Transcutaneous Electric Nerve Stimulators ( TENS unit - TNS unit - Pain Machine) are small, battery powered devices that are worn on the body and emit up to 150 electrical impulses per second to control pain. The devices were widely prescribed during the 1980's by physical therapists and medical doctors as a last means of helping the chronic pain patient. There was generally a trial period for each patient to determine if the tens unit would provide relief sufficient to justify daily, continuous use and/or purchase by an insurance company.
The big drawback was the unit was only effective if the patient was wearing it and upon turning the pain machine off the pain quickly returned. It was used for symptomatic relief, not curative relief. Constant usage also meant constant need for the electrodes which were/are a continuing cost.
Today interferential pain machines are supplanting tens unit for one simple reason, carryover pain relief. An interferential unit only requires a patient to do a 20 - 40 minute treatment possibly going from 2x daily to as little an once per month to stop the pain transmission. The machine emit up to 8,150 impulses per second, which is considerably more than a simple tens unit. This is changing the way chronic pain patients are being treated.
The carryover, or residual, pain relief is lasting for days/weeks and apparently is changing the underlying chemical composition of pain stimulus so the treatment is becoming curative, not treating the symptom of pain. Interferential patients are needing only 4 - 8 electrodes per month due to the infrequent use and this is saving pain patients and insurance companies enough to justify the higher cost of the interferential unit up front, but not as costly as continuous operation of a tens unit.
Here is a video on what interferential is: http://bit.ly/vsop9