Yes you can apply for SSI. SSI is usually for people that dont have enough work credits (40 credits) to get benefits from SSDI. The max amount you can recieve from SSI is $623.00 per month. You can recieve both SSI and SSDI if your SSDI is under $623.00 per month (they base it on your past work history) If your SSDI check is more then that then you will not be able to receive SSI. Granted if you have children the rules are different.

Quoted from the SSA Website:
"The Social Security Administration is responsible for two major programs that provide benefits based on disability: Social Security Disability Insurance (SSDI), which is based on prior work under Social Security, and Supplemental Security Income (SSI). Under SSI, payments are made on the basis of financial need."

"Supplemental Security Income (SSI) is a program financed through general revenues. SSI disability benefits are payable to adults or children who are disabled or blind, have limited income and resources, meet the living arrangement requirements, and are otherwise eligible. The monthly payment varies up to the maximum federal benefit rate, which may be supplemented by the State or decreased by countable income and resources."

It doesnt matter what condition you have if you have a doctor that will back you up and say that you cant work then you have hope.

You might want to check out the Social Security website it will answer alot of youe questions. SSI Questions Page