Perianal swelling

A perianal swelling is a lump, mass or area of induration at or around the anal verge. Most causes are benign and common (thrombosed external haemorrhoid, perianal abscess, skin tag, haemorrhoidal prolapse), but this area also hides necrotising infection, fistula, inflammatory bowel disease, sexually transmitted infection and anal cancer.

Anatomy. The dentate line divides the anal canal into upper (columnar mucosa, visceral innervation, painless) and lower (squamous epithelium, somatic innervation, painful). External haemorrhoids arise distal to the line, and internal haemorrhoids proximal to it. Anal glands open at the dentate line and lie in the intersphincteric space. Spaces around the anus are the perianal, ischioanal, intersphincteric and supralevator spaces.

Cryptoglandular theory. Infection of the anal glands causes about 90% of anorectal abscesses, and the tract left behind forms a fistula in about a third of patients.

Fistula classification. Intersphincteric, transsphincteric, suprasphincteric and extrasphincteric (the Parks system). Simple fistulas are low, single and involve little sphincter. Complex fistulas involve over 30 to 50% of the sphincter, are high, multiple, anterior in women, recurrent, or associated with Crohn disease or incontinence.

Haemorrhoid grading (internal). Grade I: bleed, no prolapse. Grade II: prolapse and reduce spontaneously. Grade III: prolapse and need manual reduction. Grade IV: prolapse and cannot be reduced.

Key distinctions. A constant throbbing pain that worsens with sitting and is not related to defecation, with fever and a tender fluctuant swelling, suggests an abscess. A sudden, tender, blue-purple lump after straining suggests a thrombosed external haemorrhoid. Pain during and after defecation, with fresh bleeding, suggests an anal fissure with a sentinel tag. A painless, hard, ulcerated or fixed lump needs biopsy.

History.

• Onset and duration (abrupt over hours versus gradual over weeks), pain pattern, fever, discharge, bleeding, itching, prolapse, relation to defecation and sitting.
• Previous abscess, fistula, anal surgery or childbirth, inflammatory bowel disease, constipation, straining, anal intercourse or sexual exposure, diabetes, HIV infection, tuberculosis, immunosuppression, neutropenia, radiotherapy, portal hypertension.
• Bowel habit change, weight loss, rectal bleeding, faecal incontinence, urinary retention.

Examination.

• General: temperature, pulse, blood pressure, signs of sepsis, glycaemic status.
• Inspect the perianal skin with the patient in the left lateral position, buttocks parted: erythema, swelling, fluctuance, induration, external opening, skin tags, warts, ulcer, prolapse, scars. Goodsall rule: a fistula with an external opening posterior to the transverse anal line usually tracks in a curve to the posterior midline, and one anterior to it usually tracks straight.
• Palpate the swelling and the tract (induration extending to the anus).
• Digital rectal examination when tolerated (induration, mass, tenderness, sphincter tone, internal opening), or under anaesthesia when pain prevents it. Proctoscopy or sigmoidoscopy to identify haemorrhoids, fissure, proctitis or tumour.
• Groin nodes, abdomen, and in women the vagina and vulva.

Red flags.

• Rapidly spreading erythema, skin discolouration, bullae, crepitus, severe pain out of proportion, systemic toxicity or fever with perineal or scrotal involvement (necrotising infection, Fournier gangrene).
• Diabetes, immunosuppression, neutropenia, cirrhosis or HIV infection with perianal pain.
• Hard, fixed, ulcerated or non-healing lesion, bleeding with weight loss or change in bowel habit, or inguinal nodes.
• Urinary retention, new faecal incontinence or neurological signs.
• Multiple, recurrent or complex fistulas in a young patient (Crohn disease).

Special groups.

• Pregnancy and postpartum: haemorrhoids and thrombosed external haemorrhoids are common, and fissures and perineal tear complications occur.
• Infants and young children: perianal abscess and fistula in boys under 1 year are common, and recurrent disease raises the question of inflammatory bowel disease or immunodeficiency.
• Region specific points: HIV related and tuberculous perianal disease, cutaneous amoebiasis, schistosomal granulomas, and hidradenitis suppurativa.

Infective and inflammatory.

• Perianal abscess, ischioanal abscess, intersphincteric abscess, supralevator abscess, anal fistula.
• Necrotising fasciitis of the perineum (Fournier gangrene), neutropenic perianal sepsis, cellulitis.
• Pilonidal abscess, hidradenitis suppurativa, furuncle, infected epidermoid cyst, infected Bartholin gland cyst (anterior in women).
• Crohn disease with perianal disease, tuberculosis, actinomycosis, cutaneous amoebiasis.
• Condylomata acuminata (anogenital warts), condylomata lata (secondary syphilis), molluscum contagiosum, genital herpes, lymphogranuloma venereum.

Haemorrhoidal and vascular. Thrombosed external haemorrhoid, perianal haematoma, prolapsed or thrombosed internal haemorrhoids, perianal varices (portal hypertension).

Fissure related. Anal fissure with sentinel pile, hypertrophied anal papilla, anal skin tag.

Prolapse. Full thickness rectal prolapse, mucosal prolapse, prolapsed rectal polyp, prolapsed intussusception.

Neoplastic.

• Anal squamous cell carcinoma, anal intraepithelial neoplasia, low rectal cancer, anal adenocarcinoma.
• Perianal Bowen disease, basal cell carcinoma, melanoma, extramammary Paget disease, lymphoma, gastrointestinal stromal tumour, leiomyosarcoma, presacral tumour, and metastasis.

Other. Epidermoid cyst, lipoma, dermoid cyst, perineal hernia, retained foreign body, trauma with haematoma, and pruritus ani with skin tags.

Step 1. Clinical diagnosis for most lesions. Establish whether sepsis, necrotising infection or an abscess needs drainage.

Step 2. First line tests when sepsis, diabetes or immunosuppression is possible: full blood count, CRP, glucose and HbA1c, urea, creatinine, electrolytes, lactate if unwell, blood cultures if systemically unwell, and HIV test. Pus swab culture when antibiotics will be used or the abscess is recurrent.

Step 3. Imaging for diagnostic doubt, recurrence, suspected deep or complex sepsis or fistula.

• MRI pelvis is the best test to define fistula anatomy, secondary tracts, and horseshoe or supralevator collections (sensitivity about 90%).
• Endoanal ultrasound for sphincter integrity and the primary tract.
• CT with contrast for suspected supralevator or necrotising infection (gas in tissues) when the patient is stable enough, without delaying surgery in shock.

Step 4. Examination under anaesthesia is the reference test for assessment of fistula and abscess, with biopsy of any ulcer, hard lesion or persistent mass.

Step 5. Targeted tests.

• Colonoscopy and faecal calprotectin for suspected inflammatory bowel disease or rectal cancer.
• Biopsy for anal cancer, with HIV test and staging MRI and CT of the chest, abdomen and pelvis.
• Sexual health tests: nucleic acid amplification tests (chlamydia including lymphogranuloma venereum serovars, gonorrhoea), herpes PCR, syphilis serology, HIV and hepatitis tests.
• Tuberculosis tests for chronic fistulas in endemic settings (sputum, tissue culture, interferon gamma release assay).

Limitations. Examination is limited by pain, so early examination under anaesthesia is more accurate. MRI misses small tracts. Swab culture of perianal abscess usually grows gut flora and does not change management unless the patient is immunosuppressed.

Perianal or ischioanal abscess.

• Incision and drainage as soon as possible, under local anaesthesia for small superficial abscesses and general or regional anaesthesia for deep, large or recurrent abscesses. The incision is made close to the anal verge to keep any later fistula short. Do not delay drainage for antibiotics or imaging.
• Antibiotics are not needed after adequate drainage in healthy patients. Give them for spreading cellulitis, systemic sepsis, diabetes, immunosuppression, valvular heart disease, prosthetic material, or incomplete drainage: amoxicillin with clavulanic acid 500/125 mg orally every 8 hours for 5 days (IV 1.2 g every 8 hours if unwell; maximum 3.6 g daily IV); monitor liver tests, diarrhoea and rash; reduce to every 12 hours oral (or IV 1.2 g then 600 mg every 12 hours) for eGFR 10 to 30 mL/min. Penicillin allergy: ciprofloxacin 500 mg orally every 12 hours (maximum 1.5 g daily; check QTc, tendon pain and glucose) plus metronidazole 400 mg orally every 8 hours (maximum 1.5 g daily) for 5 days.
• Examine for a fistula at the time of drainage (about a third develop one). Define and treat it electively.
• Analgesia: paracetamol 1 g orally every 6 hours (maximum 4 g daily; 3 g daily if weight under 50 kg, malnutrition, alcohol excess or liver disease). Warm sitz baths for 10 to 15 minutes two to three times daily after drainage. Avoid opioids where possible because they cause constipation, and give a stool softener when they are needed.
• Neutropenic or immunosuppressed patients: piperacillin with tazobactam 4.5 g IV every 6 hours (maximum 18 g piperacillin daily; every 8 hours for CrCl 20 to 40 mL/min; every 12 hours for CrCl under 20 mL/min) and early surgical review.

Necrotising perineal infection (Fournier gangrene).

• Emergency resuscitation, critical care, and wide surgical debridement within hours, with repeat debridement at 24 to 48 hours.
• Piperacillin with tazobactam 4.5 g IV every 6 hours (maximum 18 g piperacillin daily; every 8 hours for CrCl 20 to 40 mL/min and every 12 hours for CrCl under 20 mL/min) or meropenem 1 g IV every 8 hours (1 g every 12 hours for CrCl 26 to 50 mL/min, 500 mg every 12 hours for CrCl 10 to 25 mL/min, 500 mg every 24 hours under 10 mL/min), plus clindamycin 600 to 900 mg IV every 8 hours (maximum 2.7 g daily) to suppress toxin production, plus vancomycin 25 mg/kg IV loading dose (maximum 3 g) then maintenance dosed by kidney function and levels when MRSA is possible; monitor creatinine and levels.
• Urinary diversion, faecal diversion, wound care, nutrition, and glucose control (7.8 to 10 mmol/L).

Anal fistula.

• Simple low fistula: fistulotomy. Complex fistula: seton drainage (loose seton), then staged repair (advancement flap, ligation of the intersphincteric tract, fistula plug or glue), chosen by the colorectal surgeon.
• Crohn fistula: control sepsis, seton drainage, and medical therapy under gastroenterology direction (anti-tumour necrosis factor treatment is usual).

Thrombosed external haemorrhoid or perianal haematoma.

• Presentation within 72 hours with severe pain: excision of the thrombosed pile under local anaesthesia (lidocaine 1% with adrenaline 1 in 200,000, maximum 7 mg/kg and not more than 500 mg; plain lidocaine maximum 4.5 mg/kg and not more than 300 mg; monitor for toxicity), because excision reduces recurrence. Simple incision and clot evacuation has high recurrence.
• Presentation after 72 hours or settling pain: conservative care with warm sitz baths two to three times daily, paracetamol 1 g orally every 6 hours (maximum 4 g daily; 3 g daily if weight under 50 kg, malnutrition, alcohol excess or liver disease), and topical lidocaine 5% ointment as a thin layer to the perianal skin up to three times daily for up to 7 days.
• Prevent constipation: ispaghula husk 3.5 g sachet in 150 mL water once to twice daily (maximum 3 sachets daily; give separately from other medicines by 2 hours; maintain fluid intake), macrogol 1 sachet orally once to twice daily, fluids and fibre (25 to 30 g daily). Avoid straining and codeine.

Prolapsed internal haemorrhoids.

• Gentle manual reduction with analgesia and the patient in the lateral or head down position. Strangulated or thrombosed prolapse with necrosis needs urgent haemorrhoidectomy or conservative care with analgesia.
• Grade I to III: rubber band ligation (one to three sites at one sitting; avoid in thrombocytopenia, coagulopathy, anticoagulation without specialist direction, and immunosuppression), or sclerotherapy or infrared coagulation. Grade III to IV or failure: haemorrhoidectomy or stapled or Doppler directed techniques.

Anal fissure with sentinel pile.

• Stool softeners and fibre (ispaghula husk 3.5 g sachet in 150 mL water once to twice daily, maximum 3 sachets daily, or macrogol 1 sachet orally once to twice daily, with fibre 25 to 30 g daily), sitz baths, and topical treatment for 6 to 8 weeks: glyceryl trinitrate 0.4% rectal ointment, 2.5 cm applied to the anal canal every 12 hours (maximum 2 applications daily; headache is common; avoid with sildenafil and in hypotension), or diltiazem 2% ointment, 1 cm applied every 12 hours (maximum 2 applications daily; monitor for perianal itching and headache).
• Chronic fissure: botulinum toxin injection or lateral internal sphincterotomy.

Anogenital warts. Cryotherapy, imiquimod 5% cream applied three times weekly at night, washed off after 6 to 10 hours, for up to 16 weeks (avoid in pregnancy; monitor local reaction), or surgical excision. Test for other sexually transmitted infections, offer HIV testing, and discuss HPV vaccination.

Other causes.

• Anal cancer: urgent specialist referral, MRI staging and oncology led chemoradiotherapy.
• Herpes simplex: aciclovir 400 mg orally every 8 hours for 7 to 10 days for a first episode (maximum 1.2 g daily at this dose; reduce to every 12 hours for CrCl under 10 mL/min; ensure hydration).
• Secondary syphilis (condylomata lata): benzathine benzylpenicillin 2.4 million units IM as a single dose.
• Lymphogranuloma venereum: doxycycline 100 mg orally every 12 hours for 21 days.
• Hidradenitis suppurativa: doxycycline 100 mg orally every 12 hours for 12 weeks, topical clindamycin 1%, and surgery for chronic disease.
• Pilonidal abscess: incision and drainage with hair removal.
• Rectal prolapse: referral for surgical repair, treat constipation and straining.

Monitoring. Wound review at 48 to 72 hours after drainage and at 1 to 2 weeks, then until healed. Check temperature, glucose, spread of cellulitis and continence. Repeat blood tests in sepsis.

Referral and escalation. Colorectal or general surgery for abscess, fistula, thrombosed haemorrhoids needing excision, prolapse and suspected necrotising infection. Critical care for sepsis. Gastroenterology for inflammatory bowel disease. Sexual health, oncology and infectious diseases depending on the cause.

Admission criteria. Necrotising infection, systemic sepsis, deep or supralevator abscess, diabetes or immunosuppression with significant sepsis, neutropenia, inability to drain under local anaesthesia, urinary retention, uncontrolled pain, or suspected malignancy needing urgent work up.

Discharge and safety netting. Discharge after drainage with wound care advice (sitz baths, a soft dressing, no packing unless instructed), analgesia, laxatives and follow up in 1 week. Patients are told to return for fever, spreading redness, increasing pain, foul discharge, difficulty passing urine, or no improvement in 48 to 72 hours.