The unnamed illness plaguing Madeline manifests itself in "a settled apathy, a gradual wasting away of the person, and frequent although transient affections of a partially cataleptical character," the last referring to muscle rigidity and unresponsiveness. It is possible that these symptoms comprise Madeline's own particular brand of the "constitutional and family evil" from which her twin brother Usher suffers. For Usher, the illness shows itself in "a morbid acuteness of the senses;" certain sounds and smells are painful for him, foods with strong flavors are too overwhelming, and he can only wear one texture of clothing.
Madeline suffers from a mysterious illness that is like catalepsy. When she has an attack, she goes into a catatonic, comatose-like state and is unable to react to outside stimuli.