Oh gailm there is absolutely nothing I can say to make you feel better about that, it is simply terrible. Is there another neurologist who can go over your notes who could write the letter in place of Dr. S? I think that is the least the PA could arrange for you. Failing that, would your GP phone the PA to ask what treatment has been suggested because it must be written down.
You must be so deflated and upset right now but if you feel up to it maybe one of those options will work I hope. Can you see or speak to your GP directly to ask for his/her help with this? They have to do something because you have been more than patient. It cannot be allowed to go on any longer so either the PA or your GP will have to step up and help you, which ever one you feel most comfortable contacting first would be a good place to start I think.
I hope you get a good result and can reply that all is well very soon. X