TENS( Transcutaneous Electrical Nerve Stimulation ) .

TENS( Transcutaneous Electrical Nerve Stimulation ) Definition .

TENS( Transcutaneous Electrical Nerve Stimulation) is frequently used to relieve muscle pain in the neck, back or joint pain of knee, shoulder, etc, arising from work or sport related injuries, e.g. carpal tunnel syndrome, RSI (repetitive strain injuries), as well as, postural musculoskeletal problems related to faulty working environment. It is a modern, non-invasive, drug-free pain management modality, designed to provide afferent stimulation, used for relief of acute or chronic pain.

It is a form of peripheral electrical nerve stimulation through the skin, which is used to obtain electroanalgesia. In the past, TENS equipment were also used as nerve tracers for the search of percutaneous nerves, for maintenance of muscle activity after the stroke and for muscle development. Today, TENS is one of the most commonly used electrotherapeutic modality for pain relief.

Physiological Effect of TENS .

Two theories are used to justify the relief of pain achieved by TENS.

A. The Gate Control Theory .

The Gate Control theory by Malzack and Wall in 1972 postulated that Activation of A-beta fibres simulates the inhibitory interneuron in substantia gelatinosa located in the dorsal horn of the spinal cord. The activated interneuron produces inhibition of transmission through pain carrying A-delta and C fibres. This pre-synaptic inhibition of the T-cells closes the spinal pain gate to prevent the painful impulses from reaching the sensory cortex, where the pain is felt. Pain modulation is thus achieved by activation of central inhibition of pain transmission.

B. The Endorphin Release Theory .

The Endorphin Release theory states that Noxious stimulus causes production of endorphins in the pituitary gland. Endogenous opiates are also synthesized in peri-aquductal grey matter, midbrain and thalamus. Painful stimulus causes release of these opiates in the pain receptor sites in the brain. Pain modulation can thus be achieved through the descending pathways generating body’s own pain killing chemicals or endogenous opiates. Medium Frequency Currents

Equipment & The Nature of TENS Current .

Clinical Features of TENS . 

Externally applied electrical potential from TENS must produce evolved potentials in underlying peripheral sensory nerve(s) or receptors on the skin.  To do this, there must be an adequate stimulus to cause depolarization of the nerve cell membrane. Relief of pain by selectively stimulating the nerve fibres of choice, either large diameter A-Beta fibres or small diameter A-Delta fibres by adjusting the parameters of the machine.

Characteristics of A-Beta Fibres .

Large diameter, densely myelinated, with rapid conduction velocity. Low capacitance—does not maintain depolarized state for long. Fibres remain excitable to pulse widths as short as two microseconds. To excite A-beta fibres, high-frequency impulses with short pulse widths is used.    Physical Principles of Light

Characteristics of A-Delta Fibres .

1. Light myelination, slower conduction velocity.
2. High capacitance—longer latency.
3. Fibres are unexcitable with pulse widths below 10 microseconds.
4. To excite A-delta fibres, low-frequency stimulation, with longer pulse width is used.

Classification of  TENs based on Clinical application .

1. High Rate TENS .

Most common mode of TENS, used in acute or chronic stage of pain syndrome. Modulation of pain takes place by activation of the gate control mechanism by inhibition of pain carrying fibres by large diameter fibres. Effect is rapid but transient, i.e. no pain is felt as long as the current is on.

Operating Parameter .

1. Frequency rate: 50-100 Hz.
2. Pulse width: 50-100 microseconds.
3. Pulse shape: Bipolar asymmetric spike potentials.
4. Duration of treatment: 20-60 min in one sitting may be given continuously for 8 hrs if required.
5. Intensity: Between the first threshold of tingling and less than an intensity at which slight discomfort/muscle contraction is felt.
6. Frequency of treatment: Relief only as long as current applied, hence repeated application, particularly while working is recommended.
7. Uses: Relief of acute and postoperative pain, and in areas of hypersensitivity or increased muscle tone.

2. Low Rate TENS .

This is acupuncture like strong low-frequency stimulus, useful in relief of chronic pain. Modulation of pain takes place through release of endogenous opiates, which suppress the pain receptors in the cerebral cortex. Effect is relatively slow but lasts longer, i.e. more than hours after treatment.

Operating Parameter .

1. Frequency rate: 1-5 Hz.
2. Pulse width: 150-300 microseconds.
3. Pulse shape: Monophasic pulses.
4. Intensity: Sufficient to cause visible muscle twitches within comfortable tolerance level of the patient.
5. Duration of treatment: 20-30 minutes per sitting.
6. Frequency of treatment: Once or twice a day, depending on duration of pain relief.
7. Uses: Relief of chronic pain, effective over area of tissue or skin disturbance where destruction of large fibres means that a long pulse width is needed to achieve the effect, e.g. diabetic Neuropathy, neuralgia.

3. Brief Intense TENS .

Potentially painful, intense stimulation used to provide rapid short-term pain relief during painful procedures like tooth extraction, wound debridement and dressing of wounds, deep friction massage, forced passive movements of joints or passive stretching of soft tissue contractures. Relief of pain is temporary. Low frequency Currents

Operating Parameter.

1. Frequency rate: 80-150 Hz.
2. Pulse width: 50-250 microseconds.
3. Pulse shape: Monophasic pulses.
4. Intensity: Strong to the level of pain threshold.
5. Duration of treatment: 15 minutes.
6. Frequency of treatment: S.O.S.
7. Uses: To suppress pain during potentially painful procedures.

4. Burst Mode TENS .

This form combines the characteristics of high and low TENS, leading to release of endogenous opiates. The stimulation is well tolerated by patients, even on prolonged application, with slower onset of pain relief as compared to low TENS.

1. Rate: 50-100 Hz, delivered in bursts, of 1–4 pulses per second.
2. Pulse width: 50-200 microseconds.
3. Pulse shape: Asymmetrical biphasic.
4. Intensity: Comfortable with intermittent tingling sensation.
5. Duration of treatment: 20-30 minutes.
6. Uses : Suitable for relief of chronic muscle spasm or a combination of musculoskeletal and neurogenic pain of chronic nature, e.g. sciatic syndrome.

Waveforms of  TENS .

There is no definitive work or publication to support the claim that one waveform is better than the others. Both basic waveforms, asymmetrical biphasic or monophasic pulses, are used in TENS stimulation. The spike and the square waveforms are most effective, having a sharp rate of rise, which sufficiently depolarizes the target sensory nerve. Most popular waveforms used are biphasic and are balanced so that there is a net zero DC component, to prevent buildup of ion concentrations beneath the electrodes.

Modulation of TENS .

Modulation of electrical parameters (pulse rate, pulse width) is offered on some TENS machines in order to prevent accommodation of the nerve and receptors to the stimulus. Its clinical value remains to be assessed, apart from a pulsation, which some patients find more comfortable.

Parameter for Optimal Stimulation .

An optimal stimulation/site should be:

1. Strong enough to stimulate the CNS.
2. Suitable for proper placement of electrodes, free of bony or hairy area.
3. Segmentally-related to the source, as well as, the site of pain.
4. Anatomically-distinct, like specific spinal segment or the nerves.

Electrode Placement of  TENS .

Since the modality of TENS is still under exploration, placement of electrodes in TENS is controversial. A basic thumb rule is that the negative electrodes should be placed distal to the positive electrodes.

Negative Electrode of Placement .

The negative electrodes may be located at:

1. Acupuncture sites.
2. Dermatome of the involved nerves.
3. Over the painful spot.
4. Proximal and distal to the pain site.
5. Segmentally-related myotomes.
6. Trigger points.

The exception to the rule is pain due to surgical incisions, where the electrodes must be placed parallel to each other on either sides of the suture line. Placements rules have to be adapted with multiple channel application to ensure maximum coverage of large affected area .

Indications for TENS .

TENS is used commonly for electroanalgesia in many conditions.

1. Joints pain .

Rheumatoid arthritis, osteoarthritis, intra-articular hemorrhage, etc. Acute pain: It can also be used in the treatment of the acute pain, such as obstetric or labor pain, acute trauma, acute orofacial pain, postoperative pain, and primary dysmenorrhea.

2. Muscle pain .

It is also used in the treatment of pain due to various muscle disorders, such as muscle spasm, spastic torticollis, myositis, myalgia, and muscle strain.

3. Spinal pain .

Spinal cord injury, dorsal root compression syringomyelitis, arachanoiditis, postcordotomy, spinal nerve compression can be very well treated. Medium Frequency Currents

4. Neoplastic pain .

It can be used in the treatment of neoplastic conditions. In severe neoplastic pain, TENS can be used for 24 hours a day with a portable device. It can be applied with self-adhesive electrodes so that the patient can perform his ADL while receiving TENS.

5. Nerve disorders .

Peripheral nerve injuries, traumatic neuromas, trigeminal neuralgia, causalgia, brachial neuralgia, intercostals neuritis, mononeuritis, polyneuritis and neuropathies. Miscellaneous conditions: Itch, angina pectoris, functional abdominal pain and pancreatitis.

6. Nonanalgesic indications .

TENS can also be used in dysmenorrhea, Raynaud’s disease, Buerger’s disease, wound healing and following reconstructive surgeries.

7. Psychogenic and phantom pain .

These pains also can be treated with TENS.

Dosimetry of TENS .

In acute pain, high frequency and low or perceptible intensity TENS can be applied for 20 minutes and in chronic pain, low frequency  TENS can be applied with high or tolerable intensity for 30 minutes. However, if the intensity of pain is very severe then TENS can even be applied for 8 to 24 hours.

Benefits of TENS .

Due to reduced pain, exercise program can progress, activities of daily living may be improved, faster reduction in pain facilitates early return to work, there is no side effect with TENS, TENS reduces cost of medications, early ambulation in postoperative cases can be achieved, TENS is noninvasive, nontoxic and nonpharmacological.

Contraindications of TENS .

TENS is an inexpensive patient friendly modality having a wide range of application with superb safety record. However in some types of cases the application of the modality should be withheld.

1. Patients with demand type pace makers.
2. Over the chest wall of cardiac patients.
3. Over the eyes, larynx, pharynx over mucosal membrane.
4. Over the head or neck of a patient with recent history of stroke or epilepsy.

 

REFERENCES.

1. Walsh D. TENS: Clinical Applications and Related Theory. Churchill Livingstone, 1997.

2. Ellis B. A retrospective study of long-term users of TNS. British Journal of Therapy and Rehabilitation 1996;3(2):88-93.

3. Walsh D, Baxter D. Transcutaneous electrical nerve stimulation—A review of experimental studies. European Journal of Medical Rehab
1996;6(2):42-50.

4. Roche P, Wright A. An investigation into the value of TENS for arthritic pain. Physiotherapy. Theory and Practice 1990;6:25-33.

 

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