What problem existed for one of our main characters that necessitated a visit from the narrator?

We learn that the Narrator and Usher were childhood friends. Recently, the Narrator received a letter from Usher. In the letter, Roderick described a certain "mental disorder" that was plaguing him, and he communicated a desperate desire to see his old companion. Due to the urgent tone of the letter, the Narrator never thought twice; without hesitation he obeyed this "very singular summons."