I’m quite surprised you even got a diagnosis of CIS, with no brain abnormalities on the scan at all!
The fact (which somehow rarely seems to be made clear) is that there are strict evidential rules for an MS diagnosis - it’s not just a matter of what anybody thinks, but of what they can prove. With a clear LP and no visible brain lesions at all, after two successive scans, a confirmed MS diagnosis would be controversial, if not reckless.
No disrespect to your MS nurse, but she’s not qualified to diagnose MS (neither is your GP, for that matter).
In retrospect, I believe my consultant knew or believed I had MS from the very first time he met me, but even if he had been sure enough to stake his (no doubt very nice!) house on it, he still wouldn’t have been free to diagnose me until all the evidential guidelines were met - which was some eight months later.
That was despite having six brain lesions and at least one spinal cord lesion (possibly two) right from the initial scan. I had to wait 'til a second scan showed even more, thus proving it couldn’t have been a one-off (i.e. CIS), and was a recurring or ongoing problem. So even having multiple lesions on MRI doesn’t usually, by itself, lead to an MS diagnosis.
It is possible, but extremely rare for MS to present with lesions ONLY in the spinal cord (not brain), which is why the brain is almost always chosen as the most logical place to look. Because of the unlikeliness (is that a word?) of finding lesions in your spinal cord, but not your brain, I doubt you would gain anything by pressing for a spinal MRI at the moment. Additionally, if it did detect lesions, I’m not sure if this would move you closer to an MS diagnosis, or actually further away, because of the atypical presentation. There are some rarer “MS-like” conditions, that disproportionately attack the spinal cord, rather than the brain, but as my neuro once rather memorably put it: “Common things are common, rare things are rare.” Stating the obvious, of course, but what it means in practical terms is it’s unlikely you would have a rare thing that attacks ONLY the spinal cord, rather than common old MS. Not that MS itself is at all common in the population at large, but if you’ve reached the point a demyelinating disease is already suspected, it’s a lot more common than most of the alternatives.
Many people find themselves in this frustrating: “Wait and see” scenario. It’s probably not much consolation, but they do like to be as sure as possible of the diagnosis, which is why the evidential threshold is very high, and it can take a long time to get there.
Because there’s no “yes or no” test for MS, it relies on continuing to collect evidence until the scales are tipped. In a few cases, there’s enough evidence almost immediately, but more commonly, as with you, the symptoms are there but NOT the proof, so they (and you) just have to keep waiting and looking.
On the positive side, regardless of the opinion of your nurse, as long as you remain undiagnosed, there’s still a chance it might not be MS, which can only be a good thing. I would imagine your consultant believes there’s a high probability it will turn out to be MS, as I think a referral to an MS nurse would be unusual otherwise. But if he continues to be unable to prove it, there’s a chance you might be lucky!
One thing’s for sure - if it is MS, it doesn’t stay invisible forever - one of these days, something will show on MRI. In general, the more time passes with still nothing on the scans, the less likely it is MS.