I have CD and AS too.
Your Dr. is right it can affect any where along your spine.
My AS was caught early and the MTX keeps it in check - somewhat I find the Humira not as effective, but thats just me. I cant say that I have every heard that its irreversable.
My AS is maanageable so long as I stay active - walking and yoga help for me.
Good luck with your blood tests and find yourself a good Rheumy
Let us know how you go.