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Changes in burning pain threshold induced by acupuncture in man
Brain Research, 104 (1976) 335-340
335
© Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands
Changes in burning pain threshold induced by acupuncture in man
S. CROZE, C. ANTONIETTI AND R. DUCLAUX Universitd Claude Bernard, Facultd de Mddecine, Lyon Sud-Ouest, et Laboratoire de Physiologie, B.P. 12, 69600 Oullins (France)
(Accepted December 1st, 1975)
Since 1958 (ref. 12) acupuncture, a Chinese technique of healing several thousand years old, has also been used to produce analgesia in surgery. Although acupuncture is commonly employed in China 12, the mechanism of its analgesic action is still unknown; it may not be due to acupuncture itself but to the psychoprophylactic preparation of the patients 13 which usually precedes the operation. One can formulate three hypotheses to explain the mechanism of analgesia induced by acupuncture. (a) Analgesic action could be due to the stimulation of nerve terminations by the needles inserted into specific cutaneous loci. As a matter of fact, in man 1 and monkey 2° objective studies have shown a hypoalgesic effect during electrical acupuncture of loci described in Chinese reports as clinically effective. (b) It could be explained by the misdirection of attention by the conscious sensation due to the needles. The fact is that cutaneous pain perception can be changed by various stimulations, painfully felt2,9,11 or not 7,8,11,15. (c) Finally, suggestion could be the essential factor of hypoalgesia, since it may have a powerful influence on the intensity of perceived pain 14. In order to test the validity of these hypotheses, the burning pain threshold in man has been measured under acupuncture of various loci, known to be clinically effective or not, and under 'white noise' used as a diverting stimulus. The belief of the subject in the therapeutic value of acupuncture has been evaluated by a sample survey. The burning pain threshold was measured with a dolorimeter constructed in the laboratory on the principle described by Hardy et aL to. Tbe pain stimulus was produced by an infrared beam emitted from a 100 W bulb, focussed on a hole of 4.5 mm diameter in the front face of the apparatus. The subject placed his thenar eminence against this hole. The skin was blackened with Indian ink before the experiment, to decrease its reflectivity. The hand skin temperature was measured continually with a copper-constantan thermocouple. Stimulation was started and stopped by the subject himself. He was instructed to stop the stimulus when he felt the very first painful sensation. The duration of the stimulus was measured precisely using an electronic timer coupled with the stimulator. This duration allowed measurement of the pain perception threshold. Eight voluntary healthy subjects, 4 men and 4 women, 23-30 years old, were involved in this experiment.
336
Fig. 1. Location of the three different needling spots. A : traditional acupunture points. • : adjoining spots. A: distant spots. The arrow points to the thenar eminence to which radiant heat is applied.
They had previously been trained to recognize their pain threshold every 2 min, 20 min each day, over 2 or 3 weeks. The experiment started when the pain threshold remained at the same level from day to day. For each subject, the experiment was divided into 5 different trials. Each trial was undertaken on different days at 1 p.m. The order of these trials was drawn for. The trial itself consisted of the measurement of the pain threshold during 3 consecutive periods: a control period of 20 min, an analgesic period of 40 min, a control period of 40 min. As indicated below, the analgesic procedure used in the second period differentiated each trial from the others. Trial 1: control trial during which no analgesic procedure was applied to the subjects. Trial 2: white sound listening (waterfall recording) of 100 dB. Trial 3: acupuncture needling on two contralateral Chinese spots (Fig. 1): Shou San Li and Z~ San Li (ref. 18), one 7 cm below the external extremity of the bend of the elbow and anotber 4 cm under the anterior tibialis tuberosity. Needling of these spots is prescribed for aching in both hands. Trial 4: acupuncture on spots a p a r t from the previous ones. At the arm the point was situated 1 cm externally, at the leg it was situated 1 cm internally. Both points were away from any Chinese meridian. In classical Chinese theory, meridians are defined as subcutaneous channels, each canalizing the 'energy' emitted by an organ. Acupuncture loci are situated along these meridians. Trial 5: acupuncture on contralateral spots away from any meridian and at a distance from the spots needled in trial 2. One is on the abdomen, 3 cm laterally at the right of the navel, and the other 5 cm above the right side of the patella (Fig. 1). Acupuncture was done with sterile flexible stainless steel needles (0.3 m m diam-
337 +4' v
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.....
~
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•
•
'
-.-
-
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TIME
(m~n)
Fig. 2. Evolution of the pain threshold over 100 min. On the ordinate the time of the stimulation is the difference between the mean duration of the stimulus and the mean duration of the first 10 stimulations, set at zero. Threshold values are therefore compared with the first control period (0-20 rain). - - : control trial on 8 subjects. The grey area represents 95 % confidence interval for the threshold mean value calculated every 2 rain. - . . . . . . . . : between the 20th and 60th min, the 8 subjects heard a white noise. No significant effect is observed.
eter a n d 4 c m in length). These were inserted vertically to a d e p t h r a n g i n g f r o m 0.5 to 2 c m d e p e n d i n g on the thickness o f the s u b c u t a n e o u s a d i p o s e tissue. T h e insertion t o o k place at the b e g i n n i n g o f the second p e r i o d o f the trials. Needles were m a n i p u l a t e d at a frequency o f a b o u t 2/sec b y r o t a t i o n a n d b y push a n d pull m o v e m e n t s , every 2 min d u r i n g the whole d u r a t i o n o f each m e a s u r e m e n t o f the p a i n threshold, i.e. f r o m 3 to 10 sec. In o r d e r to test the r e p r o d u c i b i l i t y o f the results, 3 subjects p a r t i c i p a t e d 3 m o n t h s later in a second series o f the trials 3 a n d 5. D u r i n g the 100 min o f the c o n t r o l trial the p a i n threshold r e m a i n e d a t the s a m e level in the 8 subjects (Fig. 2); the h e a r i n g o f waterfall sounds d i d n o t m o d i f y the p a i n t h r e s h o l d as is shown on the s a m e figure. T h e v a r i a t i o n o f the p a i n t h r e s h o l d d u r i n g a c u p u n c t u r e allowed distinction o f two p a t t e r n s o f response. (1) 4 subjects (Fig. 3) did n o t change their p a i n t h r e s h o l d , whichever spots were needled a n d (2) in the 4 o t h e r subjects, on the c o n t r a r y , the ACUPUNCTURE +4 03
I..-
m
0
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i
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Fig. 3. Mean pain threshold of 4 subjects during acupuncture compared with the control threshold values in grey. The ordinate is identical to that in Fig. 3. - traditional acupuncture points. . . . . . . . . . : adjoining spots. : distant spots. In these subjects acupuncture has no effect.
338 ACUPUNCTURE ~+4
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Fig. 4. M e a n pain threshold o f 4 subjects during a c u p u n c t u r e c o m p a r e d with the control values in grey. T h e ordinate is identical to that in Fig. 3. - : traditional a c u p u n c t u r e points. - . . . . . . . . . : adjoining spots. : distant spots. In these subjects, a c u p u n c t u r e in Chinese a n d adjoining spots increases the pain threshold.
burning pain threshold was raised when Chinese spots and neighbouring ones were needled. When the Chinese spots were stimulated (Fig. 4) the hypoalgesia was more pronounced and persisted after the needles were removed. During the increase of the pain threshold, the 4 subjects described a change in their painful sensation. Without acupuncture it was identified as a sharp pricking. Under effective acupuncture, i, was preceded by a plateau of gentle heat, and the pricking was more diffuse. This modification persisted as long as the threshold remained high. The puncture of distant spots (trial 5) did not change the pain threshold significantly in 3 of these subjects. A small increase was, however, observed in the fourth one (Fig. 4). To test reproducibility of the results, 3 subjects participated in a second selies of experiments 3 months later. In two of them, acupuncture again proved its efficacy. In the third subject, this technique remained ineffective. Two examples of these results are shown in Fig. 5.
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Fig. 5. E v o l u t i o n o f the pain threshold in t w o subjects during acupuncture in the Chinese spots,
s h o w i n g reproducibility o f the results. Subject 1 : - - - - , M a r c h 4, 1975 a n d 1975. Subject 2: - - - - - January, 14, 1975 a n d . . . . . . . . . . : April, 15, 1975.
J u n e 24,
339 The sample survey showed that 7 subjects believed in the virtue of acupuncture and that they would accept acupuncture as a therapy. Only 4 of them increased their pain threshold. In 4 subjects, acupuncture increased the time necessary to raise the skin temperature of the thenar eminence from its initial level up to the pain threshold. This increase was not due to a drop in the adapting cutaneous temperature of the hand since its value remained constant during acupuncture. Therefore, any modification in the duration of the heating represents an actual variation of the pain threshold. In half of the subjects tested acupuncture increased the burning pain threshold. However, effective acupuncture did not produce analgesia but hypoalgesia since none of the subjects allowed himself to be burnt. The mechanism of this hypoalgesia is not a diversion of attention since the 3 different needling locations, which have certainly the same distracting power, did not show the same efficacy. Moreover, acupuncture does not need to be painful to possess hypoalgesic property, since in the present experiment no correlation has been found between the painful character of needle stimulation and its efficacy. The distraction induced by white noise failed to modify the pain threshold. This result agrees with the observations of Camp et aL 3. Some authors 6,1a assume that the suggestion that acupuncture is good is determinant in the induction of acupuncture hypoalgesia. Although the subjects had no indication as to the nature of the expected results, suggestion may have played some part because the placing and the manipulation of the needles was performed by a professional acupuncturist who of course knew which sites could be effective. He may have conveyed his belief to the subjects in some unknown fashion, although all precautions have been taken for avoiding this. According to the survey responses, faith in the virtue of acupuncture seems to be a necessary condition if one is to observe an increase in the threshold during acupuncture. However, it is obvious that this condition is not sufficient since 3 of the subjects who believed in the efficacy of the method did not show any hypoalgesia. Besides, if suggestion was determinant, each needling location should result in the same hypoalgesic action. As suggestion is not sufficient, it is necessary to assume that the acupunture action is specific of the spots stimulated by the needles. Acupuncture analgesia may be conceived as a case of hyperstimulation analgesia 14. The information from the punctured spots to the central nervous system, responsible for the decrease in the conscious sensation of pain, is transmitted by afferent nerves since the hypoalgesic effect can be completely abolished by infiltration of procaine in the deep tissues around the spot T M and is not present when the affected limb of hemiplegic or paralytic patients 16 is needled. This information coming from acupunctured spots may inhibit impulse transmission in pain pathways. This possible mechanism has been demonstrated in animals4,17,19, zl at various levels of the central nervous system. Under the conditions of our experiments, this inhibition could take place in the dorsal horn of the spinal cord since the Shou San Li spot is innervated by the radial nerve, which enters the cord at the level of C6, as does the cutaneous palmal branch of the median nerve which innervates the thenar eminence. In this hypothesis, the absence of an effect of acupuncture should be due to anatomic variations since the conditions of acupuncture were constant in all subjects. The 4 subjects could have been needled out-
340
side a specific area centred on the Chinese spot. It is also possible that the suggestion may have been less powerful in these subjects. One of them indeed, did not believe that acupuncture could produce any effect. In some patients anaesthesia by drugs may be dangerous and the use of acupuncture could then be prescribed. A rapid measurement of the pain threshold during acupuncture before surgery could permit the selection of subjects who could benefit by acupuncture anaesthesia. This research was supported by the Centre National dc ta Recherche Scientifique (C.N.R.S.) and the Institut National de la Sant6 et de la Recherche M6dicale (I.N.S.E.R.M.).
1 ANDERSSON,S. A., ERICSON,T., HOLMGREN,E., AND LINDOUIST,G., Electro-acupuncture: effect on
pain threshold measured with electrical stimulation of teeth, Brain Research, 63 (1973) 393-396. 2 BERLIN, L., GOODELL, H., AND WOLFF, H. G., Studies on pain. Relation of pain perception and central inhibitory effect of noxious stimulation to phenomenon of extinction, Arch. Neurol. Psychiat. (Chic.), 80 (1958) 533-543. 3 CAMP, W., MARTIN,R., AND CHAPMAN,L. F., Pain threshold and discrimination of pain intensity during exposure to intense noise, Science, 135 (1962) 788. 4 CHANGHSlANG-TUNG,Integrative action of thalamus in the process of acupuncture for analgesia, Scientia siniea, 16 (1973) 25-59. 5 CHIANG CHEN-Yu, CHANG CHING-TSAI, CHU HSIU-LING, AND YANG LIAN-FANG, Peripheral afferent pathway for acupuncture analgesia, Scientia Sinica, 16 (1973) 210-217. 6 CRAWFORD,CLARK,W., AND YANG, J. C., Acupunctural analgesia? Evaluation by signal detection theory, Science, 184 (1974) 1096-1098. 7 ELLIS, M., The relief of pain by cooling of the skin, Brit. med. J., (1961) 250-252. 8 GAMMON,G. D., AND STARR, 1., Studies on the relief of pain by counter-irritation, J. clin. Invest., 20 (1941) 13-20. 9 GUTTMAN,E., AND MAYER-GRoss, W., The psychology of pain, Lancet, 244 (1943) 225-227. 10 HARDY, J. D., GOODELL,H., AND WOLFF, H. G., Pain Sensations and Reactions, The Williams and Wilkins Co., Baltimore, Md., 1952, 435 pp. 11 HARDY, J. D., WOLFF, H. G., AND GOODELL, H., Studies on pain. A new method for measuring pain threshold: observations on the spatial summation of pain, J.clin. Invest., 19 (1940) 649-657. 12 KAADA,B., HOEL, E., LESETH,K., NYGAARD-OSTBY,B., SETEKLEIV,J., AND STOVNER,J., Acupuncture analgesia in the People's Republic of China, T. norske Laegeforen, 94 (1974) 417-442. 13 KROGER, W. S., Acupunctural analgesia: its explanation by conditioning theory, autogenic training and hypnosis, Amer. J. Psychiat., 130 (1973) 855-860. 14 MELZACK,R., The Puzzle of Pain, Penguin Books Ltd., Hardmondsworth, 1973, 189 pp. 15 NATHAN, P. W., Improvement in cutaneous sensibility associated with relief of pain, J. Neurol. Neurosurg. Psychiat., 23 (1960) 202-206. 16 PEKINGACUPUNCTUREANAESTHESIACOORDINATINGGROUP, Preliminary study on the mechanism of acupuncture anaesthesia, Scientia sinica, 16 (1971) 447~156. 17 ROHNER, J. J., ET PLANCHE, D., Modification des potentiels 6voqu6s associatifs par stimulation des points d'acupuncture chez le Chat, C.R. Soc. Biol. (Paris), 167 (1973) 1180-1185. 18 RUmN, M., Manuel d'Acupuncture Fondamentale, Mercure de France, Paris, 1974, 364 pp. 19 SHANGHAIINSTITUTEOF PHYSIOLOGY,Electrical response to nocuous stimulation and its inhibition in nucleus centralis lateralis of thalamus in rabbits, Chin. med. J., 3 (1973) 131-135. 20 VIERCK, C. J., LINEBERRY,C. G., LEE, P. K., AND CALDERWOOD,H. W., Prolonged hypalgesia following 'acupuncture' in monkeys, Life Sci., 15 (1974) t277-1290. 21 Wu CHIEN-PING,CHAP CHIH-CHI, AND WEI JEN-Y~3, Inhibitory effect produced by stimulation of afferent nerves on responses of cat dorsolateral fasciculus fibers to nocuous stimulus, Scientia siniea, 17 (1974) 688-697.
335
© Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands
Changes in burning pain threshold induced by acupuncture in man
S. CROZE, C. ANTONIETTI AND R. DUCLAUX Universitd Claude Bernard, Facultd de Mddecine, Lyon Sud-Ouest, et Laboratoire de Physiologie, B.P. 12, 69600 Oullins (France)
(Accepted December 1st, 1975)
Since 1958 (ref. 12) acupuncture, a Chinese technique of healing several thousand years old, has also been used to produce analgesia in surgery. Although acupuncture is commonly employed in China 12, the mechanism of its analgesic action is still unknown; it may not be due to acupuncture itself but to the psychoprophylactic preparation of the patients 13 which usually precedes the operation. One can formulate three hypotheses to explain the mechanism of analgesia induced by acupuncture. (a) Analgesic action could be due to the stimulation of nerve terminations by the needles inserted into specific cutaneous loci. As a matter of fact, in man 1 and monkey 2° objective studies have shown a hypoalgesic effect during electrical acupuncture of loci described in Chinese reports as clinically effective. (b) It could be explained by the misdirection of attention by the conscious sensation due to the needles. The fact is that cutaneous pain perception can be changed by various stimulations, painfully felt2,9,11 or not 7,8,11,15. (c) Finally, suggestion could be the essential factor of hypoalgesia, since it may have a powerful influence on the intensity of perceived pain 14. In order to test the validity of these hypotheses, the burning pain threshold in man has been measured under acupuncture of various loci, known to be clinically effective or not, and under 'white noise' used as a diverting stimulus. The belief of the subject in the therapeutic value of acupuncture has been evaluated by a sample survey. The burning pain threshold was measured with a dolorimeter constructed in the laboratory on the principle described by Hardy et aL to. Tbe pain stimulus was produced by an infrared beam emitted from a 100 W bulb, focussed on a hole of 4.5 mm diameter in the front face of the apparatus. The subject placed his thenar eminence against this hole. The skin was blackened with Indian ink before the experiment, to decrease its reflectivity. The hand skin temperature was measured continually with a copper-constantan thermocouple. Stimulation was started and stopped by the subject himself. He was instructed to stop the stimulus when he felt the very first painful sensation. The duration of the stimulus was measured precisely using an electronic timer coupled with the stimulator. This duration allowed measurement of the pain perception threshold. Eight voluntary healthy subjects, 4 men and 4 women, 23-30 years old, were involved in this experiment.
336
Fig. 1. Location of the three different needling spots. A : traditional acupunture points. • : adjoining spots. A: distant spots. The arrow points to the thenar eminence to which radiant heat is applied.
They had previously been trained to recognize their pain threshold every 2 min, 20 min each day, over 2 or 3 weeks. The experiment started when the pain threshold remained at the same level from day to day. For each subject, the experiment was divided into 5 different trials. Each trial was undertaken on different days at 1 p.m. The order of these trials was drawn for. The trial itself consisted of the measurement of the pain threshold during 3 consecutive periods: a control period of 20 min, an analgesic period of 40 min, a control period of 40 min. As indicated below, the analgesic procedure used in the second period differentiated each trial from the others. Trial 1: control trial during which no analgesic procedure was applied to the subjects. Trial 2: white sound listening (waterfall recording) of 100 dB. Trial 3: acupuncture needling on two contralateral Chinese spots (Fig. 1): Shou San Li and Z~ San Li (ref. 18), one 7 cm below the external extremity of the bend of the elbow and anotber 4 cm under the anterior tibialis tuberosity. Needling of these spots is prescribed for aching in both hands. Trial 4: acupuncture on spots a p a r t from the previous ones. At the arm the point was situated 1 cm externally, at the leg it was situated 1 cm internally. Both points were away from any Chinese meridian. In classical Chinese theory, meridians are defined as subcutaneous channels, each canalizing the 'energy' emitted by an organ. Acupuncture loci are situated along these meridians. Trial 5: acupuncture on contralateral spots away from any meridian and at a distance from the spots needled in trial 2. One is on the abdomen, 3 cm laterally at the right of the navel, and the other 5 cm above the right side of the patella (Fig. 1). Acupuncture was done with sterile flexible stainless steel needles (0.3 m m diam-
337 +4' v
+3
go ~-~
.....
~
'~
~:5"~
•
•
'
-.-
-
ku
TIME
(m~n)
Fig. 2. Evolution of the pain threshold over 100 min. On the ordinate the time of the stimulation is the difference between the mean duration of the stimulus and the mean duration of the first 10 stimulations, set at zero. Threshold values are therefore compared with the first control period (0-20 rain). - - : control trial on 8 subjects. The grey area represents 95 % confidence interval for the threshold mean value calculated every 2 rain. - . . . . . . . . : between the 20th and 60th min, the 8 subjects heard a white noise. No significant effect is observed.
eter a n d 4 c m in length). These were inserted vertically to a d e p t h r a n g i n g f r o m 0.5 to 2 c m d e p e n d i n g on the thickness o f the s u b c u t a n e o u s a d i p o s e tissue. T h e insertion t o o k place at the b e g i n n i n g o f the second p e r i o d o f the trials. Needles were m a n i p u l a t e d at a frequency o f a b o u t 2/sec b y r o t a t i o n a n d b y push a n d pull m o v e m e n t s , every 2 min d u r i n g the whole d u r a t i o n o f each m e a s u r e m e n t o f the p a i n threshold, i.e. f r o m 3 to 10 sec. In o r d e r to test the r e p r o d u c i b i l i t y o f the results, 3 subjects p a r t i c i p a t e d 3 m o n t h s later in a second series o f the trials 3 a n d 5. D u r i n g the 100 min o f the c o n t r o l trial the p a i n threshold r e m a i n e d a t the s a m e level in the 8 subjects (Fig. 2); the h e a r i n g o f waterfall sounds d i d n o t m o d i f y the p a i n t h r e s h o l d as is shown on the s a m e figure. T h e v a r i a t i o n o f the p a i n t h r e s h o l d d u r i n g a c u p u n c t u r e allowed distinction o f two p a t t e r n s o f response. (1) 4 subjects (Fig. 3) did n o t change their p a i n t h r e s h o l d , whichever spots were needled a n d (2) in the 4 o t h e r subjects, on the c o n t r a r y , the ACUPUNCTURE +4 03
I..-
m
0
.J
4'0
I
i
TIME (min)
Fig. 3. Mean pain threshold of 4 subjects during acupuncture compared with the control threshold values in grey. The ordinate is identical to that in Fig. 3. - traditional acupuncture points. . . . . . . . . . : adjoining spots. : distant spots. In these subjects acupuncture has no effect.
338 ACUPUNCTURE ~+4
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-
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L/'., ".-
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TIME (min)
Fig. 4. M e a n pain threshold o f 4 subjects during a c u p u n c t u r e c o m p a r e d with the control values in grey. T h e ordinate is identical to that in Fig. 3. - : traditional a c u p u n c t u r e points. - . . . . . . . . . : adjoining spots. : distant spots. In these subjects, a c u p u n c t u r e in Chinese a n d adjoining spots increases the pain threshold.
burning pain threshold was raised when Chinese spots and neighbouring ones were needled. When the Chinese spots were stimulated (Fig. 4) the hypoalgesia was more pronounced and persisted after the needles were removed. During the increase of the pain threshold, the 4 subjects described a change in their painful sensation. Without acupuncture it was identified as a sharp pricking. Under effective acupuncture, i, was preceded by a plateau of gentle heat, and the pricking was more diffuse. This modification persisted as long as the threshold remained high. The puncture of distant spots (trial 5) did not change the pain threshold significantly in 3 of these subjects. A small increase was, however, observed in the fourth one (Fig. 4). To test reproducibility of the results, 3 subjects participated in a second selies of experiments 3 months later. In two of them, acupuncture again proved its efficacy. In the third subject, this technique remained ineffective. Two examples of these results are shown in Fig. 5.
ACUPUNCTURE i
1
+4
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.
.
.
.
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.
6
.
.
.
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.
.
"'"
Xo
6'o
X./'"
TIME
"v"
"
(min)
Fig. 5. E v o l u t i o n o f the pain threshold in t w o subjects during acupuncture in the Chinese spots,
s h o w i n g reproducibility o f the results. Subject 1 : - - - - , M a r c h 4, 1975 a n d 1975. Subject 2: - - - - - January, 14, 1975 a n d . . . . . . . . . . : April, 15, 1975.
J u n e 24,
339 The sample survey showed that 7 subjects believed in the virtue of acupuncture and that they would accept acupuncture as a therapy. Only 4 of them increased their pain threshold. In 4 subjects, acupuncture increased the time necessary to raise the skin temperature of the thenar eminence from its initial level up to the pain threshold. This increase was not due to a drop in the adapting cutaneous temperature of the hand since its value remained constant during acupuncture. Therefore, any modification in the duration of the heating represents an actual variation of the pain threshold. In half of the subjects tested acupuncture increased the burning pain threshold. However, effective acupuncture did not produce analgesia but hypoalgesia since none of the subjects allowed himself to be burnt. The mechanism of this hypoalgesia is not a diversion of attention since the 3 different needling locations, which have certainly the same distracting power, did not show the same efficacy. Moreover, acupuncture does not need to be painful to possess hypoalgesic property, since in the present experiment no correlation has been found between the painful character of needle stimulation and its efficacy. The distraction induced by white noise failed to modify the pain threshold. This result agrees with the observations of Camp et aL 3. Some authors 6,1a assume that the suggestion that acupuncture is good is determinant in the induction of acupuncture hypoalgesia. Although the subjects had no indication as to the nature of the expected results, suggestion may have played some part because the placing and the manipulation of the needles was performed by a professional acupuncturist who of course knew which sites could be effective. He may have conveyed his belief to the subjects in some unknown fashion, although all precautions have been taken for avoiding this. According to the survey responses, faith in the virtue of acupuncture seems to be a necessary condition if one is to observe an increase in the threshold during acupuncture. However, it is obvious that this condition is not sufficient since 3 of the subjects who believed in the efficacy of the method did not show any hypoalgesia. Besides, if suggestion was determinant, each needling location should result in the same hypoalgesic action. As suggestion is not sufficient, it is necessary to assume that the acupunture action is specific of the spots stimulated by the needles. Acupuncture analgesia may be conceived as a case of hyperstimulation analgesia 14. The information from the punctured spots to the central nervous system, responsible for the decrease in the conscious sensation of pain, is transmitted by afferent nerves since the hypoalgesic effect can be completely abolished by infiltration of procaine in the deep tissues around the spot T M and is not present when the affected limb of hemiplegic or paralytic patients 16 is needled. This information coming from acupunctured spots may inhibit impulse transmission in pain pathways. This possible mechanism has been demonstrated in animals4,17,19, zl at various levels of the central nervous system. Under the conditions of our experiments, this inhibition could take place in the dorsal horn of the spinal cord since the Shou San Li spot is innervated by the radial nerve, which enters the cord at the level of C6, as does the cutaneous palmal branch of the median nerve which innervates the thenar eminence. In this hypothesis, the absence of an effect of acupuncture should be due to anatomic variations since the conditions of acupuncture were constant in all subjects. The 4 subjects could have been needled out-
340
side a specific area centred on the Chinese spot. It is also possible that the suggestion may have been less powerful in these subjects. One of them indeed, did not believe that acupuncture could produce any effect. In some patients anaesthesia by drugs may be dangerous and the use of acupuncture could then be prescribed. A rapid measurement of the pain threshold during acupuncture before surgery could permit the selection of subjects who could benefit by acupuncture anaesthesia. This research was supported by the Centre National dc ta Recherche Scientifique (C.N.R.S.) and the Institut National de la Sant6 et de la Recherche M6dicale (I.N.S.E.R.M.).
1 ANDERSSON,S. A., ERICSON,T., HOLMGREN,E., AND LINDOUIST,G., Electro-acupuncture: effect on
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