These "FDA approved" treatments are probably variations on TENS devices. The Rebuilder is sold thru chiros now, and some are using Calmare treatments which is also a type of TENS device. The devices are approved safe by the FDA along with TENS but they are not FDA approved in the same way as FDA approved drugs. They do not have to provide evidence of effectiveness or "cure".

Some Chiros also use cold laser treatments too. So in order to get more information you need to contact one that advertises and get more details, as they may be pushing different modalities. All of them cost alot. Rebuilder is more than $600 and the Calmare is also quite expensive, but done in the office.

This PDF explains in detail:
https://www.bcbsnc.com/assets/servic...stimulator.pdf

Usually this claim that "FDA approved" is a come on of sorts.
The specifications for the application of electrical energy is screened by FDA regulations so people won't be injured. But that approval does not claim therapeutic success as it does with drugs approved by the FDA.

Recently some TENs devices are available without a prescription for home use. The Rebuilder is one of those. (if you search the name here on PN forum you will find threads with people who had some success with it, failed with it and some even who got stiffed by the guarantee offered.