I'm sorry, I don't know much about permanent nerve damage. Although after my double osteotomy (tibia and femur - right leg) I had almost total loss of sensation from my thigh down to my ankle on the front part of my leg. Then it progressed (as the nerves that were severed during the surgery formed new bits) to a dreadful sensation something akin to a SEVERE sunburn whenever ANYTHING touched the area. My Physical Therapist told me the only solution was to deliberately rub the area with varying types of textures (rough towel, fingernails - like scratching an itch, smooth cotton, even the pumice stone I use for my feet) to acclimate the area until new nerve branches developed. It felt AWFUL (shaving my legs was torture!), but I have to say that, 3 months down the road, it's SO MUCH BETTER!!! However, in your case I don't want to assume that's the right protocol, since your damage was brain stem damage - not severed nerves.
The other thing is that my doctor has now prescribed a drug called "Neurontin," which is specifically meant to treat chronic nerve pain. You may want to ask your doctor about this or something like it.
I'm so sorry you're suffering in this way, but please accept my warmest best wishes in your recovery, and lots of hugs!