A perianal abscess is a collection of pus near the anus, usually starting from an infected anal gland. After drainage, about a third of patients develop a fistula, an abnormal tunnel between the anal canal and the skin that keeps leaking.
Key symptoms
- Constant throbbing pain near the anus, worse when sitting
- Painful red swelling
- Fever and feeling unwell
- Fistula: recurrent drainage of pus or blood and skin irritation
What you find on exam
- Tender, warm, fluctuant swelling beside the anus
- External opening of a fistula with discharge
- Gentle rectal exam; deep abscesses may only cause tenderness
- Look for signs of Crohn disease or spreading cellulitis
Tests
- Usually a clinical diagnosis
- CT or MRI of the pelvis for deep or complex abscesses
- MRI of the pelvis to map a complex fistula
- Glucose and blood count if diabetic or very ill
Treatment basics
- Prompt incision and drainage; antibiotics alone are not enough
- Antibiotics for cellulitis, diabetes, weak immunity or sepsis
- Sitz baths, pain control and stool softeners
- Fistula: surgery such as fistulotomy or a seton
- Check for Crohn disease when fistulas are complex or recurrent
Red flags
- Fever, spreading redness or signs of sepsis
- Severe pain, crackling under the skin or dark skin (Fournier gangrene)
- Diabetes or weak immunity with a perianal infection
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Sources: NCCPA PANCE Content Blueprint; NBOME COMLEX-USA clinical presentations; AAMC Core Entrustable Professional Activities (Core EPAs). Written by Making Synapses. Making Synapses is not affiliated with these organizations.