Perianal Abscesses & Fistulae

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Perianal Abscesses & Fistulae


Perianal abscess– a pus collection in the perianal region

-Abscesses classified according to location: submucous, subcutaneous, intersphinteric, ischiorectal and pelvirectal/supralevator



Perianal fistula- an abnormal chronically infected tract communicating between the perianal skin and either the anal canal or rectum

-Classification (Park’s): superficial, intersphinteric, transsphinteric, suprasphinteric or extrasphinteric

Image result for perianal fistula

Or: low anal (below puborectalis), or high anal (at or above puborectalis) and pelvirectal (involving levator ani)

rectumAetiology/ risk factors:

Obstruction and stasis of anal crypt glands leads to superinfection that spreads to perianal tissues.

-Abscesses are usually caused by gut organisms, rarely Staphylococcus or TB.

-Fistulae may develop once abscess discharges or has been evacuated.

-Fistulae and abscesses are associated with Crohn’s disease, diabetes and malignancy (rectal carcinoma)

-Fistulae are also caused by perianal sepsis, TB, diverticular disease, immunocompromised

Epidemiology: common, peak incidence 20-40, more common in men (abscesses approx. 8x more in men)


-constant throbbing pain in the perineum

-with fistulae = intermittent, mucus or blood-stained discharge near the anal region


-localised tender perianal swelling or a small skin opening with discharge near the anus (fistula opening)

Generally, if the external fistula opening is anterior to the anal canal, the fistula tract runs in a straight line into the anal canal. If the external fistula opening is posterior to the anal canal, the fistula tract follows a curved path and opens internally in the posterior midline.

PR exam: an area of induration may be felt (abscess or fistula tract)

-A PR exam is not always possible due to pain or sphincter spasm and examination under general anaesthesia may be needed.


Blood: FBC, CRP, ESR, blood cultures if pyrexial

MRI- useful for complex fistulae, allowing detailed study of the tracts, for surgical planning with complete excision

Endoanal ultrasound- less useful than MRI


Treatment is under general anesthetic.


Open drainage of abscess = incision + drainage, cavity is irrigated and gently packed

Antibiotics useful if there is surrounding cellulitis.



Laying open of fistula = probe used to gently explore tract, hydrogen peroxide or methylene blue injected into the external opening to demonstrate the internal opening

Low fistulae = fistulotomy + excision= cutting down on and laying open the tract, allowing to heal

High fistulae (involving upper half of sphincter complex)– muscle division would cause incontinence:

Seton = non-absorbable suture that is threaded through the fistula tract, allows drainage of sepsis and gradually cuts through the sphincter in a manner that preserves continence

Image result for perianal fistula seton

Advancement flap = external part of fistula is excised and the internal opening is closed by a mucosal advancement plug

Fistula plug = xenograft made from porcine intestinal submucosa inserted into the tract to encourage closure or fibrin glue can obliterate tract but long-term results are poor.



-severe sepsis if untreated

-fistula surgery- injury to the anal sphincter and incontinence

Prognosis: high recurrence rate without complete excision.

References: Rapid Surgery, Cheese & Onion

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