Transcutaneous Electrical Nerve Stimulation (TENS)

Transcutaneous electrical nerve stimulation is a specialized form of

electrical stimulation designed to reduce pain, either acute or chronic in

contrast to other forms of electrical stimulation. The other forms are

utilized either to produce muscle contractions or to introduce chemicals

into the body (iontophoresis). Initial reports comparing TENS with

narcotic-managed pain indicated that it as an effective alternative to

known narcotics to avoid patient’s addiction. Due to its wide range of

clinical applications, TENS quickly becomes known in the medical and

allied health professions since its early use in 1970. So, TENS is a tool,

which should be utilized in a logical manner to produce favorable results.

Physics and Physiology:

Gate control theory:

Through using TENS, an electrical current is applied to the

cutaneous nerve ending, which travels toward the brain along selective

fibers (A fibers). According to Melzack and Wall (1965), these fibers

pass through the substantia gelatinosa (SG), which contains specialized

cells (T-cells) involved in the neural transmission. These T-cells are also

responsible for transmission of sensory nerve fibers carrying slow pain (C

fibers) toward the thalamus or pain-center of the brain. Both fibers, A and

C, pass through the same T-cells in the spinal cord, taking into

consideration that C fibers are considerably slower than A fibers. As T-

cells are considered as a gate through which these signals must pass,

transmission of large fast A fibers could block the incoming slow, small

C fibers transmission to the brain. In this manner, pain signals could be

effectively blocked by the gating mechanism, which in turn leads to pain

relief.

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In order to utilize the gating effect, transmission of A fibers should

be increased without similar increase in transmission of C fibers carrying

pain. As A fibers respond to a greater extent to phasic input, while C

fibers react best to continuous wave forms, the skin surface should be

stimulated with phasic input device to stimulate A fibers without

disturbing C fibers. This is the basic concept for TENS mode of action.

Precautions:

- Electrode should not be placed in the area of the carotid sinus in the

anterolateral region of the neck.

- TENS must not be used with pacemakers’ patients.

- TENS should not be applied for undiagnosed pain.

- TENS should be applied with great care with patients under the

influence of narcotic medication.

Parameters:

* Wave forms:

Current TENS models favor the biphasic waveform, containing

both the positive and negative phases. These waveforms can be square,

rectangular, sine wave or triangular/spiked. There has been no clinical

evidence of physiologic benefit of any specific waveform, other than a

trial to provide patient comfort. The following waves may be used:

- Spike waves: They are generally more irritating to the skin, which

requires frequent movement of electrodes or shorter treatment time. They

are recommended for intense or hyper-irritating stimulation such as with

acute pain.

- Square, rectangular or sine waves: They are longer-duration

waveforms, with which the skin irritation is less. So, they are indicated

when some nerve damage has been associated with the pain pathology.

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Generally, the intense stimulation with spike waves does not

produce as long lasting a relief as that provided by the longer duration

square or rectangular ones.

* Frequency:

Frequency or rate of impulses indicates the number of stimuli

being transmitted each second. As with waveforms, the frequency

depends on the type of pain.

- High frequencies in the range of 80 to 120 Hz are selected if the

condition is acute, so an immediate relief is needed.

- Lower frequencies in the range of 1 to 20 Hz, are more applicable in

case of chronic pain, as a long-lasting relief of pain is the goal of

treatment.

* Pulse width (Duration):

Pulse width is the length of time the current is actually acting on

the patient, during each individual pulse. In current models, the pulse

width ranges between 50 and 400 msec. When TENS is applied to a

normal neuromuscular system, a range of 100 to 150 msec is

recommended, whereas in patients with neurological damage, wider

widths are indicated in range of 200 to 300 msec. This is because of the

less-than-normal status of the damaged nerve.

* Amplitude (intensity):

Most TENS units are ranging from 1 ma to 100 ma in amplitude.

Treatment should be based on sensation rather than on ma readout as the

ideal intensity for TENS administration is still controversial. The low

amplitude which is barely sensed by the patient is more preferable. As the

high-amplitude administration offers an immediate relief of pain, being

too-short lived as compared with the longer-lasting relief, provided by the

lower intensities.

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Modulation of parameters:

As the human body can get used to anything, the electrical

stimulation may become less effective if the body accommodates itself to

the current passage. So, slight alterations in the treatment parameters may

be needed to vary the current in order to avoid accommodation:

- Frequency modulation: 10% periodically (100 Hz, 90 Hz, 100 Hz, 90

Hz, etc).

- Pulse width modulation: 10% periodically (150 msec, 135 msec, 150

msec, 135 msec, etc).

- Amplitude modulation: 10 % periodically (10 ma, 9 ma, 10 ma, 9 ma,

etc).

Treatment procedures:

The use of TENS for pain control is advised to be for one hour per

session, four times daily. As pain decreases, sessions may be gradually

lessened as needed. Prolonged use is also recommended in postoperative

use, painful scars and obstetrics.

1. Preparing the patient:

- Skin in the area of electrode placement should be clean, clear of lesions.

- A conduction medium (gel or spray) is recommended.

- Tapes are required to fix electrodes in position to maintain contact

throughout the period of stimulation.

2. Electrode placement:

There is a great controversy about the exact site for electrode

placement. Several techniques suggested are based on nerve root,

acupuncture points or trigger points. Although all of these techniques are

valuable, they vary with each individual case, as several key anatomical

points should be established to suit each case.

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a) Electrode placement for upper limb:

- C3: C7 nerve roots/dermatomes.

- Point of pain

- Tip of acromion.

- Web space (between thumb and forefingers)

- Wrist-watch position

- Tip of lateral epicondyle.

b) Electrode placements for lower limb:

- L1 - S1 nerve roots / dermatomes.

- Gluteus maximus center.

- Popliteal space.

- Posterior to lateral malleolus.

- Head of fibula

- Medial/lateral knee.

c) Electrode placements for lower back:

- Associated nerve roots/dermatomes.

- Gluteus maximum center.

- Popliteal space.

- Crossed pattern: Para-vertebral al L1 and L5, in a box-like pattern, with

the circuits crossing at L3.

d) General configurations:

- Associated nerve roots/dermatomes.

- Point of pain

- Acupuncture point proximal to the point of pain.

- Acupuncture point distal to the point of pain.

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e) Certain techniques:

- Transarthral placements: Shoulder, elbow wrist, knee and ankle joints.

- Bilateral placement: Midback and low back.

- Contralateral placements: Non-accessible pain sites as in amputation

dressings, open wounds and casts.

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