Cochlear Implant / Medicare

I would like to know if Medicare pays any or all of the cost for a 
cochlear implant?

I have tried top brand hearing aids and with speech discrimination at 
less than 20%, I have been informed by my audiologist that a CI is the 
only chance I have of hearing and understanding speech again.

I do not have insurance other than through Medicare and would like to 
know what they cover.

Thanks.

Don