A perianal fistula (fistula-in-ano) refers to an abnormal connection between the anal canal and the perianal skin.
The majority are associated with anorectal abscess formation, with one third of patients with an anorectal abscess having an associated perianal fistula at the time of presentation.
The formation of an perianal fistula typically occurs as a consequence of an perianal abscess (over 90% secondary to a perianal fistula).
However, other risk factors for their formation include:
- Inflammatory bowel disease – Crohn’s disease or ulcerative colitis
- Systemic diseases – Tuberculosis, diabetes, HIV
- History of trauma to the anal region
- Previous radiation therapy to the anal region
Anal fistulae usually present with either (1) recurrent perianal abcesses (2) intermittent or continuous discharge onto the perineum, including mucus, blood, pus, or faeces.
On examination, an external opening on the perineum may be seen; these can be fully open or covered in granulation tissue. A fibrous tract may be felt underneath the skin on digital rectal examination.
The Goodsall Rule
The Goodsall rule can be used clinically to predict the trajectory of a fistula tract, depending on the location of the external opening:
- External opening posterior to the transverse anal line – fistula tract will follow a curved course to the posterior midline
- External opening anterior to the transverse anal line – fistula tract will follow a straight radial course to the dentate line
Proctoscopy can be used to visualise the opening of the tract in the anal canal. For complex fistula, MRI imaging is often required to visualise the anatomy of the tract.
Park’s classification system divides anal fistulae into four distinct types (Fig. 2):
- Inter-sphincteric fistula (most common)
- Trans-sphincteric fistula
- Supra-sphincteric fistula (least common)
- Extra-sphincteric fistula
The definitive management for an anal fistula depends largely on the cause and site. Indeed, if the patient has no symptoms, a conservative approach may be used. Varying surgical options are available for those deemed not suitable for conservative approach
A Cochrane Review concluded that there is no difference in recurrence rates between the various techniques used in the surgical treatment for anal fistulae. The most common methods employed are laying open of the tract or seton insertion.
- A fistulotomy (suitable for superficial / submucosal fistulae) involves laying the tract open and allowing it to heal by secondary intention. A probe is passed along the tract, allowing the the skin and subcutaneous tissue* to be divided in turn, to lay the tract open.
- The placement of a seton (a rubber sling) though the fistula attempts to bring together and close the tract, passing out at opening of the perianal skin adjacent to the external opening
It is quite common for patients with complex anal fistulas to require several procedures over months or years.
*If the fistula has a low track course (whereby the tract travels through less subcutaneous tissue and muscle) continence is rarely impaired post-operatively, however if the fistula has a high tract course then there is a higher chance of impairment in continence
- An anal fistula is an abnormal connection between the anal canal and the perianal skin
- Most fistulae can be investigated via proctoscopy, to visualise the opening of the tract within the anal canal
- Any patient with an anal fistula or recurrent peri-anal abscesses should be investigated for possible Crohn’s disease
- There are multiple surgical management options available, none of which have been shown to be superior in outcomes