I started with a diagnosis of reactive arthritis (still called Reiter's Syndrome at the time), after an intestinal infection. After 10 years of feeling fine and unmedicated, but with occasional flare-ups, I suddenly went downhill and my diagnosis was changed to ankylosing spondylitis (modified New York criteria). To my knowledge, there is very little research on the separation or progression between reactive arthritis and ankylosing spondylitis.

That's brutal. I hope you're doing alright these days.

I loved your username "spondylosaurus", it made me laugh, thank you. I wish you well too. Cheers!

I cannot really add anything useful, but my progression was so similar in all senses that I thought I should comment: it started after an intestinal infection I got in India, many different hypotheses until they settled on Reiter's syndrome/reactive arthritis. Then for 12 years I was without real symptoms or medication, though with almost chronic back pain. A new bout 12 years later (knees like grapefruits etc), diagnosed now as ankylosing spondylitis. In fact I had understood that this was a specific form of reactive arthritis (rather than something different), but probably you're right.