In terms of specific neurological disorders, one might think about certain types of movement disorders, such as Parkinson's disease, which could have a variable presentation lasting from several years to dozens of years, in which case a person has to go through life with those symptoms and try to manage as best they can. Some of the neurodegenerative problems and certain of the less common dementias may have presentations like that.
On the second question, for people we have not yet determined have a grievous and irremediable medical condition, in most serious neurological disorders the answer is time. In serious neurological disorders, things get worse over time. The example, once again, is something like motor neurone disease or ALS, where you're starting off with a twitch in a foot and ending up in a wheelchair two years later, fed by a tube.