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Anal Abscess and Anal Fistula

An anal abscess is a sore, bulging, often fluctuating, abscess in the anal area, developing as a result of infection of an anal gland.
An anal fistula is a tract from the anal canal to the skin developing as a result of an anal abscess.
Anal abscesses and fistulas are treated surgically.

In 90% of the cases the abscess is of cryptoglandular origin, i.e. arising from infection of the anal glands.
10% of the cases have another aetiology: e.g. Crohn's disease, trauma, HIV, radiation therapy, neoplasia.

An anal abscess causes severe acute pain or discomfort, often with fever. However, fever is not always present.
A deep abscess can cause difficulties in voiding.

Inspection of the anal region usually reveals the evident diagnosis: a tender mass varying in size is observed near the anus.
Per rectum examination may be impossible because of the pain but perineal palpation can be done and may reveal the location of a painful mass in these patients.
If needed, the diagnosis can be confirmed by perineal ultrasonography.
The abscess may be located in the anal canal between the sphincter muscles (intersphincteric abscess) or deep in the ischiorectal space, in which case it is palpable on digital rectal examination or visible by endoanal ultrasonography, CT or MRI.
If the patient has several abscesses or if there is a suspicion of Crohn's disease Crohn's Disease for some other reason, MRI or CT should be done to investigate the extent of the disease, and, after treatment, an ileocolonoscopy should be carried out to confirm Crohn's disease.
An anal abscess should not be confused with a thrombosed haemorrhoid / perianal haematoma, which shows as a taut, dark haematoma in the haemorrhoid or under the skin. A pilonidal sinus abscess is associated with a diseased hair follicle, usually located high in the anal cleft; the sinus openings are located in the midline of the anal cleft.

An anal abscess always requires surgical incision and drainage.
Small, superficial abscesses can be incised under local anaesthesia.
Deeper and larger abscesses should be incised under general or spinal anaesthesia so as to be able to drain the abscess cavity completely.
All abscesses should be incised within 24 hours 1.
If a fistula tract is found during abscess incision, it can be treated at the same time Incision and Drainage of Perianal Abscess with or Without Treatment of Anal Fistula.
If a fistula was not found during the operation, the wound should still be monitored until it has healed.
One patient in three with anal abscess will be found to have a fistula at the time of incision or will subsequently develop a fistula 2.

An anal fistula is a chronic inflammatory tunnel from the anal canal to the skin, developing as a result of an anal abscess.
Other possible causes of an anal fistula include Crohn's disease, trauma, HIV, radiation therapy and neoplasia.
In patients with Crohn's disease, fistulas may develop due to inflammation of the mucosa without a preceding abscess.

There is purulent discharge from the external opening of the fistula, usually close to the anus.
An anal fistula is often palpable underneath the skin.
The external opening of the fistula may be situated further away from the anus, in women in the vagina (retrovaginal fistula) or in men in the scrotum.

The treatment of fistulas is surgical.
There are several surgical techniques available; recurrence is quite common. Methods with a low risk of affecting continence are the first choice.
However, if the fistula recurs after surgery, at the next stage, a more risky technique may need to be used. In difficult cases, creation of a temporary stoma may be necessary.

Tarasconi A, Perrone G, Davies J, et al. Anorectal emergencies: WSES-AAST guidelines. World J Emerg Surg 2021;16(1):48 [PubMed]
Sahnan K, Askari A, Adegbola SO, et al. Natural history of anorectal sepsis. Br J Surg 2017;104(13):1857-1865 [PubMed]