Hi, I think this may have just happened to me! I just got out of an exam under anesthesia today so I need to get more info from my doc (he spoke to my partner but not me, as I was still under). But I had a draining seton for 12 weeks for middle-depth fistula that crossed through both sphincter muscles. Today we were planning to do a LIFT procedure but when I awoke I had a cutting seton. The doc said the fistula is now only about 2mm deep, so cutting seton should work well and quickly. I am not sure of the muscle involvement or why he didn't lay it open with a regular fistulotomy (I assume there is still muscle involvement), but I will ask him when I am slightly less high from the anesthesia...
I assume that the fistula must have gotten more shallow over the last 12 weeks, as an MRI showed it was deeper before the draining seton. I did think the string seemed a bit longer towards the end, and I would have muscle spasms sometimes that felt like stuff was changing in there.
Hope this info is helpful!
I don't have IBS or Crohns, by the way. i had a perianal abscess that turned into a fistula, likely due to trauma from another surgery.