The pathology of abscess and fistula are the same, abscess representing the acute phase and fistula the chronic sequelae.
- 25-50% associated with fistula depending on what you read.

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All ages, incl. infants / children
- most common ~20-50yrs.
Men > Women for ischiorectal.

Risk Factors

Diabetes, Crohn's, Immunocompromise.

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Cryptoglandular Infection

Most (90%) arise from a blocked anal gland.
- these are the mucus secreting glands in the columns of Morgagni at the dentate line.
This subsequently becomes infected.
- 60% grow E.coli
- 23% grow S.aureus.
- Few are pure bacteroides, strep, proteus strains.
- Many are mixed.
Hence broad spectrum antibiotics required
Other causes include perforation at this point, eg fish-bone, blood-borne infection, Crohn's, thrombosed external pile when haematoma is not drained.

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- Refer Netters plates 364-.

There are four anatomical types depending on the course of spread from an infected anorectal gland:
Sorry, Picture Unavaliable

Picture note: bowel lumen is to the left, sphincters are block outlines.

i) perianal (60%)

Spreads superficially to lie in the region of the subcut portion of the external sphincter.
The pus can expand the tissues fairly easily here - pain and systemic features less.

ii) ischiorectal (30%)
Extend laterally into the deeper ischioanal fossa.
- the fat here is vulnerable as the blood supply is poor.
- thus it is not long before the whole space is involved.
- eventually will 'horseshoe' around rectum to involve whole posterior part of anal canal and contralateral ischioanal fossa.
- may extend above the levator ani muscles ('pelvi-rectal' or 'supralevator'):

iii) intersphincteric
Tracking between the sphincters either up or down.

iv) submucosal
Internal to the internal sphincter, above the dentate line, beside the anal glands.
May occur after injection of haemorrhoid tissue
- usually esolves spontaneously.

v) supralevator
Pus tracks vertically upwards
- deep and difficult to diagnose.


May discharge spontaneously to skin
If communication to skin is established, a fistula-in-ano may result.
- this occurs in ~25% of patients.

May horse-shoe around circumferentially from one side to the other.
- via the intersphincteric space, supralevator space or ischiorectal fossa:
Sorry, Picture Unavaliable

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Gradual onset, increasing severity over days.
Throbbing & constant.
Worse with defecation.
Associated tender swelling.
May vary from local pain and tenderness (perianal) to a deep seated rectal pain with no local tenderness (ischiorectal)


Often found in cases where abscess is ischiorectal or deeper.
May be severe; temp to 39 is not uncommon.


Localised inflammation.
An acutely tender, rounded, cystic, cherry-sized lump may be seen at the anal verge.
Seen below the dentate line.
Ensure no fasciitis or cellulitis.


 - large, tender and brawny if ischiorectal type.
May be fluctuant.


Usually painful and not contributory.
Perform if no external evidence
- may show an intersphincteric or supralevator abscess.
Don't do a sigmoidoscopy or anoscopy.

Beware the immunocompromised
Pain without other features may occur.
Localised abscess formation will not occur in pronounced leukopaenia.
EUA may ne required, ensure culture performed.

Intersphincteric Abscesses
Severe throbbing pain like a fissure.
Should be suspected if pain does not settle after treatment of a coexisting fissure.

Key Point

Perianal and ischiorectal abscesses often present as characteristic fluctuant masses.
Intershpincteric or suprelevator abscesses may have few findings, except tenderness on DRE.

May be confused with a pilonidal, Bartholin's gland or Cowper's gland, hidradenitis, crohn's etc.

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Do a FBC, CRP and look for elevated glucose if suspected.

Pelvic CT or MRI helps direct surgical draining in non-obvious cases.
Important for identifying occult abscesses (supralevator, deep ischiorectal, intersphincterics).
Especially in the morbidly obese.

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No. Cannot reach the contents of the abscess in enough concentration.
- not to be relied upon alone
Are not required in routine drainage of a perianal abscess
- Class II, Grade A evidence (Practice Parameters)
Do consider, however, when cellulitis / fasciitis / high risk pt / immunosuppressed / cardiac prophylaxis / diabetes / complex comorbidities.
- obtain a culture in such situations.


Incise and drain
- optimal preservation of underlying sphincters is essential.
- make a radial incision over area of greatest induration.
- if uncertain where, needle aspiration will help plan site.
- as close to anus as possible in case subsequent fistulotomy is required
- allow drainage of whole cavity
- remove necrotic tissue lining walls with a finger wrapped in gauze.
- consider biopsy of the wall if needed.
- insert kaldistat packing to the mouth only to achieve haemostasis (removed by pt in bath the next day).
- inserting a drain for three days post-op is not usually necessary.
- packing is generally unnecessary and painful.
- several days of frequent soaks and sitz baths to help to irrigate the cavity.

- often helps to use a needle to localize the abscess if necessary.
- incise as above
- explore the cavity, gently breaking down septae.
- remove necrotic tissue lining walls with a finger wrapped in gauze.
- biopsy the wall and send the pus for culture if necessary.
- don't be over-enthusiastic with probes so as to go through the levator.
- perform sigmoidoscopy to examine rectal mucosa (exclude Crohn's).

Drain into the rectal cavity
Incise the internal sphincter directly over the cavity to release pus

Supralevator & intermuscular
Drain depending on sepsis tract to develop least-complex fistula.

Sorry, Picture Unavaliable

But the patient is not responding?
- consider immunocompromise, residual infection / inadequate drainage and recurrent abscesses.

My patient has a big horseshoe abscess.
- crypt of origin may be located in the posterior midline.
- surgically conservative approach is a good idea.
- provide counter-drainage + consider inserting drain bilaterally if large horse-shoe
- consider small posterior midline incision from subcut ext sphincter over abscess to tip of coccyx
- pass hemostats gently into the posterior midline, draining seton around the posterior sphincter.
--> thus unroofing the post-anal space and its ischioanal extension.
--> can deal with the horseshoe fistula later.
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Cameron 10th
Sabistons 17th.
Am Society of Colon & Rectal Surgeons. Practice Parameters.  Dis Col Rect 2005;48:1337-42.