I think a change from SP to PP or vice versa is fairly common, maybe more so in the case of a change from RR/SP to PPMS, because they do appear to follow different disease patterns.
The use of lumbar puncture tends to be most useful in diagnosis of PPMS, although a positive LP is used as part of the evidence for all types of MS.
The reason a change of diagnosis is more commonly made from the RR/SP type to PP is that in relapsing remitting MS, and therefore secondary progressive too, there are more inflammatory attacks (ie relapses). This makes the diagnosis usually slightly easier than for PP. In primary progressive, there may be one or more inflammatory attacks, but quite often there are none at all. This is why an LP is useful in diagnosis of PPMS.
Do you recall having relapses, or episodes that may now retrospectively be considered relapses?
Ultimately, there may be little difference in the two diagnoses, it’s not that one diagnosis is ‘worse’ than the other; the symptoms are often pretty similar. In PPMS, there may be more spinal than brain lesions.
With regard to how essential the LP is, if your MS can be shown to have ‘dissemination in space and time’ (lesions that appear in different areas and are either lesions of different ages, or evidence of previous relapses), then the LP doesn’t add much value really. If it can’t be seen to have this dissemination in space and time, the presence of Oligoclonal bands in the CSF are quite important for diagnosis, which is why it’s so useful for PPMS.
An LP is quite different to a colonoscopy. (For one thing, it involves different regions of the body!) The lumbar puncture involves a tiny needle being inserted into your spine under a local anaesthetic to remove a small amount of cerebrospinal fluid. It doesn’t hurt, but it can feel rather strange. Just the thought of someone sticking a needle into your spine makes many people feel a bit squirmy!
The most important thing to remember about having an LP is to stay laying flat for as long as possible after the test - at least an hour. People are usually recommended to drink lots of caffeinated drinks, eg full strength coke (through a straw as you’re laying down). Both the coke and the staying flat are to reduce the likelihood of a terrible headache. Some people don’t have the benefit of this advice and are fine (I didn’t know this and was fine), other people are unlucky and get the headache from hell anyway.
If your MS diagnosis is already made and is a firm diagnosis, you, shouldn’t ‘need’ to have the LP. So theoretically you can refuse to have it. But it’s quite hard to say no to doctors, so you might find your new neurologist wants to do it for perfectly valid reasons and you end up having the test.
Basically, it’s not an overly invasive, risky or painful test (so long as you stay laying flat after it). So having it perhaps isn’t a bad thing.