Jouma, Printed
of Psychosomnrrc I” Great Brltaln.
Rrsevrch.
Vol
36. No
I. pp. 47-53.
1992. d
0022-3999192 $5 00 + .oO 199 I Pergan1on Press plc
DIFFERENCES IN PAIN PERCEPTION AND SENSORY DISCRIMINATION BETWEEN CHRONIC LOW BACK PAIN PATIENTS AND HEALTHY CONTROLS MADELON (Received
L.
12 March
PETERS
and ANTON
J.
M.
SCHMIDT
1991; accepted in revised form 7 May
1991)
Abstract-Pain perception threshold (PPT), maximal pain tolerance (MPT) and pain discrimination of CLBP patients and controls were tested. Pain perception threshold was significantly higher in the patient group for two different pain stimuli (electrical and pressure pain). Maximal pain tolerance was significantly higher in CLBP patients only for the pressure pain stimulus. There was no difference between the groups in their capacity to discriminate between painful stimuli of different intensity, as measured by a forced-choice task. It is concluded that CLBP patients have a decreased sensitivity for experimental pain. Two theories which might explain this decreased sensitivity are discussed.
lNTRODUCTION
years, research has been directed towards the question of whether patients with various chronic pain syndromes have a different perception of experimental laboratory pain than healthy controls. The result of these studies seems to depend at least partly on such factors as the pain syndrome selected, the experimental pain stimulus used and on the pain variable measured (e.g. threshold vs tolerance) [ 11. The pain syndrome most extensively studied in relation to this question has been chronic low back pain (CLBP). For CLBP patients an increased pain perception threshold and decreased discrimination between stimuli of different intensity was found [2-51, and also decreased pain tolerance [6, 71. Studies using patients with other pain syndromes have obtained less consistent results 18-121. Moreover, the methods used and patients selected have been too diverse to allow for any definite conclusions. But also pertaining to CLBP patients conclusions should be drawn with caution because pain perception threshold and pain discrimination on the one hand and pain tolerance on the other, have been measured by means of different pain stimuli (radiant heat vs cold pressor pain) and the three pain variables have never all been measured in the same subjects. Therefore, a study measuring pain perception threshold, pain tolerance and pain discrimination by means of the same stimulus and in the same group of patients was badly needed. The present study wanted to provide for this by measuring all three variables in CLBP patients and controls using an electrical pain stimulus. Another improvement upon previous studies was the inclusion of a second pain stimulus to test for the generalizability of the results from one pain stimulus to another. A stimulus of a completely different nature than the electrical stimulus was used, namely pressure pain. Whereas for the electrical stimulus, pain threshold and tolerance were measured by means of discrete steps of increasing and decreasing intensity, the pressure pain stimulus continuously increased in intensity and pain threshold and pain tolerance were FOR SOME
Address correspondence to: M. L. Peters, University P.O. Box 616, 6200 MD Maastricht, The Netherlands. 47
of Limburg,
Dept of Medical
Psychology.
48
M. L. PEIERS and A. J. M.
SCHMIDT
measured in seconds. Also, the body location of application varied for the two stimuli. To measure pain discrimination. a forced-choice discrimination task was chosen, instead of the often used sensory decision theory (SDT) procedure. The discrimination index obtained by this last procedure, relies on intensity ratings of a stimulus, and contrary to what is often suggested, can therefore be influenced by response tendencies. A forced-choice paradigm is free of these influences, and in previous studies binary decision procedures have been found to obtain better discrimination scores than the SDT method [131. A forced-choice auditory discrimination task was also included, to control for nonspecific discrimination deficits in CLBP patients. METHODS
Forty-three subjects participated in the study, 20 CLBP patients (12 men. 8 women) and 23 controls (13 men, 10 women) of comparable age (mean age for rhe patient group: 42.0 yr (range: 24-53 yr) and for the control group 43.9 yr (range 25-59 yr). Chronic low back pain patients were selected by means of an advertisement in a local newspaper. Inclusion criteria were: low back pain for at least 1 year; daily continuous pain; and age between 20 and 60 yr. Exclusion criteria were: malignant diseases; cardiovascular problems; general poor health: use of analgesics: prior participation in pain research; and hearing problems. Control subjects consisted of the partners. relatives or friends of the CLBP patients. Inclusion criteria for the control group were: no pain complaints; general good health; and age between 20 and 60 yr. The exclusion criteria were the same a5 for the patient group. All subjects received a small tinancial reward after participation.
Electrical pain stimuli were delivered by a constant current generator (Siemens neuroton 627). The output intensity of the current ranged from 0 to I2 mA. Every stimulus consisted of a four-second train containing rectangular pulses of 20 msec with 20 msec interval time. Shocks were delivered to the degreased skin of the right ankle by means of Beckman 8 mm Ag/AgCI electrodes. filled with Spectra 360 electrode gel. Pressure pain stimuli were applied to the middle phalanx of the second finger by a slightly modified Forgione-Barber pressure stimulator [141. A 1.7 mm wide plexiglass wedge with a pressure of 1850 grams was placed on the finger, causing a gradually increasing pain. Tones of 400 Hz were produced by a tone generator and recorded on tape. During the experiment the subjects listened to this tape through stereo headphones.
Before the experiment, subjects completed the Spielberger trait and state anxiety questionnaire (STAI). Chronic low back pain patients, in addition, completed a back pain questionnaire. The experiment took place in a soundproof room of constant temperature. The subject was seated at a table and the shock electrodes were attached to the right ankle. First, pain perception threshold (PPT) and maxima1 pain tolerance (MPT) for electrical stimulation were assessed. Stimulus intensity started at 0 mA. increased in steps of 0.1 mA, and above I mA increased in steps of 0.2 m.4. Pain perceptlon threshold was measured in three ascending and three descending series of stimuli. Maximal pain tolerance was measured by means of three ascending series. The auditory discrimination task was presented next. The subjects received taped instructions. and subsequently a simple hearing test was presented to make sure that all subjects could actually detect low intensity 400 Hz tones. During the discrimination task. a total of 24 pair5 of 3 set tones of 400 HI were presented. The subject had to detect which of the two was the louder one. For every pair, a choice was required (forced-choice task). The tones differed by either I dB (lox), 2 dB (10x) or 4 dB (4x). Mean intensity and order of the louder tone varied randomly. The intensity of all tones was kept at a level which was well audible for all subjects, but non-aversive. Hereafter, the pain discrimination task consisting of 24 pairs of electrical stimuli, was presented. The difference in intensity between the stimuli was 0.2 mA for all pairs. A pilot study had previously shown
Differences
in pain perception
49
that for most subjects the difference was difficult to detect, though not impossible. Which of the two stimuli was the most intense varied randomly. Three kinds of pairs were constructed, on the basis of the previously assessed PPT and MPT measures. The most intense pair was 0.1 and 0.3 mA below MPT (8x). the least intense pair 0.1 and 0.3 mA above PPT (8 x), and the intermediate pair was exactly in between those pairs (8x). Thus, stimulus intensity was matched for individual sensitivity. After this test, the shock electrodes were removed, and the PPT and MPT for the pressure pain stimulus were assessed. The wedge of the pressure stimulator was placed on the second finger of the left hand, and time until the report of first perception of pain was measured in seconds. The total amount of time the subject tolerated the stimulus was taken as MPT.
RESULTS
Descriptive
analysis
of the CLBP patient
group
All CLBP patients reported having continuous back pain. The average pain duration had been 14 yr (range: 2-30 yr). Most patients had pain on both sides of the lower back, with radiation into the leg or the shoulders. Only two of the patients were currently under medical treatment for their back pain. Diagnosis were obtained from the patients themselves. Ten patients indicated to have undiagnosed back pain. Four patients said that the pain had continued after an operation for disc herniation and in one patient the pain had started after trauma (a fall). Two patients indicated a muscular origin for their pain, two indicated a structural spinal abnormality, one indicated intervertebral disc degeneration. Twelve patients still had full-time or parttime occupations, five had been declared disabled (one only for 25%) and three patients were unemployed. Usually the pain had begun gradually, and was now described as ‘considerable’ (10 cases) or ‘moderate’ (7 cases). Three subjects only reported weak pain. PPT and A4PT Pain perception threshold for electrical stimuli was defined as the average of the least intensity at which a stimulus was experienced as painful during two of the ascending series, and the three descending series. The first ascending series was considered a test trial, to familiarize the subject with the sensation and the procedure. Maximal pain tolerance for electrical stimuli was defined as the average of the maximal intensity the subject was willing to accept during the second and third ascending series. Pain perception threshold and MPT for pressure were measured in seconds. The results are presented in Table I. The distribution of scores markedly differed from normality, and non-parametric statistics were used (Mann-Whitney U-test). Chronic low back pain patients had a significantly higher PPT for both pain tests. Contrary to our expectation, MPT was also found to be higher in the patient group. This difference only reached significance for the pressure pain stimulus. Pain and tone discrimination There were no difference between CLBP patients and controls on the pain discrimination test. The mean number of incorrect choices was 5.4 for the CLBP patient group and 5.0 for the control group. This was not significantly different (MannWhitney U-test, z = 0.67, p = 0.26, one-tailed). Because CLBP patients received on average stimuli of greater intensity, also an analysis of covariance was performed, with intensity as covariate. This neither indicated a significant difference between the
M. L. PETERS and A. J. M. SCHMIDT
50 TARLE I.-MEAN -rOLERAN
of Psychosomnrrc I” Great Brltaln.
Rrsevrch.
Vol
36. No
I. pp. 47-53.
1992. d
0022-3999192 $5 00 + .oO 199 I Pergan1on Press plc
DIFFERENCES IN PAIN PERCEPTION AND SENSORY DISCRIMINATION BETWEEN CHRONIC LOW BACK PAIN PATIENTS AND HEALTHY CONTROLS MADELON (Received
L.
12 March
PETERS
and ANTON
J.
M.
SCHMIDT
1991; accepted in revised form 7 May
1991)
Abstract-Pain perception threshold (PPT), maximal pain tolerance (MPT) and pain discrimination of CLBP patients and controls were tested. Pain perception threshold was significantly higher in the patient group for two different pain stimuli (electrical and pressure pain). Maximal pain tolerance was significantly higher in CLBP patients only for the pressure pain stimulus. There was no difference between the groups in their capacity to discriminate between painful stimuli of different intensity, as measured by a forced-choice task. It is concluded that CLBP patients have a decreased sensitivity for experimental pain. Two theories which might explain this decreased sensitivity are discussed.
lNTRODUCTION
years, research has been directed towards the question of whether patients with various chronic pain syndromes have a different perception of experimental laboratory pain than healthy controls. The result of these studies seems to depend at least partly on such factors as the pain syndrome selected, the experimental pain stimulus used and on the pain variable measured (e.g. threshold vs tolerance) [ 11. The pain syndrome most extensively studied in relation to this question has been chronic low back pain (CLBP). For CLBP patients an increased pain perception threshold and decreased discrimination between stimuli of different intensity was found [2-51, and also decreased pain tolerance [6, 71. Studies using patients with other pain syndromes have obtained less consistent results 18-121. Moreover, the methods used and patients selected have been too diverse to allow for any definite conclusions. But also pertaining to CLBP patients conclusions should be drawn with caution because pain perception threshold and pain discrimination on the one hand and pain tolerance on the other, have been measured by means of different pain stimuli (radiant heat vs cold pressor pain) and the three pain variables have never all been measured in the same subjects. Therefore, a study measuring pain perception threshold, pain tolerance and pain discrimination by means of the same stimulus and in the same group of patients was badly needed. The present study wanted to provide for this by measuring all three variables in CLBP patients and controls using an electrical pain stimulus. Another improvement upon previous studies was the inclusion of a second pain stimulus to test for the generalizability of the results from one pain stimulus to another. A stimulus of a completely different nature than the electrical stimulus was used, namely pressure pain. Whereas for the electrical stimulus, pain threshold and tolerance were measured by means of discrete steps of increasing and decreasing intensity, the pressure pain stimulus continuously increased in intensity and pain threshold and pain tolerance were FOR SOME
Address correspondence to: M. L. Peters, University P.O. Box 616, 6200 MD Maastricht, The Netherlands. 47
of Limburg,
Dept of Medical
Psychology.
48
M. L. PEIERS and A. J. M.
SCHMIDT
measured in seconds. Also, the body location of application varied for the two stimuli. To measure pain discrimination. a forced-choice discrimination task was chosen, instead of the often used sensory decision theory (SDT) procedure. The discrimination index obtained by this last procedure, relies on intensity ratings of a stimulus, and contrary to what is often suggested, can therefore be influenced by response tendencies. A forced-choice paradigm is free of these influences, and in previous studies binary decision procedures have been found to obtain better discrimination scores than the SDT method [131. A forced-choice auditory discrimination task was also included, to control for nonspecific discrimination deficits in CLBP patients. METHODS
Forty-three subjects participated in the study, 20 CLBP patients (12 men. 8 women) and 23 controls (13 men, 10 women) of comparable age (mean age for rhe patient group: 42.0 yr (range: 24-53 yr) and for the control group 43.9 yr (range 25-59 yr). Chronic low back pain patients were selected by means of an advertisement in a local newspaper. Inclusion criteria were: low back pain for at least 1 year; daily continuous pain; and age between 20 and 60 yr. Exclusion criteria were: malignant diseases; cardiovascular problems; general poor health: use of analgesics: prior participation in pain research; and hearing problems. Control subjects consisted of the partners. relatives or friends of the CLBP patients. Inclusion criteria for the control group were: no pain complaints; general good health; and age between 20 and 60 yr. The exclusion criteria were the same a5 for the patient group. All subjects received a small tinancial reward after participation.
Electrical pain stimuli were delivered by a constant current generator (Siemens neuroton 627). The output intensity of the current ranged from 0 to I2 mA. Every stimulus consisted of a four-second train containing rectangular pulses of 20 msec with 20 msec interval time. Shocks were delivered to the degreased skin of the right ankle by means of Beckman 8 mm Ag/AgCI electrodes. filled with Spectra 360 electrode gel. Pressure pain stimuli were applied to the middle phalanx of the second finger by a slightly modified Forgione-Barber pressure stimulator [141. A 1.7 mm wide plexiglass wedge with a pressure of 1850 grams was placed on the finger, causing a gradually increasing pain. Tones of 400 Hz were produced by a tone generator and recorded on tape. During the experiment the subjects listened to this tape through stereo headphones.
Before the experiment, subjects completed the Spielberger trait and state anxiety questionnaire (STAI). Chronic low back pain patients, in addition, completed a back pain questionnaire. The experiment took place in a soundproof room of constant temperature. The subject was seated at a table and the shock electrodes were attached to the right ankle. First, pain perception threshold (PPT) and maxima1 pain tolerance (MPT) for electrical stimulation were assessed. Stimulus intensity started at 0 mA. increased in steps of 0.1 mA, and above I mA increased in steps of 0.2 m.4. Pain perceptlon threshold was measured in three ascending and three descending series of stimuli. Maximal pain tolerance was measured by means of three ascending series. The auditory discrimination task was presented next. The subjects received taped instructions. and subsequently a simple hearing test was presented to make sure that all subjects could actually detect low intensity 400 Hz tones. During the discrimination task. a total of 24 pair5 of 3 set tones of 400 HI were presented. The subject had to detect which of the two was the louder one. For every pair, a choice was required (forced-choice task). The tones differed by either I dB (lox), 2 dB (10x) or 4 dB (4x). Mean intensity and order of the louder tone varied randomly. The intensity of all tones was kept at a level which was well audible for all subjects, but non-aversive. Hereafter, the pain discrimination task consisting of 24 pairs of electrical stimuli, was presented. The difference in intensity between the stimuli was 0.2 mA for all pairs. A pilot study had previously shown
Differences
in pain perception
49
that for most subjects the difference was difficult to detect, though not impossible. Which of the two stimuli was the most intense varied randomly. Three kinds of pairs were constructed, on the basis of the previously assessed PPT and MPT measures. The most intense pair was 0.1 and 0.3 mA below MPT (8x). the least intense pair 0.1 and 0.3 mA above PPT (8 x), and the intermediate pair was exactly in between those pairs (8x). Thus, stimulus intensity was matched for individual sensitivity. After this test, the shock electrodes were removed, and the PPT and MPT for the pressure pain stimulus were assessed. The wedge of the pressure stimulator was placed on the second finger of the left hand, and time until the report of first perception of pain was measured in seconds. The total amount of time the subject tolerated the stimulus was taken as MPT.
RESULTS
Descriptive
analysis
of the CLBP patient
group
All CLBP patients reported having continuous back pain. The average pain duration had been 14 yr (range: 2-30 yr). Most patients had pain on both sides of the lower back, with radiation into the leg or the shoulders. Only two of the patients were currently under medical treatment for their back pain. Diagnosis were obtained from the patients themselves. Ten patients indicated to have undiagnosed back pain. Four patients said that the pain had continued after an operation for disc herniation and in one patient the pain had started after trauma (a fall). Two patients indicated a muscular origin for their pain, two indicated a structural spinal abnormality, one indicated intervertebral disc degeneration. Twelve patients still had full-time or parttime occupations, five had been declared disabled (one only for 25%) and three patients were unemployed. Usually the pain had begun gradually, and was now described as ‘considerable’ (10 cases) or ‘moderate’ (7 cases). Three subjects only reported weak pain. PPT and A4PT Pain perception threshold for electrical stimuli was defined as the average of the least intensity at which a stimulus was experienced as painful during two of the ascending series, and the three descending series. The first ascending series was considered a test trial, to familiarize the subject with the sensation and the procedure. Maximal pain tolerance for electrical stimuli was defined as the average of the maximal intensity the subject was willing to accept during the second and third ascending series. Pain perception threshold and MPT for pressure were measured in seconds. The results are presented in Table I. The distribution of scores markedly differed from normality, and non-parametric statistics were used (Mann-Whitney U-test). Chronic low back pain patients had a significantly higher PPT for both pain tests. Contrary to our expectation, MPT was also found to be higher in the patient group. This difference only reached significance for the pressure pain stimulus. Pain and tone discrimination There were no difference between CLBP patients and controls on the pain discrimination test. The mean number of incorrect choices was 5.4 for the CLBP patient group and 5.0 for the control group. This was not significantly different (MannWhitney U-test, z = 0.67, p = 0.26, one-tailed). Because CLBP patients received on average stimuli of greater intensity, also an analysis of covariance was performed, with intensity as covariate. This neither indicated a significant difference between the
M. L. PETERS and A. J. M. SCHMIDT
50 TARLE I.-MEAN -rOLERAN
