According to Catholicism, how can Catholics be faithful to an abusive spouse?

In old age due to dementia a person can become irritable and abusive, wholly unlike their prior self. As a married person will have taken vows to be faithful; what guidance is there to a spouse in the instance their partner becomes abusive? I bring up dementia as an example as the persons behaviour maybe completely outside of their own will. Does the Catholic Church have some sort of official guidelines on how to deal with these issues in mental health situations and/or pastoral care training for priests?