I too worried a little bit about this at first. In nursing schoool and now 13 months working in downtown Detroit, it has yet to happen.
Also when I was sitting for almost two years, with some reallyyyy wacky pt's, it didn't happen beyond maybe some minor scratches or something from an old lady or two. I think the more you are calm, not scarred, don't take things personally, the more a situation won't escalate.
That being said you do have to keep your wits about you, bc I have heard some crazy stories. And it might help I'm a very tall, if not skinny, young male.
The good news is that it is rare, unless maybe if you work in the ER