TENS in treating

chronic pain Dennis E McDonnell, MD

P ain usually serves a useful pur- pose in warning us tha t our bodies are being injured, and it

spurs us to take corrective action to pre- vent further injury. We quickly move our hand away from an unexpectedly hot frying pan handle. The absence of pain sensation can lead to severe injury. Decubitus ulcer formation in a n anesthetic-paralyzed patient suffering from a spinal cord injury demonstrates this dramatically.

Chronic pain and postoperative pain, on the other hand, serve no such protec- tive purpose. Chronic pain is personal, individual, and difficult to measure. Apart from the initiating disease or in- jury, other factors, such as personality type, family conflict, emotional stres- ses, or monetary gain, aggravate and augment the chronicity of pain. Chronic pain can become a disease, and it is dif- ficult to treat successfully.

Prior to the proposal of the gate theory for pain in 1965, surgical ap- proaches to alleviate pain were destruc- tive, such as cutting nerves and making lesions in the brain (lobotomy) or spinal cord (cordotomy). These procedures can produce relief but at the cost of neuro- logic deficits such as numbness or mus- cle weakness. Often such procedures fail t o give lasting relief.

For acute postoperative pain, narcot- ics or their synthetic derivatives have often been used. But along with their analgesic effect, these agents bring un- wanted side effects, such as respiratory depression, nausea and vomiting, con-

Dennis E McDonnell, MD, is associate pro- fessor of neurosurgery, Uniformed Services University of Health Sciences School of Medicine at the Walter Reed Army Medical Center, Washington DC At the time this arti- cle was written, he was associate professor of neurosurgery, University of Iowa Hospitals and Clinics He is a graduate of Creighton Uni- versity School of Medicine, Omaha

AOR,V Jo l i rnn / . September 1980. Vol 32, N o 3 401

stipation, mood alterations and depen- dence.2

Recently, approaches for pain treat- ment have been more physiologic, ap- plying nondestructive electrical stimu- lation, such as transcutaneous electri- cal nerve stimulation (TENS), to alter or modify pain mechanisms and trans- mission function in the nervous system without damaging it.

The afferent system of the nervous system, particularly with reference to pain perception, is extremely complex and still incompletely understood. The rationale for using extraneous electri- cal stimulation to relieve pain will be- come clearer with an understanding of pain-signaling mechanisms in t h e peripheral and central nervous sys- tems.

Peripheral pain receptors, scattered throughout the body, are complex struc- tures with relatively high thresholds to ti mu la ti on.^ These are activated by noxious or painful stimuli. These recep- tors transmit pain impulses via small- diameter, finely myelinated A-delta fibers and unmyelinated C fibers. These fibers conduct slowly and have high thresholds of stimulation. They adapt

Fig 1. Two-channel transcutaneous stimulator unit showing two pairs of electrodes, regular size 3.75 x 5 cm and large size 5 x 10 em. Powered by three AA size dry cell batteries.

slowly or not a t all to painful ~ t i m u l i . ~ These fibers transmit pain impulses to the dorsal horn, the posterior column of gray matter in the spinal cord. A-delta fibers evoke sharp, pricking pain, and C fibers evoke deep aching or burning pain.

Larger, heavily myelinated A fibers transmit other types of sensation such as impulses of touch, vibration, and po- sition. They conduct electrical impulses rapidly. The central input from A fibers tends to balance that from A-delta and C fibers, possibly inhibiting the trans- mission of pain perceptions from these fiber^.^

All of the impulses coming into the spinal cord are modulated initially by neurons arranged segmentally along the spinal cord in the dorsal horn gray matter, known as the substantia gel- atinosa, before being transmitted to the brain.6 In general, neurons of the substant ia gelatinosa inhibit pain transmission.'

Transmissions from more specific re- ceptors, such as those from precise touch sensation, travel via the larger, rapidly conducting A fibers to cell bodies in the dorsal root ganglia, which in turn

402 AORN Journal, September 1980, Vol32, No 3

transmit their impulses to the same pool of neurons in the dorsal horn. If the stimulation intensity far exceeds the normal threshold of these specific recep- tors, centrally perceived pain results. Thus, there is no specific system for transmitting pain; rather, the entire peripheral nervous system is involved.8

It has been proposed that the large A fibers provide major inhibition of pain transmissions in the dorsal horn gray matter, whereas the small A-delta and C fibers excite the neurons and provide relatively little inhibition. Influences from the brain also affect this modula- t i ~ n . ~

This balanced circuitry of excitation and inhibition has been demonstrated histochemically, with the identification of a pain-receptive neurotransmitter (substance P) and an endogenous anal- gesic transmitter (endorphin).lo

Fibers transmitting pain impulses take several routes to the brain. Pain transmission involves most sensory f i - bers in the peripheral nerves as well as a dffuse fiber system in the spinal cord. This explains why some destructive procedures such as cordotomy may fail to relieve pain over the long term."

The clinical importance of these prin- ciples becomes obvious with trans- cutaneous or direct spinal cord stimula- tion. Augmenting the large fiber input by applying extraneous stimulation to the large fibers of peripheral nerves or the dorsal columns of the spinal cord increases the pain inhibition influences on pain perception neurons in the dorsal horn.

Chronic pain occurs when there is an imbalance, with a dominance of C fiber input tending to block inhibitory action of the substantia gelatinosa cells, open-

Fig 2. Trial areas of transcutaneous electrical nerve stimulation. Xs mark areas stimulated and 0 s show areas where stirnulation trials failed to relieve the patient's low back and right sciatic pain.

ing the gate for transmission of pain. By adding an external source of stimula- tion to increase the input of the A fiber system, the inhibitory action of the sub- stantia gelatinosa is augmented, and the gate is closed. External electrical stimulation to peripheral nerves stimu- lates the fast-conducting fibers to close the gate and inhibit pain.'* This theory is particularly attractive because i t suggests that the application of simple, safe electricity through the skin to a peripheral nerve may relieve pain. Electrical stimulation for the treatment of pain is not new and has been used for over 100 years. l3 There was no scientific explanation for the mechanism of its ac- tion, however, until Melzack and Wall's gate theory for pain was published in 1965.14

This theory was tested initially by stimulating the largest concentration of heavily myelinated fibers in the ner- vous system, the posterior column of the spinal ~ 0 r d . l ~ Results of stimulating the

AORN Journal, September 1980. Vo132, No 3 403

Fig 3-A. A small, compact, two-channel transcutaneous stimulator, light enough to be carried on underclothes for continuous stimulation during normal daily activity.

spinal cord for pain relief were initially encouraging. TENS developed from a need for a screening device to determine patients who would be candidates for eventual spinal cord stimulation. Transcutaneous stimulation, however, was found to be effective in its own right in relieving pain and therefore sup- planted the need for stimulator im- plants in some cases.16

Transcutaneous stimulators a r e transistorized electronic devices that generate a pulsating electrical current (Fig 1). The voltage, pulse width, and frequency can be easily adjusted by the patient. They are designed to develop a maximum current of approximately 75 milliamps. The voltage can be regulated from 0 to 100 volts. The electrical wave

Fig 3-6. Nonreactive adhesive discs secure electrodes with a minimum amount of adhesive contact to the skin, also prevent drying of electrode jelly, and ensure effective electrode contact during normal daily activity.

form varies with the type of unit and manufacturer. The wave length (pulse duration) can be adjusted from 0.1 to 0.8 milliseconds. Some units have controls with which the patient may adjust wave length, although newer models have the pulse duration pre-fixed." The stimulator unit is powered by dry cell batteries; most newer models have re- chargeable battery packs as a standard power source. The electrodes are car- bonized silicone rubber and have a min- imal area of 4 sq cm.l* Smaller elec- trodes, which are flat and conform com- fortably to the skin surface, are avail- able for special purposes. An aquaphilic gel is used to facilitate current trans- mission from the electrode through skin resistance to the peripheral nerve or

404 AORN Journal, September 1980. Vol32, No 3

painful trigger point. Electrode positioning is critical for

success. Ideally, the electrodes should be placed proximal to the site of the pain close to the nerve trunk innervating the area of pain. Bipolar electrodes should be positioned from within 2 to 20 cm apart. The electrodes should be posi- tioned as far proximal to the pain as possible. Frequently, however, stimula- tion of points directly over the painful areas is more effective. For postopera- tive pain, sterile electrodes may be posi- tioned immediately after skin closure in the OR. These may be applied to the skin on either side of the incision about 1.0 cm away from the suture line.l9 The de- cision for electrode position requires time and patience, both on the part of the physician as well as the patient (Fig 2). Success is directly proportional to the amount of time the physician or a trained assistant is able to spend with the patient.

Electrical stimulation should evoke a tingling paresthesia that is a t a com- fortable level and not strong enough to provoke discomfort a t the electrode site or a muscle contraction around the elec- trode. Excitation of t h e peripheral nerve trunk should be enough so that paresthesias are perceived along the nerve, yet not strong enough to evoke a motor response.

The pulse rate of the current is usu- ally adjusted unt i l pain relief is achieved. In general, more rapid pulse frequencies a re more effective, al- though this is an individual response and is determined empirically. The fre- quency of application is determined by the patient. Persistent severe, intracta- ble pain requires constant stimulation to achieve satisfactory relief. The use of stimulation at night for sleep depends on individual needs, but it is usually not necessary.

Factors that tend to influence results are: skin resistance. the location of the

Fig 4. Small-sized electrodes positioned bilaterally over the supraorbital and infraorbital nerves to relieve intractable facial neuralgia of multiple sclerosis. This unit is a compact two-channel transcutaneous stimulator wih rechargeable battery pack.

electrodes, the distance between the electrodes, and the tolerance of the in- dividual patient. Emotional factors, the patient’s cooperation, his understand- ing of the principles involved, and de- sire for relief are important for SUC-

cess.*O Each patient requires frequent follow up for reinstruction in electrode positioning, encouragement on con- tinued use of the stimulator, and treat- ment of local skin irritation resulting from the electrodes, tape, or electrode gel.

Skin irritation is the only major com- plication of TENS. It can be serious enough to force discontinuation of stimulation. This can be avoided by al- tering or changing the positioning of the electrodes. Rotating electrode positions can help if there are alternate areas where TENS is effective in relieving

AORN Jourrcal, September 1980, Vol 32. N o 3 405

Table 1 Study group response results

Diagnosis Angina Amputation stump Arm edema Atypical face pain L-urn scar Arthritis Cancer Causalgia Central dysesthesia Chest wall pain Chest trauma Headache Herpes zoster-thoracic Herpes zoster-other Low-back pain-etiology linknown Low-back pain-postsurylual Lumbar spondylosis Migraine Neck, shoulder, arm pain Neuroma Perineal pain Peripheral neuropathy Phantom limb pain Root avulsion Surgical scar Torticollis Traumatic neuropathy Trigeminal neuralgialtic

douloureux Visceral pain Total

Result

Good Partial 4 0 3 0 1 0 2 1 0 0 1 0 8 2 0 0

12 1 5 0 4 0 2 1 2 0 2 2

10 3 26 3 2 0 2 0 5 0 2 0 0 0 3 0 0 0 0 0 9 0 1 0 8 0

0 0 1 0

115 13

Failed lr 3 1 0

10 2 5 5 2 3 6 1

15 31 3 0 5 2 2 4 2 1

18 0

Total 5 6 2 6 1 1

20 2

18 10 6 6 8 5

28 60 5 2

10 4 2 7 2 1

27 1

12

4 7

268

pain. Application of 0.25% hydrocor- tisone water-soluble ointment to skin stimulation areas will help relieve any inflammatory reaction.

New TENS models are small and compact. This allows concealment dur- ing stimulation (Fig 3-A). The elec- trodes can be attached to the skin with preformed plastic adhesive, which effec- tively secures the electrodes and delays

gel desiccation (Fig 3-B). This tends to reduce skin irritation, allows for more prolonged stimulation, and improves convenience and comfort while using TENS during daily activity.

Electrodes are preferably positioned over subcutaneous peripheral nerves that can be easily stimulated. Position- ing over the common peroneal nerve, where it crosses just below the head of

406 AORN Journal, September 1980, Vol32, No 3

Table 2

Overview of study group

Ages Age Men 179 1 7-81 49 years

Result

Good

79

Average 1

268 17-82 50 years 115

I 36 I 89 I 21-82 I 50 years

I Total I

Partial I Failed I 9 I 91

I 49 4

13 I 140

the fibula, is good for distal leg and foot pain.

Facial pain can be treated by position- ing electrodes over the supraorbital and infraorbital nerves (Fig 4). The greater occipital nerve can be used for posterior neck and occipital pain. In this case, some of the hair behind the ear must be shaved to allow close apposition of elec- trodes to the skin surface. Hair over the stimulated surface forms air pockets be- tween the skin and electrode surface that interfere with transcutaneous elec- trical impulse conduction.

TENS has been successful in treating acute and short-term pain, such as post- operative incision pain and the first stages of labor.21 Treatment for acute pain was more successful in patients who had not been on narcotics preopera- tively.22 The majority of chronic pain patients, in contrast, have been on nar- cotics prior to TENS and still gain bene- ficial response. This suggests that the endorphine system may be involved with pain relief using TENS, but the mechanisms may be different for acute versus chronic pain with respect to re- sponse to TENS.23

For a broad survey of unselected pa-

tients, the Neurosurgery Service at the University of Iowa and a t the Veterans Administration Hospital, both in Iowa City, evaluated 268 patients for re- sponse to TENS. These patients suf- fered from chronic intractable pain due to a variety of causes and could not ob- tain relief from other modes of therapy (Table 1). Each patient had a complete physical, neurologic, and psychometric examination. TENS and proper use of the equipment were carefully explained to the patients. Each was then given a TENS unit to use on loan a t home for a four-week period. Some patients were admitted to the hospital for two to three days for closer supervision and rein- forced instruction.

Pat ients returned for follow-up evaluation after the trial, and TENS was either discontinued or prescribed for use on a continued basis. Those pa- tients continuing TENS beyond the ini- tial four-week trial period were followed for a t least three months. Most patients have been followed for up to four years (Table 2).

Eventual response of these patients was categorized as good, partial, and failed. The good response category is

AORN Journal, September 1980, Vol32, No 3 407

Table 3

Response percentages

No Patients YO of Group ~~

Result

Good 115 4 3 O/o

~~ -.

Partial 13 5%

Failed 140 52%

Total 268 100% ~~

composed of 115 patients, 92 of whom had excellent response to TENS. These patients had good relief with continuing use of TENS or stopped TENS because the pain stopped. Patients in this group may have chosen a stimulator implant for more convenience. Twenty-three pa- tients had good pain relief but a de- creased response with continued use. They continued to use TENS regularly but were not fully satisfied and may have chosen a stimulator implant to gain a better response. The partial category was composed of 13 patients who had some relief of pain with TENS but still required the usual dosage of analgesic or were using TENS only oc- casionally with less than satisfactory relief. The failure category was com- posed of two groups. Thirty-six patients initially had some positive relief of their pain with TENS, but after a few days failed to gain any response. This group probably was composed of those re- sponding to a placebo effect of TENS. There were 104 patients who failed to gain any relief with TENS during the initial trial period. These patients may even have had aggravation of their pain by TENS (Table 3).

During the follow-up period, 48% of the group continued t o use TENS and were gaining some relief with it. This compares favorably with other similar

studies using high-frequency, low- intensity, long-term TENS.24 One hundred sixty-four patients, or 61% of the group, had initial response to TENS, but 36 patients in this group had fail- ures after a few days, reflecting a prob- able placebo effect. Including t h e placebo group, 140 patients, or 52% of the group, failed to respond to TENS.

The most common patient complaint included in this study was low-back pain, either of unknown etiology or fol- lowing lumbar surgery. Of 88 patients, 36 gained good relief, 6 gained partial relief, and 46 failed to gain relief with TENS. This reflects a similar percen- tage response compared with the entire group of patients complaining of chronic pain regardless of diagnosis. One diag- nostic category in which response was at variance with other reports of similar series was a group of 18 patients with central dysesthesia. Of this group, 12 had good response, 1 had partial re- sponse, and only 5 failed to respond to TENS. Other series report a much higher failure rate. 25 The results of this study, therefore, would be more en- couraging for the use of TENS in the treatment of various central dyses- thesia syndromes. Patients complain- ing of pain in well-healed surgical scars had a rather disappointing response to TENS, with only 9 of 27 patients gain- ing a good response and 18 failing t o respond at all to TENS.

Patients with traumatic neuropathy responded better than the group as a whole, with seven of ten patients gain- ing relief with TENS. The opportunity to stimulate the involved nerve trunk directly and apply stimulation proximal to the site of injury may allow for more effective stimulation of the painful area, resulting in the better success rate in this group of patients.

The 268 patients in this study were treated by conventional TENS a t high frequency (10 to 100 Hz) at low intensity

408 AORN Jvurrinl, September 1980, Vol.?2, No 3

with constant current or constant volt- age type outputs. One or two pairs of carbonized rubber patch electrodes re- quiring conductive gel and adhesive tape for skin application were used. Re- cent reports suggest that modification in these standard elements can signifi- cantly improve effectiveness and suc- cess of TENS in masking pain percep- tion.26

Constant energy stimulation output flows to the shortest path to give high current density in the skin layers where the neural network is richer for recep- tion of electric stimulation. Using con- stant energy output with a spike wave form and multiple electrode pairs ac- tivated in sequence has been shown to produce better relief for longer periods of time.27

Acupuncture-like TENS is a high- intensity low pulse ra te (1 t o 4Hz) stimulation that activates motor nerves to produce muscle contraction.28 Pa- tients with chronic pain tend not to tol- erate stimuli strong enough to elicit muscle twitches near the painful area. However, when short trains of repeti- tive impulses are substituted for a single repetitive impulse, i t is possible to decrease the stimulation strength to tolerable levels. 29 Muscle twitches can be maintained and analgesia gained in patients with chronic pain who failed with conventional high frequency Stimulation. This improved effect sug- gests that high threshold sensory fibers (part of which are the C fibers) must be included in the sensory stimulation (der- matomes). Muscle contraction activates muscle afferents and joint afferents (myo- tomes) to augment sensory input.30 This may be an added factor in better response to acupuncture-like TENS.

TENS at very high intensities has in- creased threshold to experimental pain perception, but moderate intensities failed to alter response to experimental painful stimuli.31 Sequential groups of

higher intensity impulses combined with acupuncture-like TENS may affect fiber threshold peripherally as well as central pain modulation in the spinal cord. Deleterious effects may occur, however, with prolonged high intensity extraneous stimulation of the nervous system .32

Recent innovations in electrode de- sign have improved comfort and conve- nience in the use of TENS. Silicone rub- ber with inherent adhesive qualities enclosing a thin, broad, malleable me- tallic electrode adheres to the skin for stimulation without the need for elec- trode gel or adhesive. Such electrodes, packed sterile, are presently used to re- lieve pain in fresh surgical wounds. They also are used for the continuous stimulation treatment of chronic pain patients.

This series of 268 patients, along with other reported series, supports the effec- tiveness of TENS in relieving pain due to a variety of causes. New technical advances, along with better under- standing of pain mechanisms, will further improve results. However, an important element for success will al- ways be continued patient support and follow-up along with reinstruction on equipment use and electrode place- ment. These instructions, along with positive encouragement offered by knowledgeable and interested health professionals, will always be an indis- pensable component of the regimen for relieving the patient's pain. 0 Notes

1 Ronald Melzack, Patrick Wall, "Pain rnecha- nisms: A new theory," Science 150 (Nov 19, 1965)

2. RoberlA Solomon, Mary C Viernstein, Donlin M Long, "Reduction of postoperative pain and nar- cotic use by transcutaneous electrical nerve stimula- tion,'' Surgery 87 (February 1980) 142.

3. J R Bloedel, D B McCreery, "Organization of peripheral and central pain pathways," Surgical Neurology 4 (July 1975) 65.

4. Stephen G Dennis, Ronald Melzack, "Pain-

971 -979).

AORN Jourrial. September 1980, Vol32, No 3 409

signalling systems in the dorsal and ventral spinal cord," Pain 4 (December 1977) 114.

5. Melzack, Wall, "Pain mechanisms," 975. 6. Bloedel, McCreery, "Organization of

peripheral and central pain pathways," 70; Dennis, Melzack, "Pain-signalling systems," 108; Frederick W L Kerr, "Pain: A central inhibitory balance theory," Mayo Clinic Proceedings 50 (December 1975) 689.

7. Kerr, "Pain: A central inhibitory balance theory," 686.

8. Bloedel, McCreery, "Organization of peri- pheral and central pain pathways," 67.

9. Dennis, Melzack, "Pain-signalling systems," 122.

10. David J Mayer, Donald D Price, "Central nerv- ous system mechanisms of analgesia," Pain 2 (De- cember 1976) 393, 395.

11. Bloedel, McCreery, "Organization of peripheral and central pain pathways," 74.

12. Ronald J Ignelzi, Judith K Nyquist, "Direct effect of electrical stimulation on peripheral nerve evoked activity: Implications in pain relief," Journal of Neurosurgery 45 (August 1976) 159-165.

13. Charles Burton, Donald Maurer, "Pain sup- pression by transcutaneous electronic stimulation," lEEE Transactions on Biomedical Engineering BME-21 (March 1974) 81.

14. Melzack, Wall, "Pain mechanisms," 971-979. 15. Burton, Maurer, "Pain suppression," 81. 16. Burton, Maurer, "Pain suppression," 81 ;

Joseph C Cauthen, Elisabeth J Renner, "Trans- cutaneous and peripheral nerve stimulation for chronic pain states," Surgical Neurology 4 (July 1975) 103.

17. C Norman Shealy, Donald Maurer, "Trans- cutaneous nerve stimulation for control of pain," Surgical Neurology 2 (January 1974) 45.

18. Burton, Maurer, "Pain suppression," 85. 19. Solomon, Viernstein. Long, "Reduction of

postoperative pain," 143. 20. John D Loeser, Richard G Black, Amalia

Christman, "Relief of pain by transcutaneous stimu- lation," Journal of Neurosurgery 42 (March 1975) 313.

21. Gary D VanderArk, Kathleen A McGrath, "Transcutaneous electrical stimulation in treatment of postoperative pain," The American Journal of Surgery 130 (September 1975) 338-340; Lars-Erik Augustinsson et al, "Pain relief during delivery by transcutaneous electrical nerve stimulation," Pain 4

22. Solomon, Viernstein, Long, "Reduction of postoperative pain," 144.

23. Margareta B E Eriksson, Bengt H Sjolund, Soren NielzBn, "Long-term results of peripheral conditioning stimulation as an analgesic measure in chronic pain," Pain 6 (June 1979) 335-347: B Sjolund, L Terenius, M Eriksson, "Increased cere-

(1 977) 59-65.

brospinal fluid levels of endorphins after electro- acupuncture," Acta Physiologica Scandinavica 100

24. Cauthen, Renner, "Transcutaneous and peripheral nerve stimulation," 102-104; Mark Linzer, Donlin M Long, "Transcutaneous neural stimulation for relief of pain," I€€€ Transactions on Biomedical Engineering BME-23 (July 1976) 341 -345; Loeser, Black, Christman, "Relief of pain by transcutaneous stimulation," 308-314; Don M Long, James N Campbell, Gunduz Gucer, "Transcutaneous electri- cal stimulation for relief of chronic pain," in Ad- vances in Pain Research and Therapy, vol 3, ed. John J Bonica et al (New York: Raven Press, 1979) 593-599.

25. Long, Campbell, Gucer, "Transcutaneous electrical stimulation," 593-599.

26. Jesmond Birkhan et al, "Modifications of TENS by constant-energy stimulation delivered through multiple electrodes: Method and evalua- tion," in Advances in Pain Research and Therapy, vol 3, ed. John J Bonica et al (New York: Raven Press, 1979) 607-613; Eriksson, Sjolund, Nielzbn, "Long-term results of peripheral conditioning stimu- lation," 335-347; Sjolund, Terenius, Eriksson, "ln- creased cerebrospinal fluid levels," 382-384.

27. Birkhan et al, "Modifications of TENS by constant-energy stimulation," 61 0.

28. Eriksson, Sjolund, NielzBn, "Long-term re- sults of peripheral conditioning stimulation," 335.

29. lbid, 336. 30. lbid, 338. 31. Clifford J Woolf, "Transcutaneous electrical

nerve stimulation and the reaction to experimental pain in human subjects," Pain 7 (1979) 115-127.

32. Robert H Pudenz, "Adverse effects of electri- cal energy applied to the nervous system," Neurosurgery 1 (SeptemberiOctober 1977) 190- 191.

(July 1977) 382-384.

410 AORN Journal, September 1980, Vol32, No 3