Another UCer who's had a fistula. I'd only recently been diagnosed with UC when I noticed a lump just off to one side "down there", and my GP (not my usual one) calmly told me it was just an anal fissure and not to worry. Only much later when I was investigating did I notice that lumps aren't part of the presentation, and what he'd failed to diagnose was a whacking great abscess which spread and tunnelled its way out - as anyone who's had a fistula is aware, that's an interesting day when it finally breaks out o_O.
I was quite lucky because it was a low one, and my surgeon didn't muck about once it was actually diagnosed - in for surgery the following week - they "lay open" the tunnel so that it can heal from inside out, which helps prevent re-infection, and as a bonus they put a seton in - this is a rubber band which slowly cuts through the sphincter muscle, it causes less lasting damage and is less likely to cause complications like incontinence than just cutting through in one go. That was back in Feb, and I've still got the seton in - but it's manageable and the horrible pain I was in before surgery is gone, so it's all worthwhile. Plus not leaking all the time does tend to let the confidence come back a bit - like I said to my doc, I can deal with UC, I can deal with a fistula, but not both...
The way my Gastro doc sees it, is UC when you're in a flare causes a lot of strain down there, leading to tearing in the lining of the colon - and that's where the infection gets a chance to start having a dig about, which seems to make sense to me. At least now I know what to look out for, though hopefully I won't get another one.
Hope the surgery goes well, and thought it's painful for a bit afterwards, I found it wasn't as painful as the abscess itself had been - just find a good soft pillow!
Feel better soon,