I'm very sorry about your wife. Welcome to the forum.
Amazingly, the doc at the clinic tells us this is a dermatological problem and the Internal med doc says it's a "nerve ending" problem, as she sees no skin problems whatsoever. I've been trying to monitor where the itching occurs with the progress of the atrophy. I could be wrong, but the itching seems to be where he is losing/has lost muscle mass.
This seems to be a common problems among pALS, so I really think it has something to do with nerve irritation.
If you page down past these posts, you will see where other threads address this problem, also.
I use a hair brush on my husband, as well as the nail brush tomby mentioned!
Good luck!