Effects of Unilateral and Bilateral Auricular Transcutaneous Electrical Nerve Stimulation on Cutaneous Pain Threshold

ANN WOODWARD KRAUSE, JO ANN CLELLAND, CHERYL J. KNOWLES, and JAMES R. JACKSON

This study compared the effects of unilateral and bilateral auricular transcuta­neous electrical nerve stimulation on cutaneous pain threshold. Auricular acu­puncture points were stimulated with low frequency, high intensity TENS for 45 seconds. Sixty healthy, adult subjects were assigned randomly to one of two treatment groups or to a control group. The two treatment groups received low frequency, high intensity TENS either unilaterally or bilaterally. The control group did not receive auricular stimulation. Experimental pain threshold at the left wrist was determined with a painful stimulus before and after auricular stimulation. Both unilateral and bilateral auricular stimulation groups exhibited a significant increase (p < .05) in experimental pain threshold, but the control group did not. The mean change values between the unilateral and bilateral stimulation groups were not statistically different. These results suggest that both unilateral and bilateral auricular TENS can increase pain threshold.

Key Words: Ear, Electric stimulation, Pain, Physical therapy.

Approaches to pain management have improved over the years, but be­cause of the serious side effects, many modern methods still are inadequate. A noninvasive and nonaddictive tech­nique to relieve pain would be a wel­come addition to current pain control methods. Transcutaneous electrical nerve stimulation is an example of a relatively new noninvasive and nonad­dictive treatment for pain management.

Low frequency, high intensity TENS applied over acupuncture points is re­ferred to as "acupuncture-like" TENS. Acupuncture points on the auricle of the ear are sometimes the sites for this

type of TENS application. Auriculother-apy in various forms has been practiced for thousands of years.1,2 Many ques­tions remain concerning the use of all types of auriculotherapy, including au­ricular TENS, for the treatment of pain.

Pain is the most common symptom causing patients to seek medical treat­ment.3 Researchers have posed many theories concerning the mechanism un­derlying pain perception and its possible management. The specificity theory proposes that a mosaic of specific pain receptors in body tissues project to a main center in the brain.3 The pattern theory is based on stimulus intensity and central summation being the critical de­terminants of pain,4 but this theory has been challenged because it does not ex­plain the existence of specific end or­gans.3 Melzack and Wall suggested that the substantia gelatinosa acts as a gate control system that modulates the syn­aptic transmission of nerve impulses from peripheral fibers to central cells.4

Transcutaneous electrical nerve stim­ulation is being used increasingly when other means do not yield pain relief. Two types of TENS, conventional (low intensity, high frequency) and acupunc­ture-like (high intensity, low frequency) have been shown to decrease pain.5"8

Acupuncture-like TENS is linked to the

release of endorphins,3,9 and the gate control theory is implicated with con­ventional TENS.4

Investigators have studied the endog­enous opiate system and its relationship to pain relief. Sjolund and Eriksson9 and Eriksson et al10 found that analgesia produced by acupuncture-like TENS was mediated by inhibitory mechanisms releasing endogenous morphine-like substances. Sjolund et al also evaluated endorphin levels in cerebrospinal fluid following electroacupuncture delivered with surface electrodes for 45 minutes.6

The CSF was collected by lumbar puncture 30 minutes after stimulation. This experiment showed a release of endorphins during electroacupuncture. Increased amounts of β-endorphins in peripheral blood during electroacupunc­ture were found by Malizia and co­workers.11 Abbate and associates inves­tigated the β-endorphin immunoreac-tivity in 12 patients undergoing thoracic surgery who were anesthetized by elec­troacupuncture and found an increase in β-endorphin levels in all of the pa­tients.12

Transcutaneous electrical nerve stim­ulation of acupuncture points on the auricle of the ear also has been used therapeutically.8,13-15 Some of the points on the ear that are suggested for stimu-

Ms. Krause is a physical therapist at Visana In-stitut, Tessinstrasse 15, Basel, Switzerland. At the time of this study, she was a master's degree candi­date, Division of Physical Therapy, School of Com­munity and Allied Health, The University of Ala­bama at Birmingham, Birmingham, AL 35294.

Ms. Celland is Associate Professor and Associate Director, Division of Physical Therapy, School of Community and Allied Health, The University of Alabama at Birmingham.

Ms. Knowles is Instructor, Division of Physical Therapy, School of Community and Allied Health, The University of Alabama at Birmingham.

Dr. Jackson is Assistant Professor of Education in Medicine, Office of Educational Development, School of Medicine, The University of Alabama at Birmingham.

This article was submitted September 9, 1985; was with the authors for revision 22 weeks; and was accepted July 1, 1986. Potential Conflict of Interest: 4.

Volume 67 / Number 4, April 1987 507

lation vary between Chinese and Euro­pean auriculotherapists. Nogier, a French neurologist, developed auricu-lotherapy and mapped auricular acu­puncture loci that correspond to a so-matotopic representation of an inverted fetus on the auricle.8 Although basically the same, the auricular acupuncture points identified by the Chinese and by Nogier vary in their somatotopic loca­tion of the lower back, leg, heart, kidney, spleen, adrenal glands, and certain parts of the nervous system.1

Oleson et al evaluated the accuracy of the somatotopic mapping of the body on the external ear, using 40 patients with musculoskeletal pain as subjects.15

A physician with no prior knowledge of the patients' medical conditions evalu­ated the ears for areas of increased con­ductivity or tenderness. The authors hy­pothesized that tender auricular points would coincide with the area of the pa­tients' pain. Concordance between the established medical diagnosis and the auricular diagnosis was 75.2%. This finding supports the hypothesis of a so­matotopic organization of the body rep­resented on the auricle.

Auriculotherapy is viewed skeptically by many practitioners of western medi­cine. Research shows both benefi­cial8,13,14,16-18 and ineffective results from the treatment.19 Auriculotherapy has been shown to be effective in treating a wide variety of disorders such as reflex sympathetic dystrophy in a child,13 an­kle inversion sprains,14 migraine head­aches,16 dental pain,17 arthritis, and scia­tica.

A question that arises in the use of auriculotherapy, including auricular TENS, concerns the appropriate ear or ears to be treated. Traditional acupunc­ture laws prescribe stimulating the ears bilaterally.16 Katide and Hyodo con­ducted a study to determine whether bilateral electroacupuncture of appro­priate auricular points increased pain threshold. Their results showed that pain threshold was increased when ears were stimulated bilaterally with elec­troacupuncture.20 In a case study on the treatment of a child with reflex sympa­thetic dystrophy, TENS was adminis­tered bilaterally to the auricles and symptoms were alleviated.13 The child was rechecked three months later and remained asymptomatic. Paris and co­workers stimulated the auricles and an­kles bilaterally for ankle inversion sprains, and the patients receiving auric­ulotherapy had decreased rehabilitation times and achieved normal range of mo­

Fig. 1. Selected auricular points for treatment groups.

tion sooner than patients not receiving auriculotherapy.14 Chun and Heather used the site of the pain as the deciding factor in which ear to treat.8 With uni­lateral pain, the ipsilateral ear was used for treatment; in bilateral involvement or a midline lesion, both ears were used for treatment. Nogier suggests treating the side according to the dominant cer­ebral hemisphere of the individual, whereas Chinese acupuncturists stimu­late the left side predominantly.16

The purpose of our study was to com­pare the effects of unilateral and bi­lateral auricular TENS (acupuncture­like) on cutaneous pain threshold. The following hypotheses were posed: 1) Changes in experimental pain thresh­old, measured at the wrist, after either a unilateral or bilateral auricular TENS treatment to appropriate auriculother­apy points will be significantly greater than changes observed in the pain threshold of a control group measured after a rest period and 2) changes in experimental pain threshold, measured at the wrist, after a bilateral auricular TENS treatment to appropriate auricu­lotherapy points will be significantly greater than changes in pain threshold after a unilateral auricular TENS treat­ment to appropriate auriculotherapy points.

METHOD

Subjects Sixty healthy, adult men and women,

participated in our study. The majority of subjects were university faculty and staff members and students. All were free from neurological abnormalities

that might have influenced their re­sponse to TENS. Because transcutane­ous electrical nerve stimulation usage is contraindicated in people who are preg­nant or use a pacemaker3, people with these conditions were excluded from our study. The subjects were naive to the procedure and the anticipated effects. Data were collected between the hours of 1 and 7 PM. The study was approved by the Institutional Review Board for Human Use at The University of Ala­bama at Birmingham.

Instrumentation

A TECA CH3 chronaxie meter* was used to determine experimental pain threshold. A pencil electrode with a di­ameter of about 2 mm was the stimulat­ing electrode, and a 10- × 8-cm moist pad electrode was the dispersive elec­trode. A Staodyn Insight† stimulator was used to deliver TENS to auricular points. This unit was equipped with a spring-loaded probe tip for stimulation and a 4- × 5-cm dispersive electrode.

Procedure

We randomly assigned the subjects to one of three groups. Group 1 subjects received unilateral TENS to four points on the auricle: wrist, shen-men, lung, and dermis (Fig. 1). The shen-men point is associated with sedative, analgesic, and tranquilizing effects20; the lung and dermis points often are associated with reducing skin hypersensitivity.14,15

* TECA Corp, 3 Campus Dr, Pleasantville, NY 10570.

† Staodynamics, Inc, PO Box 1379, Longmont, CO 80502.

508 PHYSICAL THERAPY

RESEARCH

Fig. 2. Location of the stimulating electrode for determination of experimental pain threshold.

TABLE 1 Characteristics of Total Sample (N = 57)

Characteristics

Number of female subjects Number of male subjects Number of left-handed subjects Number of right-handed subjects Mean age (yr)

Group 1 (Unilateral) (n = 20)

9 11 2

18 26.2

Group 2 (Bilateral) (n = 19)

9 10 3

16 24.0

Group 3 (Control) (n = 18)

6 12 0

18 23.8

Group 2 subjects received bilateral TENS to the same four points. Group 3 functioned as a control group and did not receive auricular TENS. The exper­imental pain thresholds of the Group 3 subjects were measured before and after a 10-minute rest period.

Subjects were positioned supine on a treatment table for the experimental pain threshold determination and auric­ular TENS treatment. Their skin at all electrode sites was wiped with cotton gauze and isopropyl alcohol to reduce skin impedance.

Threshold measurement. The investi­gator (A.W.K.) placed the dispersive electrode of the chronaxie meter behind the subject's neck, from the level of the seventh cervical vertebra to the upper thoracic vertebrae, and held the stimu­lating electrode, which was moistened with conductive gel, on the skin overlay­ing the volar surface of the distal end of the left radius, avoiding the acupuncture point LI 5 (Fig. 2). The subjects were allowed to feel the current, before meas­urement, on the right wrist to familiarize them with the sensation. The stimulus consisted of 100-Hz rectangular waves of 5-msec duration. The intensity of the stimulus was increased systematically, stopping every 0.25 mA for about one second. The subjects were asked to re­port verbally the moment they felt any electric current at the wrist and the mo­ment they experienced a painful "pin­prick" sensation. The intensity at which a painful pinprick sensation was felt was

recorded as the "pain threshold," and these threshold values were recorded three consecutive times during both the pretreatment and posttreatment ses­sions. The threshold values were aver­aged, and the mean pain threshold value for each subject was recorded.

Treatment. The dispersive electrode of the auricular TENS unit was placed in the subject's left hand. Any earrings or necklaces were removed by the sub­jects. Auricular stimulation points were located using acupuncture charts and by finding areas of decreased skin resist­ance through the aid of a visual signal from the stimulating unit. The auricular stimulus consisted of positive polarity direct current with a maximum output of 1,000 µA delivered at a frequency of 1 Hz. The width of the stimulus was constant, as preset in the unit. Each subject was asked to respond verbally the first moment the stimulus was felt by saying, "Feel it." The stimulus was increased until the subject's pain thresh­old was reached, at which time the sub­ject said, "Stop." Each auricular point was stimulated for 45 seconds, with the intensity maintained as high as the sub­jects could tolerate. Group 1 subjects received treatment to the left ear; Group 2 subjects received treatments to both ears. Group 3 subjects rested on the treatment table for 10 minutes, the max­imum time required to complete an au­ricular TENS treatment. After the auric­ular TENS treatment or rest, pain threshold was remeasured at the wrist in

all subjects. This treatment technique is similar to that used by Oliveri and co­workers.21

Data Analysis

Three subjects—two from Group 2 and one from Group 3—were excluded from the data analysis because their pain threshold values decreased more than three standard deviations from the means of their respective groups. De­scriptive statistics were calculated for the pretreatment and posttreatment meas­urements and for the changes in mean pain threshold for each group. A two-factor split-plot analysis of variance (ANOVA) was used to test the research hypotheses for statistical significance. Because both hypotheses were stated in terms of change, a split-plot design was used with an interaction effect that tested the differences among the pre­treatment and posttreatment changes in mean pain threshold for the three groups. Assuming a statistically signifi­cant interaction, the split-plot design also provides tests of simple main ef­fects, which can be used to test the dif­ferences among the groups based on their pretreatment and posttreatment means and to test the difference between the pretreatment and posttreatment means within each group. Pair-wise comparisons between groups were made with Duncan's new multiple range test.22 An alpha level of .05 was selected for all tests.

RESULTS

The group characteristics regarding sex, hand dominance, and age are shown in Table 1. Descriptive statistics for pain threshold measurements by group are shown in Table 2. Mean pain threshold values for each group at pre­treatment and posttreatment are shown in Figure 3. The ANOVA showed a statistically significant interaction be­tween group and pretreatment-post-treatment effects (Tab. 3); therefore, a statistically significant difference existed among the groups in terms of mean pretreatment and posttreatment pain threshold changes. Follow-up pair-wise comparisons using Duncan's new mul­tiple range test showed statistically sig­nificant differences (p < .05) between the mean change for Group 3 and the mean changes for Groups 1 and 2 (Tab. 2). Thus, our first hypothesis was sup­ported. Because a statistically significant difference did not exist between the

Volume 67 / Number 4, April 1987 509

TABLE 2 Mean Pain Threshold Measurements (mA)

Group

1—Unilateral Points (n = 20) Pretreatment Posttreatment Change

2—Bilateral Points (n = 19) Pretreatment Posttreatment Change

3—Control (n = 18) Pretreatment Posttreatment Change

3.92 4.40 0.47

3.87 4.54 0.67

3.37 3.44 0.06

Pain Threshold

s

1.00 1.08 0.60

1.17 1.28 0.45

0.94 0.90 0.55

Range

(2.16-6.17) (2.81-7.17)

(-0.58-1.92)

(2.16-5.83) (2.83-6.50) (0.00-1.75)

(2.00-5.00) (1.92-5.08)

(-0.50-1.67)

Treatment control unilateral bilateral

Fig. 3. Changes in experimental pain threshold for the three groups.

mean changes of Groups 1 and 2, our second hypothesis was not supported. Tests of simple main effects showed no statistically significant differences among the pretreatment means of the three groups, indicating a random as­signment (Tab. 3). The differences among posttreatment means, however,

were statistically significant (p < .05), with the control group mean differing from the means of each stimulation group. Tests of simple main effects also showed statistically significant pretreat­ment and posttreatment pain threshold changes for Groups 1 and 2, but not for Group 3 (Tab. 3).

DISCUSSION Other investigators have studied the

effects of acupuncture,20 acupressure, and auriculotherapy on experimental pain thresholds in both animals and healthy adults and have found a signifi­cant increase in pain threshold with treatment.20,23,24 This investigation also supports the hypothesis that auricular stimulation increases pain threshold.

The results of this study suggest that unilateral auricular stimulation may be as effective as bilateral auricular stimu­lation for an increase in experimental pain threshold. Katide and Hyodo stim­ulated the ears bilaterally and found pain threshold was increased.20 Chun and Heather stimulated only the ears bilaterally if the patient's pain was bilat­eral or produced by a midline lesion.8 If a patient's pain was unilateral, Chun and Heather stimulated only the ipsilat-eral ear. Sixty-four percent of their pa­tients experienced at least 50% relief of their original pain when treated by this method.

Comparisons are difficult to make be­tween our study and that of Melzack and Katz,19 which reported that auricu­lotherapy was not an effective therapeu­tic procedure for chronic pain. An im­portant factor in our study was the use of high intensity TENS. Although Mel­zack and Katz stimulated auricular acu­puncture points, the stimulation was not applied to the level of the patients' pain tolerance. Their stimulator produced a maximum current output of 200 µA, whereas the one used in our study pro­duced a maximum current output of 1,000 µA.

Further research is needed to assess the most effective sites for administering TENS. Auricular TENS often is used in conjunction with stimulation at other body sites.13,14 In the treatment of ankle inversion sprains, Paris and associates stimulated six auricular acupuncture points in addition to six ankle acupunc­ture points.14 Patients receiving this treatment had decreased rehabilitation times and achieved normal ROM sooner than those receiving only stand­ard physical therapy consisting of cryotherapy, elevation, and nonweight bearing. Leo treated a case of reflex sym­pathetic dystrophy by stimulation of six auricular acupuncture points and three body acupuncture points. The results were full passive ROM and increased active ROM of the involved extremity.13

Duration of stimulation also could be a factor in the use of auricular TENS. Oliveri et al unilaterally stimulated, for

510 PHYSICAL THERAPY

RESEARCH

TABLE 3 Analysis of Variance for Pain Threshold

Source

Group Error Group at pretreatmentb

Group at posttreatmentb

Error Pretreatment-posttreatment Error Pretreatment-posttreatment (Group 1)b

Pretreatment-posttreatment (Group 2)b

Pretreatment-posttreatment (Group 3)b

Error Interaction Error

df 2

54 2 2

108 1

54 1 1 1

54 2

54

SS

14.710 116.364

3.302 13.196

124.272 4.774 7.908 2.266 4.265 0.032 7.908 1.788 7.908

MS

7.355 2.155 1.651 6.598 1.151 4.774 0.146 2.266 4.265 0.032 0.146 0.894 0.146

F

3.413a

1.435 5.734a

32.603a

15.472a

29.121a

2.21

6.105a

90 seconds, the same four acupuncture points used in our study and found a 16.3% increase in mean pain thresh­old.21 In our study, unilateral auricular stimulation led to a 12.2% increase in mean pain threshold, and bilateral au­ricular stirhulation led to a 17.4% in­crease in mean pain threshold. The find­ing that all of these values for increases in pain threshold are in a narrow range suggests that 45 seconds of unilateral

stimulation at each point is sufficient to increase pain threshold significantly. Further studies are needed to assess whether less than 45 seconds of stimu­lation would increase pain threshold.

The increases in pain threshold achieved in our study were small, and how these increases in experimental pain threshold might compare with clin­ical pain was not determined. Studies involving patients who are experiencing

pain have yielded various re­sults.5,10,13,14,16,19 The suppression of pain sensibility in a healthy laboratory sub­ject may be a quite different phenome­non than the treatment-induced relief of patients who suffer from chronic pain.23

Increased knowledge about the most ef­fective means of pain suppression in the experimental setting, however, could be an important step toward understanding the most effective means of pain suppression in the clinical setting.

CONCLUSIONS

In two groups of healthy subjects, the use of low frequency, high intensity au­ricular TENS administered either uni­laterally or bilaterally led to small, but statistically significant, increases in pain threshold. A control group that received no auricular stimulation did not dem­onstrate an increase in pain threshold. These results suggest that unilateral au­ricular stimulation may be as effective as bilateral auricular stimulation for in­creasing experimental pain threshold. Further research about auriculotherapy and its role in pain relief with patients is needed.

REFERENCES 1. Oleson TD, Kroening RJ: A comparison of

Chinese and Nogier auricular acupuncture points. American Journal of Acupuncture 11:205-223, 1983

2. Veith I: The Yellow Emperor's Classic of Inter­nal Medicine. Berkeley, CA, University of Cali­fornia Press, 1966, p 5

3. Coffey GH, Mahon MV: Pain: Theories and a new approach to treatment. J Natl Med Assoc 74:147-153, 1982

4. Melzack R, Wall PD: Pain mechanisms: A new theory. Science 150:971-979, 1965

5. Stratton SA: Role of endorphin in pain modu­lation. Journal of Orthopaedic and Sports Physical Therapy 3:200-205, 1982

6. Sjolund BH, Terenius L, Eriksson MB: In­creased CSF levels of endogenous morphine after electroacupuncture. Acta Physiol Scand 100:382-384,1977

7. Anderson SA, Hansson G, Holmgren E, et al: Evaluation of pain suppressive effect of differ­ent frequencies of peripheral electrical stimu­lation in chronic pain conditions. Acta Orthop Scand 47:149-157, 1976

8. Chun S, Heather AJ: Auriculotherapy: Micro­current application on the external ear—Clini­cal analysis of a pilot study of 57 chronic pain syndromes. Am J Chin Med 2:399-405, 1974

9. Sjolund BH, Eriksson MB: Electroacupuncture and endogenous morphines. (Letter to the ed­itor.) Lancet 2:1085, 1976

10. Eriksson MB, Sjolund BH, Nielsen S: Long-term results of peripheral conditioning stimu­lation as an analgesic measure for chronic pain. Pain 6:335-347, 1979

11. Malizia E, Andreucci G, Paolucci D, et al: Elec­troacupuncture and peripheral beta-endorphin and ACTH levels. Lancet 2:535-536, 1979

12. Abbate D, Santamaria A, Brambilla A, et al: β-Endorphin and electroacupuncture. Lancet 2:1309, 1980

13. Leo KC: Use of electrical stimulation at acu­puncture points for the treatment of reflex sym­pathetic dystrophy in a child: A case report. Phys Ther 63:957-959, 1983

14. Paris DL, Baynes F, Gucker B: Effects of the neuroprobe in the treatment of second-degree ankle inversion sprains. Phys Ther 63:35-40, 1983

15. Oleson TD, Kroening RJ, Bresler DE: An ex­perimental evaluation of auricular diagnosis: The somatotopic mapping of musculoskeletal pain at ear acupuncture points. Pain 8:217-229, 1980

16. Kajdos V: Experiences with auricular acupunc­ture. American Journal of Acupuncture 4:130-136, 1976

17. Brandiem A, Corlos J: Cutaneous and trans­cutaneous electroacupuncture. American Jour­nal of Acupuncture 4:161-164, 1976

18. Leung CY, Spoerel WE: Effect of auriculoacu-puncture on pain. Am J Chin Med 2:247-260, 1974

19. Melzack R, Katz J: Auriculotherapy fails to relieve chronic pain: A controlled crossover study. JAMA 251:1041-1043, 1984

20. Katide T, Hyodo M: The effects of stimulation of ear acupuncture on the body's pain thresh­old. Am J Chin Med 7:241-252, 1979

21. Oliveri AC, Clelland JA, Jackson J, et al: Effects of auricular transcutaneous electrical nerve stimulation on experimental pain threshold. Phys Ther 66:12-16, 1986

22. Kirk RE: Experimental Design: Procedures for the Behavioral Sciences. Belmont, CA, Wads-worth Publishing Co, 1968, pp 93-94

23. Chapman CR, Benedetti C, Colpitts YH, et al: Naloxone fails to reverse pain thresholds ele­vated by acupuncture: Acupuncture analgesia reconsidered. Pain 16:13-31, 1983

24. Ha H, Wu RS, Contreras RA, et al: Measure­ment of pain threshold by electrical stimulation of tooth pulp afferents in the monkey. Exp Neurol 61:260-269, 1978

a p < .05. b Test of simple main effect.

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