AKT · Gastroenterology/Anorectallow yield

Perianal abscess & fistula

Infection of the anal glands (cryptoglandular) → abscess; chronic tract → fistula-in-ano

Overview

Infection of the anal glands (cryptoglandular theory) producing a perianal/ischiorectal ABSCESS — a painful, throbbing, fluctuant perianal swelling needing incision and drainage. A chronic epithelialised tract between the anal canal and skin = fistula-in-ano (recurrent discharge). Recurrent/complex/atypical perianal sepsis → think CROHN'S DISEASE. Antibiotics alone do not treat an abscess.

Recognise

Throbbing, constant perianal PAIN with a tender, red, fluctuant swelling; fever/malaise; pus discharge
Fistula-in-ano: recurrent discharge of pus/blood from a perianal opening, intermittent abscesses
Recurrent/complex/multiple → Crohn's disease; also diabetes, immunosuppression

Red flags

Spreading sepsis / Fournier gangrene (necrotising perineal infection — diabetic/immunocompromised, systemic toxicity, crepitus) → emergency surgery
Recurrent/complex perianal sepsis → investigate for Crohn's

Differentials & how to tell them apart

Anal fissurepain on defecation with a tear, not a fluctuant abscess

Thrombosed haemorrhoidtense tender lump but not an abscess/pus

Crohn's perianal diseaserecurrent/complex fistulae, skin tags — underlying IBD

Pilonidal/hidradenitisnatal cleft / apocrine sites

Investigations

Clinical (fluctuant swelling). MRI pelvis for fistula anatomy/complex disease; examination under anaesthesia. Screen for Crohn's/diabetes if recurrent.

Management

Incision & drainage of the abscess (surgery ± seton for fistula)

1Drain the abscess (incision and drainage) — this is a surgical, not antibiotic, problem.Gate: Antibiotics alone do NOT treat a perianal abscess (it needs DRAINAGE); recurrent/complex perianal sepsis or fistulae → investigate for CROHN'S DISEASE; necrotising infection (Fournier) is a surgical emergency
2Fistula-in-ano → MRI + fistulotomy/seton (preserving continence); treat underlying Crohn's (biologics); manage diabetes/immunosuppression.

Incision and drainage (the treatment for an abscess) — antibiotics ALONE do not cure an abscess

Antibiotics — adjunct for surrounding cellulitis/immunosuppression/systemic upset

Fistula surgery (fistulotomy/seton) ± treat Crohn's — definitive fistula management

Key points

Throbbing fluctuant perianal swelling = abscess → drain it (antibiotics aren't enough). Recurrent/complex fistulae = Crohn's until excluded. Fournier gangrene in the diabetic is the emergency.

Monitor & prognosis

Healing; recurrence → Crohn's work-up.

Good after drainage; fistulae can recur.

Source: ACPGBI; NICE