But what is "atypical post traumatic stress disorder"?

I'm looking at the discharge summary prepared by my former psychiatrist, and he wrote that (on top of dysthymic disorder and ruling out personality disorders) but there's no explanation of what it is, and I can't find information about it?

What makes something "atypical"?

(I'm also horribly curious as to what he thinks caused this diagnosis. I'm not disagreeing with it, since I've gone through some stuff. But I'm wondering what would you do to figure out what CAUSED it in the first place if your entire life is just a accumulation of issues...)