Transverse myelitis is the nonspecific term used to denote inflammation and demyelination in the spinal cord, and because it is non specific it has a variety of causes. Transverse myelitis can occur in patients with vasculitis, multiple sclerosis, mycoplasmal infections, Lyme disease, syphilis, TB, or viral meningoencephalitis and in patients using antiparasitic or antifungal drugs etc.
If your demyelination is limited to your spinal cord and not your brain, you would probably be pushed into the TM category. You would have most of your problems from your chest downward.
This is from what I have read. MSers would "overlap" with TM but TM may overlap with MS only if it progressed to MS much like MS overlaps with ON but ON may not overlap with MS. That is to say, MSers can experience ON and TM but those with ON or TM only experience MS if their case develops into MS.