The opposite happens as well, I have a family member with MS that left the U.S. twice for treatments. The first one he flew to Russia to get a stem cell treatment where even though they use the stem cells from his own body, was still considered illegal in the U.S. at the time. (May still be). That slowed/paused his MS and he didn't have any large degredations for 15 years after that. The second time he flew to South America, can't remember where for a different experimental surgery. That was about 5 years ago. All in all it took about 30 years post diagnosis to finally put him in a wheelchair, though I believe he still can stand up for short stints.