The risk of paralysis is approximately 1in 150,000, see link below
http://www.reddinganesthesia.com/spi...%20anesthesia?
Have looked after patients who were paralyzed from epidurals. I believe the theory behind the paralysis in the patients I saw was; the epidural caused the b/p to drop really low, which lead to an inadequate blood supply to the spinal cord, caused spinal cord ischemia and permanent parlysis.