Here is an article...this is not to say you should do all these things. It is however to consider there IS other things that need to be done, some basic, some herbal.
This addresses Neuromuscular/immune disorders...not specifically your issue. Despite a syndrome being named in your dog, there is a similarity in all immune and neuro disorders.
What this addresses is B-12 (specifically methylcobalamin - not "cyano" -- this is artifical and contains coal tar - which is what is found in kibble)
Magnesium - for smooth muscle contraction, neuro function and 300 other processes in the body
Detoxing heavy metals and pesticides (this is likely what the immune system is attacking or is stimulating the activity) like mercury (in rabies vax.), aluminum (in other vax.)
There are herbals, collodial silver (for bacteria). RAw food has the enzymes and all essential amino acids which are the building blocks of protein and building muscle.
Lots to read through (incl. links inside the link)...you will need to enlist a good holistic vet, as I didn't go through all....just to give you hope as a lot of these things listed are safe - just not sure about all noted
Natural Remedies for Myasthenia Gravis
Also, not sure if it would be helpful, but makes sense to consider, an enzyme called serratiopeptidase (called other shorter names)...this is an anti-inflamm enzyme that is touted to break up scar tissue, cysts as well as other properties w/o damaging the physical muscle.
I used a brand that had 17,500 UI for my dog and was using wobenzyme (also a consideration)...he had a bulge protruding from his carpel joint...it's gone now, he had it for about a year...I am not sure if it was the serro, or the wobby's...seemed to coincide w/the serro though I only used one bottle before going back to wobenzyme.
Google both (the serro product was called "Infla-Heal Plus" by New Roots
The serro comes in different units of measurement, like 80,000IU's which I believe is 50mg , but don't quote me.
NO MORE vaccines either - this is hugely important re: detox.
Milk thistle for the liver too
Vit C is also a heavy metal chelator
Good luck, I will forward anything else I happen across