Perianal Abscess and Anal Fistula

Meet the patient

A 42-year-old man rocks into A&E at 3am, unable to sit. For three days a throbbing perianal pain has stopped him sleeping, passing urine, and thinking straight; he is febrile, and the perineum is hot, red and exquisitely tender to the lateral touch you barely attempt. There is a fluctuant lump at the anal margin.[1]

Two questions will decide his next hour, and they decide every perianal case: is this pus that needs a knife tonight? and how much sphincter sits between the sepsis and the skin? Hold those two questions and the whole topic falls into place. The first protects him from Fournier's gangrene; the second protects him from a wet sock for the rest of his life.[1]

One disease, two stages — the cryptoglandular cascade

The abscess is the acute stage; the fistula is the chronic stage — and both begin at one obstructed anal gland. This is the cryptoglandular theory, and it accounts for over 90 percent of fistulas.[1]

The cascade, walked step by step the way a viva wants it:[1]

1. The anal canal carries 6 to 12 anal glands opening into the crypts of Morgagni at the dentate line, some diving into the internal sphincter or the intersphincteric plane.
2. A duct is obstructed — by inspissated stool, oedema, debris, or a faecolith — and the trapped secretion becomes a culture medium.
3. Mixed aerobic and anaerobic flora (E. coli, Bacteroides, streptococci, staphylococci, enterococci) generate pus; the obstructed gland distends into a small intersphincteric abscess — the seed of every cryptoglandular collection.
4. Pus spreads along the path of least resistance: downward to a perianal abscess (commonest), laterally across the external sphincter into an ischiorectal abscess, upward above levator into a supralevator abscess, or circumferentially through the deep postanal space into a horseshoe.
5. When the abscess drains — spontaneously or at surgery — a communicating tract may persist between the dentate-line internal opening and the skin. Both openings re-epithelialise continuously, the tract lining becomes granulation then epithelium, and the tract cannot close on its own. That persistent epithelialised tunnel is an anal fistula.[1]

The number examiners love sits at step 5: 30 to 50 percent of drained abscesses go on to fistulise. Warn every abscess patient at discharge, or you will be the one meeting them again in clinic.[1]

The map before the operation — applied anatomy

Every classification, every operation, and every incontinence risk reduces to one relationship: the tract versus the sphincter complex. Learn the anatomy and the surgery teaches itself.[1]

The anal canal is about 4 cm long, with two examinable definitions: the surgical (anatomical) canal runs from the anorectal ring to the anal verge, and the embryological (histological) canal runs from the dentate line to the verge, marking the endoderm-to-ectoderm transition.[1]

The dentate (pectinate) line sits about 2 cm proximal to the verge and is the most important landmark in the canal. Here a ring of 6 to 12 anal glands opens upward into the anal sinuses; the glands penetrate the internal sphincter and some reach the intersphincteric plane. This is the structural flaw the cryptoglandular theory exploits — a duct can block, and the gland sitting within or between the sphincters is primed to suppurate.[1]

The sphincter complex is two concentric muscles with very different temperaments:[1]

• Internal anal sphincter (IAS) — involuntary smooth muscle, a continuation of the rectum's circular layer, in continuous tonic contraction; it generates 70 to 85 percent of resting anal pressure.
• External anal sphincter (EAS) — voluntary striated muscle in subcutaneous, superficial and deep parts; it generates squeeze pressure, and it is the muscle whose division causes incontinence.[1]

Between them lies the intersphincteric plane — the surgeon's motorway. Dissecting here disturbs no functional muscle, which is exactly why fistulotomy and LIFT both travel this plane. Posteriorly, puborectalis (part of levator ani) slings the anorectal junction in a U, pulling it forward into the anorectal angle. Its palpable upper border is the anorectal ring — the fistulotomy safety line.[1]

The perirectal spaces decide where pus goes, and therefore which abscess you are dealing with:[1]

[1]

Etymology for viva gold: the crypts of Morgagni carry the name of the 18th-century Italian anatomist Giovanni Battista Morgagni, who taught that disease lives in anatomy. He was right about this one — the crypt is where the whole story starts.[1]

Park's quartet — 70, 25, 5, 1

Park's classification, published in 1976, remains the global standard because it is built around the muscle you must protect. Memorise the four types and their frequencies as a single chord — 70, 25, 5, 1.[1]

[1]

The simple-versus-complex split is the bedside discriminator that dictates the whole operation. A fistula is simple when it is intersphincteric or low transsphincteric, involves under 30 percent of the external sphincter, and has a single tract in an otherwise normal anus. It is complex when it involves more than 30 percent of the EAS, has a high internal opening, is recurrent or horseshoe, coexists with Crohn's disease, has failed previous surgery, or sits in an irradiated or already-incontinent patient.[2]

The radiologist writes a different language on the MRI report — the St James's University Hospital classification, grading fistulas from simple intersphincteric (Grade 1) up to horseshoe and extrasphincteric disease (Grade 5). Pair the two: Park's is the surgeon's operative map, St James's is the radiologist's roadmap.[2][3]

[1]

Goodsall's transverse line — anterior straight, posterior to six

Goodsall's rule predicts the internal opening from the external one — invaluable in theatre when you cannot see where the tract goes. Draw an imaginary transverse line across the anus.[1]

• An external opening anterior to the line runs a short, straight (radial) tract directly to the nearest anal crypt.
• An external opening posterior to the line runs a curved tract that opens at the posterior midline (6 o'clock) crypt.[1]

The exception that earns marks: an anterior opening more than 3 cm from the anal verge usually behaves like a posterior one — its tract curves to the 6 o'clock midline — because it originates from a posterior midline gland.[1]

Goodsall is right in about 80 percent of cases, but is less reliable in recurrent fistulas and in Crohn's disease, where MRI and EUA take over. Apply it at the bedside, confirm it on the table, and never let it override imaging in a complex tract.[1]

[1]

Who gets it, and why

Perianal sepsis is common — about 15 to 20 per 100,000 per year, with a lifetime prevalence around 1 in 10,000, peaking at 30 to 50 years and with a striking male predominance of 2 to 4:1 (the androgenic effect on anal gland secretion).[1]

The risk factors sort into two piles — those that obstruct the gland duct and those that impair healing or immunity:[1]

• Crohn's disease — the single most important systemic association; perianal fistula can be the presenting feature, and up to a third of Crohn's patients develop perianal disease over a lifetime. Think Crohn's whenever fistulas are multiple, recurrent, complex, or come with diarrhoea.
• Diabetes mellitus — higher risk of severe, necrotising infection and Fournier's gangrene; an undiagnosed diabetic can walk in with a perianal abscess as the first symptom.
• Immunosuppression — HIV/AIDS, chemotherapy, long-term steroids, post-transplant — atypical, aggressive infection and poor healing.
• Prior anorectal surgery or trauma, including obstetric injury.
• Tuberculosis in endemic regions — a thin, watery, "tubercular fistula".
• Malignancy — rarely, a chronic non-healing fistula hides an anal or rectal cancer.
• Smoking — strongly linked to Crohn's perianal disease and poor healing.
• Chronic diarrhoea or constipation — mechanically promotes duct obstruction.[1]

The abscess at 3am — recognise it

The abscess announces itself with severe, constant, throbbing perianal pain that keeps the patient awake — the pain of pus under pressure. It is worse on sitting, walking, and defecation, and it comes with swelling, redness, fever, and malaise; urinary retention from intense pelvic-floor spasm can be the presenting complaint.[1]

On examination: a tender, fluctuant, erythematous swelling with surrounding induration and cellulitis, sometimes inguinal lymphadenopathy, and systemic signs of sepsis when the collection is large. Digital rectal examination is often too painful to complete — and when it is, you are looking for an internal component, induration, or a high abscess.[1]

The fistula that keeps coming back

The fistula presents as chronic, relapsing perianal discharge — purulent, bloody, or mucopurulent, soiling underwear, demanding a pad. The history that is almost diagnostic is "the same lump that bursts and comes back": the tract blocks, an abscess reforms, it discharges, the swelling settles, and the cycle repeats.[1]

Examination shows a small external opening — a punctum or papule with surrounding skin discolouration and a bead of visible discharge — and sometimes a palpable subcutaneous cord running toward the anus. On DRE, feel for the internal opening as a pit or indurated nodule at the dentate line, and gauge the sphincter tone — the single most important factor in choosing the operation. Apply Goodsall's rule at the bedside before you ever reach theatre.[1]

The low-versus-high fork that decides the operation:[1]

[1]

Not everything perianal is cryptoglandular

Every perianal lump or chronic sinus is not a cryptoglandular abscess — and one of the mimics is a surgical emergency. Name the close relations and the discriminator for each.[1]

[1]

The discriminator line: if the sinus is not connected to the anal canal, it is not a cryptoglandular fistula — work up the alternative.[1]

Drain the pus — the operation beats the antibiotic

Incision and drainage is the definitive treatment of an abscess; the operation is more important than any antibiotic. Pus under pressure does not respond to penicillin — it responds to a knife.[1]

[1] [1] [1]

Resuscitation is reserved for the large abscess with systemic sepsis or suspected necrotising infection: IV access and fluids, IV opioid analgesia, broad-spectrum IV antibiotics (co-amoxiclav 1.2 g every 8 hours, or ceftriaxone 1 to 2 g daily plus metronidazole 500 mg every 8 hours), urgent surgical assessment, and — in the diabetic — strict glucose control with an insulin sliding scale and a low threshold for imaging and ICU.[1]

The anorectal ring — the fistulotomy safety line

Fistulotomy (lay-open) is the operation for a low, simple fistula, and it works because the tract crosses little or no functional muscle. Success is over 90 percent and recurrence under 10 percent — the best numbers in fistula surgery, earned only when you have correctly identified a low tract.[1]

Pass a probe from the external to the internal opening, divide the overlying tissue along the entire tract, convert the tunnel into a gutter, accept a small amount of internal sphincter in the floor, and let it heal by secondary intention over 2 to 6 weeks with sitz baths and dressings. Incontinence is minimal for a truly low fistula and is proportional to the amount of muscle divided.[1]

The rule that protects continence: a fistulotomy below the anorectal ring is safe; above the ring it divides puborectalis and is catastrophic. If the tract climbs high, stop, leave a loose seton, and choose a sphincter-sparing definitive procedure.[1]

LIFT, seton, and the advancement flap — spare the sphincter

For any fistula that crosses a significant portion of external sphincter, the goal shifts from "lay it open" to "eliminate the source while leaving the sphincter intact." Four techniques do that, and the choice follows the anatomy.[2]

Loose (draining) seton — the cornerstone of the high or complex fistula. A non-absorbable suture or silicone vessel loop passes through the tract and is tied loosely, maintaining drainage, preventing abscess recurrence, and inducing gradual peritract fibrosis — it deliberately does not divide the sphincter. Left for weeks to months, it is often the first stage of a planned procedure or the long-term answer in Crohn's.[1][3]

LIFT (ligation of the intersphincteric fistula tract) — for transsphincteric fistulas. The tract is identified in the intersphincteric plane through a small perianal incision, ligated and divided at both ends, and the distal tract is curetted. No functional muscle is divided. The original series reported a success rate of about 70 to 80 percent with minimal continence disturbance, and LIFT is now a first-line sphincter-sparing option.[4]

Endorectal advancement flap — for complex or high fistulas. A broad-based mucosal-muscular flap is raised from the rectum, the internal opening is closed, and the flap is advanced down to cover it, eliminating the source of sepsis while leaving the sphincter intact. Success is 60 to 80 percent and the flap can be repeated.[1]

Cutting seton, fibrin glue, plug, and VAAFT — an honest summary: the cutting seton is progressively tightened to slowly divide the sphincter behind a fibrosing tract, but it risks incontinence and is falling out of favour. Fibrin glue and the anal fistula plug have low morbidity but disappointing durability (15 to 60 percent). VAAFT (video-assisted anal fistula treatment) visualises the tract with a fistuloscope, diathermies it, and closes the opening — sphincter-sparing, and assessed by NICE as an option with appropriate consent.[2][3]

[1]

The Crohn's fork — seton and anti-TNF, never the knife

Crohn's perianal disease demands a multidisciplinary, sphincter-conserving strategy: the surgeon controls sepsis, the gastroenterologist treats the inflammation. Divide the sphincter here and you trade a fistula for an incontinence pad, because inflamed, immunosuppressed tissue does not heal.[3]

The sequence:[3]

• Control sepsis first — EUA, drain abscesses, place loose setons.
• Anti-TNF therapy is the evidence-based medical treatment. Infliximab 5 mg/kg IV at weeks 0, 2, and 6, then 5 mg/kg every 8 weeks — Present's 1999 induction RCT proved that about two-thirds of infliximab-treated patients had at least a 50 percent reduction in draining fistulas versus about a quarter on placebo, and the 5 mg/kg dose was best tolerated.[5]
• Maintenance infliximab keeps fistulas closed — ACCENT II showed roughly twice as many patients maintained fistula closure on infliximab as on placebo (about 36 percent versus 19 percent at week 54).[6]
• Adalimumab (160 mg subcutaneously at week 0, 80 mg at week 2, then 40 mg every other week) is the alternative for patients who lose response to or cannot tolerate infliximab.
• Antibiotics (metronidazole 400 mg three times daily, ciprofloxacin 500 mg twice daily) cover secondary infection and bridge to biologic response.
• For severe, refractory disease, a defunctioning stoma (loop ileostomy) or ultimately proctocolectomy may be required.[3]

[6] [1] [5]

The horseshoe and the supralevator — image before you cut

Two subtypes bite the unwary because their anatomy is not what it looks like on the skin. Both demand imaging before drainage.[1]

The horseshoe abscess and fistula involves both ischiorectal fossae through the deep postanal space, with a posterior midline internal opening. The operation is the Hanley procedure — bilateral drainage plus posterior midline unroofing (counter-incision) of the deep postanal space, often with setons. Drain only one side and recurrence is guaranteed.[1]

The supralevator abscess is the trap within the trap: it arises either from upward extension of a cryptoglandular abscess or from an intra-abdominal source (diverticulitis, appendicitis, Crohn's, pelvic sepsis). Imaging is mandatory to identify the source, because the route of drainage must match the origin — draining a supralevator abscess of pelvic origin through the sphincter builds an iatrogenic extrasphincteric fistula.[1]

The recurrent and atypical tracts:[3]

• Recurrent fistula after surgery — reassess with MRI and EUA, and actively hunt for Crohn's, tuberculosis, HIV, or malignancy; biopsy the tract.[3]
• Tubercular fistula (endemic regions) — thin watery discharge, multiple tracts, underlying pulmonary TB, caseating granulomas on histology; anti-tubercular therapy (rifampicin, isoniazid, pyrazinamide, ethambutol) comes first, surgery only for non-healing disease after adequate ATT.
• HIV-related perianal disease — higher risk of atypical infection and anal intraepithelial neoplasia; manage conservatively, biopsy every atypical lesion, CD4-guided therapy.
• Fistula-associated cancer — a chronic, long-standing fistula (especially in Crohn's) rarely hides squamous cell carcinoma or mucinous adenocarcinoma; a non-healing tract must be biopsied.[3]

Fournier's gangrene — the perineum that cannot wait

Perianal sepsis with skin necrosis, crepitus, or rapidly spreading erythema and systemic toxicity is Fournier's gangrene — a surgical emergency, not a swollen abscess. Mortality is 20 to 40 percent, higher with every hour of delay, and the diabetic and immunocompromised are at the highest risk.[1]

The bundle, without hesitation:[1]

• Immediate IV resuscitation and broad-spectrum triple-antibiotic cover for polymicrobial infection — a Gram-negative agent, anaerobe cover, and often a penicillin or carbapenem.
• Urgent, aggressive surgical debridement — wide removal of all necrotic skin, subcutaneous tissue, and fascia until healthy bleeding tissue is reached. Multiple returns to theatre are the rule. A defunctioning colostomy and, rarely, orchiectomy may be required.
• The LRINEC score can support — but never delay — the decision to operate.[1]

Etymology for viva gold: Fournier's gangrene is named for Jean-Alfred Fournier, the Parisian dermatologist who in 1883 described the rapidly necrotising perineal infection in young men. The eponym survived because the disease has not changed — only the patients have, now older, diabetic, and immunosuppressed.[1]

Investigations — MRI is the gold standard for the complex tract

The diagnosis of an abscess is clinical — you drain it, you do not image it. MRI earns its place for the complex, recurrent, or deep tract, and for any abscess that is not what it seems.[1]

For the abscess: blood glucose (exclude diabetes), full blood count, blood cultures if septic, and a pus swab at drainage. Reserve CT or MRI for the complex, recurrent, deep, or supralevator collection, suspected necrotising infection (gas in soft tissues), or the immunocompromised host whose examination is unreliable.[1]

For the fistula:[2]

• MRI of the pelvis and perineum (endoanal or surface coil) is the gold standard for the complex or recurrent fistula — it maps the primary tract, its relationship to the sphincter, secondary extensions, horseshoeing, and any associated collection. Essential before any second operation.
• Endoanal ultrasound (EAUS) is quicker and cheaper, less detailed than MRI but useful in theatre; hydrogen peroxide injected into the tract enhances definition.
• Examination under anaesthesia (EUA) with probing and dye injection (methylene blue, hydrogen peroxide) is often both diagnostic and therapeutic.
• Fistulography is largely obsolete, used only where MRI is unavailable.
• Colonoscopy and ileoscopy if Crohn's is suspected; biopsy of any atypical tract at EUA is mandatory to exclude Crohn's, tuberculosis, or malignancy.[3]

The classic traps — ways to harm a patient

Most perianal morbidity is avoidable, and it follows a short list of recurring errors. Memorise the list and you will not make them.[1]

• Treating an abscess with antibiotics alone — pus must be evacuated; the IV line is adjunct, not treatment.
• Dividing sphincter blindly in a high fistula — assess the tract with MRI and EUA first; incontinence is forever.
• Missing a horseshoe and draining only one side — recurrence is then inevitable.
• Missing Fournier's gangrene — crepitus, skin necrosis, or systemic toxicity is a surgical emergency.
• Draining a supralevator abscess of pelvic origin trans-sphincterically — you have just built an extrasphincteric fistula.
• Not suspecting Crohn's in multiple or recurrent fistulas — colonoscopy is mandatory.
• Not biopsying an atypical or non-healing fistula — missing malignancy or TB.[1]

The mantra, and the memory aids

The mantra: eliminate the septic source while preserving the sphincter. Every decision in this topic is a working-out of that single line.[1]

[1]

Prognosis and special situations

Perianal abscess does well after I and D; recurrence is 10 to 15 percent and a fistula follows in 30 to 50 percent. Most are managed as a day case and warned at discharge about the symptoms of recurrence. Anal fistula prognosis tracks complexity — low fistulas: fistulotomy success over 90 percent, recurrence under 10 percent, minimal incontinence; high or complex fistulas need staged procedures and long follow-up. Crohn's fistulas are chronic and recurrent; the realistic aim is control of sepsis and preservation of continence, not cure.[2]

The special populations:[1]

• Diabetics — strict glucose control with an insulin sliding scale, broader antibiotic cover, low threshold for imaging, debridement, and ICU; highest risk of necrotising infection.
• Immunocompromised (HIV, chemotherapy, transplant, steroids) — atypical aggressive organisms and poor healing; favour conservative surgery, broad cover, biopsy everything, early MDT input.
• Crohn's — avoid sphincter division, control sepsis with setons, treat with anti-TNF, involve gastroenterology early.[3]
• Pregnancy — an abscess is drained (safe in pregnancy); defer fistulotomy to the postpartum if possible; a symptomatic fistula gets a loose seton as the safe interim option.
• Children — usually simple and cryptoglandular; fistulotomy is well tolerated, but always consider an underlying cause in atypical or recurrent paediatric disease.[1]

Guidelines and regional differences

German S3 (Ommer, 2017) is the topic's backbone: drain every abscess surgically, reserve antibiotics for systemic infection or high-risk patients, fistulotomy for low fistulas, seton or LIFT for high fistulas, MRI for all complex or recurrent fistulas, and anti-TNF plus seton drainage for Crohn's disease.[1]

ASCRS 2016 (Vogel) and 2022 (Gaertner) mirror those recommendations and emphasise sphincter preservation in high fistulas, LIFT and advancement flaps as sphincter-sparing options, MRI as the imaging standard, and anti-TNF as the evidence-based medical treatment for Crohn's perianal fistula — supported by ACCENT II.[2][3][6]

Ward-round test — three stems

Stem 1 — the man at 3am (answer)

A 42-year-old diabetic has 72 hours of worsening throbbing perianal pain, fever, and a fluctuant tender lump at the anal margin. What is the operation, and what is the role of antibiotics? Model: This is a perianal abscess — incision and drainage under GA, radial or cruciate over the most fluctuant point, break all loculations, send pus, loose pack or a mushroom catheter for a deep cavity, do not primarily close. Antibiotics are not routine — add them here only because he is diabetic (a high-risk host), with co-amoxiclav or ceftriaxone plus metronidazole. Strict glucose control with an insulin sliding scale, and warn him that 30 to 50 percent develop a fistula. The IV line is adjunct, not treatment — drain the pus tonight.[1]

Stem 2 — the recurrent discharging sinus (answer)

A 38-year-old man has a recurrent perianal discharge. The external opening lies at 3 o'clock, 1 cm from the verge. Where is the internal opening, and what is the operation? Model: By Goodsall's rule, an anterior external opening within 3 cm of the verge runs a straight radial tract to the nearest anal crypt at about 3 o'clock on the dentate line. At EUA, if the tract is low intersphincteric, the operation is fistulotomy (lay-open) with recurrence under 10 percent. If EUA or MRI shows it crosses significant external sphincter, do not lay it open — place a loose seton and plan a LIFT or advancement flap. MRI first if there is any doubt, or any prior surgery.[1][4]

Stem 3 — the diabetic with crepitus (answer)

A 55-year-old diabetic presents with perianal pain, fever, perineal skin necrosis with crepitus, and rapidly spreading erythema. What is this, and what do you do in the next hour? Model: This is Fournier's gangrene — necrotising fasciitis of the perineum, a surgical emergency with 20 to 40 percent mortality. Resuscitate, start broad-spectrum triple-antibiotic cover, and take him to theatre for urgent aggressive debridement of all necrotic tissue to healthy bleeding muscle, with planned returns. Insulin sliding scale, ICU, and a low threshold for a defunctioning colostomy. The LRINEC score supports but never delays the operation — the skin is the timer.[1]

References

1. [1]Ommer A, Herold A, Berg E, et al. German S3 guidelines: anal abscess and fistula (second revised version) Langenbecks Arch Surg, 2017.PMID 28251361
2. [2]Vogel JD, Johnson EK, Morris AM, et al. Clinical Practice Guideline for the Management of Anorectal Abscess, Fistula-in-Ano, and Rectovaginal Fistula Dis Colon Rectum, 2016.PMID 27824697
3. [3]Gaertner WB, Burgess PL, Davids JS, et al. The American Society of Colon and Rectal Surgeons Clinical Practice Guidelines for the Management of Anorectal Abscess, Fistula-in-Ano, and Rectovaginal Fistula Dis Colon Rectum, 2022.PMID 35732009
4. [4]Rojanasakul A, Pattanaarun J, Sahakitrungruang C, Tantiphlachiva K. Total anal sphincter saving technique for fistula-in-ano; the ligation of intersphincteric fistula tract J Med Assoc Thai, 2007.PMID 17427539
5. [5]Present DH, Rutgeerts P, Targan S, et al. Infliximab for the treatment of fistulas in patients with Crohn's disease N Engl J Med, 1999.PMID 10228190
6. [6]Sands BE, Anderson FH, Bernstein CN, et al. Infliximab maintenance therapy for fistulizing Crohn's disease N Engl J Med, 2004.PMID 14985485