I don't appear to have spinal lesions, just brain lesions, but when it comes to determining the difference between PPMS and SPMS, it is my understanding that it has to do with the time between flares, and lesions presenting. More so than the location. PPMS is more like 'the highway to hell,' and SPMS is the 'city streets to hell.' You get more stop lights with SPMS.

Then there's the lucky groups in PRMS, and even luckier SRMS. Where the PRMS get to go hurling down the highway, pull off backtrack a bit taking in the sights before jumping back on, and those taking the true scenic route, weaving forwards and backwards distracted by butterflies and leaves along the way. (My aunt is fortunate enough to have that form, has a little flare, rebounds, meanders for a bit, oops little flare... Ahh we're better now). Insert complete jealousy and envy here:

Granted that's just my understanding, I had a great uncle with PPMS who went from a giant larger than life hero! To a frail shadow of himself in just a few years... Long before they had any treatment to speak of. My aunt by marriage has the SRMS, and my mother was an EMT for a private ambulance that often transported people with MS on a regular basis for dr appt, and such. We didn't see much of the regressive group, but it got fairly easy to tell the SPMS from the PPMS patients, even when both had lost their mobility. The rapidity of decline was too marked. Which might be why your doctor is denying PPMS dx, even if your still creating lesions.

And the presence of lesions also doesn't seem to matter as much as how they effect you, again from my understand, my aunt has numerous lesions in her brain and spine but even when she's in regression they don't really 'go away' but they aren't affecting her. Either they aren't in an area to pose a problem for everyday living or her body is capable of creating new pathways for the information to travel through. So yeah, tricky disorder to try and make sense of, and how doctors classify you, it seems largely guess work on their part as well.