Two Types of E-Stim

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August 2004

By Richard Kosses, DPT

Electrical stimulation has many uses in physical therapy. Perhaps the most familiar use for electrical stimulation is the control of pain. The most common ways electrical stimulation is used to control pain are trans-cutaneous electrical nerve stimulation (TENS) and interferential current.

However, there appears to be quite a bit of confusion surrounding TENS and interferential current. When should each be used? What frequencies work best? Should a muscle contraction be seen? Does it make a difference where I place the electrodes? By the end of this brief report, some of the questions that you may have about TENS and interferential current will be answered.

E-STIM AND PAIN
Since it was first used, the effectiveness of electrical stimulation to control pain has been questioned. Some research has shown it to be an effective modality, while other studies have shown it to be no more effective than a placebo.^1,2 Much of this debate likely stems from the fact that pain is a subjective description by the patient that varies across gender, cultures, socioeconomic upbringing, as well as the patient's personal life experiences.

Both TENS and interferential current may be used to control several different types of pain including acute, chronic, phantom limb, postoperative, obstetric, cardiopulmonary, and neurological pain. TENS may also be effective for use during potentially painful treatments such as wound debridement by increasing one's pain threshold.³

The way in which electrical stimulation, and in particular TENS, functions to control pain has been debated since its inception. However, two major theories have evolved: gate control and opiate-mediated control. The gate control theory states that small-diameter, slow-conducting, nociceptive nerve fibers that have little or no myelin (A-delta, C fibers) transmit painful stimuli to the spinal cord where they are then transmitted to the brain. The activity of these fibers can be inhibited by large-diameter, fast-conducting, highly myelinated proprioceptive sensory nerve fibers (A-beta fibers). Electrical stimulation may diminish the sensation of pain by increasing the activation of the A-beta fibers, thereby reducing the transmission of pain to the spinal cord.⁴

The opiate-mediated control theory is based on the presence of natural opiates in the body. These opiates, which act as the body's natural pain suppressor, are produced in the pituitary gland as beta-endorphins and in the spinal cord as enkephalins. Stimulation of the sensory nerves promotes the release of these opiates. These endorphins then bind to specific receptor sites in the central and peripheral nervous system where they block the perception of pain.^3,5 It is most likely that what is occurring is a combination of these theories, with perhaps one being more prevalent than the other depending on which method, and the exact settings of the electrical stimulation, that is applied.

MAJOR DIFFERENCES
Electrical stimulation in general has been found to have relatively few contraindications, which include the following: demand-type pacemakers, application over the carotid sinus, and pregnancy.

The following precautions exist for the use of TENS and interferential current: placement on the anterior neck secondary to the proximity to the carotid sinus, stimulation over the chest in patients with cardiac disease, over the head and neck in patients with epilepsy or any neurological disorder including history of CVAs, over the eyes, patients with impaired mentation who may not be able to safely operate the device, directly in open wounds (for wound debridement, the electrodes are placed around the wound to decrease pain during the procedure), poor circulation or sensation, and placement over cancerous tissue.

There are several differences between TENS and interferential current. The first is that interferential currents are believed to penetrate to deeper tissues than TENS. This is because the individual channels of interferential currents operate at higher frequencies. The typical 4,000-5,000 Hz frequency that interferential currents are typically set at results in less interference from the skin than the 20-200 Hz frequencies that TENS operates at (1,000 ohms vs 50 ohms).⁶ The second difference is that because of the decreased resistance from the skin at higher frequencies, patients may better tolerate interferential current than TENS.

When the individual channels of interferential are crossed to create the interference pattern that is needed for the current, a beat frequency of around 10-100 Hz develops, depending on the exact settings of these channels. The beat frequency then operates in a manner that is similar to TENS in terms of its effect on surrounding tissues. The beat frequency can be adjusted for stimulation of the sensory nerves at higher frequencies (approximately 100 Hz), or titanic muscle contractions at lower frequencies (approximately 10 Hz). Most interferential units allow for modulations of the beat frequency so that multiple effects can occur during the same treatment. The ability to adjust the frequency during the treatment is not present on most TENS devices.

TENS can be applied in several different ways including sensory level, motor level, subsensory, and noxious level. The different methods of stimulation are produced by the altering of three parameters of the TENS unit: pulse rate/frequency, pulse width/duration, and intensity/amplitude. It is unclear from the available literature whether manipulation of these variables has any effect on the patient's experience of analgesia.^6,7

Sensory level TENS is the most commonly used method; hence it is often referred to as "conventional TENS." This method is considered to work via the gate control mechanism. Conventional TENS is typically used during the acute stages of an injury, but it may also be used for controlling chronic pain. Pain relief is typically expected within 5 minutes of initiation, but typically does not last more than 1 hour after the treatment is stopped. Interferential current at beat frequencies of about 80 Hz or higher is also believed to work via gate control; however, the pain relief that is associated with interferential currents usually lasts longer than that of TENS. Typical parameters for conventional TENS are as follows:

Pulse rate: 50-200 pulses per second (pps)
Pulse width: 20-100 µs
Intensity: maximum tolerated tingling, no muscle contractions.

Motor level TENS, also known as "acupuncture-like TENS," is more commonly used to control chronic pain. Unlike sensory TENS, motor level TENS should produce visible muscle contractions. Motor level stimulation is believed to work via the opiate-mediated mechanism of pain control, with the low rate stimulation causing the release of endorphins. Likely this mechanism is also what occurs with interferential currents in which the beat frequencies are about 10 Hz or lower. The lower frequency interferential current may or may not produce a muscle contraction. Pain relief with motor TENS should be expected to take longer than with conventional TENS (15-60 minutes), but the relief likely will last longer (> 1 hour). The longer period of pain relief associated with motor level TENS may be attributed to the time it takes the released endorphins to reabsorb into the blood stream. Typical parameters for motor level TENS are as follows:

Pulse rate: < 20 pps
Pulse width: 50-600 µs
Intensity: Maximum tolerated contraction.

Noxious level TENS stimulation is often the last treatment choice in the application of TENS, because as the name implies, it is not a comfortable procedure for patients. There is no counterpart for noxious TENS in interferential current. Typically, noxious stimulation is used for chronic pain. This method of TENS, similar to motor level stimulation, is believed to work via the opiate-mediated theory. Onset of analgesia is usually swift, typically within seconds or minutes. Also, because this method of TENS is thought to work via the opiate-mediated theory of pain control, pain relief is expected to last more than 1 hour.

However, while sensory level and motor level TENS may be applied for 1 hour or greater, noxious level TENS is usually only applied for several minutes because of the difficulty associated with tolerating it. This shorter application time may result in shorter periods of pain relief compared to motor level TENS. The goal of applying noxious stimulation to an area is to produce the maximum amount of discomfort that the patient can tolerate to allow for a quicker onset of analgesia. Noxious level stimulation may or may not result in a muscle contraction, depending on the location of the electrodes and the exact parameters that are programmed. Typical parameters for noxious level TENS are as follows:

Pulse rate: 100-200 pps
Pulse width: > 200 µs
Intensity: maximum tolerated tingling or contraction

Because of the severe amount of discomfort associated with noxious stimulation, it is recommended that it be applied to patients only by a clinician who is experienced in the clinical application of TENS.

Most interferential units that are used today have the capacity to perform a "premodulated" current. The amplitude of the single channel that is used is altered to produce a current that is similar to the beat frequency produced by true interferential current. However, this form of stimulation operates similarly to TENS because the units cannot produce the same field effects as when the two channels in interferential cross.

WHEN TO USE WHAT
The decision on whether to use TENS or interferential current will likely be based on the depth of the tissue being targeted, and experimenting on what works best for the patient. Deeper tissues are likely better targeted via interferential currents while more superficial tissues may respond better to TENS. Also, there are fewer varieties of home interferential units available than TENS units, and those that are available are more bulky than TENS. It has also been my experience that it is more difficult to obtain insurance coverage for home interferential units than for TENS.

Whether you are using TENS or interferential current, the location of the electrodes will have a major impact on the success of the modality. Typically, the electrodes are placed either directly over or immediately surrounding the painful area. When using interferential stimulation, the currents must be made to cross in the area being targeted. However, these locations may not be appropriate for everyone. Some patients may be in too much pain to tolerate any stimulation in that area, the location is contraindicated, or placing the electrodes over the area does not provide the maximum amount of pain relief. In this case, other locations should be attempted. Other locations where the electrodes may be placed are along neurologically related areas including involved nerve roots, along the course of a specifically involved peripheral nerve, trigger points, and acupuncture or acupressure points.

The best results will likely be found by trying to place the electrodes over different appropriate areas and finding what works best for your patient. The distance between the electrodes is related to the intensity that the patient is usually able to tolerate. Decreasing the distance typically decreases the tolerance for the current. This may be due to the fact that when the electrodes are placed closer together, the current travels more superficially than when the electrodes are spaced farther apart. Thus, the electrodes should never be placed less than 1 cm apart to avoid blistering and burning of the skin.

Most available electrical stimulation devices allow for the manipulation of the variables mentioned earlier. Thus, selection of an appropriate unit will instead be based on questions such as the size/weight/shape of the unit, ease of use, and price. Also, some therapists may find useful other features that are becoming more common in home devices, such as patient compliance and effectiveness recording.

Electrical stimulation is a modality that is designed to control patients' pain so that they can continue with their lives with fewer restrictions or limitations. Unfortunately, home use is often considered only when all other treatment options have been exhausted and both the therapist and the patient are at their wit's end. Usually the device is used on the patient's last day with only one setup of the unit being attempted. There are so many different ways that TENS can be applied and so many variables to consider that one training visit is likely to be insufficient. Thus, I would recommend planning on using two or three therapy visits to properly ensure that each patient is getting the maximum benefit from the TENS device.

Both TENS and interferential current can be an effective modality to control pain while in the clinic, allowing the therapist to treat the source of the patient's pain. For example, if a patient is being treated for a shoulder condition for which joint mobilization is indicated, but the patient is unable to tolerate the treatment, then perhaps sensory level TENS applied 5 to 10 minutes before the mobilizations are attempted will decrease pain and lead to a more successful intervention.

CONCLUSION
Hopefully, this clarifies some of the confusion surrounding the use of TENS and interferential current. Like any other intervention that we provide, the more that we understand the concepts behind it and the more that we practice, the better we will become at utilizing it. Hopefully, with a better understanding of how TENS works and how it may benefit our patients, we as clinicians will feel more comfortable using this modality.

Richard Kosses, DPT, is a staff physical therapist at Spaulding Rehabilitation Hospital Outpatient Center in Framingham, Mass.

REFERENCES

Fishbain DA, Chabel C, Abbott A, Heine LW, Cutler R. Transcutaneous electrical nerve stimulation (TENS) treatment outcome in long-term users. Clin J Pain. 1996;12:201-214.
Leo KC, Dostal WF, Bossen DG, Eldridge VL, Fairchild ML, Evens RE. Effect of transcutaneous electrical nerve stimulation characteristics on clinical pain. Phys Ther. 1986;66:200-205.
Hecox B, Mehreteab TA, Weisburg J. Physical Agents: A Comprehensive Text for Physical Therapists. East Norwalk, Conn: Appleton & Lange; 1994:299-305.
Melzack R, Wall PD. Pain mechanisms: a new theory. Science. 1965;150:1971-979.
Cameron MH. Physical Agents in Rehabilitation from Research to Practice. Philadelphia: WB Saunders Company; 1999:387-390.
Wells PE, Frampton V, Bowsher D. Pain Management by Physical Therapy. 2nd ed. Oxford, England: Butterworth-Heinemann Ltd; 1994:145-150.
Foster NE, Baxter F, Walsh DM, Baxter GD, Allen JM. Manipulation of transcutaneous electrical nerve stimulation variables has no effect on two models of experimental pain in humans. Clin J Pain. 1996;12:301-310.
Chesterton LS, Foster NE, Wright CC, Baxter GD, Barlas P. Effects of TENS frequency, intensity and stimulation site parameter manipulation on pressure pain thresholds in healthy human subjects. Pain. 2003;106:76-80.

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August 2004