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Pain, 51(1992) 281-287 0 1992 Elsevier Science Publishers B.V. All rights reserved 0304-3959/92/$05.00
PAIN 02171
Relations between experimentally induced tooth pain threshold changes, psychometrics and clinical pain relief following TENS. A retrospective study in patients with long-lasting pain E.G. Widerstrijm Departments
a, P.G. Aslund
of a Physiology and b Psychology,
b, L.-E. Gustafsson a, C. Mannheimer and S.A. Andersson a
‘, S.G. Carlsson
University of Giiteborg, Giiteborg (Sweden) and c Department dtra Hospital, Giiteborg (Sweden)
b
of Medicine, Pain section,
(Received 19 November 1991, revision received 29 June 1992, accepted 7 July 1992)
Summary The present study investigates the relationships between clinical pain relief, physiological and psychological parameters. Out of 50 patients with long-lasting musculoskeletal neck- and shoulder-pain treated with transcutaneous electrical nerve stimulation (TENS), 21 were selected and classified as responders (n = 13) or non-responders (n = 8). Tooth pain thresholds (PT) were measured before and after an experimental TENS treatment and the relative change in PT following the stimulation was calculated. Three psychometric self-inventories were administered: Zung Depression Scale, Spielberger’s Trait Anxiety Scale and the Multidimensional Health Locus of Control Scale. Responders (R) and non-responders (NR) differed significantly from each other in the PT measurements as well as on the psychometric scales. NR exhibited higher levels of anxiety and depression, a more pronounced powerful other orientation and no change or a decrease in PT following TENS compared to R. These findings indicate relationships and interactions between physiological and psychological factors in patients with long-lasting pain. Key words: Chronic pain; TENS; Pain relief; Psychometric measures; Experimental
Introduction Chronic pain is a complex condition involving multiple somatic and psychological factors. Although our understanding of chronic pain has advanced greatly during the last years, it is still far from complete. In the clinical situation this becomes apparent when trying to design the most appropriate treatment for the individual patient. Some patients may respond well to e.g., pharmacological or behavioral treatment whereas in others transcutaneous electrical nerve stimulation (TENS) or acupuncture is more suitable. U&ally different therapies need to be tested before the best
Correspondence too: Dr. Eva Widerstriim, Department of Physiology, University of Giiteborg, Medicinaregatan 11, S-413 90 GGteborg, Sweden. Tel.: (+ 46-31) 853-513; Fax: (+ 46-31) 853-512.
pain
treatment strategy is found, a procedure which sometimes can be inordinately time consuming. It is our belief that a multidisciplinary collaboration between, for example, physicians, psychologists and researchers in basic science is beneficial for achieving a better understanding and treatment of the chronic pain patient. Mechanical or electrical stimulation of somatic afferents by methods such as TENS or electro-acupuncture can induce analgesia in humans (Andersson et al. 1973; Olausson et al. 1986) and anti-nociceptive effects in animals (Shyu et al. 1982; J&urn and Shyu 1988). In this study we have focused on TENS which is a method commonly used in the treatment of chronic pain. The effects can be explained by the activation of pain inhibitory systems (Melzack and Wall 1965; Basbaum and Fields 1984) and by psychological mechanisms (Fields 1987). TENS is proven to be effective in differ-
282
em chronic pain conditions, with a success rate of 50-80% in the short-term outcome (Taylor et al. 1981), whereas the long-term effect falls within the range of 20-30% (Bates and Nathan 1980). The pain-relieving effect may vary considerably according to the type of pain, the selection of patients, the type of stimulation, the skill of the therapist (Parry 1980) and probably psychological and social patient factors (Mendelsson 1991). Success with this method is often acquired in conditions with musculoskeletal pain (Wolf et al. 1980). TENS can also be an effective treatment in pains of neurogenic origin, like causalgia and peripheral nerve injury (Eriksson et al. 1979). In contrast, it is of little or no benefit in the treatment of visceral or psychogenic pain (Nielzen et al. 1982). In experimental evaluations of the effectiveness of TENS, pain thresholds are sometimes used as a measure of the analgesia produced. An increase in the tooth pain threshold (PT) when low-frequency TENS is applied segmentally has been reported (Andersson et al. 1973). This effect may partly be related to the activation of endogenous opioid systems (Olausson et al. 1986). However, the effect varies between individuals, possibly indicating a varying degree of activity in these systems. Whether the experimentally induced changes in PT are related to clinical pain relief is a matter of uncertainty. Long-lasting pain is frequently associated with psychological disorders (Magni 1987; Benjamin et al. 1988) which can be assessed by the use of standardized questionnaires, e.g., Minnesota Multiphasic Personality Inventory (MMPI). Cox et al. (1978) reported that chronic pain patients show significantly elevated scores on hysteria, depression and hypochondriasis (‘the neurotic triad’) on MMPI and this finding has been confirmed by others (Sternbach 1973; Franz et al. 1986). The impact of a chronic disease on an individual, as well as his responses to treatment, is related not only to psychopathology but to psychological characteristics in general. Several psychological concepts have proven to be relevant in these respects. An interesting example is the Multidimensional Health Locus of Control Scale (MHLC) (Wallston et al. 1987) which consists of 3 subscales: internal (I-LOCI, powerful others (P-LO0 and chance locus of control (C-LO0 Patients with a more pronounced internal locus of control orientation (a high score on I-LOCI have a greater belief in their own capacity to cope with or reduce the pain. Individuals who assume that their health situation primarily depends on luck, fate or chance score highly on C-LOC, while those who mainly rely on health care providers are more powerful-others oriented (P-LOC) (Skevington 1983; Crisson and Keefe 1988). The literature shows a somewhat conflicting picture with, on the one hand, studies that state the relation between a successful treatment and an internal locus of control orienta-
tion (Wallston and Wallston 1981; Harklpal et al. 1991) and, on the other hand, investigations showing correlations between a powerful-other orientation and a good treatment outcome (Nagy and Wolfe 1984). The material presented here is part of a larger investigation concerning the development of test batteries which can be used clinically to select patients with long-lasting pain who are likely to benefit from therapies stimulating afferent fibers like TENS and acupuncture. The present study is explorative and the focus of interest is aimed at the relationships between clinical pain relief, physiological and psychological parameters.
Methods
Selecting subjects The experimental subjects were recruited consecutively from patients attending the pain section at the Gstra hospital in Goteborg. Fifty patients suffering from musculoskeletal pain in the neck and shoulder region were treated with TENS during a period which varied between 2 to 6 months. They were seeing a physiotherapist on a regular basis and were carefully instructed in the use of the apparatus. Different frequencies, intensities and electrode placements were tested in order to obtain the best pain relieving effect. The treatment was partly self-administered as the patients received a TENS stimulator to use at home. Twenty-three subjects were selected from the treated patients in order to form 2 subgroups: one consisting of patients with a substantial pain relief due to the treatment, denoted responders (RI, and the other including patients who did not benefit at all, denoted non-responders (NRI. This evaluation was made by an experienced clinician and based on an interview. Two of the selected patients were not willing to participate in the study. Of the remaining 21, 13 were responders and 8 were non-responders, As can be seen in Table I, there were only minor differences in background characteristics between the 2 subgroups. All patients participated on a voluntary basis and were also informed that they could withdraw from the study at any time. The study was approved by the Ethical Committee of the University of Goteborg.
TABLE SAMPLE
I CHARACTERISTICS
Number of patients Male Female Age distribution (years) Median Mean Pain duration (years) Median Mean
OF CHRONIC
PAIN PATIENTS
All patients
R
NR
21 6 15 25-67 50.0 49.0 0.5-38 7.0 9.0
13
8
10 25-65 50.0 49.3 0.5-38 7.0 9.6
36-67 48.5 48.6 2.5-20 8.0 8.6
283
Experimental test
Psychometric measures
After completion of the clinical TENS treatment, the selected patients entered the experimental study which consisted of two 60-min sessions. The experimental tests were conducted blind, i.e., the experimental leaders were not aware of the clinical effectiveness of the TENS treatment in their subjects. At the first occasion the patient was carefully informed about the experimental procedure and introduced to the methods. A suitable tooth was selected for the tooth pain threshold (PT) measurement. PTs were repeatedly measured during a period of 20 min in order to train the patients in establishing stable values. In this way the patients became familiar with the experimental setting as a way to reduce anxiety and stress levels in the test situation, which otherwise might have influenced the results (Cornvall and Donderi 1988). At the end of the first session they filled out the psychometric forms. The second session started with a period of 10 min during which PT were measured in order to ascertain a baseline level. This was followed by 30 min of low-frequency TENS stimulation. PT were recorded immediately after the stimulation and again after 20 min.
At the end of the first experimental session each patient completed the psychometric forms described below. They were seated in a room adjacent to the laboratory and before they were left alone, the therapist made sure that the patient fully understood how to complete the task. After 30 min the therapist entered the room, and if all the questions were answered and nothing was unclear the forms were collected and the session terminated. Multidimensional Health Locus of Control (MHLC). Locus of control beliefs were assessed using a Swedish translation of the Multidimensional Health Locus of Control (MHLC) Scales (Wallston et al. 1987). MHLC, which consists of 18 items, is developed to assess the patients beliefs whether their state of health primarily is a matter of chance, or in the hands of powerful others, such as health care professionals, or internally controlled by themselves. State-Trait Anxiety Inventory (STAI-Yj. A Swedish translation of the Trait Anxiety Scale was used. It consists of 20 statements and evaluates how anxious the patient ‘generally’ is (Spielberger et al. 1970). Zung Self-Rating Depression Scale. This scale consists of twenty items measuring both somatic and affective components of depression (Zung 1965). We have used the SDS index which is a conversion of the raw score ranging from 25 to 100. The index is developed by Zung as a total indication of how depressed the patient is and there are cut-off levels for discrimination between different degrees, or lack of, depression.
Tooth pain threshold measurement During the first session an upper healthy incisor was selected for the testing, based on reproducible thresholds in response to electrical stimulation. To keep the electrode in the same location during the whole experiment, a dental splint made of hard silicone (Provil putty paste) was individually constructed for each subject. By keeping a cotton roll under the upper lip, moisture was prevented from interfering with the current flow and thus distorting the measurements. The electrical stimuli were distributed via a coal rubber electrode (cathode) placed in the dental splint against the buccal surface of the tooth. Conducting gel was used to reduce the resistance between the tooth and the electrode. The other electrode (anode) was held by the patient as a current switch. A constant current stimulator including a security device preventing the current to exceed 100 PA was used to deliver unipolar square-wave pulses of 2-msec duration at a frequency of 20 Hz. The current was increased in steps of 0.33 PA. The number of pulses were 2-6 on each step and randomly varied by a computer in order to produce 5 different speeds by which the intensity increased. The patient was instructed to break the current by using the switch in his/her hand as soon as a pain sensation was perceived in the tooth (Sessle 1979; Ahlquist et al. 1984; Virtanen et al. 1987). The current required to produce a pain sensation ranged from 5 to 25 kA. The randomized procedure prevented the time factor from biasing the patients response as the time required for the current to reach threshold varied in each trial. The patients were not informed about the purpose of the tooth pulp stimulation procedure. The mean of 3 successive measurements was defined as the pain threshold.
Experimental TENS stimulation A constant-current TENS stimulator was used to deliver a train of 8 stimuli with an inter-pulse interval of 14 msec (71.43 Hz) at a frequency of 1.7 Hz. Each stimulus had a duration of 0.2 m sec. Six electrodes (size 50 X 30mm) were used. One was applied over the left and another was placed over the right infra-orbital foramen. A pair of electrodes was placed on each upper limb, one on the hand just distal to the junction of the metacarpal bones between the thumb and the index finger and the other on the arm between the lateral epicondyle and the crease of the elbow. The limb placements were chosen because intense stimulation of an area between the 1st and 2nd, and the 4th and the 5th finger has been shown to produce dental analgesia (Taub et al. 1977; Melzack and Bentley 1983). The intensity was gradually increased during the session and varied between 15 and 40 mA. The stimulation was set to give strong but not painful contractions in the mpscles of the face and the arms during 30 min, while the patient was comfortably seated in a dental chair.
Statistics The statistical significance of the results was evaluated by using the Mann-Whitney U test and the Wilcoxon signed ranks test (Siegel and Castellan 1988).
Results Comparison of pain thresholds between responders and non-responders
The results from the 2 subgroups differed from each other as patients with good clinical pain relief CR) significantly increased their PT after stimulation and patients with no effect clinically (NR) showed no PT elevation at all. In fact there was a significant PT decrease at the last measurement. In Fig. 1 the change in PT is measured from a baseline value equal to zero. Responders (n = 13) showed a median increase of 19.7% (range: -4.1 to + 100.0) immediately after 30 min of TENS and after another 20 min it returned to base level (median: - 0.4%; range: - 27.0 to + 23.8). The increase directly after stimulation was statistically significant (P < 0.01, 2-tailed Wilcoxon). Non-responders (n = 8) showed no significant change (median: - 3.2%; range: - 24.0 to + 9.5) in PT directly after stimulation. However, a significant (P < 0.02, 2-tailed Wilcoxon) decrease in the PT of 10.3% (range: - 19.0 to -4.1) compared to base level appeared at the end of the session. The PT changes in the 2 subgroups differed significantly from each other directly after stimulation (P < 0.01, 2-tailed Mann-Whitney U> but not at the last measurement.
284
30 x 2 8
m
RESPONDERS
m
NON-RESPONDERS
(n = 13) Cn - 81
20-
Z
Fig. 2. Individual rank sums for responders (n = sponders (n = 8). A high rank sum indicates a high and depression together with a more pronounced orientation on MHLC and a lack of increase, or a See text for more details.
5 2
‘0
0 5 E E
0
I 20
t
1 40
I 60 mr
13) and non-relevel of anxiety powerful other decrease, in PT.
-10 ---TENS-stimulation
these parameters significantly discriminated between the 2 subgroups of patients. Because the number of patients is too small for a formal discriminant analysis, we chose to demonstrate tentatively the discrimination ability of these parameters in the following manner. The individual scores from each of the tests were sorted separately in successive order and given a rank number. Low scores on the psychometric scales and high percentage of PT increase directly after completed stimulation corresponded to low-iank numbers. By adding the ranks from each of the selected tests an individual total rank score was calculated for every patient. The patients were ordered according to their total rank sum in a diagram (Fig. 2), with an obvious polarization of R to the left corresponding to low scores and NR to the right corresponding to high scores.
1 Fig. 1. Tooth pain threshold (PT) change in percent following 30 min of low-frequency TENS in responders (R) and non-responders (NR) compared to baseline. PT increased significantly in R directly after stimulation (P < 0.01). The PT in NR decreased significantly (P < 0.02) at the last measurement. The difference between the 2 subgroups was significant (P < 0.01) directly after stimulation, but not 20 min later.
Comparison of psychometric sponders and non-responders
parameters
between re-
The medians and ranges for the patients on measures of MHLC, STAI-Y and Zung appear in Table II. There were no significant differences between responders and non-responders on the Internal and Chance dimensions of the MHLC scale. However, in the Powerful Others Locus of Control orientation as well as in the Anxiety and the Depression measures, the non-responders scored significantly higher.
Discussion Discrimination between responders and non-responders by combining physiological and psychological variables
An interesting question to be asked is if a chronic pain condition can cause psychophysiological changes and, if so, are such changes reflected in pain threshold measurements? The effect of TENS on PT differed significantly between the 2 categories of patients. R showed a median PT increase of 19.7% (range: - 4.1 to + 100.0) whereas NR did not show any increase. In fact, their thresholds decreased below the base level 20
An attempt was made to determine whether the power to discriminate between responders and non-responders might be improved by combining the results of the physiological and psychological tests. The following parameters were selected: the Anxiety and Depression scales, the Powerful-other orientation on MHLC and the degree of experimental analgesia, as each of TABLE
II
MEDIANS AND RANGES ON THE MULTIDIMENSIONAL ZUNG DEPRESSION SCALE (SDS-INDEX). RESPONDERS Responders
Locus of control dimension Internal Powerful other Chance STAI-Y Zung ’ P < 0.02, 2-tailed, ’ P < 0.05, 2-tailed,
Mann-Whitney. Mann-Whitney.
HEALTH LOCUS OF CONTROL AND NON-RESPONDERS
SCALE,
Non-responders
(n = 13)
SPIELBERGER’S
(n = 8)
Median
Range
Median
Range
22 15 a I8 38 a 42 ’
17-23 8-29 11-30 21-54 26-56
20.5 23 .’ 15 s3 a 55 h
6-25 15-36 13-18 36-75 40-91
STAI-Y
AND
285
min after stimulation. The PT elevation induced in the responder group is parallel to our own results in an earlier study where low-frequency TENS was given infra-orbitally to healthy volunteers (Olausson et al. 1986). In that investigation the mean PT increased to 19.3% (SD. = 19.1; range: - 13.7 to +61.9) after 30 min of stimulation. A chronic pain condition may cause modifications, possibly reversible, in the processing of sensory stimuli. This has been suggested in several studies in which pain perception or level of endorphins have been investigated in chronic pain patients and healthy normals (Almay et al. 1978; Malow and Olson 1981; Lipman et al. 1990). Malow and Olson (1981) compared pain perception in chronic pain patients before and after various treatments. After completed pain therapy the threshold to painful stimuli increased in patients who were improved, whereas the threshold was unchanged in the unsuccessfully treated patients. They also noted similarities between improved patients and healthy normals in their response to painful stimuli. Evidence for a relationship between chronic pain and changes in endogenous pain modulatory systems are provided by Lipman et al. (1990) who demonstrated a reduced peak B-endorphin concentration in CSF in chronic pain patients compared to normals. Almay et al. (1978) found a correlation between low levels of endorphins in CSF and organic pain of long duration. The lack of analgetic response in NR in our study may partly be related to reversible malfunctions in pain inhibitory systems since R and healthy subjects show similar increase in PT to an afferent stimulation. Since the present study was retrospective, one must also consider the possibility of influence of expectancy. For example, positive beliefs concerning the ability of acupuncture to relieve pain have been shown to correlate to .the degree of clinical pain relief (Kreitler et al. 1987) and to experimental pain tolerance (Norton et al. 1984) following acupuncture. However, this seems to be a matter of controversy (Moore and Berk 1976; Chapman et al. 1982). It can be assumed that patients who have undergone a successful treatment differ from those patients with an unsuccessful treatment in what they expect from an additional and similar treatment. In this study precautions were taken, concerning the pain threshold procedure, to minimize the patient response bias (see Methods). To what extent expectations has influenced the PT is difficult to estimate. The relation between chronic pain and psychological factors has been investigated in many studies. Benjamin et al. (1988) studied 106 outpatients in a pain clinic and observed that mental illness was diagnosed in about one-half of the patients. The majority of these suffered from depression, whereas the remainder had somatic disorders. This relation has also been emphasized by Chaturvedi (1987) in a study of 500 psychiatric
patients. He found chronic pain to be a common complaint especially among patients diagnosed as having anxiety neurosis, neurotic depression or hysteria. Such psychological parameters are also important in the experimental situation. Anxiety increases pain ratings when pain is experimentally induced in humans (Cornvall and Donderi 1988). Stimulation induced analgesia can most certainly be influenced by psychological factors and this has been demonstrated in an interesting study in animals by J&-urn and Shyu (1988). They investigated the relation between changed pain sensitivity induced by low-frequency electrical nerve stimulation and behavior in the rat. Anti-nociceptive effects were induced in rats that were quiet during the stimulation, whilst this did not occur in rats showing signs of distress. The behavioral signs of distress were interpreted as possibly equivalent to human anxiety. The study implies that analgesia can be counteracted by emotions, e.g., anxiety. The elevated level of anxiety and the unchanged or decreased PT following TENS in NR, in the present investigation, parallels their results. Our results are in line with the frequently stated view that patients with long lasting pain often exhibit high degrees of anxiety and depression. The NR in our study had significantly higher scores on the depression and trait anxiety tests than R. Similar results have been reported earlier by Hossenlopp et al. (1976). The levels in R were comparable with data obtained from healthy volunteers (Zung 1965; Spielberger et al. 19701. In conclusion, it is reasonable to assume that the emotional state of the patient or subject interferes with analgesia in the clinical or experimental situation. The only dimension on the MHLC scale that significantly differed between R and NR was the powerfulothers orientation (P-LOC) which was more pronounced in NR. Buckelew et al. (19901 suggest that patients with a high degree of P-LOC, i.e., those who are more reliant upon medical professionals, exhibit fewer coping strategies. It can be presumed that patients with a poor treatment outcome have a low ability to develop coping strategies. To a large extent TENS is self-administered so that the motivation and active participation of the patients play a crucial role in the outcome. From that point of view it might have been expected to find a more internally oriented locus of control in R compared to NR. However, we failed to replicate the relationship between treatment success and I-LOC reported by others (Wallston and Wallston 1981; Harkapla et al. 1991). Many of the psychological parameters may influence the perception of pain, both in the experimental as well as in the clinical situation. This was apparent when combining the results from Zung, STAI-Y, PLOC and degree of PT change. This emphasizes the close connection between physiological and psychological factors.
286
In conclusion, the findings of the present study indicate that NR and R differ from each other when using our test battery. This difference may have been due to the polarized sample of patients and show that physiological and psychological factors may influence the clinical outcome. Whether these differences are apparent in a more homogenous group of patients and in a prospective study are currently under investigation.
This work was supported by the Swedish Medical Research Council (Project 55) and Stiftelsen for Acupuncture and Alternative biological treatment methods. The authors would like to thank Nina Hedner, Dan Magnusson and Stina Morelius for their skilful technical and administrative assistance, Dr Eva Esbjornsson for valuable discussions and Dr Brian Noga for constructive comments on the manuscript.
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Pain, 51(1992) 281-287 0 1992 Elsevier Science Publishers B.V. All rights reserved 0304-3959/92/$05.00
PAIN 02171
Relations between experimentally induced tooth pain threshold changes, psychometrics and clinical pain relief following TENS. A retrospective study in patients with long-lasting pain E.G. Widerstrijm Departments
a, P.G. Aslund
of a Physiology and b Psychology,
b, L.-E. Gustafsson a, C. Mannheimer and S.A. Andersson a
‘, S.G. Carlsson
University of Giiteborg, Giiteborg (Sweden) and c Department dtra Hospital, Giiteborg (Sweden)
b
of Medicine, Pain section,
(Received 19 November 1991, revision received 29 June 1992, accepted 7 July 1992)
Summary The present study investigates the relationships between clinical pain relief, physiological and psychological parameters. Out of 50 patients with long-lasting musculoskeletal neck- and shoulder-pain treated with transcutaneous electrical nerve stimulation (TENS), 21 were selected and classified as responders (n = 13) or non-responders (n = 8). Tooth pain thresholds (PT) were measured before and after an experimental TENS treatment and the relative change in PT following the stimulation was calculated. Three psychometric self-inventories were administered: Zung Depression Scale, Spielberger’s Trait Anxiety Scale and the Multidimensional Health Locus of Control Scale. Responders (R) and non-responders (NR) differed significantly from each other in the PT measurements as well as on the psychometric scales. NR exhibited higher levels of anxiety and depression, a more pronounced powerful other orientation and no change or a decrease in PT following TENS compared to R. These findings indicate relationships and interactions between physiological and psychological factors in patients with long-lasting pain. Key words: Chronic pain; TENS; Pain relief; Psychometric measures; Experimental
Introduction Chronic pain is a complex condition involving multiple somatic and psychological factors. Although our understanding of chronic pain has advanced greatly during the last years, it is still far from complete. In the clinical situation this becomes apparent when trying to design the most appropriate treatment for the individual patient. Some patients may respond well to e.g., pharmacological or behavioral treatment whereas in others transcutaneous electrical nerve stimulation (TENS) or acupuncture is more suitable. U&ally different therapies need to be tested before the best
Correspondence too: Dr. Eva Widerstriim, Department of Physiology, University of Giiteborg, Medicinaregatan 11, S-413 90 GGteborg, Sweden. Tel.: (+ 46-31) 853-513; Fax: (+ 46-31) 853-512.
pain
treatment strategy is found, a procedure which sometimes can be inordinately time consuming. It is our belief that a multidisciplinary collaboration between, for example, physicians, psychologists and researchers in basic science is beneficial for achieving a better understanding and treatment of the chronic pain patient. Mechanical or electrical stimulation of somatic afferents by methods such as TENS or electro-acupuncture can induce analgesia in humans (Andersson et al. 1973; Olausson et al. 1986) and anti-nociceptive effects in animals (Shyu et al. 1982; J&urn and Shyu 1988). In this study we have focused on TENS which is a method commonly used in the treatment of chronic pain. The effects can be explained by the activation of pain inhibitory systems (Melzack and Wall 1965; Basbaum and Fields 1984) and by psychological mechanisms (Fields 1987). TENS is proven to be effective in differ-
282
em chronic pain conditions, with a success rate of 50-80% in the short-term outcome (Taylor et al. 1981), whereas the long-term effect falls within the range of 20-30% (Bates and Nathan 1980). The pain-relieving effect may vary considerably according to the type of pain, the selection of patients, the type of stimulation, the skill of the therapist (Parry 1980) and probably psychological and social patient factors (Mendelsson 1991). Success with this method is often acquired in conditions with musculoskeletal pain (Wolf et al. 1980). TENS can also be an effective treatment in pains of neurogenic origin, like causalgia and peripheral nerve injury (Eriksson et al. 1979). In contrast, it is of little or no benefit in the treatment of visceral or psychogenic pain (Nielzen et al. 1982). In experimental evaluations of the effectiveness of TENS, pain thresholds are sometimes used as a measure of the analgesia produced. An increase in the tooth pain threshold (PT) when low-frequency TENS is applied segmentally has been reported (Andersson et al. 1973). This effect may partly be related to the activation of endogenous opioid systems (Olausson et al. 1986). However, the effect varies between individuals, possibly indicating a varying degree of activity in these systems. Whether the experimentally induced changes in PT are related to clinical pain relief is a matter of uncertainty. Long-lasting pain is frequently associated with psychological disorders (Magni 1987; Benjamin et al. 1988) which can be assessed by the use of standardized questionnaires, e.g., Minnesota Multiphasic Personality Inventory (MMPI). Cox et al. (1978) reported that chronic pain patients show significantly elevated scores on hysteria, depression and hypochondriasis (‘the neurotic triad’) on MMPI and this finding has been confirmed by others (Sternbach 1973; Franz et al. 1986). The impact of a chronic disease on an individual, as well as his responses to treatment, is related not only to psychopathology but to psychological characteristics in general. Several psychological concepts have proven to be relevant in these respects. An interesting example is the Multidimensional Health Locus of Control Scale (MHLC) (Wallston et al. 1987) which consists of 3 subscales: internal (I-LOCI, powerful others (P-LO0 and chance locus of control (C-LO0 Patients with a more pronounced internal locus of control orientation (a high score on I-LOCI have a greater belief in their own capacity to cope with or reduce the pain. Individuals who assume that their health situation primarily depends on luck, fate or chance score highly on C-LOC, while those who mainly rely on health care providers are more powerful-others oriented (P-LOC) (Skevington 1983; Crisson and Keefe 1988). The literature shows a somewhat conflicting picture with, on the one hand, studies that state the relation between a successful treatment and an internal locus of control orienta-
tion (Wallston and Wallston 1981; Harklpal et al. 1991) and, on the other hand, investigations showing correlations between a powerful-other orientation and a good treatment outcome (Nagy and Wolfe 1984). The material presented here is part of a larger investigation concerning the development of test batteries which can be used clinically to select patients with long-lasting pain who are likely to benefit from therapies stimulating afferent fibers like TENS and acupuncture. The present study is explorative and the focus of interest is aimed at the relationships between clinical pain relief, physiological and psychological parameters.
Methods
Selecting subjects The experimental subjects were recruited consecutively from patients attending the pain section at the Gstra hospital in Goteborg. Fifty patients suffering from musculoskeletal pain in the neck and shoulder region were treated with TENS during a period which varied between 2 to 6 months. They were seeing a physiotherapist on a regular basis and were carefully instructed in the use of the apparatus. Different frequencies, intensities and electrode placements were tested in order to obtain the best pain relieving effect. The treatment was partly self-administered as the patients received a TENS stimulator to use at home. Twenty-three subjects were selected from the treated patients in order to form 2 subgroups: one consisting of patients with a substantial pain relief due to the treatment, denoted responders (RI, and the other including patients who did not benefit at all, denoted non-responders (NRI. This evaluation was made by an experienced clinician and based on an interview. Two of the selected patients were not willing to participate in the study. Of the remaining 21, 13 were responders and 8 were non-responders, As can be seen in Table I, there were only minor differences in background characteristics between the 2 subgroups. All patients participated on a voluntary basis and were also informed that they could withdraw from the study at any time. The study was approved by the Ethical Committee of the University of Goteborg.
TABLE SAMPLE
I CHARACTERISTICS
Number of patients Male Female Age distribution (years) Median Mean Pain duration (years) Median Mean
OF CHRONIC
PAIN PATIENTS
All patients
R
NR
21 6 15 25-67 50.0 49.0 0.5-38 7.0 9.0
13
8
10 25-65 50.0 49.3 0.5-38 7.0 9.6
36-67 48.5 48.6 2.5-20 8.0 8.6
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Experimental test
Psychometric measures
After completion of the clinical TENS treatment, the selected patients entered the experimental study which consisted of two 60-min sessions. The experimental tests were conducted blind, i.e., the experimental leaders were not aware of the clinical effectiveness of the TENS treatment in their subjects. At the first occasion the patient was carefully informed about the experimental procedure and introduced to the methods. A suitable tooth was selected for the tooth pain threshold (PT) measurement. PTs were repeatedly measured during a period of 20 min in order to train the patients in establishing stable values. In this way the patients became familiar with the experimental setting as a way to reduce anxiety and stress levels in the test situation, which otherwise might have influenced the results (Cornvall and Donderi 1988). At the end of the first session they filled out the psychometric forms. The second session started with a period of 10 min during which PT were measured in order to ascertain a baseline level. This was followed by 30 min of low-frequency TENS stimulation. PT were recorded immediately after the stimulation and again after 20 min.
At the end of the first experimental session each patient completed the psychometric forms described below. They were seated in a room adjacent to the laboratory and before they were left alone, the therapist made sure that the patient fully understood how to complete the task. After 30 min the therapist entered the room, and if all the questions were answered and nothing was unclear the forms were collected and the session terminated. Multidimensional Health Locus of Control (MHLC). Locus of control beliefs were assessed using a Swedish translation of the Multidimensional Health Locus of Control (MHLC) Scales (Wallston et al. 1987). MHLC, which consists of 18 items, is developed to assess the patients beliefs whether their state of health primarily is a matter of chance, or in the hands of powerful others, such as health care professionals, or internally controlled by themselves. State-Trait Anxiety Inventory (STAI-Yj. A Swedish translation of the Trait Anxiety Scale was used. It consists of 20 statements and evaluates how anxious the patient ‘generally’ is (Spielberger et al. 1970). Zung Self-Rating Depression Scale. This scale consists of twenty items measuring both somatic and affective components of depression (Zung 1965). We have used the SDS index which is a conversion of the raw score ranging from 25 to 100. The index is developed by Zung as a total indication of how depressed the patient is and there are cut-off levels for discrimination between different degrees, or lack of, depression.
Tooth pain threshold measurement During the first session an upper healthy incisor was selected for the testing, based on reproducible thresholds in response to electrical stimulation. To keep the electrode in the same location during the whole experiment, a dental splint made of hard silicone (Provil putty paste) was individually constructed for each subject. By keeping a cotton roll under the upper lip, moisture was prevented from interfering with the current flow and thus distorting the measurements. The electrical stimuli were distributed via a coal rubber electrode (cathode) placed in the dental splint against the buccal surface of the tooth. Conducting gel was used to reduce the resistance between the tooth and the electrode. The other electrode (anode) was held by the patient as a current switch. A constant current stimulator including a security device preventing the current to exceed 100 PA was used to deliver unipolar square-wave pulses of 2-msec duration at a frequency of 20 Hz. The current was increased in steps of 0.33 PA. The number of pulses were 2-6 on each step and randomly varied by a computer in order to produce 5 different speeds by which the intensity increased. The patient was instructed to break the current by using the switch in his/her hand as soon as a pain sensation was perceived in the tooth (Sessle 1979; Ahlquist et al. 1984; Virtanen et al. 1987). The current required to produce a pain sensation ranged from 5 to 25 kA. The randomized procedure prevented the time factor from biasing the patients response as the time required for the current to reach threshold varied in each trial. The patients were not informed about the purpose of the tooth pulp stimulation procedure. The mean of 3 successive measurements was defined as the pain threshold.
Experimental TENS stimulation A constant-current TENS stimulator was used to deliver a train of 8 stimuli with an inter-pulse interval of 14 msec (71.43 Hz) at a frequency of 1.7 Hz. Each stimulus had a duration of 0.2 m sec. Six electrodes (size 50 X 30mm) were used. One was applied over the left and another was placed over the right infra-orbital foramen. A pair of electrodes was placed on each upper limb, one on the hand just distal to the junction of the metacarpal bones between the thumb and the index finger and the other on the arm between the lateral epicondyle and the crease of the elbow. The limb placements were chosen because intense stimulation of an area between the 1st and 2nd, and the 4th and the 5th finger has been shown to produce dental analgesia (Taub et al. 1977; Melzack and Bentley 1983). The intensity was gradually increased during the session and varied between 15 and 40 mA. The stimulation was set to give strong but not painful contractions in the mpscles of the face and the arms during 30 min, while the patient was comfortably seated in a dental chair.
Statistics The statistical significance of the results was evaluated by using the Mann-Whitney U test and the Wilcoxon signed ranks test (Siegel and Castellan 1988).
Results Comparison of pain thresholds between responders and non-responders
The results from the 2 subgroups differed from each other as patients with good clinical pain relief CR) significantly increased their PT after stimulation and patients with no effect clinically (NR) showed no PT elevation at all. In fact there was a significant PT decrease at the last measurement. In Fig. 1 the change in PT is measured from a baseline value equal to zero. Responders (n = 13) showed a median increase of 19.7% (range: -4.1 to + 100.0) immediately after 30 min of TENS and after another 20 min it returned to base level (median: - 0.4%; range: - 27.0 to + 23.8). The increase directly after stimulation was statistically significant (P < 0.01, 2-tailed Wilcoxon). Non-responders (n = 8) showed no significant change (median: - 3.2%; range: - 24.0 to + 9.5) in PT directly after stimulation. However, a significant (P < 0.02, 2-tailed Wilcoxon) decrease in the PT of 10.3% (range: - 19.0 to -4.1) compared to base level appeared at the end of the session. The PT changes in the 2 subgroups differed significantly from each other directly after stimulation (P < 0.01, 2-tailed Mann-Whitney U> but not at the last measurement.
284
30 x 2 8
m
RESPONDERS
m
NON-RESPONDERS
(n = 13) Cn - 81
20-
Z
Fig. 2. Individual rank sums for responders (n = sponders (n = 8). A high rank sum indicates a high and depression together with a more pronounced orientation on MHLC and a lack of increase, or a See text for more details.
5 2
‘0
0 5 E E
0
I 20
t
1 40
I 60 mr
13) and non-relevel of anxiety powerful other decrease, in PT.
-10 ---TENS-stimulation
these parameters significantly discriminated between the 2 subgroups of patients. Because the number of patients is too small for a formal discriminant analysis, we chose to demonstrate tentatively the discrimination ability of these parameters in the following manner. The individual scores from each of the tests were sorted separately in successive order and given a rank number. Low scores on the psychometric scales and high percentage of PT increase directly after completed stimulation corresponded to low-iank numbers. By adding the ranks from each of the selected tests an individual total rank score was calculated for every patient. The patients were ordered according to their total rank sum in a diagram (Fig. 2), with an obvious polarization of R to the left corresponding to low scores and NR to the right corresponding to high scores.
1 Fig. 1. Tooth pain threshold (PT) change in percent following 30 min of low-frequency TENS in responders (R) and non-responders (NR) compared to baseline. PT increased significantly in R directly after stimulation (P < 0.01). The PT in NR decreased significantly (P < 0.02) at the last measurement. The difference between the 2 subgroups was significant (P < 0.01) directly after stimulation, but not 20 min later.
Comparison of psychometric sponders and non-responders
parameters
between re-
The medians and ranges for the patients on measures of MHLC, STAI-Y and Zung appear in Table II. There were no significant differences between responders and non-responders on the Internal and Chance dimensions of the MHLC scale. However, in the Powerful Others Locus of Control orientation as well as in the Anxiety and the Depression measures, the non-responders scored significantly higher.
Discussion Discrimination between responders and non-responders by combining physiological and psychological variables
An interesting question to be asked is if a chronic pain condition can cause psychophysiological changes and, if so, are such changes reflected in pain threshold measurements? The effect of TENS on PT differed significantly between the 2 categories of patients. R showed a median PT increase of 19.7% (range: - 4.1 to + 100.0) whereas NR did not show any increase. In fact, their thresholds decreased below the base level 20
An attempt was made to determine whether the power to discriminate between responders and non-responders might be improved by combining the results of the physiological and psychological tests. The following parameters were selected: the Anxiety and Depression scales, the Powerful-other orientation on MHLC and the degree of experimental analgesia, as each of TABLE
II
MEDIANS AND RANGES ON THE MULTIDIMENSIONAL ZUNG DEPRESSION SCALE (SDS-INDEX). RESPONDERS Responders
Locus of control dimension Internal Powerful other Chance STAI-Y Zung ’ P < 0.02, 2-tailed, ’ P < 0.05, 2-tailed,
Mann-Whitney. Mann-Whitney.
HEALTH LOCUS OF CONTROL AND NON-RESPONDERS
SCALE,
Non-responders
(n = 13)
SPIELBERGER’S
(n = 8)
Median
Range
Median
Range
22 15 a I8 38 a 42 ’
17-23 8-29 11-30 21-54 26-56
20.5 23 .’ 15 s3 a 55 h
6-25 15-36 13-18 36-75 40-91
STAI-Y
AND
285
min after stimulation. The PT elevation induced in the responder group is parallel to our own results in an earlier study where low-frequency TENS was given infra-orbitally to healthy volunteers (Olausson et al. 1986). In that investigation the mean PT increased to 19.3% (SD. = 19.1; range: - 13.7 to +61.9) after 30 min of stimulation. A chronic pain condition may cause modifications, possibly reversible, in the processing of sensory stimuli. This has been suggested in several studies in which pain perception or level of endorphins have been investigated in chronic pain patients and healthy normals (Almay et al. 1978; Malow and Olson 1981; Lipman et al. 1990). Malow and Olson (1981) compared pain perception in chronic pain patients before and after various treatments. After completed pain therapy the threshold to painful stimuli increased in patients who were improved, whereas the threshold was unchanged in the unsuccessfully treated patients. They also noted similarities between improved patients and healthy normals in their response to painful stimuli. Evidence for a relationship between chronic pain and changes in endogenous pain modulatory systems are provided by Lipman et al. (1990) who demonstrated a reduced peak B-endorphin concentration in CSF in chronic pain patients compared to normals. Almay et al. (1978) found a correlation between low levels of endorphins in CSF and organic pain of long duration. The lack of analgetic response in NR in our study may partly be related to reversible malfunctions in pain inhibitory systems since R and healthy subjects show similar increase in PT to an afferent stimulation. Since the present study was retrospective, one must also consider the possibility of influence of expectancy. For example, positive beliefs concerning the ability of acupuncture to relieve pain have been shown to correlate to .the degree of clinical pain relief (Kreitler et al. 1987) and to experimental pain tolerance (Norton et al. 1984) following acupuncture. However, this seems to be a matter of controversy (Moore and Berk 1976; Chapman et al. 1982). It can be assumed that patients who have undergone a successful treatment differ from those patients with an unsuccessful treatment in what they expect from an additional and similar treatment. In this study precautions were taken, concerning the pain threshold procedure, to minimize the patient response bias (see Methods). To what extent expectations has influenced the PT is difficult to estimate. The relation between chronic pain and psychological factors has been investigated in many studies. Benjamin et al. (1988) studied 106 outpatients in a pain clinic and observed that mental illness was diagnosed in about one-half of the patients. The majority of these suffered from depression, whereas the remainder had somatic disorders. This relation has also been emphasized by Chaturvedi (1987) in a study of 500 psychiatric
patients. He found chronic pain to be a common complaint especially among patients diagnosed as having anxiety neurosis, neurotic depression or hysteria. Such psychological parameters are also important in the experimental situation. Anxiety increases pain ratings when pain is experimentally induced in humans (Cornvall and Donderi 1988). Stimulation induced analgesia can most certainly be influenced by psychological factors and this has been demonstrated in an interesting study in animals by J&-urn and Shyu (1988). They investigated the relation between changed pain sensitivity induced by low-frequency electrical nerve stimulation and behavior in the rat. Anti-nociceptive effects were induced in rats that were quiet during the stimulation, whilst this did not occur in rats showing signs of distress. The behavioral signs of distress were interpreted as possibly equivalent to human anxiety. The study implies that analgesia can be counteracted by emotions, e.g., anxiety. The elevated level of anxiety and the unchanged or decreased PT following TENS in NR, in the present investigation, parallels their results. Our results are in line with the frequently stated view that patients with long lasting pain often exhibit high degrees of anxiety and depression. The NR in our study had significantly higher scores on the depression and trait anxiety tests than R. Similar results have been reported earlier by Hossenlopp et al. (1976). The levels in R were comparable with data obtained from healthy volunteers (Zung 1965; Spielberger et al. 19701. In conclusion, it is reasonable to assume that the emotional state of the patient or subject interferes with analgesia in the clinical or experimental situation. The only dimension on the MHLC scale that significantly differed between R and NR was the powerfulothers orientation (P-LOC) which was more pronounced in NR. Buckelew et al. (19901 suggest that patients with a high degree of P-LOC, i.e., those who are more reliant upon medical professionals, exhibit fewer coping strategies. It can be presumed that patients with a poor treatment outcome have a low ability to develop coping strategies. To a large extent TENS is self-administered so that the motivation and active participation of the patients play a crucial role in the outcome. From that point of view it might have been expected to find a more internally oriented locus of control in R compared to NR. However, we failed to replicate the relationship between treatment success and I-LOC reported by others (Wallston and Wallston 1981; Harkapla et al. 1991). Many of the psychological parameters may influence the perception of pain, both in the experimental as well as in the clinical situation. This was apparent when combining the results from Zung, STAI-Y, PLOC and degree of PT change. This emphasizes the close connection between physiological and psychological factors.
286
In conclusion, the findings of the present study indicate that NR and R differ from each other when using our test battery. This difference may have been due to the polarized sample of patients and show that physiological and psychological factors may influence the clinical outcome. Whether these differences are apparent in a more homogenous group of patients and in a prospective study are currently under investigation.
This work was supported by the Swedish Medical Research Council (Project 55) and Stiftelsen for Acupuncture and Alternative biological treatment methods. The authors would like to thank Nina Hedner, Dan Magnusson and Stina Morelius for their skilful technical and administrative assistance, Dr Eva Esbjornsson for valuable discussions and Dr Brian Noga for constructive comments on the manuscript.
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