What kind of illness does his sister Madeline Usher have?

Before the narrator gets to the house, Madeline has fallen into a catatonic state. We really have to question if Madeline actually existed. Madeline can be seen as kind of like a doppelganger of Roderick: kind of like Norman Bates and his mother in the Hitchcock movie Psycho. Perhaps Madeline's return from the dead is only a metaphor of Roderick's fears. In any case, Madeline serves as a plot device to give a manifestation to Roderick's fears and anxieties.