Maria Lebyadkin (spoilers)

Though Mademoiselle Lebyadkin is clearly described throughout the novel as being crippled, there is never any real explanation regarding what, exactly, is wrong with her. She is plainly ambulatory, and can move from one place to another of her own accord (as is evident during the chapter in which she approaches Varvara Stavrogina in the church), though she presumably moves with a gait of some kind. As to her 'feeble mindedness,' she could readily be classified as an isolate, given the fact that her brother more or less kept her quarantined in their home and gained custody of her simply as a means to secure cash payments from Stavrogin. She could be considered the only morally upstanding character in the novel, due to the fact that she is more or less a tabula rasa. That being said, she is only ever vaguely described as a 'cripple.' Did Dostoevsky have any specific condition in mind when he wrote that character?