An anal fistula is an abnormal communication between the inside of the anus or rectum and the skin around the anus. The fistula itself is called the fistula tract and connects the internal and external openings.
It usually results from an infection in one of the glands of the anus which forms an abscess. However, complex fistulas can also arise in the setting of Crohn’s disease (an inflammatory bowel disease).
A fistula tract can be simple or very complex and can go through the muscles of the anus or between them. If the tract goes through no muscle at all or a small amount of muscle it is called a low fistula. If it goes through much of the anal muscles, it is called a high fistula.
Treatment of an anal fistula is surgical. Under general anaesthetic the anal canal is examined and the external and internal openings are assessed.
It can be very difficult at times to find the internal opening especially for high / complex fisultae.
Once the internal opening is found, a gentle attempt at finding the tract is carried out and a fistula probed is passed through it.
If the tract is deemed low without much muscle involvement, a fistulotomy is performed. This is where all of the tissues from the skin and anal lining are cut down to the fistula probe in order to open the fistula and allow it to heal from inside out. This usually involved cutting part of the muscle(s). If too much muscle is divided, this can result in incontinence.
Therefore, if it is not clear how much muscle is involved or if there is obviously a lot of muscle involvement, a “seton” is placed through the tract guided by the fistula probe. This is a rubber band that stays in place keeping the tract open to prevent further complications of the fistula.
Further imaging (with endo-anal ultrasound or MRI) can be carried out to determine how much muscle is involved.
If the fistula is deemed high, treatment is more complex and may require a procedure called a mucosal advancement flap. This is done by a specialist colorectal surgeon.