There is genetic research which suggests that IBD is a continuum, with UC at one end, Crohn's colitis in the middle, and Crohn's at the other end. https://www.sciencedaily.com/releases/2015/10/151018223039.htm
That would explain the existence of indeterminate colitis, a form of IBD that cannot be categorised as either Crohn's or UC. Most people with a diagnosis of indeterminate IBD do eventually go on to get a diagnosis of either UC or Crohn's.https://inflammatoryboweldisease.net/types-of-ibd/indeterminate-colitis/
I had the opposite happen to me. After 15 years of a Crohn's diagnosis, I had my surgeon tell me I didn't have Crohn's. She said my colon biopsies were negative for Crohn's. I asked if it was UC instead - she said no, it was colitis. We argued about
it for 5 minutes and she finally checked her notes, which stated that I had "classic Crohn's" of the terminal ileum. Diagnosis restored!
That said, I do have a distant memory of a doctor muttering to himself about
the confusingness of my diagnosis and that it wasn't obvious if it was Crohn's or UC - don't remember the exact words. But that was at my first hospital, so a long time ago now. But my official diagnosis has always been Crohn's: I suspect it's the terminal ileum inflammation which nailed it down thus.
Dx Crohn's in June 2000. (Yay )
Tried: 5-ASAs, azathioprine, 6MP, Remicade, methotrexate, Humira, diets.
1st surgery 20/2/13 - subtotal colectomy with end ileostomy.
2nd surgery 10/7/15 - ileorectal anastomosis. Stoma reversed and ileum connected to the rectum.
Current status: Chronic flare. Do I have any other kind?
Current meds: 50mg 6MP; Entyvio (started 3/11/16)