I have RSD (since '93) and do get ssi (too few work credits for ssd, as they only go 5 years back for work experience/credits).
I had a workmans comp case that took 4 years before that, so had thermography info, bone density tests, nerologists exams and documentation and a slew of other medical info.
They (S.S) also sent me to be evaluated by one of their doctors. They also reexamine you every three years or so to see if you're still disabled.
I was denied the first time, and got it the second round. I've heard a rule of thumb for SSI o SSD is usually the third times the charm.
Google International Reflex Sympathetic Dystrophy Foundation
and skim toward the middle of the page for "Social security Ruling". It's a pdf file, but it has a lot of information in it.
If ya have any more questions or I can help at all, let me know.