Journal of
J. Neurol. 217, 1--10 (1977)
Neurology © by Springer-Verlag 1977
Original Investigations Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain* H. M. Strassburg**, J.-U. Krainick, and U. Thoden The Department of Clinical Neurology and Neurophysiology, University of Freiburg, D-7800 Freiburg i. Brsg., Federal Republic of Germany Summary. Using transcutaneous nerve stimulation (TNS) simple surgical procedures such as tooth extractions and nerve biopsies can be performed without the usual anesthetics. Estimation of threshold and suprathreshold intensities of painful electrical stimuli show no significant change during TNS. Only the threshold for nonpainful electrical stimuli is slightly increased. Cortical potentials evoked by electrical peripheral nerve stimulation are not significantly modulated by TNS. Latencies of the early components 0, I - - I I I are unchanged, the amplitudes only slightly reduced. These observations are in contradiction to the 'gate-control' theory of pain. Key words: Nerve stimulation, transcutaneous - Pain suppression by electrical stimuli - Pain - Somatosensory evoked cortical potentials - Peripheral nerve stimulation.
Zusammenfassung. Unter transkutaner Nervenstimulation (TNS) sind kleinere operative Eingriffe, wie Zahnextraktionen und Nervenbiopsien ohne weitere An~isthesie durchftihrbar. Schwellenmessungen und Intensit~itsschiitzungen tiberschwelliger elektrischer Schmerzreize zeigen jedoch unter TNS keine signifikante _Anderung der Schmerzempfindung. Lediglich nicht schmerzhafte elektrische Reize sind in der Empfindungsschwelle m~iBig angehoben. Evozierte somatosensible Hirnrindenpotentiale nach elektrischer Medianusreizung ~indern sich unter TNS-Verdeckungsreiz nicht signifikant. Die Latenzen der frtihen Komponenten 0, I - - I I I sind unver~indert, die Amplituden dieser Komponenten geringgradig reduziert. Die Befunde widersprechen der 'gate-control'-Theorie des Schmerzes. * Supported by Sonderforschungsbereich Hirnforschung und Sinnesphysiologie(SFB 70) der Deutschen Forschungsgemeinschaft (DFG) and Bundesminister ftir Arbeit und Sozialordnung, Bonn-Bad Godesberg, Federal Republic of Germany ** Corresponding author
2
H.M. Strassburg et al.
Various methods of modulating pain by electrical stimulation have been investigated clinically during the last few years [2, 10--12, 17--19, 21, 26, 29, 30, 34--36]. These methods are based on recent pain theories which are supported principally by animal experiments [1, 7--9, 15, 20, 24, 25, 28, 38]. It is generally accepted that spinal neurons of the dorsal horn activated by pain signals in A delta and C fibres are inhibited by activity in fast A fibres. This inhibitory mechanism is sustained by activity in peripheral, myelinated A fibres as well as by activity of the dorsal columns and certain subcortical brain structures via descending collaterals. With careful patient screening favorable clinical results can be obtained by electrical stimulation of either peripheral nerves or the dorsal columns in special cases of chronic pain syndromes [12]. Apart from stimulation of the dorsal columns by endodural implanted or epidural percutaneously inserted electrode systems, the so-called transcutaneous nerve stimulation has given favorable results in patients with chronic intractable pain [12, 17--19, 21, 30, 35, 36]. During transcutaneous nerve stimulation the pain area itself or the innervating nerves are stimulated by skin electrodes. The stimuli are modified rectangular pulses of 0.1--1 msec duration with frequencies between 50--100 Hz. During this stimulation paresthesias are felt in the stimulated area itself or the innervated skin area of the peripheral nerve. The intensity and frequency of the electrical stimulus can be regulated by the patient himself. In the following paper the pain modulating effect of transcutaneous nerve stimulation on acute pain, its influence on pain threshold, suprathreshold pain stimuli and on somatosensory evoked potentials are discussed. Clinical Results of TNS on Acute Pain
Stimulation was performed on 30 patients during dental treatment with two skin electrodes, one at the mandible, the other at a trigeminal exit point 1. The stimulation was started a few minutes before treatment. In all 30 cases a local anesthesia would otherwise have been necessary. In 29 of the 30 cases pain control of the trigeminal nerve was adequate during and after the dental treatment. In only one case local anesthesia was also necessary [26]. Furthermore minor operations (median nerve compression syndromes and nerve and muscle biopsies) were performed on 9 patients using stimulation with two electrodes placed further up the arm or around the operating field. All procedures were possible under stimulation except manipulation on the nerve itself. This method of anesthesia was unsuccessful in only one case of a polyneuropathy with diminished sensation of all sensory qualities (Table 1). These clinical observations can be explained by the above mentioned theories of pain, assuming that there is a segmental or supraspinal modulation of pain signals by the peripheral electrical stimulation of myelinated fibers. If a hypalgesia or hypesthesia of other sensory qualities can be induced by electrical stimulation of fast conducting afferent fibers this must be expressed subjectively device: Medtronic "Neuromod"
Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain
Table 1. Effects of transcutaneous nerve stimulation on acute pain
Dental treatment Peripheral nerve surgery (ulnar and median nerve) Biopsy of muscle and nerve
n
Successful Unsuccessful
30
29
45
3 4
as a shift of pain threshold, suprathreshold pain sensation or in somatosensory evoked potentials. To define the range of pain modulation the following investigation on pain thresholds and recordings of somatosensory evoked potentials during transcutaneous nerve stimulation were performed in a control group of normal persons.
Pain Treshold and Somatosensory Evoked Cortical Potentials (SSEP) during Transcutaneous Nerve Stimulation (TNS) Methods. The following study was performed in a control group of 10 people ranging in age from 22 to 35 years. The threshold o f perception for a barely perceptible pulsing sensation and for the pain threshold were determined during bipolar stimulation of the median nerve at the wrist (rectangular impulses of 0.2 msec, frequencies changing between 0.5 and 1.5 Hz and intensities between 0.6 and 2 millamp). Moreover the threshold for a first m o t o r twitch of the thenar muscle was defined as an objective control of electrode position. These series of measurements were repeated three times. In three persons tested suprathreshold electrical painful stimuli were estimated. The median nerve was stimulated for pain, as described by Notermans [23], blpolarly at the wrist with trains of 100 msec at 50 Hz and 2 msec rectangular impulse duration. The intensity range between pain threshold and the maximal tolerable pain sensation was divided into 9 scale-units. The intensity of a given stimulation was then estimated as a scale value between 1 and 9. The different pain stimuli were applied randomly; the pain threshold stimulus was repreated as a control before each estimation. After the control the procedure was repeated during slight and during strong TNS. Somatosensory evoked cortical potentials (SSEP) were recorded with silver chloride electrodes of 4 m 5 k ~ , which were placed over the hand area of the postcentral gyrus contralateral to the median nerve stimulated [4--6, 14, 27]. A time constant of 1 msec and a filter of 1000 Hz were used for better evaluation of the components of the potentials and to reduce noise. There were 128 responses averaged over 125 or 250 msec (Averager Nicolet Model 1072). The different latencis were determined by digital scanning of the peak amplitudes I - - I V in the terminology of Shagass and Schwartz [27] (Fig. I). As a further parameter the amplitudes of the components I - - I I I were measured as the distance between the positive and negative peaks of these potentials.
4
H.M. Strassburg et al.
Fig. 1. Somatosensory evoked potential during suprathreshold electrical stimulation of median nerve with 0.2 msec single impulses on the wrist. 4 potentials, 128 repetitions. Components O--VI in the nomenclature of Shagass and Schwartz
The electrodes for TNS were fixed on the median nerve at the coresponding elbow. The nerve was stimulated continuously during analysis with two different intensities by a transcutaneous nerve stimulator (Neuromod, Medtronic). The stimulation was defined as 'slight' if a just perceptible paresthesia was felt in the projection of the median nerve, covering thus the place of test stimulation. The stimulus was defined as 'strong' at the beginning of a contraction of the thenar muscle. The stimulus frequency given by the TNS device ranged between 70 and 100 Hz, the usual frequencies for pain treatment. The SSEPs were recorded at the threshold of perception, at the motor threshold for thenar twitch and at the pain threshold. Thereafter the TNS was switched on at intensities for slight paresthesia and the recordings of SSEP repeated; the same procedure was then performed during strong TNS.
Results
1. Estimation of Pain Threshold during TNS The stimulus intensities of the median nerve at the threshold of perception, at the first twitch of thenar muscle and at the threshold of pain sensation, are presented in Table 2 without TNS, with slight, and with strong TNS (together with the standard deviation). The intensity is indicated by numerical values on a scale from 0--10 ranging from 0.6--2milliamp. The average stimulus intensity for the first perception increases from the control value of 1.9 to 2.0 during slight and to 2.3 during strong TNS. The
Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain Tabelle 2 n
Without TNS
TNS slight
TNS strong
10 10 10
1.9 ± 0.5 3.2 ± 0.8 6.1 ± 0.8
2.0 + 0.5 3.2 ± 0.7 6.1 ± 1.2
2.3 ± 0.8 3.3 + 0.9 6.1 + 1.1
Perception threshold
n
Motor threshold
n
1.5 × motor threshold
n
Pain threshold
18 ±1.4 21.0±1.8 25.3±4.7 30.3±3.9
9 9 9 9
16.4±1.4 19.4±1.6 26.1±4.1 31.3±5.4
9 10 9 9
16.2±1.3 19.3±1.6 25.2±3.4 31.1±6.0
10 10 9 9
16.3±1.0 19.4±1.5 25.3±3.4 31.3±5.3
8 9 8 8
16.8±1.4 19.6±1.7 25.8±3.0 32.9±4.7
8 10 9 9
16.6±1.2 19.5±1.4 26.3±3.6 32.1±5.6
8 10 10 9
16.2±0.6 19.4±1.8 25.7±3.1 31.2±5.6
8
16.8 + 1.1 19.8 + 1.9 25.6 + 3.2 33.2 + 6.7
9 10 9 10
16.3 + 19.4 + 26.5+ 32.6+
8 10 10 10
16.6 + 20.1 + 26.0 + 32.5+
Perception threshold Motor threshold Pain threshold
Table 3 n WithoutTNS 0 2 I 4 II 4 III 4
Withsli~tTNS 0 I II III With strong TNS 0 I
9
II III
8 8
1.1 1.2 3.7 6.2
1.2 1.9 3.7 6.1
difference b e t w e e n the c o n t r o l a n d the value during strong T N S is o f no statistical significance ( P > 0 . 1 ) . M o r e o v e r the m o t o r a n d the pain t h r e s h o l d s were not significantly c h a n g e d d u r i n g slight a n d strong T N S .
2. Estimation of Pain Stimulus Intensity during TNS T h e r e is a n e a r l y linear increase in the e s t i m a t i o n o f scaled values o f electrical p a i n stimuli b e t w e e n p a i n t h r e s h o l d a n d the level o f intolerability. W i t h s t r o n g e r stimulus intensities the curve o f e s t i m a t e d values levels o f f slightly (Fig. 2). D u r i n g slight as well as d u r i n g strong T N S s t i m u l a t i o n n o change occurs in the c o r r e l a t i o n b e t w e e n stimulus intensity and p a i n estimation. The slight t e n d e n c y t o w a r d s l o w e r e s t i m a t i o n values is not significant.
3. Somatosensory Evoked Potentials (SSEP) during Different Stimulus Intensities F i g u r e 3 shows original registrations o f SSEPs for different stimulus intensities o f the c o n t r a l a t e r a l m e d i a n nerve w i t h o u t as well as d u r i n g slight a n d s t r o n g T N S .
6
H.M. Strassburg et al.
Stimulus intensity 10
without TNS with slight TNS
x ~. e.---..e
with strong TNS
o - . - .~,
poin-threshold
threshold ot intoLerobility
Fig. 2. Stimulus-intensity curve for suprathreshold electrical pain stimulation
1
2
3
3uV 50 msec
Fig. 3. Somatosensory evoked cortical potentials after electrical stimulation of the contralateral median nerve with 0.2msec single impulses. 128 repetitions, l s t = w i t h o u t TNS, 2 n d = T N S slight, 3 r d = T N S strong, a = a t threshold of perception, b = m o t o r threshold, c--1.5 times motor threshold, d = pain threshold
Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain wilhout INS ....
0
I
II
7
III
with strong INS
-3~tV .3~V
~ 10
20
I 30 msec
Fig. 4. Amplitudes of SSEP during TNS
The latencies of the first components 0 and I are slightly reduced with increased stimulation intensities of the median nerve. The components II and III are not significantly different in latency with increased median nerve stimulation. No significant difference is observed between these latencies and the latencies during slight and strong TNS (P always > 0.2). The first potentials show only a small variability whereas the late components especially V and VI differ in amplitude and the total form of the components as least under our conditions of recording. Therefore latencies of the late components were not measured. In Figure 4 the amplitudes of the early components (0, I, II) of SSEPs are compared. During strong TNS there is a slight tendency for reduction of the first peaks (P>0.1), which is not seen with slight TNS (see Fig. 3). The later components vary strongly and therefore were not considered in the comparison.
Discussion
The electrical stimulation of fast fiber systems for the treatment of chronic intractable neurogenic pain, based on new pain concepts [2, 10--12, 17--19, 21, 26, 29, 30, 34--36], is also useful for modulating acute pain sensations [26, 30]. Simple surgical procedures such as tooth extraction [11], decompressions of the median nerve and nerve biopsies can be performed using electrical transcutaneous nerve stimulation. These observations of pain control by electrical stimulation, as well as by acupuncture [19], are mostly explained by animal experiments. A spinal modulation of pain afferent activity in A delta and C fibers in cells of the dorsal horn as first described by Melzack and Wall in their 'gate-contror theory [20] is still seen as the primary neuronal component of pain perception [8, 25, 38]. In o r d e r to correlate clinical observations with animal experiments a change of the threshold
8
H.M. Strassburg et al.
of perception of painful stimuli or of suprathreshold pain perception should be found. But the above described threshold studies and estimations of pain intensity show n o significant change of subjective pain sensation during TNS of different intensities. Only the threshold of non-painful stimuli is slightly increased. Similar observations have also been reported by Nathan [21], who found no increase of threshold for painful heat stimuli during TNS, but, in contrast to our results, a slight increase in the pain threshold for electrical stimuli. Similar threshold studies during electrical dorsal column stimulation (DCS)-based on the same concept and showing good clinical results in chronic pain syndromes [10, 12, 13J--support the above observations with TNS. During DCS the threshold for non painful cutaneous sensations such as touch and vibration is slightly increased, whereas the threshold for painful electrical and heat stimuli was unchanged [16, 34]. Moreover changes in SSEP during DCS were reported by different authors. Grieshop et al. [7] found during thoracic DCS in monkeys a nearly total attenuation of all early components up to 200msec in the SSEP evoked by stimulation of nerves in the lower extremities. Corresponding results in man and monkeys by Larson et al. [15] show an inhibition of early components as well as recovery times up to 60 min. On the contrary Blair et al. [1] found during DCS a reduction of only the late components after 100msec up to 500msec, whereas the early components seemed to be unchanged. Only during nearly intolerable DC stimulation were all the components of SSEP completely inhibited. Our studies on evoked potentials during DCS are in contrast to these observations and show no significant changes of the SSEP [34]. The SSEPs are also only slightly modulated during TNS as described above. In particular during additional TNS there is no change in the reduced latencies brought about by increasing median nerve stimulation. The amplitudes of the components 0, I and II are only slightly and not significant diminished. The later components up to 250msec show a high variability, allowing no valid comparison. A considerably stronger reduction of early components as well as extended latencies were recorded in vibratory stimuli. These mechanisms of interaction are also essentially unknown [3, 6]. Our psychophysical results cannot be explained with the original 'gatecontrol' theory [20]. According to this hypothesis pain signals should be blocked by fast fiber input on a spinal level. An essential shift of thresholds of subjective pain sensation should be expected, but could not be confirmed in man. Assuming that the clinical observations of modulation of acute pain during operations by TNS are based on neuronal mechanisms, the following interpretation may be possible. The natural pain stimulus consists of signals of different skin receptors in a certain spatial and temporal distribution. Moreover additional central summation of longer receptor activity may be essential. An alteration of single components of the whole afferent spectrum may lead to a change of the 'pain pattern' and could result in a subjectively diminished pain sensation. In contrast to this natural pain stimulus the test pain stimulus is felt unchanged because it is better focused by synchronous stimulation of all fibers in the peripheral nerve and is expected by the test person. This interpretation seems to support the concept of pattern theory [31, 37]. The data about specific pain receptors could be consistent with the pattern
Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain
9
theory, supposing an additional m o d u l a t i n g interaction in the spinal a n d supraspinal structures. A l t h o u g h the existence o f specific pain receptors a n d pain conducting A delta and C fibers is accepted, most o f the receptors w o r k multim o d a l l y in a higher threshold range a n d are able to change their stimulus behavior [9]. A similar, mainly multimodal interaction, is shown b y neurons o f the dorsal horn, especially in lamina I a n d I V - - V I I . Here an interaction o f different receptor activity o f m e c h a n o , thermal and pain receptors can take place. Because essential studies on the interaction o f different receptor populations with the pain stimuli are at present missing in man, a n d the gap between h u m a n and animal experiments is still very large, exaggeratedly theoretical discussions should be avoided.
References 1. Blair, R. D. G., Lee, G., Vanderlinden, G,: Dorsal Column Stimulation. Its Effect in the Somatosensory Evoked Response. Arch. Neurol. 32, 826--829 (1975) 2. Campbell, J. N., Taub, A.: Local analgesia from percutaneous electrical stimulation. Arch. Neurol. 28, 347--350 (1973) 3. Delwaide, P. J., Oliver, R.: Potential evoqu~ somesthique et stimulus vibratoire cutanr. Rev. neurol. 122, 453m454 (1970) 4. Desmedt, J. E., Brunks, E., Debecker, J., Carmeliet, J.: The system bandpass required to avoid distortion of" early components when averaging somatosensory evoked potentials. EEG and clin. Neurophysiol. 37, 407--410 (1974) 5. Franzen, O., Offenloch, K.: Evoked response correlates of psychophysical magnitude estimates for tacticle stimulation in man. Exp. Brain Research $, l w l 8 (1969) 6. Giblin, D. R.: Somatosensory evoked potentials in healthy subjects and in patients with lesions of the nervous system. Ann. N.Y. Acad. Sci. 112, 93--142 (1964) 7. Grieshop, J , Goldstein, F. P , Larson, S. J.: Spinal electroanaesthesia: Its relationship to somatosensory cerebral evoked potentials. In: Etectrotherapeutic Sleep and Electroanaesthesia (F. M. Wagender, S. T. Schuy, eds.). Amsterdam: Excerpta Medica 2, 33D37 (1970) 8. Handwerker, H. O., Iggo, A., Zimmermann, M.: Segmental and supraspinal actions on dorsal horn neurons responding to noxious and non noxious stimuli. Pain 1, 147--165 (1975) 9. Kenton, B., Crue, B. L., Carregat, E. J. A.: The role of cutaneous mechanorezeptors in thermal sensation and pain. Pain 2, 139--140 (1976) 10. Krainick, J.-U., Thoden, U., Riechert, T., Tenschert, G.: Elektrische Hinterstrangreizung bei chronischen Schmerzen. Klinische Erfahrung iiber 2 Jabre. Neurochirurgia 17, 162 (1974) 11, Krainick, J.-U., Krekeler, G., Thoden, U.: Vorl~ufiger Bericht zur Schmerzunterdr~ckung im Trigeminusbereich. Zahnlirzfl. Welt 83, 703 (1974) 12. Krainick, J.-U., Thoden, U.: Electrical stimulation of the spinal cord for the relief of pain, Method--patient selection--clinical results. Adv. in Neurosurgery 4, 210---215 (1975) 13. Krainick, J.-U., Thoden, U., Riechert, T.: Spinal cord stimulation in post amputation pain. Surg. neurol. 4, 167 (1975) 14, Ktihn, K., Grtte, J., Strlzel, R., Grtze, W.: Klinlsche Anwendung somato-sensorisch evozierter kortikaler Potentiale. 1. Untersuchung mit starken nnd schwachen Reizen an Gesunden. Z. J. EEG-EMG und verwandte Gebiete 4, 8t--85 (1973) 15. Larson, S. J., Sances, A., Riegel, D. H., Meyer, G. A , Dallmann, D. E., Swiontek, T.: Neurophysiological effects of dorsal column stimulation in man and monkey. J. Neurosurg. 41, 217--223 (1974) 16, Lindblom, U., Meyerson, B. A.: Influence on touch, vibration and cutaneous pain of dorsal column stimulation in man. Pain 1,257~270 (1975) 17. Loeser, J. D., Black, R. G., Christman, A.: Relief of pain by transcutaneous stimulation. J. Neurosurg. 42, 308--314 (1975)
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18. Long, D. M., Hagfors, N.: Electrical stimulation in the nervous system: The current status of electrical stimulation of the nervous system for relief of pain. Pain 1,109--123 (1975) 19. Mann, F., Bowsher, D., Mumford, J., Lipton, S., Miles, J.: Treatment of intractable pain by acupuncture. The Lancet 1973 I, 57--60 20. Melzack, R., Wall, P. D.: Pain mechanisms: A new theory. Science 150, 971--979 (1965) 21. Nathan, P. W., Wall, P. D.: Treatment of post-herpetic neuralgia by prolonged electric stimulation. Brit. Med. J. 3,645--647 (1974) 22. Nathan, P. W., Rudge, P.: Testing the gate-control theory of pain in man. J. Neurosurg. 37, 1366--1372 (1974) 23. Notermans, S. L. H.: Measurement of the pain threshold determined by electrical stimulation and its clinical application. J. Neurology 16, 1071--1086 (1966) 24. Nyquist, J. K., Greenhoot, J. H.: Responses evoked from the thalamic centrum medianum by painful input: suppression by dorsal funiculus conditioning. Exp. Neurology 39, 215--222 (1973) 25. Price, D. D., Browe, A. C.: Spinal cord coding of graded non-noxious and noxious temperature increases. Exp. Neurol. 48, 201--221 (1975) 26. Reuter, E., Krekeler, G., Krainick, J.-U., Thoden, U., Doerr, M.: Schmerzunterdrtickung im Trigeminusbereich dutch transkutane Nervenstimulation. Dtsch. zahn~irztl. Z. 31, 274--276 (1976) 27. Shagass, C.,Schwartz, M.: Recovery functions of somatosensory peripheral nerve and cerebral evoked responses in man. EEG and clin. Neurophysiol. 17, 126--135 (1964) 28. Shealy, C. N., Taslitz, N., Mortimer, J. T., Becker, D. P.: Electrical inhibition of pain: Experimental evaluation. J. Int. Anesthesia Research Society 46, 299--304 (1967) 29. Shealy, C. N., Mortimer, J. T., Hagfors, N. R.: Dorsal column electroanalgesia. J. Neurosurg. 32, 560--564 (1970) 30. Shealy, C. N., Mauerer, D.: Transcutaneous nerve stimulation for control of pain. A preliminary technical note. Surg. Neurol. 2, 45--47 (1974) 31. Sinclair, D. C.: Cutaneous sensation and the doctrine of specific energy. Brain 78, 584 (1975) 32. Spreng, M., Ichioka, M.: Langsame Rindenpotentiale bei Schmerzreizung am Menschen. Pfliagers Archiv 279, 122--133 (1964) 33. Stevens, S. S., Carton, A. S., Shickman, G. M.: A scale of apparent intensity of electric shock. J. exper. Psychol. 56, 328 (1958) 34. Thoden, U., Krainick, J.-U., Doerr, M.: Spinal cord stimulation in chronic pain syndromes. Clinical and electrophysiological results. First World Congress on Pain. Florence Sept. 5--8, 1975 35. Thoden, U., Krainick, J.-U.: Ambulante Schmerzbehandlung durch transkutane Nervenstimulation (TNS). Dtsch. med. Wschr. 99, 1692--1693 (1974) 36. Wall, P. D., Sweet, W. H.: Temporary abolition of pain in man. Science 155, 108 (1967) 37. Weddel, G.: Somesthesis and the clinical senses. Ann. Rev. Psychol. 6, 119 (1955) 38. Zimmermann, M.: Dorsal root potentials after C-Fiber stimulation. Science 160, 896--898 (1968)
Received April 18, 1977
J. Neurol. 217, 1--10 (1977)
Neurology © by Springer-Verlag 1977
Original Investigations Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain* H. M. Strassburg**, J.-U. Krainick, and U. Thoden The Department of Clinical Neurology and Neurophysiology, University of Freiburg, D-7800 Freiburg i. Brsg., Federal Republic of Germany Summary. Using transcutaneous nerve stimulation (TNS) simple surgical procedures such as tooth extractions and nerve biopsies can be performed without the usual anesthetics. Estimation of threshold and suprathreshold intensities of painful electrical stimuli show no significant change during TNS. Only the threshold for nonpainful electrical stimuli is slightly increased. Cortical potentials evoked by electrical peripheral nerve stimulation are not significantly modulated by TNS. Latencies of the early components 0, I - - I I I are unchanged, the amplitudes only slightly reduced. These observations are in contradiction to the 'gate-control' theory of pain. Key words: Nerve stimulation, transcutaneous - Pain suppression by electrical stimuli - Pain - Somatosensory evoked cortical potentials - Peripheral nerve stimulation.
Zusammenfassung. Unter transkutaner Nervenstimulation (TNS) sind kleinere operative Eingriffe, wie Zahnextraktionen und Nervenbiopsien ohne weitere An~isthesie durchftihrbar. Schwellenmessungen und Intensit~itsschiitzungen tiberschwelliger elektrischer Schmerzreize zeigen jedoch unter TNS keine signifikante _Anderung der Schmerzempfindung. Lediglich nicht schmerzhafte elektrische Reize sind in der Empfindungsschwelle m~iBig angehoben. Evozierte somatosensible Hirnrindenpotentiale nach elektrischer Medianusreizung ~indern sich unter TNS-Verdeckungsreiz nicht signifikant. Die Latenzen der frtihen Komponenten 0, I - - I I I sind unver~indert, die Amplituden dieser Komponenten geringgradig reduziert. Die Befunde widersprechen der 'gate-control'-Theorie des Schmerzes. * Supported by Sonderforschungsbereich Hirnforschung und Sinnesphysiologie(SFB 70) der Deutschen Forschungsgemeinschaft (DFG) and Bundesminister ftir Arbeit und Sozialordnung, Bonn-Bad Godesberg, Federal Republic of Germany ** Corresponding author
2
H.M. Strassburg et al.
Various methods of modulating pain by electrical stimulation have been investigated clinically during the last few years [2, 10--12, 17--19, 21, 26, 29, 30, 34--36]. These methods are based on recent pain theories which are supported principally by animal experiments [1, 7--9, 15, 20, 24, 25, 28, 38]. It is generally accepted that spinal neurons of the dorsal horn activated by pain signals in A delta and C fibres are inhibited by activity in fast A fibres. This inhibitory mechanism is sustained by activity in peripheral, myelinated A fibres as well as by activity of the dorsal columns and certain subcortical brain structures via descending collaterals. With careful patient screening favorable clinical results can be obtained by electrical stimulation of either peripheral nerves or the dorsal columns in special cases of chronic pain syndromes [12]. Apart from stimulation of the dorsal columns by endodural implanted or epidural percutaneously inserted electrode systems, the so-called transcutaneous nerve stimulation has given favorable results in patients with chronic intractable pain [12, 17--19, 21, 30, 35, 36]. During transcutaneous nerve stimulation the pain area itself or the innervating nerves are stimulated by skin electrodes. The stimuli are modified rectangular pulses of 0.1--1 msec duration with frequencies between 50--100 Hz. During this stimulation paresthesias are felt in the stimulated area itself or the innervated skin area of the peripheral nerve. The intensity and frequency of the electrical stimulus can be regulated by the patient himself. In the following paper the pain modulating effect of transcutaneous nerve stimulation on acute pain, its influence on pain threshold, suprathreshold pain stimuli and on somatosensory evoked potentials are discussed. Clinical Results of TNS on Acute Pain
Stimulation was performed on 30 patients during dental treatment with two skin electrodes, one at the mandible, the other at a trigeminal exit point 1. The stimulation was started a few minutes before treatment. In all 30 cases a local anesthesia would otherwise have been necessary. In 29 of the 30 cases pain control of the trigeminal nerve was adequate during and after the dental treatment. In only one case local anesthesia was also necessary [26]. Furthermore minor operations (median nerve compression syndromes and nerve and muscle biopsies) were performed on 9 patients using stimulation with two electrodes placed further up the arm or around the operating field. All procedures were possible under stimulation except manipulation on the nerve itself. This method of anesthesia was unsuccessful in only one case of a polyneuropathy with diminished sensation of all sensory qualities (Table 1). These clinical observations can be explained by the above mentioned theories of pain, assuming that there is a segmental or supraspinal modulation of pain signals by the peripheral electrical stimulation of myelinated fibers. If a hypalgesia or hypesthesia of other sensory qualities can be induced by electrical stimulation of fast conducting afferent fibers this must be expressed subjectively device: Medtronic "Neuromod"
Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain
Table 1. Effects of transcutaneous nerve stimulation on acute pain
Dental treatment Peripheral nerve surgery (ulnar and median nerve) Biopsy of muscle and nerve
n
Successful Unsuccessful
30
29
45
3 4
as a shift of pain threshold, suprathreshold pain sensation or in somatosensory evoked potentials. To define the range of pain modulation the following investigation on pain thresholds and recordings of somatosensory evoked potentials during transcutaneous nerve stimulation were performed in a control group of normal persons.
Pain Treshold and Somatosensory Evoked Cortical Potentials (SSEP) during Transcutaneous Nerve Stimulation (TNS) Methods. The following study was performed in a control group of 10 people ranging in age from 22 to 35 years. The threshold o f perception for a barely perceptible pulsing sensation and for the pain threshold were determined during bipolar stimulation of the median nerve at the wrist (rectangular impulses of 0.2 msec, frequencies changing between 0.5 and 1.5 Hz and intensities between 0.6 and 2 millamp). Moreover the threshold for a first m o t o r twitch of the thenar muscle was defined as an objective control of electrode position. These series of measurements were repeated three times. In three persons tested suprathreshold electrical painful stimuli were estimated. The median nerve was stimulated for pain, as described by Notermans [23], blpolarly at the wrist with trains of 100 msec at 50 Hz and 2 msec rectangular impulse duration. The intensity range between pain threshold and the maximal tolerable pain sensation was divided into 9 scale-units. The intensity of a given stimulation was then estimated as a scale value between 1 and 9. The different pain stimuli were applied randomly; the pain threshold stimulus was repreated as a control before each estimation. After the control the procedure was repeated during slight and during strong TNS. Somatosensory evoked cortical potentials (SSEP) were recorded with silver chloride electrodes of 4 m 5 k ~ , which were placed over the hand area of the postcentral gyrus contralateral to the median nerve stimulated [4--6, 14, 27]. A time constant of 1 msec and a filter of 1000 Hz were used for better evaluation of the components of the potentials and to reduce noise. There were 128 responses averaged over 125 or 250 msec (Averager Nicolet Model 1072). The different latencis were determined by digital scanning of the peak amplitudes I - - I V in the terminology of Shagass and Schwartz [27] (Fig. I). As a further parameter the amplitudes of the components I - - I I I were measured as the distance between the positive and negative peaks of these potentials.
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Fig. 1. Somatosensory evoked potential during suprathreshold electrical stimulation of median nerve with 0.2 msec single impulses on the wrist. 4 potentials, 128 repetitions. Components O--VI in the nomenclature of Shagass and Schwartz
The electrodes for TNS were fixed on the median nerve at the coresponding elbow. The nerve was stimulated continuously during analysis with two different intensities by a transcutaneous nerve stimulator (Neuromod, Medtronic). The stimulation was defined as 'slight' if a just perceptible paresthesia was felt in the projection of the median nerve, covering thus the place of test stimulation. The stimulus was defined as 'strong' at the beginning of a contraction of the thenar muscle. The stimulus frequency given by the TNS device ranged between 70 and 100 Hz, the usual frequencies for pain treatment. The SSEPs were recorded at the threshold of perception, at the motor threshold for thenar twitch and at the pain threshold. Thereafter the TNS was switched on at intensities for slight paresthesia and the recordings of SSEP repeated; the same procedure was then performed during strong TNS.
Results
1. Estimation of Pain Threshold during TNS The stimulus intensities of the median nerve at the threshold of perception, at the first twitch of thenar muscle and at the threshold of pain sensation, are presented in Table 2 without TNS, with slight, and with strong TNS (together with the standard deviation). The intensity is indicated by numerical values on a scale from 0--10 ranging from 0.6--2milliamp. The average stimulus intensity for the first perception increases from the control value of 1.9 to 2.0 during slight and to 2.3 during strong TNS. The
Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain Tabelle 2 n
Without TNS
TNS slight
TNS strong
10 10 10
1.9 ± 0.5 3.2 ± 0.8 6.1 ± 0.8
2.0 + 0.5 3.2 ± 0.7 6.1 ± 1.2
2.3 ± 0.8 3.3 + 0.9 6.1 + 1.1
Perception threshold
n
Motor threshold
n
1.5 × motor threshold
n
Pain threshold
18 ±1.4 21.0±1.8 25.3±4.7 30.3±3.9
9 9 9 9
16.4±1.4 19.4±1.6 26.1±4.1 31.3±5.4
9 10 9 9
16.2±1.3 19.3±1.6 25.2±3.4 31.1±6.0
10 10 9 9
16.3±1.0 19.4±1.5 25.3±3.4 31.3±5.3
8 9 8 8
16.8±1.4 19.6±1.7 25.8±3.0 32.9±4.7
8 10 9 9
16.6±1.2 19.5±1.4 26.3±3.6 32.1±5.6
8 10 10 9
16.2±0.6 19.4±1.8 25.7±3.1 31.2±5.6
8
16.8 + 1.1 19.8 + 1.9 25.6 + 3.2 33.2 + 6.7
9 10 9 10
16.3 + 19.4 + 26.5+ 32.6+
8 10 10 10
16.6 + 20.1 + 26.0 + 32.5+
Perception threshold Motor threshold Pain threshold
Table 3 n WithoutTNS 0 2 I 4 II 4 III 4
Withsli~tTNS 0 I II III With strong TNS 0 I
9
II III
8 8
1.1 1.2 3.7 6.2
1.2 1.9 3.7 6.1
difference b e t w e e n the c o n t r o l a n d the value during strong T N S is o f no statistical significance ( P > 0 . 1 ) . M o r e o v e r the m o t o r a n d the pain t h r e s h o l d s were not significantly c h a n g e d d u r i n g slight a n d strong T N S .
2. Estimation of Pain Stimulus Intensity during TNS T h e r e is a n e a r l y linear increase in the e s t i m a t i o n o f scaled values o f electrical p a i n stimuli b e t w e e n p a i n t h r e s h o l d a n d the level o f intolerability. W i t h s t r o n g e r stimulus intensities the curve o f e s t i m a t e d values levels o f f slightly (Fig. 2). D u r i n g slight as well as d u r i n g strong T N S s t i m u l a t i o n n o change occurs in the c o r r e l a t i o n b e t w e e n stimulus intensity and p a i n estimation. The slight t e n d e n c y t o w a r d s l o w e r e s t i m a t i o n values is not significant.
3. Somatosensory Evoked Potentials (SSEP) during Different Stimulus Intensities F i g u r e 3 shows original registrations o f SSEPs for different stimulus intensities o f the c o n t r a l a t e r a l m e d i a n nerve w i t h o u t as well as d u r i n g slight a n d s t r o n g T N S .
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H.M. Strassburg et al.
Stimulus intensity 10
without TNS with slight TNS
x ~. e.---..e
with strong TNS
o - . - .~,
poin-threshold
threshold ot intoLerobility
Fig. 2. Stimulus-intensity curve for suprathreshold electrical pain stimulation
1
2
3
3uV 50 msec
Fig. 3. Somatosensory evoked cortical potentials after electrical stimulation of the contralateral median nerve with 0.2msec single impulses. 128 repetitions, l s t = w i t h o u t TNS, 2 n d = T N S slight, 3 r d = T N S strong, a = a t threshold of perception, b = m o t o r threshold, c--1.5 times motor threshold, d = pain threshold
Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain wilhout INS ....
0
I
II
7
III
with strong INS
-3~tV .3~V
~ 10
20
I 30 msec
Fig. 4. Amplitudes of SSEP during TNS
The latencies of the first components 0 and I are slightly reduced with increased stimulation intensities of the median nerve. The components II and III are not significantly different in latency with increased median nerve stimulation. No significant difference is observed between these latencies and the latencies during slight and strong TNS (P always > 0.2). The first potentials show only a small variability whereas the late components especially V and VI differ in amplitude and the total form of the components as least under our conditions of recording. Therefore latencies of the late components were not measured. In Figure 4 the amplitudes of the early components (0, I, II) of SSEPs are compared. During strong TNS there is a slight tendency for reduction of the first peaks (P>0.1), which is not seen with slight TNS (see Fig. 3). The later components vary strongly and therefore were not considered in the comparison.
Discussion
The electrical stimulation of fast fiber systems for the treatment of chronic intractable neurogenic pain, based on new pain concepts [2, 10--12, 17--19, 21, 26, 29, 30, 34--36], is also useful for modulating acute pain sensations [26, 30]. Simple surgical procedures such as tooth extraction [11], decompressions of the median nerve and nerve biopsies can be performed using electrical transcutaneous nerve stimulation. These observations of pain control by electrical stimulation, as well as by acupuncture [19], are mostly explained by animal experiments. A spinal modulation of pain afferent activity in A delta and C fibers in cells of the dorsal horn as first described by Melzack and Wall in their 'gate-contror theory [20] is still seen as the primary neuronal component of pain perception [8, 25, 38]. In o r d e r to correlate clinical observations with animal experiments a change of the threshold
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of perception of painful stimuli or of suprathreshold pain perception should be found. But the above described threshold studies and estimations of pain intensity show n o significant change of subjective pain sensation during TNS of different intensities. Only the threshold of non-painful stimuli is slightly increased. Similar observations have also been reported by Nathan [21], who found no increase of threshold for painful heat stimuli during TNS, but, in contrast to our results, a slight increase in the pain threshold for electrical stimuli. Similar threshold studies during electrical dorsal column stimulation (DCS)-based on the same concept and showing good clinical results in chronic pain syndromes [10, 12, 13J--support the above observations with TNS. During DCS the threshold for non painful cutaneous sensations such as touch and vibration is slightly increased, whereas the threshold for painful electrical and heat stimuli was unchanged [16, 34]. Moreover changes in SSEP during DCS were reported by different authors. Grieshop et al. [7] found during thoracic DCS in monkeys a nearly total attenuation of all early components up to 200msec in the SSEP evoked by stimulation of nerves in the lower extremities. Corresponding results in man and monkeys by Larson et al. [15] show an inhibition of early components as well as recovery times up to 60 min. On the contrary Blair et al. [1] found during DCS a reduction of only the late components after 100msec up to 500msec, whereas the early components seemed to be unchanged. Only during nearly intolerable DC stimulation were all the components of SSEP completely inhibited. Our studies on evoked potentials during DCS are in contrast to these observations and show no significant changes of the SSEP [34]. The SSEPs are also only slightly modulated during TNS as described above. In particular during additional TNS there is no change in the reduced latencies brought about by increasing median nerve stimulation. The amplitudes of the components 0, I and II are only slightly and not significant diminished. The later components up to 250msec show a high variability, allowing no valid comparison. A considerably stronger reduction of early components as well as extended latencies were recorded in vibratory stimuli. These mechanisms of interaction are also essentially unknown [3, 6]. Our psychophysical results cannot be explained with the original 'gatecontrol' theory [20]. According to this hypothesis pain signals should be blocked by fast fiber input on a spinal level. An essential shift of thresholds of subjective pain sensation should be expected, but could not be confirmed in man. Assuming that the clinical observations of modulation of acute pain during operations by TNS are based on neuronal mechanisms, the following interpretation may be possible. The natural pain stimulus consists of signals of different skin receptors in a certain spatial and temporal distribution. Moreover additional central summation of longer receptor activity may be essential. An alteration of single components of the whole afferent spectrum may lead to a change of the 'pain pattern' and could result in a subjectively diminished pain sensation. In contrast to this natural pain stimulus the test pain stimulus is felt unchanged because it is better focused by synchronous stimulation of all fibers in the peripheral nerve and is expected by the test person. This interpretation seems to support the concept of pattern theory [31, 37]. The data about specific pain receptors could be consistent with the pattern
Influence of Transcutaneous Nerve Stimulation (TNS) on Acute Pain
9
theory, supposing an additional m o d u l a t i n g interaction in the spinal a n d supraspinal structures. A l t h o u g h the existence o f specific pain receptors a n d pain conducting A delta and C fibers is accepted, most o f the receptors w o r k multim o d a l l y in a higher threshold range a n d are able to change their stimulus behavior [9]. A similar, mainly multimodal interaction, is shown b y neurons o f the dorsal horn, especially in lamina I a n d I V - - V I I . Here an interaction o f different receptor activity o f m e c h a n o , thermal and pain receptors can take place. Because essential studies on the interaction o f different receptor populations with the pain stimuli are at present missing in man, a n d the gap between h u m a n and animal experiments is still very large, exaggeratedly theoretical discussions should be avoided.
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Received April 18, 1977
