As my neurosurgeon explained it, the sensors attached to my scalp during surgery (that required tiny shaved spots, ruining a perfectly good haircut) send tiny electronic signals during the operation. If the doctor's scalpel comes too close and a nerve is disturbed, a signal triggers a visual and audible signal and the surgeon will stop the surgery. In my case, a nerve-monitoring specialist was brought in (from Pennsylvania) to perform this task and he assured me, just before I surrendered to Morpheus, that he wouldn't let my neurosurgeon do any damage to my facial nerves, as this was a major concern for me. He was true to his word. No facial paralysis or serious problems post-op and what little facial irregularities there were (my smile was off a tiny bit) resolved within six months.
I assume that nerve monitoring is 'standard procedure' for AN surgery now but before going into surgery, I would verify that with the surgeon. I believe it is vital for both patient and surgeon.
Jim