As the duct from the infected anal gland leading into the anal canal is blocked, the accumulated pus may spread in other directions. It usually follows the path of least resistance. The path created will determine the type of anal fistula formed.
An anal fistula usually has two openings — an internal opening in the anal canal and an external opening outside the anal verge.
A simple standard classification of the types of anal fistula was described by Parks et al2 (as shown on the next page). This classification is particularly helpful in determining the type of treatment required by the patient.
There are basically two broad groups of anal fistula in clinical practice — simple and complex anal fistula.
The 'complexity' of the anal fistula is basically dependent on:
- The amount of anal sphincter muscle involvement
- The presence of more than one anal fistula (secondary tract) and/or with abscess cavity
- Anal fistula with tracts above the anal sphincter complex or with the internal opening in the lower rectum or with the external opening further away from the anal verge
- Anal fistula associated with other diseases (e.g. Crohn's disease, tuberculosis)
A. Inter-sphincteric Anal Fistula
B. Transphincteric Anal Fistula
C. Supra-sphincteric Anal Fistula
D. Extra-sphincteric Anal Fistula
'Complex' is a more severe form of anal fistula, and will definitely require more complicated treatment. Surgical treatment for these fistula is sometimes carried out in stages.
Fortunately, the majority of anal fistulas are of the 'simple' type and therefore easily treatable with positive outcomes and low recurrence rates.
Such simple types of fistulas usually:
- Are of the low intersphincteric or transphincteric type
- Single short tract
- Have an external opening that is close to the anal verge
- Have an internal opening that is lower (closer to the anal verge)
- Are absent of any secondary tract or abscess cavity
- Are absent of any association with other diseases
Any other anal fistulas that do not fulfil the criteria stated above are considered 'complex'. Fistulas that persist or recur despite treatment are also considered to be 'complex' as they are difficult to treat.