My understanding is that these cases are indeed a misdiagnosis. Yes, there are definitely different forms of IBD, with UC and Crohn's being the primary types. Within those types, there are subtypes that they are beginning to figure out. The belief is that the various subtypes have a "built-in" response, or lack of one, to the various treatments, explaining why there is so much variability in the response to treatment.
There is a trend in the belief that at least some types of pouchitis may be a novel type of IBD, but Crohn's of the pouch is still Crohn's disease. The idea of the novel IBD subtype of pouchitis comes from the theory that altering the ileum to be a fecal reservoir invites a new inflammatory target, because as the pouch adapts, the mucosa changes on a cellular level to resemble rectal mucosa. But, chronic pouchitis does not occur in everyone with IBD. It is most common in those with an indeterminate diagnosis and Crohn's colitis. Pouchitis is very uncommon in FAP or other non-IBD diagnoses leading to colectomy.
But, back to the initial misdiagnosis idea. Just because you never had any evidence of small bowel inflammation, it does not mean that an original diagnosis of UC was correct. Once there is good evidence of Crohn's, that actually was the real diagnosis all along. The difference is that it just had not fully manifested previously. Crohn's colitis is still Crohn's, and not cured by colectomy. There are theories that colectomy in Crohn's colitis just removes the target du jour, and the disease then moves on to something else.
So, to summarize, the diagnosis may change, but your genetic make-up has been the same since your birth (or maybe even conception, assuming no mutations during fetal growth). IBD is a disease that takes years, decades, or more to develop, and how, and if, it manifests depends on your particular environmental triggers.
Jan
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