If we can't see any external source, we have to assume it's internal.
It's reasonable to assume the mask contains an incredibly strong anesthetic of some sort, and it's slowly released into his system over time.
His mask is covered in small pipes and tubes, each one could contain some of this wonderful mystery substance. (Venom?)
Bane also wore a mask that pumped gas into his system to alleviate the pain from his injury within the prison.
