I'm not sure how it works now as I fall under the rules of discharge that were in place almost 4 years ago when I first applied. Back then, you needed to be unable to work and unable to work for at least so many years into the future (I think it was 3, or maybe it was 5?) in order to qualify as "totally and permanently disabled". The old definition of "totally and permanently disabled" required a doctor to say that you would never heal and never be able to work again for the rest of your life, but I think they realized that this requirement was too restrictive as many doctors don't feel comfortable making a projection about someone's abilities for the rest of their life, especially if they are fairly young. So, they reduced it to a certain number of years. I know when I applied, there wasn't any requirement for being approved by Social Security or falling under the 5-7 year review category, but I was told that those who are most likely to be approved do indeed meet these parameters. (I am in the 5-7 year review category.)
I know there are a number of sites where you can find out about the different types of discharge for loans. Last time I checked, the two categories were for those who are totally and permanently disabled and those who are teachers and such (in certain types of teaching) can apply for the discharge.
The disability discharge process is a bear to go through, but in the end worth it.