Anal Fistula (Fistula-in-Ano)
An anal fistula refers to an abnormal connection (a channel) between the anal canal and the perianal skin.
It typically develops following an anorectal abscess, and is more common in males than females.
In this article, we shall look at the risk factors, clinical features and management of an anal fistula.
Aetiology and Risk Factors
The formation of an anal fistula typically occurs as a consequence of an anal abscess – and this is the greatest risk factor. Between 25-50% of individuals with an anal abscess will develop an anal fistula.
Other risk factors 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
In addition, anorectal cancers can occasionally present with an anal fistula. Assessment of the patient should seek to exclude this as a cause.
Anal fistulae commonly occur in cases of chronic anal infection (e.g. an abscess). Thus, they will often present with an intermittent or continuous discharge onto the perineum, including mucus, blood, pus, or faeces.
Fistulas may also cause severe pain, swelling, change in bowel habit and systemic features of infection (e.g. fever).
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.
A rigid sigmoidoscopy can be used to visualise the opening of the tract in the anal canal. Further investigations, such as fistulography, endo-anal ultrasound or MRI imaging may be used to visualise the remaining part of the tract.
Park’s classification system divides anal fistulae into four distinct types:
- Inter-sphincteric fistula (most common)
- Trans-sphincteric fistula
- Supra-sphincteric fistula (least common)
- Extra-sphincteric fistula
Note: This system does not include not include submucosal fistula which are not cryptoglandular in origin.
The definitive management for an anal fistula depends largely on the cause. If the patient has no symptoms, a conservative approach may be used. Surgery for an anal fissure should also not be performed in the setting of an acute anorectal abscess.
A fistulotomy involves laying the tract open and allowing it to heal by secondary intention. A probe is passed into the tract and the skin, subcutaneous tissue, and internal sphincter are divided in turn, thus opening the tract.
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.
Other options include placement of a seton and/or opening the perianal skin adjacent to the external opening which promotes healing before external closure and prevents recurrence of an abscess.
A Cochrane Review concluded that there is no difference in recurrence rates between the various techniques used in the surgical treatment for anal fistulae.