Generally a tens machine was not associated with multiple sclerosis. Part of the reasoning was the tens industry claimed a tens machine would work as a muscle stimulator, or properly called Functional Electrical Stimulators. A tens machine is intended for controlling pain, not preserving or increasing function. A tens machine is not the same type machine referred to in Dr. Terry Wahl's book, "Minding My Mitochondria". The reference by Dr. Wahl's is to muscle stimulation, not pain control.
Due to the pain that is associated with multiple sclerosis a tens machine may be totally appropriate. All pain originates in the brain and tens machines are very effective for controlling pain no matter what the source of the pain. A tens machine stimulates non pain nerves and diverts the pain message away from the brain and spinal cord which is the main thoroughfare of transport to the brain. By stimulating sensory nerve fibers, not motor muscle fibers, the tens machine can help stop the pain endured by a MS patient. The mechanisms of blocking c-fiber ( pain fiber ) messages to the brain of the MS patient can be achieved with a TENS machine.
The drawback to a tens machine is since it's an active modality it generally needs to be used during waking hours when pain is present to block the pain signals. If the tens unit is turned off then the block is removed and often the pain for the MS patient returns. There is no harm in using a tens machine 24/7, 365 days per year if it is helping. The biggest concern would be skin breakdown where the tens electrodes were placed and occluded the skin causing skin irritation. The skin breakdown, if it occurs, is not a result of the electrical stimulation but the same effect seen when a band aid is worn continuously and air/light is not allowed to touch the skin.
Many MS pain patients only use a tens unit when retiring to sleep using the machine to get to sleep. Often pain is greatest at night for the MS patient and the tens machine provides enough relief for the patient to get to sleep. There is no harm using a tens machine all night while sleeping and the tens does not present and "addiction" issues for the patient.
If the multiple scleroisis patient desires a pain machine that does not require constant use then interferential therapy may be the most beneficial for MS pain. The Infrex Plus and the Infrex FRM clinical units provides "carryover pain relief" which is another way of saying constant use is not needed as the use of interferential therapy provides relief after the machine is turned off.