Often those with MS will figure out the effects of heat themselves, relating such incidents as, "I was sunbathing at the beach, and after about an hour of lying in the sand, my vision went out in my right eye just like it did last year. I went home, turned on the air conditioner, and two hours later I was back to normal." Or, "In the middle of taking a hot shower, my right leg gave way, just like three months ago. When I got out, I could only drag my leg. But one hour later it was back to normal."
What causes this kind of temporary dysfunction? The best explanation is that a set of nerve fibers has a spot of myelin damage from the past. Nerve conduction recovered, but the myelin never completely repaired itself. When the body is overheated, conduction is slowed, and the nerve impulse traveling down this set of fibers cannot jump the gap where the myelin is damaged. The part of the body supplied by that bundle of nerves stops functioning temporarily until the nerve tract cools off.