In some movies, sometimes medical conditions are shown inaccurately. For example, in Ghajini (2008 film), anterograde amnesia is shown wrongly. The main protagonist has anterograde amnesia, but he sometimes doesn't even remember his past, which is medically inaccurate.
So, are there any rules regarding the usage of medical terms/conditions/diseases in an inaccurate manner?
There exist no rules because movies are made for entertainment, and don't have any problem if they show a disease the way it is not. The main problem would be a complete rejection to the movie from the audience because it is showing something incorrectly. After some google research, I haven't found any case of movies being sued for this kind of thing.