Optic neuritis is usually caused by a viral infection, especially viruses in the herpes family. Shingles are often associated with optic neuritis.
It is also very common in multiple sclerosis, but since it was so long ago I suspect yours was viral. I had it after contracting West Nile Virus.
Because it is caused by nerve inflammation, it would not be associated with MC. Is your doctor ordering the DNA testing? That will help you know for sure what type of myotonia you're dealing with. It's always a relief to have a name for the all the weird symptoms we experience!