I think the hardest part of the initial application will be choosing a date of onset of disability. You have a progressive disorder so the date you were diagnosed is not necessarily the date of onset because you continued to work. You may have to use the date you stopped working or if you continue to work, the date that the value of the work you did for your employer was no longer valued at an amount that represents Substantial Gainful Activity. If you were paid (gainful) for work (activity) and the value of the work you did exceeded the dollar amount for the year you did it (substantial) you would be denied because you demonstrated the ability to work despite your condition and despite how it debilitated you or that you had no other way of supporting yourself.

What it comes down to, at this point, how much would your employer have to pay a new employee to do the work that you are doing and have been doing? That's the value of the work you are doing. It's probably been very good for you that you have an employer who continues to pay you despite your disabilities because many companies would not do that. That is called a work subsidy and SSA needs to put a dollar value on it. Not an easy thing to do.

Here's a reference about SGA.

https://secure.ssa.gov/apps10/poms.n...category=04240