Teddy,
The 2nd largest group of people with a myelin disease are those with a leukodystrophy. These are mainly children's diseases but all have an adult variation, or adult onset version. With a clean spinal, you could have one of these diseases (there are about 16 of them, or more.) the bad news is that they are fatal, and genetic, and have no treatment. Look down your family tree to see if there are a lot of unusual things that have happened. Many only take one parent to pass on to children...called autosomal dominant inheritance.