I had an anthropology professor in college who did some work on genetic diseases in the Amish. It is a problem. This Wikipedia has a short overview and a lot of references:
For instance, the brothers had lack of coordination, ataxia, and minor speech problems. They were constantly getting pulled over by the local cops for minor violations (like driving too slowly) and nearly every time the cops would insist that they were drunk even though they didn't have a drop of alcohol. To the untrained eye, their affect looked very similar to an intoxicated person: slurred speech, bobbing while standing in place, stumbling when walking or getting out of the car, &c. So yes this is a big problem in these communities.