yea so if they gave you the rbc, i imagine they'd be looking for blood cell abnormalities that would be seen with seriously deficient b12. but i have seen articles about b12 that make me suspect it could have more roles, at the higher serum values, than we understand at this point in time.
i compare it to how we used to think vitamin D was just for preventing rickets, and now we're starting to understand its importance to the immune system, and the optimal serum levels for immune health are much higher than the levels for rickets prevention.
in my case i had a documented history of b12 deficiency before my diagnosis, so there are good reasons why supplementing got rid of lhermitte's in my own case, but i reckon it's not expensive for others to try it when their spinal lesions are in the typical b12 spots (ie. cervical - one of mine was around C4 to C6 also - and thoracic). consider taking a 1000mcg sublingual strip of methylcobalamin each day, and complementing it with a b-100 complex. and heck, throw in some vitamin C to help it all stick
don't take more than 100mg of b6 per day for any extended period. oh and this is reminding me, at one point i got on a particular b-complex brand that ended up being the culprit for some mysterious phantom itching that drove me nuts until i started to suspect the b9 (there was more in that b-complex that you usually see) and got my folate tested and my numbers were stupidly ridiculously
high so i switched brands and the itching went away.
anyway the reason i suggest taking the b12 with the b-complex is because i hit a plateau with b12 supplementation and when i added the b-complex i improved dramatically in a short time period. fyi the b-complex regimen i used was a megadose treatment that you don't keep at for a long time. i don't imagine it would work for everyone, but it did for me. i was taking 300mg thiamine, plus 100mg each of b2, b5, b6 and b9, and a good chunk (at least 100mg) of niacin (b3) to bring on a flush, all that three times a day, plus the 1000mcg b12 and a few other things (lecithin etc) when i was really hitting it hard. i didn't stay on it long before seeing enough benefit that i dropped back to regular supplementing. it hasn't fixed my hands to perfection, but the lhermitte's is gone for sure.
see if your doctor will order you a serum cobalamin test so you have some baseline info. or maybe they have that number in your file already. and ask if they did a PCA test (they probably did). IF you're under 400 for serum cobalamin, there could be some value for you in the b12/complex regimen. if you think you need to and want to go for it, aim for serum cobalamin >/= 500 (from what my doc says, you can go far higher quite safely).
i hope this can accomplish something for you