Sorry to disappoint. I can help with most medical things but I am not well versed on insurance! I only know that dependng on what is wrong with you and depending on the insurance company it can be quite expensive.
I cannot see cauda equina syndrome affecting insurance, don't know about spinal muscular atrophy.
You have to be TOTALLY HONEST about all your conditions. I know someone who is in a chair and suffers from cancer, trying to organise a dream trip to Disneyland and the insurance company wanted £10,000 because to them you are "a risk." You must remember that it is not due to your disability they put you at risk it is what they must pay out of anything happens to you whilst there. So, to go to USA with terminal cancer, they presume something is going to go wrong. Where you are aware of having a pre-existing medical or disabling condition YOU MUST REVEAL THIS.
Can I suggest the following link:
http://www.direct.gov.uk/DisabledPeople/TravelHolidaysAndBreaks/PlanningATrip/PlanningATripArticles/fs/en?CONTENT_ID=4018312&chk=Jmeq5y
Also, this one can give quotes where you have pre-existing disabling conditions:-
http://www.allcleartravel.co.uk/Sorry, cant be more help than this.