Not much to add except to say Hancock's explanation is a great one - the brief, bullet-pointed medical sites we get so fixated on don't usually give the nuanced explanation of why symptoms happen, or in what order. Also you need to bear in mind that MND is such a rare disease that it shouldn't really be on anyone's radar unless dramatic symptoms start to present - it's not an explanation for vague feelings.

Hopefully this thread has been enough to convince you what you're describing isn't what you're fearful of.