Pain, 46 (1991) 337-342 0 1991 Elsevier Science Publishers ADONIS 030439599100188F
337 B.V. 0304-3959/91/$03.50
PAIN 01838
The effects of auricular transcutaneous electrical nerve stimulation (TENS) on experimental pain threshold and autonomic function in healthy subjects M.I. Johnson, V.K. Hajela, C.H. Ashton and J.W. Thompson Clinical Psychopharmacology Unit, Department of Pharmacological Sciences, The Medical School, lJniuersi@ of Newcastle upon Tyne, Newcastle upon Tyne NE2 4HH (U. R) (Received
1 November
1990, revision
received
22 January
1991, accepted
23 January
1991)
The present study examines the effects of auricular transcutaneous electrical nerve stimulation Summary (TENS) on electrical pain threshold measured at the ipsilateral wrist and autonomic functions including skin temperature, blood pressure and pulse rate in 24 healthy subjects. TENS was administered as low frequency trains of pulses delivered at a ‘strong but comfortable’ intensity to 1 of 3 auricular points to be examined: (i) autonomic effects (autonomic point), (ii) pain threshold effects (wrist point), and (iii) placebo effects at an unrelated point (face point). A fourth untreated group was designated as a situation control. The main finding of the study was that auricular TENS produced no significant overall effects on experimental pain threshold or autonomic functions recorded under the present conditions. However, pain threshold was found to increase by over 50% of its pretreatment baseline in 4 subjects and by 30% in 6 subjects. This rise was not dependent upon the site of auricular TENS. The possible mechanisms of such changes are discussed. Key words: Auricular
nerve stimulation
Introduction
chronic pain [2,14,15,19] but this technique is hindered by the necessity of regularly attending a trained acupuncturist. Transcutaneous electrical nerve stimulation (TENS), a self-administered non-invasive technique, has been most successful in the control of both acute and chronic pain conditions [8,3.5,391. Recently TENS has been applied to the external ear in an attempt to relieve pain at distant sites although reports on the efficacy of such auricular TENS are conflicting [20,24]. Oliveri et al. [29] reported an elevation of electrically induced pain threshold determined at the ipsilatera1 wrist before and after ear stimulation in 45 healthy subjects. Three further reports from the same source, possibly using many of the same subjects, confirmed this increase in experimental pain threshold by both unilateral and bilateral auricular TENS [.16,27] and reported its effectiveness in reducing distal extremity pain in 15 outpatients [20]. In contrast, Melzack and Katz [24] reported that auriculotherapy by TENS did
therapy; Transcutaneous electrical Autonomic functions; Pain threshold
The Yellow Emperor’s Classic of Internal Medicine, the oldest text on acupuncture, first described the therapeutic uses of auricular acupuncture. However, it was not until the 195Os, when Nogier reported that stimulation of areas on the external ear by microcurrents could alleviate pain [26], that interest in ‘auriculotherapy’ appeared in the West, although direct experimental evidence for the existence of specific auricular points is sparse [1,28]. With the advent of solid-state electronics, attention turned to the effectiveness of electrical stimulation of the ear, primarily by electro-acupuncture, to relieve
Correspondence to: Dr. C.H. Ashton, Clinical Psychopharmacology Unit, Department of Pharmacological Sciences, The Medical School, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne NE2 4HH, U.K.
(TENS);
Acupuncture;
Analgesia;
338
not relieve chronic pain any more effectively than a placebo control in 36 patients. Melzack and Katz refuted Nogier’s claim that the auricle was an effective site of stimulation to modulate pain at distant sites and suggested instead that auriculotherapy was a ‘powerful placebo.’ Auricular stimulation has also been successfully used as a treatment for rapid detoxification of narcotic addicts [17,31,37,38]. Wen and Cheung [37] reported that 39 out of 40 narcotic-addicted patients were successfully withdrawn from opium/heroin addiction within 4-8 days. The autonomic point, which lies over an area innervated by a branch of the vagus (Fig. l), is usually the site chosen for stimulation in narcotic detoxification. To our knowledge no studies have examined the effect of auricular TENS on autonomic functions. The aim of the present study was: (i> to re-examine the ability of auricular TENS to reduce experimental pain at distant sites and (ii) to examine for the first time the effect of auricular TENS on autonomic functions in normal healthy subjects. TENS was administered for 15 min at a ‘strong but comfortable’ intensity (similar to that used by patients treating chronic pain by TENS) to 3 treatment points (Fig. 1): (8 autonomic point predicted to produce autonomic effects, (ii) wrist point predicted to affect pain threshold at the wrist, and (iii) face point as an unrelated (placebo) point. A fourth group who received no TENS was designated as a situation control.
Methods
the average taken of 3 observations [7]. Subjects were asked to state when stimuli of increasing current intensity became ‘noticeable’ (sensory detection threshold) and ‘definitely painful.’ To allow for inter-subject variability in sensory detection threshold, pain threshold was recorded in mA above sensory detection. Autonomic function
The average of 2 observations was recorded for each of the following autonomic parameters: (i) systolic and diastolic blood pressure, measured at the upper right arm (over the brachial artery) using an automatic Roche arteriosonde; (ii) skin temperature, measured on the ring finger of the left hand over the palmar surface of the distal phalanx using a temperature transducer connected to a Digitec digital thermometer; (iii) heart rate, measured using manual palpation over the left radial artery for 30 sec. Auricular TENS
The sites of auricular TENS are shown in Fig. 1 [lg]. ‘Burst’ mode TENS [13] was delivered using a Microtens (model 7757) portable battery-powered stimulator. This type of stimulation delivers low frequency (2.3 Hz) packets or trains of pulses, the frequency of pulses being within the train set to 100 Hz. TENS was administered via 2 electrodes placed on either side of the ear at the appropriate auricular site. A circular Ag/AgCl electrode (20 mm*> in contact with hypertonic saline gel (Dracard) was attached to the lateral touter) surface of the ear, and a self-adhesive carbonrubber electrode (6 mm2) was applied to the medial (underlying) surface. During treatment cycles, the current intensity of TENS was raised by the subject until it
Subjects
Twenty-four paid healthy students (age range: 17-24 years; mean + S.D.: 20.5 + 2.5; female: n = 12; male: n = 12) participated in this study. Subjects were randomly allocated 1 of 4 treatment groups: autonomic TENS, wrist TENS, face TENS or control. All subjects attended the laboratory for a brief familiarisation session prior to the experiment where the protocol was explained orally.
Wrist
Pain induction: electrical pain threshold
Electrical pain thresholds were determined for the right index finger ipsilateral to the site of auricular TENS [29]. Self-adhesive carbon-rubber electrodes (Tenzcare 3M No. 6222: 20 mm X 6 mm) were attached to the surface of the skin straddling the proximal interphalangeal joint approximately 1.5 cm apart (cathode proximal). Square-wave electrical pulses (200 psec pulse width) were delivered from an isolated Grass stimulator (Sll) through a constant current device (CUlA) at a frequency of 2 Hz. Sensory and pain thresholds were determined by the method of limits,
Autonomic Face
Fig.
1, Sketch of right ear indicating
sites of electrode
application.
339
Mean&SE change in pain threshold (during and post-TENS) from pre-treatment baseline
was verbally rated as ‘strong but comfortable.’ Appropriate adjustments were made to the intensity control dial to maintain this current intensity level during the course of the experiment. Control group subjects were informed that they would receive no stimulation, although electrodes were applied to the ear (wrist area). TENS was administered (except control group) for a total of 15 min during cycles 4, 5 and 6.
T
Procedure
Subjects sat on a bed in a relaxed upright position in a temperature controlled (21” C) room. Nine 5 min experimental cycles (3 ‘pre-‘, 3 ‘during’ and 3 ‘post’TENS) were made during which pain threshold and autonomic function were recorded in the following order: temperature, heart rate, blood pressure, sensory detection threshold (X3), pain threshold (X3), temperature, heart rate, blood pressure and a rest period lasting approximately 1.5 min before the start of the next cycle. TENS was switched on prior to the start of cycle 4 and switched off at the end of cycle 6.
O
Autonomic Fig. 3. Mean i S.E. change sensory detection threshold) ing the mean pretreatment “during” and “post TENS” ences were found between (unpaired
Wrist
Face
Control
(n = 6) in pain threshold (mA above calculated for each subject by subtractbaseline (3 cycles) from the mean of cycles (6 cycles). No significant differany active TENS groups and control Student’s r test).
Results Pain threshold
The general effects of auricular TENS irrespective of site of application are shown in Fig. 2. No significant differences (unpaired Student’s t test) were found between combined active TENS groups (i.e., the mean of autonomic TENS, wrist TENS and face TENS) and control although pain threshold was found to increase when TENS was switched on in the active TENS group. No significant effects were observed between treat-
Effects of auricular TENS on experimental pain threshold ,3 (mean+SE) 1
T
1
TENS on -10
-5
0
10 5 Time (mitts)
15
20
25
30
Fig. 2. Effects of auricular TENS on experimental pain threshold (mA above sensory detection threshold). Active TENS group is the combined mean of groups which were administered TENS (Le., autonomio, wrist and face). No significant differences in pain threshold were found using an unpaired Student’s t-test between active and control groups for each cycle.
ment groups or time within the 3 pretreatment cycles (Zway ANOVA repeated measures: groups, F,.,, = 0.52; cycles, F3,h0 = 0.06) and the data within the 3 pretreatment cycles was found to be highly correlated (Pearson product-moment correlation coefficients: pretreatment cycles: 1 vs. 2, r = +0.95; 1 vs. 3, r = + 0.96; 2 vs. 3, r = +0.98X Thus the mean of the 3 pretreatment cycles was calculated for each individual as a pretreatment baseline, and the change in pain threshold for each individual was calculated by subtracting successive treatment cycles from the pretreatment baseline. The percentage change in pain threshold (compared to pretreatment baseline) was also calculated for each individual. Fig. 3 summarizes the mean change in pain threshold (calculated from the mean of 3 ‘during’ and 3 ‘post’ TENS measurements) from pretreatment baseline for the 4 groups. All active TENS groups show an increase in mean change in pain threshold when compared to ‘pre-TENS’ baseline, although this increase did not reach significance (Fig. 3). This may be attributed to the large inter-subject variability in response to auricular TENS. Face TENS showed the largest magnitude of increase in pain threshold when compared to its own pretreatment cycles (2.4 mA). Autonomic and wrist TENS groups showed a delayed onset in elevation of pain threshold of 5-10 min after TENS ‘switch on.’ Pain threshold was found to remain relatively stable throughout the course of the experiment in the control group which received no auricular TENS treatment. Examination of the individual results revealed that 6 of the 18 subjects receiving an active TENS treatment
340
showed a 30% increase in pain threshold during stimulation (mean of 3 ‘during TENS’ cycles compared with pretreatment mean) and 4 of these subjects (1 autonomic TENS, 2 face TENS, 1 wrist TENS) exhibited over 50% increase. In only one of these subjects did pain threshold return to the original pretreatment baseline when auricular TENS was switched off. Autonomic functions Variables measured as indicators of autonomic function were subject to similar analysis as pain threshold. No significant effects were observed during or after auricular TENS when compared to pretreatment baseline in any of the autonomic variables measured. Examination of individual results also showed no effects on autonomic function during auricular TENS in any subjects.
Discussion The main finding of this study was that auricular TENS at 3 auricular sites delivered at an intensity rated as ‘strong but comfortable’ produced no significant overall effects on electrically induced pain threshold at the index finger of the ipsilateral hand. However, pain threshold was found to increase by over 30%) during TENS in one third of the subjects (6) and by over 50% in 4 subjects. This rise was not dependent upon the site of auricular TENS. Auricular TENS failed to produce significant effects on any of the measures of autonomic function recorded under the present conditions. l’ain threshold The 30% rise in pain threshold observed in 6 subjects was not dependent upon the site of auricular TENS, and the prediction that the wrist TENS group would produce the largest increase in ice pain threshold was not substantiated. It is unlikely that the rise in pain threshold seen within the autonomic TENS and face TENS groups was achieved by a non-specific stimulation of several auricular points (which are described as punctate in nature), as the autonomic and face points are some distance away from the wrist point (Fig. 1). Thus the rise in pain threshold in these subjects may have been due to a combination of several mechanisms. Subjects showing increases in pain threshold may have attained high intensity auricular TENS, activating extrasegmental mechanisms consistent with acupuncture analgesia (acupuncture-like TENS, AL-TENS) as described by Eriksson et al. [6]. This possibility is substantiated by the finding that 3 subjects achieved a prolonged ‘post-TENS’ analgesic effect which is often attributed to the release of opioid peptides [6,331. The
‘distracting’ effect of auricular TENS on the subjects’ attention to the pain stimuli must also be considered as marked reductions in the perception of painful stimuli occur during distraction [251. TENS may also produce a placebo effect which may increase the pain threshold [4,24] although, in previous work in this laboratory, we have found sham TENS to increase the pain threshold only minimally [ll]. The omission of sham TENS remains a deficiency of this study, as control subjects were aware that TENS was not switched on. In the clinic, TENS is usually applied to produce a ‘strong but comfortable’ electrical paraesthesia within the site of pain [12,33] producing segmental inhibition of nociceptive input in the spinal cord [231. TENS has also been used at sites distant from the pain but, from the few reports, intense (often painful) stimulation is necessary to achieve analgesia [21] although reports are conflicting [5,10]. Discrepancies in the effectiveness of auricular TENS may also be attributed to differences in the intensity of stimulation used. Oliveri et al. [29], Krause et al. [16], Noling et al. [27] and Longobardi et al. [20] who reported the success of auricular TENS used low frequency (1 Hz)/high intensity (on or above pain threshold) stimulation whereas Melzack and Katz [24] who reported the failure of auricular TENS used low frequency/low intensity stimulation (‘at a level which was felt but was not painful’). High intensity stimulation (which can be achieved with both acupuncture, electro-acupuncture and AL-TENS) has been shown to activate small diameter afferents, promoting the release of opioid peptides via descending inhibitory pathways [22,32,34,36]. The ineffectiveness of TENS administered at auricular acupuncture points in the present study may have been due to the use of an intensity of stimulation that was insufficient to activate such a system. Autonomic functions Most of the reports which claim efficacy of auricular stimulation in narcotic detoxification have been conducted on patient addicts as opposed to experimental subjects. In addicts undergoing drug withdrawal, the autonomic nervous system is hyperexcited with increased sympathetic tone. Thus addicts are in a highly pathological state compared with healthy subjects, and this may account for their greater response to TENS. Replenishment of endogenous opioids during auricular stimulation may account for the rapid narcotic detoxification observed during therapy [3,9,30]. The lack of changes in autonomic functioning observed in the present study may also be attributed to an intensity of stimulation insufficient to activate the release of endorphins or vagal afferents in the ear. Acupuncture or brief intense TENS may achieve that aim. In summary, the present findings support the claim of Melzack and Katz [24] that auricular TENS is inef-
341
fective in relieving pain. Nevertheless, the study does not necessarily conflict with the findings of Oliveri et al. [29], Krause et al. [16], Noling et al. [27] and Longobardi et al. [201, as their effects were achieved with high intensity auricular TENS. The lack of change in autonomic functioning suggests that such treatment would be ineffective for use in narcotic detoxification programs. However, applying auricular TENS for brief periods of time at very high intensities may prove a more successful means of treatment.
Acknowledgements
The authors wish to thank Mr. D.R. Bousfield and Mr. V.R. Marsh for technical assistance. This work was supported by a generous grant No. 419279 from the Wolfson Foundation and from Smith, Kline and French Pharmaceuticals.
16
17
18 19
References 1 Bossy, J., Golewski, Cl. and Maurel, J.C., Innervation and vascularisation of the auricula correlated with the loci of auriculotherapy, Acupunct. Electrother. Res., 2 (1977) 247-257. 2 Chun, S. and Heather, A.J., Auriculotherapy: microcurrent application on the external ear - clinical analysis of a pilot study on 57 chronic pain syndromes, Am. J. Chin. Med., 2 (1974) 399-405. 3 Clement-Jones, V.. Lowry, P.J., McLaughlin, L., Besser, G.M., Rees, L.H. and Wen, H.L., Acupuncture in heroin addicts changes in met-enkephalin and P-endorphin in the blood and cerebrospinal fluid, Lancet, ii (1979) 380-383. 4 Deyo, R.A., Walsh, N.E., Martin, D.G., Schoenfeld, L.S. and Ramamurthy, S., A controlled trial of transcutaneous electrical nerve stimulation (TENS) and exercise for chronic low back pain, New Engl. J. Med., 322, Suppl. 23 (1990) 1627-1634. A. and Hansson, P., Extrasegmental transcutaneous 5 Ekblom, electrical nerve stimulation and mechanical vibratory stimulation as compared to placebo for the relief of acute oro-facial pain, Pain, 23 (1985) 223-229. M.B.E., Sjolund, B.H. and Nielzen, S., Long term 6 Eriksson, results of peripheral conditioning stimulation as an analgesic measure in chronic pain, Pain, 6 (1979) 335-347. J.W., 7 Golding, J.F., Ashton, C.H., Marsh, V.R. and Thompson, Transcutaneous Electrical Nerve Stimulation produces variable changes in somatosensory evoked potentials, sensory perception and pain threshold: clinical implications for pain relief, J. Neurol. Neurosurg. Psychiat.. 49 (1986) 1397-1406. P. and Ekblom, A., Transcutaneous Electrical Nerve 8 Hansson, Stimulation (TENS) as compared to placebo TENS for the relief of acute oro-facial pain, Pain, 15 (1983) 157-165. Ho, W.K.K., Wen, H.L., Lam, S. and Ma, L., The influence of electroacupuncture on naloxone-induced morphine withdrawal in mice: elevation of brain opiate-like activity, Eur. J. Pharm.. 49 (1978) 197-199. Jeans, M.E., Relief of chronic pain by brief, intense electrical stimulation - a double blind study. In: J.J. Bonica et al. (Eds.). Advances in Pain Research and Therapy, Vol. 3, Raven Press, New York, 1979. pp. 601-606. Johnson. M.I., Ashton, H., Bousfield, D.R. and Thompson, J.W., Analgesic effects of different frequencies of transcutaneous elec-
20
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trical nerve stimulation on cold-induced pain in normal subjects, Pain, 39 (1989) 231-236. Johnson, MI., Ashton, C.H. and Thompson, J.W., An in depth study on the use of transcutaneous electrical nerve stimulation (TENS) by chronic pain patients. Implications for clinical use of TENS, Pain, 44 (1991) 221-229. Johnson, MI., Ashton, C.H., Bousfield, D.R. and Thompson, J.W., Analgesic effects of different pulse patterns of transcutaneous electrical nerve stimulation on cold-induced pain in normal subjects, J. Psychosom. Res., 35 (1991) 313. Kajdos, V., Experiences with electroacupuncture, Am. J. Acupunct., 4 (1976) 130-136. Kitade, T. and Hyodo, M., The effects of stimulation of ear acupuncture points on the body’s pain threshold, Am. J. Chin. Med., 7, Suppl. 3 (1979) 241-252. Krause, A.W., Clelland, J.A., Knowles, C.J. and Jackson, J.R., Effects of unilateral and bilateral auricular transcutaneous electrical nerve stimulation on cutaneous pain threshold, Phys. Ther., 67 (1987) 507-511. Kroening, R.J. and Oleson, T.D., Rapid narcotic detoxification in chronic oain uatients treated with auricular acuouncture and _ naloxone; Int. J. Addict., 20 (1985) 1347-1360. Kropej, H., The Fundamentals of Ear Acupuncture, Haug Verlag, Heidelberg, 1984. Leung, C.Y. and Spoerel, W.E., Effect of auricular acupuncture on pain, Am. J. Chin. Med., 2 (1974) 247-260. Longobardi, A.G., Clelland, J.A., Knowles, C.J. and Jackson, J.R., Effects of auricular transcutaneous electrical nerve stimulation on distal extremity pain: a pilot study, Phys. Ther., 69 (1989) 10-17. Melzack, R., Prolonged relief of pain by brief, intense transcutaneous somatic stimulation, Pain, 1 (1975) 357-373. Melzack, R., Acupuncture and related forms of folk medicine. In: R. Melzack and P.D. Wall (Eds.), Textbook of Pain, Churchill Livingstone, Edinburgh, 1984, pp. 691-700. Melzack, R. and Wall, P.D., Pain mechanisms: a new theory, Science, 150 (1965) 971-979. Melzack. R. and Katz, J., Auriculotherapy fails to relieve chronic pain, J. Am. Med. Assoc., 251 (1984) 1041-1043. Miltner, W., Johnson, R., Craun. C. and Larbig. W., Somatosensory event-related potentials to painful and non-painful stimuli: effects of attention, Pain, 38 (1989) 303-312. Nogier, P.F.M., Treatise of Auriculotherapy. Maisonneuvre, Moulins-les-Metz, 1972. Noling, L.B., Clelland, J.A., Jackson, J.R. and Knowles, C.J.. Effect of transcutaneous electrical nerve stimulation at auricular points on experimental cutaneous pain threshold, Phys. Ther., 68 (1978) 328-332. Oleson, T.D., Kroening, R.J. and Bresler, D.E., An experimental evaluation of auricular diagnosis: the somatotopic mapping of musculoskeletal pain at ear acupuncture points, Pain, 8 (1980) 217-229. Oliveri, A.C., Clelland, J.A., Jackson, J. and Knowles, C., Effects of auricular transcutaneous electrical nerve stimulation on experimental pain threshold, Phys. Ther., 66 (1986) 12-16. Pert, A., Dionne. R., Ng, L., Bragin. E.. Moody, T.W. and Pert, C.B., Alterations in rat central nervous system endorphins following transauricular electroacupuncture, Brain Res., 224 (1981) 83-93. Sainsbury, M.J., Acupuncture in heroin withdrawal, Med. J. Aust., 2 (1974) 102-105. Salar, G.. Job, I., Mingrino, S., Bosio, A. and Trabucchi, M., Effects of transcutaneous electrotherapy on CSF P-endorphin content in patients without pain problems, Pain. 10 (1981) 169172. Sjolund, B. and Eriksson, M., Relief of Pain by TENS, John Wiley, New York. 1985.
342 34 Sjiilund, B., Terenius, L. and Eriksson, M., Increased cerebrospinal fluid levels of endorphins after electroacupuncture, Acta Physiol. Stand., 100 (1977) 382-384. 35 Thompson, J.W., The role of transcutaneous electrical nerve stimulation (TENS) for the control of pain. In: D. Doyle (Ed.), International Symposium on Pain Control, Roy. Sot. Med. Services, London, 1987, 27-47. 36 Thompson, J.W., Acupuncture or TENS for pain relief? Scientific and clinical comparisons, Bull. Aust. Med. Acupunct. Sot., 7, Suppl. 1 (1988) 8-23.
37 Wen, H.L. and Cheung, S.Y.C.. Treatment of drug addiction by acupuncture and electrical stimulation, Asian J. Med., 9 (1973) 138-141. 38 Wen, H.L. and Teo, S.W., Experience in the treatment of drug addiction by electro-acupuncture, Mod. Med. Asia, 1 l(6) (1975) 23. 39 Woolf, C.J., Segmental afferent fibre-induced analgesia: transcutaneous electrical nerve stimulation (TENS) and vibration. In: R. Melzack and P.D. Wall (Eds.), Textbook of Pain, Churchill Livingstone, Edinburgh, 1989, pp. 884-896.
337 B.V. 0304-3959/91/$03.50
PAIN 01838
The effects of auricular transcutaneous electrical nerve stimulation (TENS) on experimental pain threshold and autonomic function in healthy subjects M.I. Johnson, V.K. Hajela, C.H. Ashton and J.W. Thompson Clinical Psychopharmacology Unit, Department of Pharmacological Sciences, The Medical School, lJniuersi@ of Newcastle upon Tyne, Newcastle upon Tyne NE2 4HH (U. R) (Received
1 November
1990, revision
received
22 January
1991, accepted
23 January
1991)
The present study examines the effects of auricular transcutaneous electrical nerve stimulation Summary (TENS) on electrical pain threshold measured at the ipsilateral wrist and autonomic functions including skin temperature, blood pressure and pulse rate in 24 healthy subjects. TENS was administered as low frequency trains of pulses delivered at a ‘strong but comfortable’ intensity to 1 of 3 auricular points to be examined: (i) autonomic effects (autonomic point), (ii) pain threshold effects (wrist point), and (iii) placebo effects at an unrelated point (face point). A fourth untreated group was designated as a situation control. The main finding of the study was that auricular TENS produced no significant overall effects on experimental pain threshold or autonomic functions recorded under the present conditions. However, pain threshold was found to increase by over 50% of its pretreatment baseline in 4 subjects and by 30% in 6 subjects. This rise was not dependent upon the site of auricular TENS. The possible mechanisms of such changes are discussed. Key words: Auricular
nerve stimulation
Introduction
chronic pain [2,14,15,19] but this technique is hindered by the necessity of regularly attending a trained acupuncturist. Transcutaneous electrical nerve stimulation (TENS), a self-administered non-invasive technique, has been most successful in the control of both acute and chronic pain conditions [8,3.5,391. Recently TENS has been applied to the external ear in an attempt to relieve pain at distant sites although reports on the efficacy of such auricular TENS are conflicting [20,24]. Oliveri et al. [29] reported an elevation of electrically induced pain threshold determined at the ipsilatera1 wrist before and after ear stimulation in 45 healthy subjects. Three further reports from the same source, possibly using many of the same subjects, confirmed this increase in experimental pain threshold by both unilateral and bilateral auricular TENS [.16,27] and reported its effectiveness in reducing distal extremity pain in 15 outpatients [20]. In contrast, Melzack and Katz [24] reported that auriculotherapy by TENS did
therapy; Transcutaneous electrical Autonomic functions; Pain threshold
The Yellow Emperor’s Classic of Internal Medicine, the oldest text on acupuncture, first described the therapeutic uses of auricular acupuncture. However, it was not until the 195Os, when Nogier reported that stimulation of areas on the external ear by microcurrents could alleviate pain [26], that interest in ‘auriculotherapy’ appeared in the West, although direct experimental evidence for the existence of specific auricular points is sparse [1,28]. With the advent of solid-state electronics, attention turned to the effectiveness of electrical stimulation of the ear, primarily by electro-acupuncture, to relieve
Correspondence to: Dr. C.H. Ashton, Clinical Psychopharmacology Unit, Department of Pharmacological Sciences, The Medical School, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne NE2 4HH, U.K.
(TENS);
Acupuncture;
Analgesia;
338
not relieve chronic pain any more effectively than a placebo control in 36 patients. Melzack and Katz refuted Nogier’s claim that the auricle was an effective site of stimulation to modulate pain at distant sites and suggested instead that auriculotherapy was a ‘powerful placebo.’ Auricular stimulation has also been successfully used as a treatment for rapid detoxification of narcotic addicts [17,31,37,38]. Wen and Cheung [37] reported that 39 out of 40 narcotic-addicted patients were successfully withdrawn from opium/heroin addiction within 4-8 days. The autonomic point, which lies over an area innervated by a branch of the vagus (Fig. l), is usually the site chosen for stimulation in narcotic detoxification. To our knowledge no studies have examined the effect of auricular TENS on autonomic functions. The aim of the present study was: (i> to re-examine the ability of auricular TENS to reduce experimental pain at distant sites and (ii) to examine for the first time the effect of auricular TENS on autonomic functions in normal healthy subjects. TENS was administered for 15 min at a ‘strong but comfortable’ intensity (similar to that used by patients treating chronic pain by TENS) to 3 treatment points (Fig. 1): (8 autonomic point predicted to produce autonomic effects, (ii) wrist point predicted to affect pain threshold at the wrist, and (iii) face point as an unrelated (placebo) point. A fourth group who received no TENS was designated as a situation control.
Methods
the average taken of 3 observations [7]. Subjects were asked to state when stimuli of increasing current intensity became ‘noticeable’ (sensory detection threshold) and ‘definitely painful.’ To allow for inter-subject variability in sensory detection threshold, pain threshold was recorded in mA above sensory detection. Autonomic function
The average of 2 observations was recorded for each of the following autonomic parameters: (i) systolic and diastolic blood pressure, measured at the upper right arm (over the brachial artery) using an automatic Roche arteriosonde; (ii) skin temperature, measured on the ring finger of the left hand over the palmar surface of the distal phalanx using a temperature transducer connected to a Digitec digital thermometer; (iii) heart rate, measured using manual palpation over the left radial artery for 30 sec. Auricular TENS
The sites of auricular TENS are shown in Fig. 1 [lg]. ‘Burst’ mode TENS [13] was delivered using a Microtens (model 7757) portable battery-powered stimulator. This type of stimulation delivers low frequency (2.3 Hz) packets or trains of pulses, the frequency of pulses being within the train set to 100 Hz. TENS was administered via 2 electrodes placed on either side of the ear at the appropriate auricular site. A circular Ag/AgCl electrode (20 mm*> in contact with hypertonic saline gel (Dracard) was attached to the lateral touter) surface of the ear, and a self-adhesive carbonrubber electrode (6 mm2) was applied to the medial (underlying) surface. During treatment cycles, the current intensity of TENS was raised by the subject until it
Subjects
Twenty-four paid healthy students (age range: 17-24 years; mean + S.D.: 20.5 + 2.5; female: n = 12; male: n = 12) participated in this study. Subjects were randomly allocated 1 of 4 treatment groups: autonomic TENS, wrist TENS, face TENS or control. All subjects attended the laboratory for a brief familiarisation session prior to the experiment where the protocol was explained orally.
Wrist
Pain induction: electrical pain threshold
Electrical pain thresholds were determined for the right index finger ipsilateral to the site of auricular TENS [29]. Self-adhesive carbon-rubber electrodes (Tenzcare 3M No. 6222: 20 mm X 6 mm) were attached to the surface of the skin straddling the proximal interphalangeal joint approximately 1.5 cm apart (cathode proximal). Square-wave electrical pulses (200 psec pulse width) were delivered from an isolated Grass stimulator (Sll) through a constant current device (CUlA) at a frequency of 2 Hz. Sensory and pain thresholds were determined by the method of limits,
Autonomic Face
Fig.
1, Sketch of right ear indicating
sites of electrode
application.
339
Mean&SE change in pain threshold (during and post-TENS) from pre-treatment baseline
was verbally rated as ‘strong but comfortable.’ Appropriate adjustments were made to the intensity control dial to maintain this current intensity level during the course of the experiment. Control group subjects were informed that they would receive no stimulation, although electrodes were applied to the ear (wrist area). TENS was administered (except control group) for a total of 15 min during cycles 4, 5 and 6.
T
Procedure
Subjects sat on a bed in a relaxed upright position in a temperature controlled (21” C) room. Nine 5 min experimental cycles (3 ‘pre-‘, 3 ‘during’ and 3 ‘post’TENS) were made during which pain threshold and autonomic function were recorded in the following order: temperature, heart rate, blood pressure, sensory detection threshold (X3), pain threshold (X3), temperature, heart rate, blood pressure and a rest period lasting approximately 1.5 min before the start of the next cycle. TENS was switched on prior to the start of cycle 4 and switched off at the end of cycle 6.
O
Autonomic Fig. 3. Mean i S.E. change sensory detection threshold) ing the mean pretreatment “during” and “post TENS” ences were found between (unpaired
Wrist
Face
Control
(n = 6) in pain threshold (mA above calculated for each subject by subtractbaseline (3 cycles) from the mean of cycles (6 cycles). No significant differany active TENS groups and control Student’s r test).
Results Pain threshold
The general effects of auricular TENS irrespective of site of application are shown in Fig. 2. No significant differences (unpaired Student’s t test) were found between combined active TENS groups (i.e., the mean of autonomic TENS, wrist TENS and face TENS) and control although pain threshold was found to increase when TENS was switched on in the active TENS group. No significant effects were observed between treat-
Effects of auricular TENS on experimental pain threshold ,3 (mean+SE) 1
T
1
TENS on -10
-5
0
10 5 Time (mitts)
15
20
25
30
Fig. 2. Effects of auricular TENS on experimental pain threshold (mA above sensory detection threshold). Active TENS group is the combined mean of groups which were administered TENS (Le., autonomio, wrist and face). No significant differences in pain threshold were found using an unpaired Student’s t-test between active and control groups for each cycle.
ment groups or time within the 3 pretreatment cycles (Zway ANOVA repeated measures: groups, F,.,, = 0.52; cycles, F3,h0 = 0.06) and the data within the 3 pretreatment cycles was found to be highly correlated (Pearson product-moment correlation coefficients: pretreatment cycles: 1 vs. 2, r = +0.95; 1 vs. 3, r = + 0.96; 2 vs. 3, r = +0.98X Thus the mean of the 3 pretreatment cycles was calculated for each individual as a pretreatment baseline, and the change in pain threshold for each individual was calculated by subtracting successive treatment cycles from the pretreatment baseline. The percentage change in pain threshold (compared to pretreatment baseline) was also calculated for each individual. Fig. 3 summarizes the mean change in pain threshold (calculated from the mean of 3 ‘during’ and 3 ‘post’ TENS measurements) from pretreatment baseline for the 4 groups. All active TENS groups show an increase in mean change in pain threshold when compared to ‘pre-TENS’ baseline, although this increase did not reach significance (Fig. 3). This may be attributed to the large inter-subject variability in response to auricular TENS. Face TENS showed the largest magnitude of increase in pain threshold when compared to its own pretreatment cycles (2.4 mA). Autonomic and wrist TENS groups showed a delayed onset in elevation of pain threshold of 5-10 min after TENS ‘switch on.’ Pain threshold was found to remain relatively stable throughout the course of the experiment in the control group which received no auricular TENS treatment. Examination of the individual results revealed that 6 of the 18 subjects receiving an active TENS treatment
340
showed a 30% increase in pain threshold during stimulation (mean of 3 ‘during TENS’ cycles compared with pretreatment mean) and 4 of these subjects (1 autonomic TENS, 2 face TENS, 1 wrist TENS) exhibited over 50% increase. In only one of these subjects did pain threshold return to the original pretreatment baseline when auricular TENS was switched off. Autonomic functions Variables measured as indicators of autonomic function were subject to similar analysis as pain threshold. No significant effects were observed during or after auricular TENS when compared to pretreatment baseline in any of the autonomic variables measured. Examination of individual results also showed no effects on autonomic function during auricular TENS in any subjects.
Discussion The main finding of this study was that auricular TENS at 3 auricular sites delivered at an intensity rated as ‘strong but comfortable’ produced no significant overall effects on electrically induced pain threshold at the index finger of the ipsilateral hand. However, pain threshold was found to increase by over 30%) during TENS in one third of the subjects (6) and by over 50% in 4 subjects. This rise was not dependent upon the site of auricular TENS. Auricular TENS failed to produce significant effects on any of the measures of autonomic function recorded under the present conditions. l’ain threshold The 30% rise in pain threshold observed in 6 subjects was not dependent upon the site of auricular TENS, and the prediction that the wrist TENS group would produce the largest increase in ice pain threshold was not substantiated. It is unlikely that the rise in pain threshold seen within the autonomic TENS and face TENS groups was achieved by a non-specific stimulation of several auricular points (which are described as punctate in nature), as the autonomic and face points are some distance away from the wrist point (Fig. 1). Thus the rise in pain threshold in these subjects may have been due to a combination of several mechanisms. Subjects showing increases in pain threshold may have attained high intensity auricular TENS, activating extrasegmental mechanisms consistent with acupuncture analgesia (acupuncture-like TENS, AL-TENS) as described by Eriksson et al. [6]. This possibility is substantiated by the finding that 3 subjects achieved a prolonged ‘post-TENS’ analgesic effect which is often attributed to the release of opioid peptides [6,331. The
‘distracting’ effect of auricular TENS on the subjects’ attention to the pain stimuli must also be considered as marked reductions in the perception of painful stimuli occur during distraction [251. TENS may also produce a placebo effect which may increase the pain threshold [4,24] although, in previous work in this laboratory, we have found sham TENS to increase the pain threshold only minimally [ll]. The omission of sham TENS remains a deficiency of this study, as control subjects were aware that TENS was not switched on. In the clinic, TENS is usually applied to produce a ‘strong but comfortable’ electrical paraesthesia within the site of pain [12,33] producing segmental inhibition of nociceptive input in the spinal cord [231. TENS has also been used at sites distant from the pain but, from the few reports, intense (often painful) stimulation is necessary to achieve analgesia [21] although reports are conflicting [5,10]. Discrepancies in the effectiveness of auricular TENS may also be attributed to differences in the intensity of stimulation used. Oliveri et al. [29], Krause et al. [16], Noling et al. [27] and Longobardi et al. [20] who reported the success of auricular TENS used low frequency (1 Hz)/high intensity (on or above pain threshold) stimulation whereas Melzack and Katz [24] who reported the failure of auricular TENS used low frequency/low intensity stimulation (‘at a level which was felt but was not painful’). High intensity stimulation (which can be achieved with both acupuncture, electro-acupuncture and AL-TENS) has been shown to activate small diameter afferents, promoting the release of opioid peptides via descending inhibitory pathways [22,32,34,36]. The ineffectiveness of TENS administered at auricular acupuncture points in the present study may have been due to the use of an intensity of stimulation that was insufficient to activate such a system. Autonomic functions Most of the reports which claim efficacy of auricular stimulation in narcotic detoxification have been conducted on patient addicts as opposed to experimental subjects. In addicts undergoing drug withdrawal, the autonomic nervous system is hyperexcited with increased sympathetic tone. Thus addicts are in a highly pathological state compared with healthy subjects, and this may account for their greater response to TENS. Replenishment of endogenous opioids during auricular stimulation may account for the rapid narcotic detoxification observed during therapy [3,9,30]. The lack of changes in autonomic functioning observed in the present study may also be attributed to an intensity of stimulation insufficient to activate the release of endorphins or vagal afferents in the ear. Acupuncture or brief intense TENS may achieve that aim. In summary, the present findings support the claim of Melzack and Katz [24] that auricular TENS is inef-
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fective in relieving pain. Nevertheless, the study does not necessarily conflict with the findings of Oliveri et al. [29], Krause et al. [16], Noling et al. [27] and Longobardi et al. [201, as their effects were achieved with high intensity auricular TENS. The lack of change in autonomic functioning suggests that such treatment would be ineffective for use in narcotic detoxification programs. However, applying auricular TENS for brief periods of time at very high intensities may prove a more successful means of treatment.
Acknowledgements
The authors wish to thank Mr. D.R. Bousfield and Mr. V.R. Marsh for technical assistance. This work was supported by a generous grant No. 419279 from the Wolfson Foundation and from Smith, Kline and French Pharmaceuticals.
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