Chapter 05 — Suppurative Disease — Abscess, Fistula, and Fournier’s Gangrene

Purpose: drain pus the same day, protect the sphincter when a fistula follows, and recognise Fournier’s gangrene within the first hour.

🎨 INFOGRAPHIC / VISUAL BLUEPRINT — CHAPTER 05

• Chapter colour palette: Flame Orange (#C2410C) header bar, Warm Peach (#FFF7ED) panels, Emergency Crimson (#DC2626) borders for Fournier’s gangrene alerts.
• Hero graphic: (1) Coronal map of the four abscess spaces and the four Parks fistula types, with a Goodsall’s rule compass; (2) a “first-hour clock” for Fournier’s gangrene: call surgeon → resuscitate → IV antibiotics → debridement → re-look at 24–48 h.
• Icon set: Scalpel with drop (drain), stopwatch (first hour), seton loop, MRI scanner, crossed-out gauze roll (no routine packing), SGLT2 pill with warning triangle.
• Bedside aphorism banner: “Drainage cures the infection, not necessarily the disease. A seton is a strategy, not a delay.”

1. The Cryptoglandular Rule: One Disease, Two Stages

Most anorectal abscesses and fistulas are the acute and chronic phases of the same infection, which starts in an anal gland opening at the dentate line (ASCRS 2022).

• A fistula is already present in a proportion of patients at the time of drainage.

• After drainage, a fistula or recurrent abscess develops in a substantial minority — reported rates range widely with follow-up (in the PPAC2 trial, 11–15% had a fistula within months).

• Warn every patient before drainage that a later fistula reflects the biology of the gland, not a surgical error.

2. The Four Abscess Spaces — and Why Deep Pus Does Not Fluctuate

Table 5.1 — Abscess spaces

3. Abscess Management Rules (ASCRS 2022; PPAC2 2022)

1. Drain promptly — never wait for fluctuance [S][C]. Fever plus severe localised perianal or rectal pain means drainage today.
2. Incise close to the anal verge [C], so that if a fistula forms its external opening lies near the sphincter, not far out on the buttock.
3. No routine packing [S]. In the PPAC2 randomised trial (433 adults, 50 UK hospitals), packing produced higher pain scores (mean maximum VAS 38.2 vs 28.2; P < 0.0001) without reducing fistula formation (15% vs 11%) or recurrence. Pack only for haemostasis, then use an absorbent dressing. (PPAC2 excluded horseshoe abscesses, known fistulas, and Fournier’s gangrene.)
4. Antibiotics are adjuncts, not substitutes [C]. After drainage, give antibiotics selectively for surrounding cellulitis, systemic signs of infection, or immunosuppression (ASCRS 2022); many clinicians also use them in poorly controlled diabetes or with prosthetic heart valves [L].
5. Do not probe aggressively during acute inflammation [C] — false tracts through the sphincter are easily created.
6. Imaging (MRI, CT, or endoanal ultrasound) helps when the abscess is deep, recurrent, horseshoe, or Crohn’s-related, or the diagnosis is uncertain — but never delays drainage of a clinically obvious abscess [C].
7. Immunosuppressed and neutropenic patients may have little pus or fluctuance. Arrange early surgical review; the neutropenic precautions in Chapter 06 never justify delaying surgical assessment [C].

4. Fistula-in-Ano for Non-Surgeons

Goodsall’s rule — predicting the internal opening [C]

• Draw a transverse line across the anus with the patient in lithotomy. Anterior external openings usually track radially, straight to the nearest crypt. Posterior openings usually curve to the posterior midline.

• Exceptions are common: anterior openings more than about 3 cm from the verge may curve posteriorly, and the rule is less reliable in women and in Crohn’s disease.

Parks classification and sphincter risk

Table 5.2 — Parks classification

Parks’ original 1976 series (a referral population) reported roughly 45% intersphincteric, 30% trans-sphincteric, 20% suprasphincteric, and 5% extrasphincteric; suprasphincteric tracts are less frequent in most contemporary series.

“Complex” fistula — refer for imaging and a sphincter-sparing plan (ASCRS 2022)

• Involves a substantial portion of the external sphincter (commonly > 30%) or is suprasphincteric or extrasphincteric.

• Anterior fistula in a woman; multiple tracts; recurrent fistula; pre-existing incontinence; previous pelvic radiation; Crohn’s disease.

• Pelvic MRI (fistula protocol) is the usual first imaging test for complex, recurrent, or Crohn’s-related fistulas.

Why setons are a bridge, not a failure

A loose draining seton keeps the tract open, drains sepsis continuously, and allows inflammation to settle before a staged sphincter-sparing repair, such as ligation of the intersphincteric fistula tract (LIFT), endorectal advancement flap, fistulectomy with primary sphincter repair, or laser or video-assisted techniques. Fistula plugs and fibrin glue have low and variable success (ASCRS 2022). Fistulectomy with primary sphincter repair has been reported by the author’s group in a single-centre series of 71 patients with one recurrence (Lasheen et al. 2016) — encouraging but low-certainty evidence [U].

5. Surgical Emergency: Fournier’s Gangrene

Fournier’s gangrene is a polymicrobial necrotising fasciitis of the perineal, perianal, and genital region. It spreads along fascial planes — very rapid spread (up to 2–3 cm per hour) has been reported — and mortality rises with every hour of delay to debridement (WSES-AAST 2021; WSES/SIS-E 2018).

Cardinal clues — never wait for all of them

• Pain out of proportion to visible skin changes. Early skin may show only subtle redness, oedema, or a bronze tinge; numbness appears late as cutaneous nerves die.

• Subcutaneous crepitus, foul “dishwater” discharge, dusky or violaceous skin, bullae, or rapidly spreading erythema.

• Systemic toxicity: fever or hypothermia, tachycardia, hypotension, raised lactate, acute kidney injury, new confusion.

• Risk multipliers: diabetes; SGLT2 inhibitors (empagliflozin, dapagliflozin, canagliflozin, ertugliflozin — subject of a 2018 FDA safety warning; Bersoff-Matcha et al. 2019); alcohol misuse; cirrhosis; obesity; HIV or other immunosuppression; chemotherapy; recent perineal surgery, urethral instrumentation, or stricture.

• A low LRINEC score or an unremarkable CT never excludes necrotising infection — this is a clinical diagnosis (WSES-AAST 2021) [C].

Table 5.3 — The first-hour Fournier’s bundle

Algorithm 5.1 — Suppurative disease

[ACUTE PERIANAL / PERINEAL PAIN +/- FEVER OR SWELLING]
|
+-- ANY OF: pain out of proportion, spreading redness,
|   dusky skin or bullae, crepitus, foul discharge,
|   tachycardia/hypotension, confusion, SGLT2 inhibitor?
|     -> RED: SUSPECT FOURNIER'S GANGRENE
|        1 Senior surgeon + theatre NOW (minute zero)
|        2 Resuscitate; stop SGLT2i; check ketones
|        3 IV antibiotics within 1 h (broad-spectrum
|          + MRSA cover if indicated + clindamycin)
|        4 Radical debridement; re-look in 24-48 h
|        Never wait for CT, MRI, or a LRINEC score
|
+-- Localised tender swelling, fever, or deep rectal pain
|   (skin may be normal if intersphincteric/supralevator)
|     -> RED (SAME-DAY): incision and drainage close to
|        the verge; no routine packing; antibiotics only
|        if cellulitis, systemic signs, immunosuppression;
|        warn about later fistula
|
+-- Recurrent discharge from an external opening (fistula)
      -> SIMPLE (low, normal continence)
      |     -> GREEN/YELLOW: surgical clinic; fistulotomy
      |        usually curative
      -> COMPLEX (high, anterior in a woman, recurrent,
         multiple tracts, Crohn's, poor continence)
            -> YELLOW: MRI first; loose draining seton;
               staged sphincter-sparing repair

6. Dangerous Mimics

Table 5.4 — Abscess and fistula mimics

7. Role-Targeted Action Boxes

🚑 ER DOCTOR ACTION BOX

• Normal skin + severe deep rectal pain + fever = intersphincteric or supralevator abscess until proven otherwise. Do not discharge as “proctalgia” or “internal piles”; call surgery for examination under anaesthesia.
• Ask every diabetic with perineal pain whether they take an SGLT2 inhibitor, and look specifically for crepitus and skin discolouration.

🩺 FAMILY MEDICINE / GP CLINIC BOX

• Stop daily ribbon-gauze packing of drained abscesses. Advise showers or warm-water sitz baths and an absorbent pad (PPAC2).
• Never treat a perianal swelling with antibiotics alone. Refer the same day.
• Arrange follow-up after drainage: persistent discharge or a recurring lump suggests a fistula.

🏥 ER & WARD NURSE CARE AND SBAR BOX

• Mark and time the erythema border with a skin pen on arrival in any patient with perianal or perineal cellulitis.
• Escalate immediately by SBAR if redness spreads beyond the line, new dusky skin, bullae, or crepitus appear, or the patient has respiratory rate ≥ 22/min, systolic BP ≤ 100 mmHg, or new confusion (qSOFA), or a NEWS2 score at the local trigger level [L].

💊 CROSS-SPECIALTY PRESCRIBING TRAP

• SGLT2 inhibitors: stop immediately when Fournier’s gangrene is suspected; check blood ketones and anion gap even if glucose is near normal. Any restart is a specialist decision.
• Ceftriaxone alone is not adequate empirical cover for necrotising perineal infection.

⛔ NEVER DO THIS

• Never treat a perianal abscess with antibiotics alone.
• Never pack a drained abscess cavity routinely.
• Never lay open a high, anterior-female, recurrent, or Crohn’s fistula; never use a tight “cutting” seton as a quick fix.
• Never delay Fournier’s surgery for imaging or a scoring system.
• Never probe aggressively for an internal opening in acute sepsis.

📝 COMMON DOCUMENTATION ERROR

• “Abscess drained” without the anatomical site, distance from the verge, volume, whether an internal opening was seen, culture (if recurrent or immunosuppressed), and fistula counselling.
• Fournier’s gangrene without time stamps: arrival, surgical call, antibiotics, theatre start.

📨 REFERRAL URGENCY BOX

The Patient in Front of You — Chapter 05 Mini-Case

🧑‍⚕️ THE PATIENT IN FRONT OF YOU

A 56-year-old man with type 2 diabetes on empagliflozin 10 mg daily presents at 02:00 with 10/10 perineal and scrotal pain. Temperature 38.6 °C, heart rate 118/min, blood pressure 96/60 mmHg, glucose 9.2 mmol/L (165 mg/dL). The perianal skin shows only mild bronze induration and a 2 cm dusky patch, but gentle palpation reveals crepitus. The resident has requested a contrast CT (2-hour wait) and given IV ceftriaxone.

Step 1 Cancel the delay. Pain out of proportion, crepitus, tachycardia, and hypotension in a diabetic on an SGLT2 inhibitor = Fournier’s gangrene.

Step 2 Launch the first-hour bundle. Call the senior surgeon and theatre now; resuscitate; stop empagliflozin and check ketones and anion gap; broaden antibiotics to a carbapenem or piperacillin–tazobactam plus MRSA cover as indicated plus clindamycin, per local protocol.

Step 3 Source control. Straight to theatre for radical debridement, then critical care and a planned re-look within 24–48 hours.

Step 4 Document times of each step.

Board-Style Question — Chapter 05

During drainage of an acute perianal abscess in a 38-year-old man, the surgeon finds a high trans-sphincteric fistula crossing more than half of the external sphincter. What is the most appropriate management?

• A. Immediate fistulotomy of the whole tract and daily tight iodoform packing
• B. Loose draining seton, no routine cavity packing, and a planned staged sphincter-sparing repair
• C. Fibrin glue injection and primary skin closure under antibiotic cover
• D. Tight cutting seton to divide the external sphincter over 48 hours

Answer and explanation

• ❌ A Dividing a high tract risks permanent incontinence, and routine packing adds pain without benefit.
• ✅ B Correct. A loose seton controls sepsis and allows a later sphincter-sparing repair (ASCRS 2022). Omitting routine packing reduces pain without increasing fistula or recurrence (PPAC2).
• ❌ C Fibrin glue has low and variable success, especially in acute sepsis; closing an abscess cavity invites recurrence.
• ❌ D Tight or cutting setons carry a substantial risk of sphincter injury and incontinence.

Ten-Item Rapid Revision Checklist — Chapter 05

• 01 I drain abscesses the same day and never wait for fluctuance.
• 02 I know intersphincteric and supralevator abscesses can have normal skin.
• 03 I do not pack drained cavities routinely (PPAC2).
• 04 I give antibiotics after drainage only for cellulitis, systemic signs, or immunosuppression (or by local policy).
• 05 I warn every patient about the risk of a later fistula.
• 06 I can describe the Parks classification and define a complex fistula.
• 07 I request MRI for complex, recurrent, or Crohn’s fistulas.
• 08 I understand why a loose seton is a deliberate strategy.
• 09 I recognise Fournier’s gangrene clinically and call the surgeon at minute zero.
• 10 I stop SGLT2 inhibitors and check ketones in suspected Fournier’s gangrene.

Five Most Dangerous Mistakes — Chapter 05

1. ⚠️ Discharging a febrile patient with deep rectal pain and normal skin.
2. ⚠️ Prescribing antibiotics instead of draining an abscess.
3. ⚠️ Sending a patient with crepitus and hypotension for CT before calling the surgeon.
4. ⚠️ Missing euglycaemic ketoacidosis in a patient on an SGLT2 inhibitor.
5. ⚠️ Laying open a complex or Crohn’s fistula and causing incontinence.

Suggested Figure and Table Captions for the Medical Illustrator — Chapter 05

• Figure 5.1. Coronal map of perianal, ischioanal, intersphincteric, and supralevator spaces.
• Figure 5.2. Parks fistula types with Goodsall’s rule overlay.
• Figure 5.3. Loose draining seton in place.
• Figure 5.4. The first-hour Fournier’s gangrene clock.
• Table 5.1. Abscess spaces and diagnostic traps.
• Table 5.2. Parks classification and sphincter risk.
• Table 5.3. The first-hour Fournier’s bundle.
• Table 5.4. Abscess and fistula mimics.
• Algorithm 5.1. Suppurative disease.