OP, I think you need to talk to your boss and HR as some other posters have mentioned. You might want to have your needs/issues documented to make sure you have all major points addressed. I have x condition and need x treatment. Managers 1 and 2 on this date made these comments. On this date, Managers 1 and 2 made these comments, etc.
If your boss is hard to reach, can you email him? Or go ahead and meet with HR first and they might follow up with your boss (you can mention to them your issues reaching him). HR will find him
Or, you could email/call your boss and if you get his secretary say, "Okay, do you know when he'll have time to meet with me? I have an appointment with HR on x date and I wanted to be able to meet with him prior to that." That could get his attention.
But definitely go to HR. You have a valid issue to be addressed and HR will work with you and your boss to ensure everyone's needs are being met. That way, it doesn't matter what the nasty managers say to you. When you need to leave for an appointment or go to the emergency room, you go! If they make snarky comments or try to prevent you, you say "No. I am leaving. I have prior approval from both boss and HR, but I will let them know of your statements/concerns."
Health situations can be a bit of a minefield from a corporate perspective and HR departments ensure they are handled carefully. Any managers mucking around with a delicate situation like this will not make HR happy and a good HR department will push to have that behavior stopped asap.