Can my employer order me to perform a task in which my doctor states I cannot?

I'm a teleworking federal employee with vasovagal syncope triggered by a myriad of different things. I literally have passed out and been concussed causing hospital stays. My doctor gave me a directive not to drive or commute and my office established telework as an option once they discovered my issue. New leadership has arisen and is requesting me to appear in the office at the job site. Prior to this request they scheduled two meetings at my home but never showed up. I was ordered to appear in the office on 1/29/16 and made every attempt to make the appoint but while en route I collapsed and was taken to the hospital where I fainted again after arriving. My employer scheduled another home meeting claiming it was just to check on me but later my employer revealed they wanted to discuss my performance objectives, my continued telework and work status. I instantly asked for my Union Rep to be present at this meeting and my employer canceled the scheduled meeting. Now again, the same employer has ordered me to work when he is aware of my syncopation, the doctor's directive of no driving or commuting and hospitalizations due to trying to meet their demands. I'm fearful of losing my job as well as losing my life attempting to adhere to their requests. Also, they have reduced my workload and when I ask why I get no response. Again I have been teleworking for at least a year without any incident or problems with work. What can I do? Disregard my doctor's directive and risk my life or obey my employer. Rock in a hard place.