Sorry to hear about your wife's health.
Like Sandy stated, it can't hurt to at least try to fax something to Carnival. However, like Dwayne stated, a doctor's note might help as well. It would be ideal if the doctor's note was written on either a letterhead, or prescription blank, so Carnival can verify that if you get a sympathetic supervisor.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)