What kind of disease is lady Madeline suffering from? Why does Poe use this disease?

Lady Madeline is Roderick Usher's twin sister. She is sick with a disease called catalepsy. One with this disease is in a trance-like state with loss of voluntary motion and failure to react to stimuli. The narrator of the short story and Roderick assume that Madeline is dead (since she has a cataleptic shock) and proceed to entomb her for two weeks. Poe wants us to wonder if Madeline is really alive or dead.

Lady Madeline is the twin sister of Roderick, they live in the house of Usher. She is sick with a disease that they could not identify. She had Catalepsy as a side affect. Catalepsy causes her to go into a state of paralysis. But another side affect of the disease is she falls asleep for days at a time and in these periods of sleep her pulse is very faint, she essentially dies and comes back to life over several days. Do to her disease Roderick is put into a state of distress and is beginning to go crazy. Roderick convinces the narrator to preserve Madeline in the chamber so that he may have a more sane life. In the Chambers Roderick hears Madeline screaming and scratching her way out of the wooden casket. At the end of the poem the narrator and Roderick open the chamber, Madeline is standing there covered in blood, she dies falling on Roderick who then dies of fear. The narrator runs outside just in time to see the House of Usher sink into the tarn leaving no trace of the Usher family.