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