Would you consider yourself to lead a sedentary life (be immobile most of the time)? Considering that you have difficulty climbing the stairs would cause atrophy to your muscles in your legs. If you are immobile or sedentary, this would also explain the atrophy to your arms as well. Here is some information to learn more about IBM: http://www.ninds.nih.gov/disorders/inclusion_body_myositis/inclusion_body_myositis.htm as I don't think it fits your symptoms, however you should be seen by a doctor to confirm this and discuss your problems.
Perhaps some physical or water therapy might help (very gently), and when home doing gentle exercises like pumping your feet, arm rolls, foot circles, etc. to slowly build back muscle.
You should also think about alternatives to having stairs or have a chair lift installed if possible. If not, try and make as few trips as possible and wear a back pack to put items in them to carry them up the stairs to you can hold on to the railing for safety.
Let me know if you have any questions.