before another person comments that the surgeon won't make a mistake (or a kindly suggestion about avoiding error) the very nature of the chiari malformation would be why patman is concerned about the error. the risk of affecting an incorrect nerve isn't due to basic error by the doctor but the nature of the malformation, thus it is a fairly reasonable fear.
while that fear might be a valid concern, your medical situation isn't something that is outside of God's purview or control. He created your body, not in assembly line, impersonal fashion. even a doctor's errors cannot remove the control or will of God. that is a pretty amazing thing. : )
that said, patman, i have prayed for you this morning that your surgery is an unmitigated success. no medical failure can eclipse or override God's will and His attention, and i pray that you find peace before the surgery, and have a very successful recovery.