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This is called, 'Laying open of the fistula' (Fistulotomy). Remarkably, this heals even in the presence of fecal matter and without any need to change or apply new dressings.
For deeper fistulas and complex fistulous tracts some surgeons perform a defunctioning or diversion colostomy to divert the fecal matter away from the fistula. This is not routinely done for simple (superficial) fistulas.
The healing process is simple, the granulation tissue (baby tissue) here in this region forms very early, within a couple of days due to a very rich blood supply. Granulation tissue is known to have the strongest resistance to infection and this is a very good example.
Your physician would have advised you to take "Sitz baths". These are done by sitting simply in a tub full of luke warm water admixed with one to two table spoons of povidone-iodine or some other antiseptic solution. This is done for 15 to 20 minutes, three or four times a day especially after a bowel movement. This cleans the area of any bacteria that can invade.
If the canal is superficial, it will take 2 weeks approximately (sometimes 3 weeks) for the area to heal completely. Healing rates vary from person to person. But on the average it takes 15 to 20 days as I mentioned.
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Edited by Dr Uzair on 1/14/2011 at 2:25 PM EST