Transcutaneous Electrical Nerve Stimulation, shortly called TENS, is a procedure that involves electrically stimulating the sensory nervous system to provide a form of symptomatic pain relief through a pathway called the pain gate mechanism (1). Research suggests that the use of TENS brings about significant pain relief, opposing the also popular view that it acts only as a placebo. While TENS could mean any form of stimulation that is done through the skin using surface electrodes, it most commonly refers to stimulation done to provide symptomatic pain relief. Several TENS machines have been manufactured and are being marketed by various producers of medical devices. They come in different units that vary in complexity and size. TENS is a non-invasive form of pain reliever. It is either used as a stand-alone therapy or together with pain-relieving medications.

Pain Gate Theory

Transcutaneous Electrical Nerve Stimulation is purported to work through the pain gate mechanism. The pain gate mechanism (2) of pain reduction involves the stimulation of the A beta sensory fibers (Aβ). Doing this reduces the flow of noxious stimulus from the ‘c’ fibers to the brain through the spinal cord. However, it is believed that clinically, the frequency of stimulation necessary for the excitation of the A fibers lies somewhere between 90 and 130Hz. While some patients fall below or above this range, the range applies to the majority of people.

Types of Transcutaneous Electrical Nerve Stimulation

Transcutaneous Electrical Nerve Stimulation is done in these methods depending on the purpose of use;

Conventional TENS (1): This is used to stimulate large diameter and non-noxious afferent fibers to achieve segmental pain relief. It involves the use of low or high frequency at the location of pain to create a strong but bearable sensation. This type of TENS is usually administered for about thirty minutes.
Acupuncture-like TENS: This is used to create muscle twitch to stimulate small diameter motor afferents to elicit segmental pain relief. High to low frequency is used over the muscle or at acupuncture points to achieve a strong but bearable muscle contraction. This is usually applied for 20 minutes or less.
Brief Intense TENS: This is done to stimulate small diameter noxious afferent in order to cause peripheral nerve blockage and thereafter, segmental pain relief. Here, high intensity and high frequency are used to achieve maximum paraesthesia. The treatment is applied usually for less than five minutes.

Use of TENS

Many studies have shown the effectiveness of TENS in reducing pain in certain conditions. Some of them are under-listed:

Child Birth: Transcutaneous Electrical Nerve Stimulation is commonly used during childbirth to reduce labor pain. This was confirmed by research (3) conducted by Kaplan B et al that involved forty-six nulliparas alongside 58 multiparas who have all used TENS for pain reduction during delivery. Through completed questionnaires and objective evaluation based on delivery parameters and medical interventions given during labor, the study concluded that TENS is an efficient alternative pain reliever. It was further concluded that TENS reduced the time for the first stage of labor. It also reduced the administration of analgesic drugs. It was described as to be a safe option as there were no adverse effects on both the newborns and their mothers.
Muscle Pain: TENS is very effective in reducing muscle pain. Apart from its ability to reduce pain through the pain gate mechanism. Electrical impulses from the TENS machine are capable of causing local micro contractions of muscle fibers. These contractions help with pain too and also help, to a significant extent, in preserving muscle strength and other muscle properties which are prone to declination when pain is present.
Others: TENS has also been postulated to help in reducing pain associated with arthritis (4), menstrual period, pelvic pain associated with endometriosis, sports injuries, back pain, fibromyalgia, neuropathic pain, and joint pain. Research (7) conducted in 2022 proved that TENS is very effective for pain associated with primary dysmenorrhea. Most TENS machines are tiny and can be carried around in a pocket or strapped to the body. This allows people to use it throughout the day.

Side Effects of Transcutaneous Electrical Nerve Stimulation

TENS is a non-invasive, non-pharmacological approach to pain reduction. It is often without serious adverse effects when used for the right reasons, and for the right patients. However, some patients have reported some side effects associated with the use of TENS.

Allergy reaction to the adhesive placed on electrodes: Some patients have an allergy to the adhesive used to make the pads stick to the skin. As a result, they develop redness, irritation, or a rash on the skin.
Another common side effect is discomfort associated with its use usually described as a tingling and prickling sensation.

Controls in a TENS Unit

A TENS Unit has three major controls that allow one to administer it at the desired level. The three major controls in most TENS units are buttons or knobs to alter intensity, frequency, and duration.

Intensity: This dial lets users change the intensity of the electrical stimulation.
Duration: This describes the number of microseconds in which the current goes into the skin with each pulse.
Frequency: This refers to the number of electric pulses in each second. Usually, high frequency in transcutaneous electrical nerve stimulation ranges from 80 to 120 Hz while low frequency ranges from 1 to 20 Hz. Low frequency is usually used to treat chronic pain while high frequency is usually used to treat acute pain.

Factors that Influence the Effectiveness of Transcutaneous Electrical Nerve Stimulation (TENS)

TENS has been proven to be a great tool for achieving alternative pain relief. However, the effectiveness of TENS is largely dependent on the proper placement of electrodes, tolerance, and intensity of stimulation, among others.

Placement of Electrode:

TENS is more effective when the electrodes are placed at the right spots - acupuncture points. Research (9) has shown that patients who received TENS treatment at acupuncture points derive more benefit from it. It is believed that this placement of electrodes helps the body in producing more endorphins leading to improved pain reduction. Apart from placing the electrodes at acupuncture points during stimulation, individuals can also get the full benefits of Transcutaneous Electrical Nerve Stimulation by using the proper electrode placement technique during treatment. Electrode placements can be unilateral, bilateral, uni-polar, and quadripolar. Unilateral placement of electrodes causes inflammation on one limb or one-half of a muscle pair. Bilateral placement is used to stimulate both limbs or the two halves of a muscle pair. Uni-polar placement involves the placement of two essential leads and electrodes over a target area while quadripolar is used for stimulation of the agonist and antagonist part of a muscle pair at the same time. It involves using two sets of electrodes from two different channels with a concurrent bipolar circuit. It can also be used for a large area such as the back.

Tolerance

Research (9) has also shown that patients that use TENS on a daily basis, especially with the same intensity and frequency are very likely to develop a tolerance to the procedure. This simply means that they won't get the same level of pain relief from the first time(s) they used the unit. Patients are advised to avoid tolerance to their TENS unit by alternating between high frequency and low frequency in each treatment session. They can also choose to increase the duration and intensity of each day.

The Intensity of Stimulation:

It is postulated (9) that high frequency treats pain better than low frequency. Most experts have suggested that patients make use of TENS at the highest intensity they can tolerate. They believe that this is the best way to get the best pain relief from the use of TENS.

Proper Application

There must be good contact between the electrodes and the skin to achieve great results from TENS. The skin should be thoroughly cleaned before the application of electrodes. Excess hair will prevent the electrodes from sticking properly to the skin. Hence, they must be trimmed off. If they become a disturbance, the extra length of wires can be taped to the skin to prevent them from tugging electrodes away from the skin. Electrodes should not be applied over irritated or broken skin. Bad reusable electrodes should be replaced especially when they no longer stick to the skin. Both the electrodes and the TENS unit must also be kept safe from moisture and extreme temperatures.

Power in TENS Unit:

Most TENS units are powered by batteries. To achieve optimum results with the TENS unit, the batteries must be replaced from time to time with quality batteries. A bad battery equals reduced power of stimulation and reduced pain relief.

Contraindications of Transcutaneous electrical Nerve Stimulation

While TENS could be very beneficial for patients with varying forms of pain, it must be totally avoided in some other patients.

Patients with pacemakers are to avoid TENS or in fact any type of electrical stimulation. Electrical impulses could alter the proper functioning of the pacemaker, cause significant damage to it, and may result in death (4). As a rule of thumb, TENS should be avoided on any part of the body where an electric device is implanted.
TENS is not recommended for use in early pregnancy. However, TENS is completely safe for use during delivery,
Patients with heart problems or epilepsy are to avoid Transcutaneous Electrical Nerve Stimulation. Electrical impulses could cause abnormal functioning of the sinus-atrial node and could also trigger a seizure in epileptic patients.
TENS use should be avoided over parts of the body with cancer or an infection or bleeding. Using TENS over these places could lead to the worsening of the conditions, the spreading of infection, and an increase in bleeding.
TENS must also be avoided in patients with mental impairment or patients who cannot communicate freely (5). Feedback from the patient is important to ensure safety while using TENS.

Other Precaution Guiding the Use of Transcutaneous Electrical Nerve Stimulation (TENS)

Electrodes are preferably placed over skin areas without abnormal skin sensation.
Placement of electrodes of the eyes must be avoided.
Although there is no proven risk of adverse effects, in children, placement of electrodes over bone epiphyseal regions should be avoided.

Contraindication of Transcutaneous Electrical Nerve Stimulation for Vagus Nerve Stimulation.

Vagus Nerve Stimulation means exciting the vagus nerve of an individual usually through a surgically implanted stimulator, It involves the delivery of electrical impulses into an individual’s vagus nerve to treat various conditions such as epilepsy, gastrointestinal diseases, and depression (6). Some believe that Transcutaneous Electrical Nerve Stimulation can be used to stimulate the vagus nerve as a replacement to standard surgical implants and natural means of stimulating the vagus nerve such as gargling loudly with water, coughing, laughing, singing, etc. However, using TENS to stimulate the vagus nerve might be dangerous and counter-effective for these reasons;

Firstly, Vagus Nerve Stimulation is a treatment for conditions such as epilepsy. Transcutaneous Electrical Nerve Stimulation has a tendency of acting as a trigger in people with epilepsy. This means using TENS for vagus nerve stimulation in patients with epilepsy should be totally avoided.
Another issue is the problem of regulation. Many surgically implanted devices for vagus nerve stimulation can be programmed to send electrical impulses for about 30 seconds every 5 minutes (8). Most TENS devices are incapable of this level of programming. Hence, the dosage and frequency of electrical impulses cannot be adequately monitored.