According to Britannica:
"According to Roderick, Madeline suffers from a cataleptic disease that has gradually limited her mobility."
According to another source:
"In other words, Lady Madeline is said to suffer from a condition that is similar to catalepsy. Catalepsy is a nervous disorder akin to epilepsy, schizophrenia and hysteria. It also involves a loss of sensation and consciousness associated with rigidity of the body."