there is a code of ethics amongst doctors. a great deal of this ethics deals with confidentiality. most doctors (naturally there are some unethical bastards), can be told anything about your life, especially if it will aid in a diagnosis of your problem. if your pain or medical condition involves AS, it must be told to the doctor. im not saying to go start telling jokes about juice or stories about when you broke a needle in your ass or when you stole a box of pins from his office while you were waiting, but the doc needs to know what is going on with your body in order to help you.
confidentiality deals with the authorities as well, so if your worried about your doctor running to the nearest cop, that should settle your fears a bit. i wouldnt say there are many consequences other than your doctor discussing the various side effects of juice and blah blah blah. worst case scenario could be the doc referring you to another's medical care, because he doesn't know enough about AS, or doesnt feel that he can stay within his ethical boundaries.
chances are, if you do indeed have 21 inch arms, your doc probably knows whats going on, remember they goto school for many many many years...theyre not dumbasses.