Hey! I had an anorectal manectomy last July. It wasn't that bad. The worst part for me was the enema I had to administer in the morning really set of a terrible IBS attack, so I was literally running to each bathroom in the hospital with terrible diarrhea up until the procedure. Yuck.
As for the procedure itself, other than the initial discomfort of having something inserted into your rectum, you really don't feel much. They ask you to do all these things to get reads on your muscle function - you cough, you squeeze, you bear down, etc. In my case, the nurse then asked me to "poop" the catheter. That was mildly embarassing, but it's all got it's purpose ;) The most uncomfortable thing for me was the last portion of the test. The nurse blew up the balloon in my rectum to the point where I got diarrhea cramping. That was to test some sort of sensory perception.
I am very glad I had to test. From it I learned I had pelvic floor disfunction and delayed sensorial perception. I probably did this to myself after years of clenching my pelvic floor muscles because of IBS pain and fear of pooping myself because of my IBS. I went to six months of pelvic floor physical therapy which greatly improved my muscle coordination, pelvic floor strength, and reduced my abdominal pain and sense of urgency.
There is no sedation and it's definitely not the most dignifying experience, but it's not bad at all. If you've lived through the hell of IBS, you can definitely do this test! Best of luck to you!