Need To Know FDA Approved Guidelines For Prescribing Skelaxin. If

Need to know FDA approved guidelines for prescribing Skelaxin. If the mechanism of action is "not known" how can insurance deny it for treatment of spasticity/involuntary hypertonicity? This medication is helpful at controlling these symptoms and thereby reducing pain and need for higher doses of opiates. Any information would be helpful. Thank you in advance.
Sincerely
Joanne Robinson