |
Landmark Knowledge
1. The "Horseshoe" Abscess
- An abscess that tracks from one ischiorectal fossa, behind the anus (deep post-anal space), to the other side.
- Needs thorough drainage (often two incisions + counter-drainage seton).
What is an Abscess?
It is a collection of pus caused by an infection of a tiny gland inside the bottom. It's like a boil, but deeper. It is extremely painful because the pressure builds up in a sensitive area.
Why do I need Surgery?
Antibiotic tablets cannot penetrate into the pus to kill the bugs. The only way to fix it is to let the pus out ("lancing" it). You will wake up with a dressing in the wound.
What is a Fistula?
Sometimes, after the abscess drains, the tunnel it made doesn't heal up completely. It stays open as a little tube (fistula) connecting the inside of the bottom to the skin. This can leak fluid on and off.
How do we fix a Fistula?
If the tunnel is shallow, we can "unroof" it so it heals from the bottom up. If it goes through the important muscles that control bowel movements, we can't just cut it. We might put a small rubber thread (A Seton) in to keep it draining safely while we plan a more complex repair.
Primary Sources
- Vogel JD, et al. Clinical Practice Guideline for the Management of Anorectal Abscess, Fistula-in-Ano, and Rectovaginal Fistula. Dis Colon Rectum (ASCRS). 2016;59:1117-1133.
- Garg P. Understanding and treating anal fistula. Colorectal Dis. 2017.
- Parks AG, Gordon PH, Hardcastle JD. A classification of fistula-in-ano. Br J Surg. 1976;63:1-12. PMID: 1267867.
Common Exam Questions
- Surgery: "Goodsall's Rule exception?"
- Answer: Anterior openings >3cm from anal verge track to the posterior midline.
- Anatomy: "Boundary of Ischiorectal Fossa?"
- Answer: Ischial Tuberosity (lateral), Sphincter complex (medial).
- Emergency: "Patient with perianal pain, fever, no lump visible. Diagnosis?"
- Answer: Suspect Intersphincteric or Supralevator abscess (Examination Under Anaesthesia required).
- Pathology: "Fistula with non-caseating granulomas?"
- Answer: Crohn's Disease. (Caseating = TB).
Viva Points
- Antibiotic overuse: Why is it bad? It sterilises the pus but leaves the cavity. The mass remains ("Antibioma"), becomes chronic, and is harder to treat.
- Seton Types: Loose (drainage only) vs Cutting (tightened slowly to cut through muscle while fibrosis holds it together - rarely used now due to pain/incontinence).
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.