So there is a field of neurology that deals with functional brain disorders (which includes things like non-epileptic seizures). Look up David Perez at Mass General. That's what actual functional neurology is, and it is a real thing.
Then there is another, completely unique "field" that was invented by chiropractors that they call "functional neurology", that is basically a fresh coat of paint on an older field of "chiropractic neurology" which uses chiropractic manipulation to treat neurologic diseases like ADHD, Autism, and Alzheimer's Disease. This is pseudoscience.