I don't know if OS really has a brain disease as I've never fictionally spirit-guided him since I don't offer complimentary mini-tacos with each session, but if he does indeed have a brain disease he has my sympathy because I'm a compassionate fictional spirit-guide.
Now with that out of the way, why would a brain disease cause you to leave DU? As it progressed wouldn't it actually make you more DU-eligible?