1. Ask for the entire claims /appeals record. My understanding, as the patient you are entitled to all the correspondence.
2. Request/demand the NPI, name and credentials of the "decision-maker" at each stage.
The NPI is a unique identification number for covered health care providers.
https://www.cms.gov/regulations-and-guidance/administrative-simplification/nationalprovidentstand#:~:text=The%20National%20Provider%20Identifier%20(NPI,for%20covered%20health%20care%20providers
Often they have a standard "script" to deny claims, executed by non medical providers.
On appeal it is probably looked at by someone with a medical profession. It should ho to a neurosurgeon, preferably a board certified neurosurgeon, however insurance companies are notorious for not hiring qualified board certified specialists. Sabmves them money but could be a general medicine doc retired for 20 years filling out the denial.
I follow Kaiser Family Foundation news on the insurance issues, denials (I'm a geek that way)
Here's one apparently useful website to look at for further information
https://www.patientadvocate.org/explore-our-resources/insurance-denials-appeals/where-to-start-if-insurance-has-denied-your-service-and-will-not-pay/#:~:text=Reasons%20your%20insurance%20may%20not,requested%20under%20your%20health%20plan