- Email: [email protected]
Peripheral and central nervous changes in patients with rheumatoid arthritis in response to repetitive painful stimulation
International Journal of Psychophysiology 37 Ž2000. 177]183
Peripheral and central nervous changes in patients with rheumatoid arthritis in response to repetitive painful stimulation Thomas Hummela,U , Christine Schiessl b, Jorg ¨ Wendler c , Gerd Kobal b a
Department of Otorhinolaryngology, Uni¨ ersity of Dresden Medical School, Fetscherstrasse 74, 01307 Dresden, Germany b Department of Experimental and Clinical Pharmacology and Toxicology, Uni¨ ersity of Erlangen-Nurnberg, ¨ Krankenhausstrasse 9, 91054 Erlangen, Germany c Department of Medicine III, Institute of Clinical Immunology and Rheumatology, Uni¨ ersity of Erlangen-Nurnberg, ¨ Krankenhausstrasse 12, 91054 Erlangen, Germany Received 16 August 1999; received in revised form 12 January 2000; accepted 19 January 2000
Abstract It has been observed that patients with rheumatoid arthritis ŽRA. respond differently to repetitive painful stimulation. The present study investigated whether this is related to the peripheral or central nervous nociceptive system. EEG-derived potentials and the negative mucosal potential ŽNMP. from the respiratory epithelium were recorded in response to painful intranasal stimulation with gaseous CO 2 . Differences between groups Ž12 RA patients, 12 controls. were found when stimuli were presented at short intervals. While the NMP did not differ between groups, patients had larger cortical responses to the first stimuli of a series of painful stimuli. This may indicate that in RA central nervous changes of nociceptive processing are present. Q 2000 Elsevier Science B.V. All rights reserved. Keywords: Rheumatoid arthritis; Irritation; Nociception; Negative mucosal potential; Peripheral; Event-related potential
U
Corresponding author. Tel.: q49-351-458-4189; fax: q49-351-458-4326. E-mail address: [email protected] ŽT. Hummel. 0167-8760r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 7 6 0 Ž 0 0 . 0 0 0 8 7 - 8
178
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
1. Introduction Chronic pain is a key symptom of rheumatoid arthritis ŽRA.. Consecutively to arthritis the nociceptive system undergoes changes on peripheral Že.g. Schaible and Grubb, 1993. and central nervous levels Že.g. Coderre et al., 1993.. Results obtained by means of event-related potentials indicated that chronic inflammatory joint pain produces generalized changes in the processing of nociceptive information ŽWendler et al., 2000.. In comparison with healthy controls, RA patients exhibited an increased response amplitude when trains of painful stimuli were presented with an interstimulus interval of 2 s; this was interpreted in terms of a decreased adaptation to painful stimuli which in turn might contribute to chronic pain in RA. The present study was aimed to find out whether these changes are related to the peripheral or central nervous nociceptive system. The gaseous CO 2 stimulus used in the presently employed model fulfills most of the criteria of an ideal painful stimulus. CO 2 stimuli are considered to specifically excite the nociceptive system ŽThurauf et al., 1991; Steen et al., 1992.. ¨ They produce controllable, repeatable sensations, excite a restricted group of primary afferents and exhibit precisely controllable onset and termination. The model allows the recording of the negative mucosal potential ŽNMP. from the nasal respiratory epithelium in response to trigeminal stimuli such as CO 2 . This response can be extinguished by means of capsaicin ŽThurauf et al., ¨ 1991., and is thought to arise from the activation of nociceptors ŽKobal, 1985.. In addition, selective stimulants of the olfactory nerve, e.g. H 2 S or vanillin, fail to elicit NMPs. Evidence from both experimental animals ŽThurauf et al., 1991. and ¨ investigations in volunteers ŽThurauf et al., 1993. ¨ indicates that the NMP is not an epiphenomenon of autonomic reflexes. NMP recordings have been shown to correlate with intensity ratings of painful stimuli ŽKobal, 1985; Hummel et al., 1998.; they are also sensitive towards effects of non-steroidal anti-inflammatory drugs such as ibuprofen ŽLotsch ¨ et al., 1997a.. In addition, using CO 2 stimuli it is also possible to simultaneously record cortical chemo-somatosensory event-related potentials
ŽCSSERPs.. These EEG derived, late nearfield potentials correlate with the subjects’ pain ratings; they have been shown to be sensitive to analgesic drug effects Žsee Handwerker and Kobal, 1993., while they are relatively robust against non-nociceptive influences such as sedation ŽHummel et al. 1994b; Thurauf et al., 1994.. ¨
2. Methods RA patients Žeight female, four male subjects; age range from 27 to 59 years, mean age 41 years. were compared with age- and sex-matched healthy controls Žeight female, four male, age range from 27 to 58 years, mean age 42 years.. The Ethics Committee of the University of Erlangen-Nurn¨ berg approved the study which was performed in accordance to the Declaration of HelsinkirSummerset West. Patients were diagnosed according to criteria of the American Rheumatism Association; they had an active disease ) 1 year. Intake of analgesics was discontinued for 3 days prior to measurements. Each patientrindividual participated in a single experiment composed of two sessions separated by 5]10 min. During the first session responses to different intensities of 24 painful stimuli were analyzed Ž40, 50, 60 and 70% vrv CO 2 ; randomized sequence, stimulus duration 500 ms, interval f 60 s; duration of session f 25 min.. During the second session repetitive painful stimulation ŽHummel et al., 1994a. was performed with a series of four stimuli of equal intensity Ž60% vrv CO 2 ; interseries interval f 60 s; 12 series with each interstimulus interval, duration of session f 25 min.. The interstimulus interval ŽISI. of individual trains of stimuli alternated between 2 and 6 s. As described previously ŽKobal, 1985. presentation of the stimuli did not elicit any sensations related to changes in pressure or temperature. Stimuli were applied to the right nostril in a constantly flowing airstream of controlled temperature and humidity Ž8 lrmin, 368C, 80% RH; inner diameter of 10-cm long Teflon tubing connected to the stimulator outlet 2.6 mm.. During experiments subjects were comfortably seated in an air-conditioned chamber. White noise of approximately 80 dB HL ŽERA
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
stimulator, Tonnies, Germany. masked switching ¨ clicks of the stimulator. Prior to the experiments subjects were thoroughly acquainted with the experimental procedures including a special breathing technique Žvelopharyngeal closure: Kobal and Hummel, 1989. by means of which respiratory air-flow in the nose is avoided. As the study focused on recordings from the periphery, EEG was obtained from position Cz referenced to linked earlobes; blink artifacts were monitored from Fp2 ŽSIR amplifier, Germany.. The band-pass ranged between 0.2 and 30 Hz Žpassive RC low-pass filter, dampening at 31 Hz 66%.; EEG was recorded ŽCED 1401; Cambridge Electronic Devices, UK. over a period of 32.7 s including a pre-stimulus period of 2 s or 6 s Žsampling frequency of 62.5 Hz.. EEG-records were averaged off-line to yield pain-related late nearfield event-related potentials ŽHandwerker and Kobal, 1993.. Single responses contaminated by artifacts Žblinking, muscle activity. were discarded from the average. CSSERP were discarded when less than five individual responses were available for averaging. Latencies of the CSSERP peaks N1 and P2 were measured relative to stimulus onset Žsee insert in Fig. 2.. In addition, amplitudes P1N1 and N1P2 were analyzed. As the peak latencies exhibited a high interindividual variability, the temporal search windows for components were set at 100]550 ms for P1, 250]750 ms for N1 and 400]1300 ms for P2. Peak identification was performed heuristically by a single, experienced experimenter. The P2 wave typically served as a landmark; in case N1 was ill-defined, the last negative wave before the beginning of the P2 positivity was measured ŽBarz et al., 1997.. NMPs were recorded by means of tubular AgrAgCl electrodes Žreference: left bridge of the nose, Teflon tubing filled with 1% Ringer-agar; impedance 2]10 k V ŽKobal, 1985... Electrode placement was performed under endoscopic control Ž58 Pandview, 1.9 mm outer diameter, Wolf, Germany.; after the electrode had been positioned it was kept in place by adjustable clips mounted on a frame similar to lensless glasses ŽHummel et al., 1996b.. The NMP Žband-pass DC 30 Hz, DC amplifier, Tonnies, Germany. was ¨
179
analyzed similarly to the CSSERP Žsee above.. After averaging, response amplitude and latencies of onset and peak maximum were measured Žsee insert in Fig. 2.. Subjects used a computerized visual analog scale ŽKobal et al., 1990; Hummel et al., 1994b, 1996a; Lotsch et al., 1995, 1996, 1997b. to rate ¨ the perceived intensity in relation to a standard stimulus Ž60% vrv CO 2 . presented at the beginning of each session Žsee insert in Fig. 2.. The intensity of this standard stimulus was defined as 100 estimation units ŽEU.. To describe the intensity of the actual stimulus, using a joystick subjects adjusted the length of a red bar Žactual stimulus. in relation to a blue bar Žstandard stimulus, i.e. prescribed modulus. the length of which was fixed. When no pain had been perceived, subjects let the red bar disappear Ž0 EU.; when the actual stimulus was as intense as the standard they did not change the red bar’s length Ž100 EU.. By providing an additional point of the scale Ž100 EU. which is different from ‘no painful intensity’, this scale is thought to reduce interindividual variance due to differences in the sensitivity to painful stimuli. This is different from other VAS where only one endpoint of scale is defined Ž‘no painful intensity’. while the description of the other endpoint typically is much less precise Že.g. ‘unbearable pain’, or ‘worst pain ever’. ŽMelzack and Katz, 1999.. During the 60-s intervals between series or single stimuli including stimulus presentation and 2]3 s following the stimulation, subjects were requested to perform a tracking task on a video screen ŽHummel et al., 1994a.; using a joystick they had to keep a small square inside a larger one which moved around at random. The patients’ performance was assessed by measuring for how long the subjects had lost track of the independently moving square Žrange from 0 to 100% success in tracking.. To test for differences between patients and controls ŽSPSS 6.1.3 for WINDOWSTM ., data were submitted to analyses of variance ŽMANOVA, repeated measurements design; between subject factor ‘group’.. To test for differences between stimulus concentrations or the four repetitive stimulations, respectively, the
180
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
within-subject factors ‘stimulus concentration’ or ‘stimulus number’ were included. Results were analyzed separately for the two interstimulus intervals used. As the number of subjects varied between analyses Ždue to varying degrees of artifacts., degrees of freedom are presented with individual F values.
3. Results No significant main effects of the factor ‘group’ were found for the NMP Ž F1,20 F 0.92, PG 0.35.. NMP amplitudes increased with increasing stimulus concentration Ž F3,60 s 46.2, P- 0.001.; this was accompanied by a shortening of the NMP onset Ž F3,60 s 43.9, P- 0.001. and the maximum
NMP peak latency Ž F3,60 s 4.52, Ps 0.006. ŽFig. 2.. Response amplitudes decreased throughout the series of stimuli ŽISI 2 s: F3,60 s 24.5, P- 0.001; ISI 6 s: F3,60 s 42.2, P- 0.001. ŽFig. 1.. Latencies of response onset increased in relation to the repetitive stimulation ŽISI 2 s: F3,60 s 19.4, P0.001; ISI 6 s: F3,60 s 8.57, P- 0.001.. This was less pronounced when an ISI of 6 s was used. Latencies of the NMPs’ maximum also behaved differently in relation to the ISI used, i.e. at an ISI of 2 s peak latencies exhibited an increase in relation to repetitive stimulation Ž F3,60 s 2.91, P s 0.042., while this was not significant at an ISI of 6 s Ž F3,60 s 2.31, Ps 0.086.. CSSERP parameters did not significantly differ between groups Ž F1,14 F 1.53, PG 0.23.. However, as indicated by the significant interaction between
Fig. 1. Examples of NMP and CSSERP recordings after repetitive painful stimulation with four 500 ms pulses Žonset of 60% vrv CO 2 stimuli indicated by arrows. presented at interstimulus intervals ŽISI. of 2 s and 6 s. Localization of the NMPs septal recording site is indicated in the insert ŽB; outline of turbinates are indicated in gray for better orientation.. The CSSERP recording site Cz is indicated in the right insert close to the CSSERP graph; the response to the first stimulus obtained at an ISI of 2 s is presented in the left insert using a different scaling of the X-axis Žthe part of the curve that was magnified is indicated by a box with broken line border.. When stimuli were presented at an ISI of 2 s the response amplitude of both NMP and CSSERP decreased throughout the series; this decrease was less pronounced when an ISI of 6 s was used.
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
factors ‘stimulus number’ and ‘group’ Ž F3,48 G 4.05, PF 0.012., the first stimuli of the trains of four stimuli elicited higher amplitudes in RA patients than in controls ŽFig. 2.. CSSERP amplitudes increased and latencies shortened with an increase in stimulus concentration Žamplitude P1N1: F3,57 s 7.49, P- 0.001; amplitude N1P2: F3,57 s 8.87, P- 0.001; latency N1: F3,57 s 13.7, P0.001; latency P2: F3,57 s 11.2, P- 0.001.. At an ISI of 2 s amplitudes decreased throughout series of painful stimuli Žamplitude P1N1: F3,42 s 7.2, P s 0.001; amplitude N1P2: F3,42 s 16.8, P 0.001.. Latencies P2 were prolonged in relation to repetitive stimulation Ž F3,42 s 2.84, Ps 0.049.. At
181
an ISI of 6 s amplitudes also decreased with repeated stimulation Žamplitude P1N1 and N1P2: F3,48 G 15.6, P- 0.001.. For intensity ratings there were no significant effects of the factor ‘group’ Ž F1,19 F 1.84, PG 0.19.. Ratings increased with rising stimulus concentration Ž F3,57 s 135.1, P- 0.001., and they decreased with repetitive stimulation at an ISI of 6 s Ž F3,57 s 10.9, P - 0.001.; they remained unchanged at an ISI of 2 s Ž F3,57 s 1.37, Ps 0.26.. Tracking performance was analyzed in the six of the 12 patients who did not complain of difficulties when carrying out the motor task. Tracking scores did not differ significantly between
Fig. 2. Means and standard errors Ž6 F n F 12. of investigated parameters Žintensity ratings, in estimation units, EU; amplitude of the negative mucosal potential, in mV wNMPx; amplitudes of the chemo-somatosensory event-related potential, in mV wCSSERPx. obtained in patients ŽB. and healthy controls Žv.. Results are shown for session 1 Žstimulation with different stimulus concentrations; 40, 50, 60 and 70% vrv CO 2 , interstimulus interval wISIx 60 s. and session 2 Žrepetitive stimulation with series of four stimuli of 60% vrv CO 2 , at an ISI of 2 or 6 s.. The inserts indicate the computerized visual analog used for intensity ratings in comparison with a standard stimulus Ž60% vrv CO 2 . Žinsert left top., a schematic drawing of a NMP where peaks Žitalic. are indicated by arrows Žinsert left middle., and a schematic drawing of a CSSERP with peaks Žitalic. indicated by arrows Žinsert left bottom.. A significant interaction between factors ‘group’ and ‘stimulus number’ Ž Ps 0.012. emerged for CSSERP amplitudes at an ISI of 6 s, no such difference was found for the peripheral response, the NMP, or the intensity ratings.
182
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
groups ŽRA patients: Ms 77.9%, S.E.M. 7.2; controls: Ms 89.9%, S.E.M.s 3.0; t-test, d.f.s 6.8, Ps 0.17.; this indicated that patients and controls appeared to be equally motivated to carry out the instructions.
4. Discussion As indicated by the recordings from the epithelium the present data suggest that CSSERP changes in RA patients are not the result of peripheral changes. The results also partly confirm previous observations ŽWendler et al., 2000. that patients with chronic inflammatory joint pain respond differently to repetitive painful stimulation. Specifically, patients had larger cortical responses to the first stimuli of a series of painful stimuli; throughout the series response amplitudes decreased to the range of controls. It has to be noted that this was only the case when CO 2stimuli were presented at a relatively high repetition rate of 0.17 Hz. In the previous study ŽWendler et al., 2000. this difference between patients and controls was seen at an interstimulus interval of 2 s. However, in the present study this became significant only at an ISI of 6 s. This difference may largely relate to the low signal-to-noise ratio of the presently recorded CSSERP. In the present study the experimental paradigm was clearly focused on the NMP and not on the recording of CSSERP; this is why the recording conditions for ERPs were not as good as in typical ERP studies. The common finding of the two studies, however, is that differences between controls and RA patients seem to appear only when relatively short interstimulus intervals of less than 8 s are used. They are not found when single stimuli are applied, i.e. when the interstimulus interval is longer than 30 s. The patients’ responses to CO 2 stimuli throughout this session did not decrease to the same degree as they decreased in healthy subjects, i.e. habituation to painful stimuli appeared to be less pronounced in patients when CO 2Stimuli were presented at relatively high repeti-
tion rates during a short period of time. In this context it is important to keep in mind that there are carryover effects between series of painful stimuli ŽHummel et al., 1994a. such that even the response to first of the four stimuli of a series decreases throughout the session ŽHummel and Kobal, 1998.. However, here it is difficult to determine whether the larger response to the first of the four stimuli is subject to reduced habituation or short-term dishabituation ŽFruhstorfer et al., 1969.. The present results support research performed in patients with non-inflammatory chronic pain Žchronic low back pain, headache, temporomandibular pain, temporomandibular dysfunction., where Flor et al. Ž1993. analyzed both ERPs and event-related magnetic fields in response to tonic and phasic painful stimuli applied at a painful site Žthermal and electrical stimulation.. As with the present results an increase of the patients’ responses was only found with trains of phasic painful stimuli but not with single tonic stimuli. As the site of painful stimulation was not the target of inflammatory processes in our RA patients, the present study extends these findings. Although Flor et al. Ž1993. observed a marked increase of cortical responses when the stimulation was applied at the site of clinical pain, our data indicate that these changes appear to exist independent from the location of the pathological events. In other words, they appear to be more generalized; this may reflect neuroplasticity of the nociceptive system ŽElbert et al., 1994.. The observed phenomenon may reflect adaptive processes of the brain, i.e. processes which may help to control pain states. In fact, Flor et al. Ž1995. reported data suggesting that cortical reorganization and phantom-limb pain are positively related. Taken together, the results confirm previous observations insofar as patients with chronic inflammatory joint pain respond differently to repetitive painful stimulation. These changes do not appear to be due to peripheral effects. Future studies will investigate when these changes occur first during the course of the disease, and whether they can be reversed with appropriate treatment.
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
Acknowledgements This research was supported by DFG grant SFB 353 ŽA7. and by grant P01 DC 00161 from the National Institute on Deafness and Other Communication Disorders, National Institute of Health, USA. References Barz, S., Hummel, T., Pauli, E., Majer, M., Lang, C.J.G., Kobal, G., 1997. Chemosensory event-related potentials in response to trigeminal and olfactory stimulation in idiopathic Parkinson’s Disease. Neurology 49, 1424]1431. Coderre, T.J., Katz, J., Vaccarino, A.L., Melzack, R., 1993. Contribution of central neuroplasticity to pathological pain: review of clinical and experimental evidence. Pain 52, 259]285. Elbert, T., Flor, H., Birbaumer, N. et al., 1994. Extensive reorganisation of the somatosensory cortex in adult humans after nervous system injury. NeuroReport 5, 2593]2595. Flor, H., Birbaumer, N., Furst, M., Lutzenberger, W., Elbert, T., Braun, C., 1993. Evidence of enhanced peripheral and cortical responses to painful stimulation in states of chronic pain. Psychophysiology 30, 9. Flor, H., Elbert, T., Knecht, S. et al., 1995. Phantom-limb pain as a perceptual correlate of cortical reorganization following arm amputation. Nature 375, 482]484. Fruhstorfer, H., Jarvilehto, T., Soveri, P., 1969. Short-term habituation and dishabituation of the sensory evoked response in man. Acta Physiol. Scand. 76, 14A]15A. Handwerker, H., Kobal, G., 1993. Psychophysiology of experimentally induced pain. Physiol. Rev. 73, 639]671. Hummel, T., Kobal, G., 1998. Chemosensory event-related potentials to trigeminal stimuli change in relation to the interval between repetitive stimulation of the nasal mucosa. Eur. Arch. Otorhinolaryngol. 256, 16]21. Hummel, T., Gruber, M., Pauli, E., Kobal, G., 1994a. Eventrelated potentials in response to repetitive painful stimulation. Electroencephalogr. Clin. Neurophysiol. 92, 426]432. Hummel, T., Hummel, C., Friedel, I., Pauli, E., Kobal, G., 1994b. Assessment of antinociceptive effects in man: comparison of imipramine, tramadol, and anpirtoline. Br. J. Clin. Pharmacol. 37, 325]333. Hummel, T., Roscher, S., Pauli, E. et al., 1996a. Assessment of analgesia in man: tramadol controlled release formula vs. tramadol standard formulation. Eur. J. Clin. Pharmacol. 51, 31]38. Hummel, T., Schiessl, C., Wendler, J., Kobal, G., 1996b. Peripheral electrophysiological responses decrease in response to repetitive painful stimulation of the human nasal mucosa. Neurosci. Lett. 212, 37]40.
183
Hummel, T., Kraetsch, H.-G., Pauli, E., Kobal, G., 1998. Responses to nasal irritation obtained from the human nasal mucosa. Rhinology 36, 168]172. Kobal, G., 1985. Pain-related electrical potentials of the human nasal mucosa elicited by chemical stimulation. Pain 22, 151]163. Kobal, G., Hummel, T., 1989. Brain responses to chemical stimulation of the trigeminal nerve in man. In: Green, B.G., Mason, J.R., Kare, M.R. ŽEds.., Chemical Senses, 2: Irritation. Marcel-Dekker, New York, pp. 123]139. Kobal, G., Hummel, T., Hoesl, M., 1990. Pain-related electrical evoked potentials by chemical stimuli: effects of fentanyl. In: Lipton, S., Tunks, E., Zoppi, M. ŽEds.., Advances in Pain Research and Therapy. Raven Press, New York, pp. 95]98. Lotsch, J., Geisslinger, G., Mohammadian, P., Brune, K., ¨ Kobal, G., 1995. Effects of flurbiprofen enantiomers on pain-related chemo-somatosensory evoked potentials in human subjects. Br. J. Clin. Pharmacol. 40, 339]346. Lotsch, J., Mohammadian, P., Hummel, T. et al., 1996. Effects ¨ of azapropazone on pain-related brain activity in human subjects. Br. J. Clin. Pharmacol. 40, 545]552. Lotsch, J., Hummel, T., Kraetsch, H.G., Kobal, G., 1997a. The ¨ negative mucosal potential: separating central and peripheral effects of NSAIDs in man. Eur. J. Clin. Pharmacol. 52, 359]364. Lotsch, J., Kobal, G., Stockmann, A., Brune, K., Geisslinger, ¨ G., 1997b. Lack of analgesic activity of morphine-6glucuronide after short-term intravenous administration in healthy volunteers. Anesthesiology 87, 1348]1358. Melzack, R., Katz, J., 1999. Pain measurement in persons in pain. In: Wall, P.D., Melzack, R. ŽEds.., Textbook of Pain, 4th ed Churchill Livingstone, Edinburgh, pp. 409]426. Schaible, H.G., Grubb, B.D., 1993. Afferent and spinal mechanisms of joint pain. Pain 55, 5]54. Steen, K.H., Reeh, P.W., Anton, F., Handwerker, H.O., 1992. Protons selectively induce lasting excitation and sensitization of nociceptors in rat skin. J. Neurosci. 12, 86]95. Thurauf, N., Friedel, I., Hummel, C., Kobal, G., 1991. The ¨ mucosal potential elicited by noxious chemical stimuli: is it a peripheral nociceptive even. Neurosci. Lett. 128, 297]300. Thurauf, N., Hummel, T., Kettenmann, B., Kobal, G., 1993. ¨ Nociceptive and reflexive responses recorded from the human nasal mucosa. Brain Res. 629, 293]299. Thurauf, N., Ditterich, W., Kobal, G., 1994. Different sensitiv¨ ity of pain-related chemosensory potentials evoked by stimulation with CO 2 , tooth pulp event-related potentials, and acoustic event-related potentials to the tranquilizer diazepam. Br. J. Clin. Pharmacol. 38, 545]555. Wendler, J., Hummel, T., Reissinger, M., et al., 2000. Patients with rheumatoid arthritis adapt differently to repetitive painful stimuli compared to healthy controls. J. Clin. Neurosci. in press.
Peripheral and central nervous changes in patients with rheumatoid arthritis in response to repetitive painful stimulation Thomas Hummela,U , Christine Schiessl b, Jorg ¨ Wendler c , Gerd Kobal b a
Department of Otorhinolaryngology, Uni¨ ersity of Dresden Medical School, Fetscherstrasse 74, 01307 Dresden, Germany b Department of Experimental and Clinical Pharmacology and Toxicology, Uni¨ ersity of Erlangen-Nurnberg, ¨ Krankenhausstrasse 9, 91054 Erlangen, Germany c Department of Medicine III, Institute of Clinical Immunology and Rheumatology, Uni¨ ersity of Erlangen-Nurnberg, ¨ Krankenhausstrasse 12, 91054 Erlangen, Germany Received 16 August 1999; received in revised form 12 January 2000; accepted 19 January 2000
Abstract It has been observed that patients with rheumatoid arthritis ŽRA. respond differently to repetitive painful stimulation. The present study investigated whether this is related to the peripheral or central nervous nociceptive system. EEG-derived potentials and the negative mucosal potential ŽNMP. from the respiratory epithelium were recorded in response to painful intranasal stimulation with gaseous CO 2 . Differences between groups Ž12 RA patients, 12 controls. were found when stimuli were presented at short intervals. While the NMP did not differ between groups, patients had larger cortical responses to the first stimuli of a series of painful stimuli. This may indicate that in RA central nervous changes of nociceptive processing are present. Q 2000 Elsevier Science B.V. All rights reserved. Keywords: Rheumatoid arthritis; Irritation; Nociception; Negative mucosal potential; Peripheral; Event-related potential
U
Corresponding author. Tel.: q49-351-458-4189; fax: q49-351-458-4326. E-mail address: [email protected] ŽT. Hummel. 0167-8760r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 7 6 0 Ž 0 0 . 0 0 0 8 7 - 8
178
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
1. Introduction Chronic pain is a key symptom of rheumatoid arthritis ŽRA.. Consecutively to arthritis the nociceptive system undergoes changes on peripheral Že.g. Schaible and Grubb, 1993. and central nervous levels Že.g. Coderre et al., 1993.. Results obtained by means of event-related potentials indicated that chronic inflammatory joint pain produces generalized changes in the processing of nociceptive information ŽWendler et al., 2000.. In comparison with healthy controls, RA patients exhibited an increased response amplitude when trains of painful stimuli were presented with an interstimulus interval of 2 s; this was interpreted in terms of a decreased adaptation to painful stimuli which in turn might contribute to chronic pain in RA. The present study was aimed to find out whether these changes are related to the peripheral or central nervous nociceptive system. The gaseous CO 2 stimulus used in the presently employed model fulfills most of the criteria of an ideal painful stimulus. CO 2 stimuli are considered to specifically excite the nociceptive system ŽThurauf et al., 1991; Steen et al., 1992.. ¨ They produce controllable, repeatable sensations, excite a restricted group of primary afferents and exhibit precisely controllable onset and termination. The model allows the recording of the negative mucosal potential ŽNMP. from the nasal respiratory epithelium in response to trigeminal stimuli such as CO 2 . This response can be extinguished by means of capsaicin ŽThurauf et al., ¨ 1991., and is thought to arise from the activation of nociceptors ŽKobal, 1985.. In addition, selective stimulants of the olfactory nerve, e.g. H 2 S or vanillin, fail to elicit NMPs. Evidence from both experimental animals ŽThurauf et al., 1991. and ¨ investigations in volunteers ŽThurauf et al., 1993. ¨ indicates that the NMP is not an epiphenomenon of autonomic reflexes. NMP recordings have been shown to correlate with intensity ratings of painful stimuli ŽKobal, 1985; Hummel et al., 1998.; they are also sensitive towards effects of non-steroidal anti-inflammatory drugs such as ibuprofen ŽLotsch ¨ et al., 1997a.. In addition, using CO 2 stimuli it is also possible to simultaneously record cortical chemo-somatosensory event-related potentials
ŽCSSERPs.. These EEG derived, late nearfield potentials correlate with the subjects’ pain ratings; they have been shown to be sensitive to analgesic drug effects Žsee Handwerker and Kobal, 1993., while they are relatively robust against non-nociceptive influences such as sedation ŽHummel et al. 1994b; Thurauf et al., 1994.. ¨
2. Methods RA patients Žeight female, four male subjects; age range from 27 to 59 years, mean age 41 years. were compared with age- and sex-matched healthy controls Žeight female, four male, age range from 27 to 58 years, mean age 42 years.. The Ethics Committee of the University of Erlangen-Nurn¨ berg approved the study which was performed in accordance to the Declaration of HelsinkirSummerset West. Patients were diagnosed according to criteria of the American Rheumatism Association; they had an active disease ) 1 year. Intake of analgesics was discontinued for 3 days prior to measurements. Each patientrindividual participated in a single experiment composed of two sessions separated by 5]10 min. During the first session responses to different intensities of 24 painful stimuli were analyzed Ž40, 50, 60 and 70% vrv CO 2 ; randomized sequence, stimulus duration 500 ms, interval f 60 s; duration of session f 25 min.. During the second session repetitive painful stimulation ŽHummel et al., 1994a. was performed with a series of four stimuli of equal intensity Ž60% vrv CO 2 ; interseries interval f 60 s; 12 series with each interstimulus interval, duration of session f 25 min.. The interstimulus interval ŽISI. of individual trains of stimuli alternated between 2 and 6 s. As described previously ŽKobal, 1985. presentation of the stimuli did not elicit any sensations related to changes in pressure or temperature. Stimuli were applied to the right nostril in a constantly flowing airstream of controlled temperature and humidity Ž8 lrmin, 368C, 80% RH; inner diameter of 10-cm long Teflon tubing connected to the stimulator outlet 2.6 mm.. During experiments subjects were comfortably seated in an air-conditioned chamber. White noise of approximately 80 dB HL ŽERA
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
stimulator, Tonnies, Germany. masked switching ¨ clicks of the stimulator. Prior to the experiments subjects were thoroughly acquainted with the experimental procedures including a special breathing technique Žvelopharyngeal closure: Kobal and Hummel, 1989. by means of which respiratory air-flow in the nose is avoided. As the study focused on recordings from the periphery, EEG was obtained from position Cz referenced to linked earlobes; blink artifacts were monitored from Fp2 ŽSIR amplifier, Germany.. The band-pass ranged between 0.2 and 30 Hz Žpassive RC low-pass filter, dampening at 31 Hz 66%.; EEG was recorded ŽCED 1401; Cambridge Electronic Devices, UK. over a period of 32.7 s including a pre-stimulus period of 2 s or 6 s Žsampling frequency of 62.5 Hz.. EEG-records were averaged off-line to yield pain-related late nearfield event-related potentials ŽHandwerker and Kobal, 1993.. Single responses contaminated by artifacts Žblinking, muscle activity. were discarded from the average. CSSERP were discarded when less than five individual responses were available for averaging. Latencies of the CSSERP peaks N1 and P2 were measured relative to stimulus onset Žsee insert in Fig. 2.. In addition, amplitudes P1N1 and N1P2 were analyzed. As the peak latencies exhibited a high interindividual variability, the temporal search windows for components were set at 100]550 ms for P1, 250]750 ms for N1 and 400]1300 ms for P2. Peak identification was performed heuristically by a single, experienced experimenter. The P2 wave typically served as a landmark; in case N1 was ill-defined, the last negative wave before the beginning of the P2 positivity was measured ŽBarz et al., 1997.. NMPs were recorded by means of tubular AgrAgCl electrodes Žreference: left bridge of the nose, Teflon tubing filled with 1% Ringer-agar; impedance 2]10 k V ŽKobal, 1985... Electrode placement was performed under endoscopic control Ž58 Pandview, 1.9 mm outer diameter, Wolf, Germany.; after the electrode had been positioned it was kept in place by adjustable clips mounted on a frame similar to lensless glasses ŽHummel et al., 1996b.. The NMP Žband-pass DC 30 Hz, DC amplifier, Tonnies, Germany. was ¨
179
analyzed similarly to the CSSERP Žsee above.. After averaging, response amplitude and latencies of onset and peak maximum were measured Žsee insert in Fig. 2.. Subjects used a computerized visual analog scale ŽKobal et al., 1990; Hummel et al., 1994b, 1996a; Lotsch et al., 1995, 1996, 1997b. to rate ¨ the perceived intensity in relation to a standard stimulus Ž60% vrv CO 2 . presented at the beginning of each session Žsee insert in Fig. 2.. The intensity of this standard stimulus was defined as 100 estimation units ŽEU.. To describe the intensity of the actual stimulus, using a joystick subjects adjusted the length of a red bar Žactual stimulus. in relation to a blue bar Žstandard stimulus, i.e. prescribed modulus. the length of which was fixed. When no pain had been perceived, subjects let the red bar disappear Ž0 EU.; when the actual stimulus was as intense as the standard they did not change the red bar’s length Ž100 EU.. By providing an additional point of the scale Ž100 EU. which is different from ‘no painful intensity’, this scale is thought to reduce interindividual variance due to differences in the sensitivity to painful stimuli. This is different from other VAS where only one endpoint of scale is defined Ž‘no painful intensity’. while the description of the other endpoint typically is much less precise Že.g. ‘unbearable pain’, or ‘worst pain ever’. ŽMelzack and Katz, 1999.. During the 60-s intervals between series or single stimuli including stimulus presentation and 2]3 s following the stimulation, subjects were requested to perform a tracking task on a video screen ŽHummel et al., 1994a.; using a joystick they had to keep a small square inside a larger one which moved around at random. The patients’ performance was assessed by measuring for how long the subjects had lost track of the independently moving square Žrange from 0 to 100% success in tracking.. To test for differences between patients and controls ŽSPSS 6.1.3 for WINDOWSTM ., data were submitted to analyses of variance ŽMANOVA, repeated measurements design; between subject factor ‘group’.. To test for differences between stimulus concentrations or the four repetitive stimulations, respectively, the
180
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
within-subject factors ‘stimulus concentration’ or ‘stimulus number’ were included. Results were analyzed separately for the two interstimulus intervals used. As the number of subjects varied between analyses Ždue to varying degrees of artifacts., degrees of freedom are presented with individual F values.
3. Results No significant main effects of the factor ‘group’ were found for the NMP Ž F1,20 F 0.92, PG 0.35.. NMP amplitudes increased with increasing stimulus concentration Ž F3,60 s 46.2, P- 0.001.; this was accompanied by a shortening of the NMP onset Ž F3,60 s 43.9, P- 0.001. and the maximum
NMP peak latency Ž F3,60 s 4.52, Ps 0.006. ŽFig. 2.. Response amplitudes decreased throughout the series of stimuli ŽISI 2 s: F3,60 s 24.5, P- 0.001; ISI 6 s: F3,60 s 42.2, P- 0.001. ŽFig. 1.. Latencies of response onset increased in relation to the repetitive stimulation ŽISI 2 s: F3,60 s 19.4, P0.001; ISI 6 s: F3,60 s 8.57, P- 0.001.. This was less pronounced when an ISI of 6 s was used. Latencies of the NMPs’ maximum also behaved differently in relation to the ISI used, i.e. at an ISI of 2 s peak latencies exhibited an increase in relation to repetitive stimulation Ž F3,60 s 2.91, P s 0.042., while this was not significant at an ISI of 6 s Ž F3,60 s 2.31, Ps 0.086.. CSSERP parameters did not significantly differ between groups Ž F1,14 F 1.53, PG 0.23.. However, as indicated by the significant interaction between
Fig. 1. Examples of NMP and CSSERP recordings after repetitive painful stimulation with four 500 ms pulses Žonset of 60% vrv CO 2 stimuli indicated by arrows. presented at interstimulus intervals ŽISI. of 2 s and 6 s. Localization of the NMPs septal recording site is indicated in the insert ŽB; outline of turbinates are indicated in gray for better orientation.. The CSSERP recording site Cz is indicated in the right insert close to the CSSERP graph; the response to the first stimulus obtained at an ISI of 2 s is presented in the left insert using a different scaling of the X-axis Žthe part of the curve that was magnified is indicated by a box with broken line border.. When stimuli were presented at an ISI of 2 s the response amplitude of both NMP and CSSERP decreased throughout the series; this decrease was less pronounced when an ISI of 6 s was used.
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
factors ‘stimulus number’ and ‘group’ Ž F3,48 G 4.05, PF 0.012., the first stimuli of the trains of four stimuli elicited higher amplitudes in RA patients than in controls ŽFig. 2.. CSSERP amplitudes increased and latencies shortened with an increase in stimulus concentration Žamplitude P1N1: F3,57 s 7.49, P- 0.001; amplitude N1P2: F3,57 s 8.87, P- 0.001; latency N1: F3,57 s 13.7, P0.001; latency P2: F3,57 s 11.2, P- 0.001.. At an ISI of 2 s amplitudes decreased throughout series of painful stimuli Žamplitude P1N1: F3,42 s 7.2, P s 0.001; amplitude N1P2: F3,42 s 16.8, P 0.001.. Latencies P2 were prolonged in relation to repetitive stimulation Ž F3,42 s 2.84, Ps 0.049.. At
181
an ISI of 6 s amplitudes also decreased with repeated stimulation Žamplitude P1N1 and N1P2: F3,48 G 15.6, P- 0.001.. For intensity ratings there were no significant effects of the factor ‘group’ Ž F1,19 F 1.84, PG 0.19.. Ratings increased with rising stimulus concentration Ž F3,57 s 135.1, P- 0.001., and they decreased with repetitive stimulation at an ISI of 6 s Ž F3,57 s 10.9, P - 0.001.; they remained unchanged at an ISI of 2 s Ž F3,57 s 1.37, Ps 0.26.. Tracking performance was analyzed in the six of the 12 patients who did not complain of difficulties when carrying out the motor task. Tracking scores did not differ significantly between
Fig. 2. Means and standard errors Ž6 F n F 12. of investigated parameters Žintensity ratings, in estimation units, EU; amplitude of the negative mucosal potential, in mV wNMPx; amplitudes of the chemo-somatosensory event-related potential, in mV wCSSERPx. obtained in patients ŽB. and healthy controls Žv.. Results are shown for session 1 Žstimulation with different stimulus concentrations; 40, 50, 60 and 70% vrv CO 2 , interstimulus interval wISIx 60 s. and session 2 Žrepetitive stimulation with series of four stimuli of 60% vrv CO 2 , at an ISI of 2 or 6 s.. The inserts indicate the computerized visual analog used for intensity ratings in comparison with a standard stimulus Ž60% vrv CO 2 . Žinsert left top., a schematic drawing of a NMP where peaks Žitalic. are indicated by arrows Žinsert left middle., and a schematic drawing of a CSSERP with peaks Žitalic. indicated by arrows Žinsert left bottom.. A significant interaction between factors ‘group’ and ‘stimulus number’ Ž Ps 0.012. emerged for CSSERP amplitudes at an ISI of 6 s, no such difference was found for the peripheral response, the NMP, or the intensity ratings.
182
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
groups ŽRA patients: Ms 77.9%, S.E.M. 7.2; controls: Ms 89.9%, S.E.M.s 3.0; t-test, d.f.s 6.8, Ps 0.17.; this indicated that patients and controls appeared to be equally motivated to carry out the instructions.
4. Discussion As indicated by the recordings from the epithelium the present data suggest that CSSERP changes in RA patients are not the result of peripheral changes. The results also partly confirm previous observations ŽWendler et al., 2000. that patients with chronic inflammatory joint pain respond differently to repetitive painful stimulation. Specifically, patients had larger cortical responses to the first stimuli of a series of painful stimuli; throughout the series response amplitudes decreased to the range of controls. It has to be noted that this was only the case when CO 2stimuli were presented at a relatively high repetition rate of 0.17 Hz. In the previous study ŽWendler et al., 2000. this difference between patients and controls was seen at an interstimulus interval of 2 s. However, in the present study this became significant only at an ISI of 6 s. This difference may largely relate to the low signal-to-noise ratio of the presently recorded CSSERP. In the present study the experimental paradigm was clearly focused on the NMP and not on the recording of CSSERP; this is why the recording conditions for ERPs were not as good as in typical ERP studies. The common finding of the two studies, however, is that differences between controls and RA patients seem to appear only when relatively short interstimulus intervals of less than 8 s are used. They are not found when single stimuli are applied, i.e. when the interstimulus interval is longer than 30 s. The patients’ responses to CO 2 stimuli throughout this session did not decrease to the same degree as they decreased in healthy subjects, i.e. habituation to painful stimuli appeared to be less pronounced in patients when CO 2Stimuli were presented at relatively high repeti-
tion rates during a short period of time. In this context it is important to keep in mind that there are carryover effects between series of painful stimuli ŽHummel et al., 1994a. such that even the response to first of the four stimuli of a series decreases throughout the session ŽHummel and Kobal, 1998.. However, here it is difficult to determine whether the larger response to the first of the four stimuli is subject to reduced habituation or short-term dishabituation ŽFruhstorfer et al., 1969.. The present results support research performed in patients with non-inflammatory chronic pain Žchronic low back pain, headache, temporomandibular pain, temporomandibular dysfunction., where Flor et al. Ž1993. analyzed both ERPs and event-related magnetic fields in response to tonic and phasic painful stimuli applied at a painful site Žthermal and electrical stimulation.. As with the present results an increase of the patients’ responses was only found with trains of phasic painful stimuli but not with single tonic stimuli. As the site of painful stimulation was not the target of inflammatory processes in our RA patients, the present study extends these findings. Although Flor et al. Ž1993. observed a marked increase of cortical responses when the stimulation was applied at the site of clinical pain, our data indicate that these changes appear to exist independent from the location of the pathological events. In other words, they appear to be more generalized; this may reflect neuroplasticity of the nociceptive system ŽElbert et al., 1994.. The observed phenomenon may reflect adaptive processes of the brain, i.e. processes which may help to control pain states. In fact, Flor et al. Ž1995. reported data suggesting that cortical reorganization and phantom-limb pain are positively related. Taken together, the results confirm previous observations insofar as patients with chronic inflammatory joint pain respond differently to repetitive painful stimulation. These changes do not appear to be due to peripheral effects. Future studies will investigate when these changes occur first during the course of the disease, and whether they can be reversed with appropriate treatment.
T. Hummel et al. r International Journal of Psychophysiology 37 (2000) 177]183
Acknowledgements This research was supported by DFG grant SFB 353 ŽA7. and by grant P01 DC 00161 from the National Institute on Deafness and Other Communication Disorders, National Institute of Health, USA. References Barz, S., Hummel, T., Pauli, E., Majer, M., Lang, C.J.G., Kobal, G., 1997. Chemosensory event-related potentials in response to trigeminal and olfactory stimulation in idiopathic Parkinson’s Disease. Neurology 49, 1424]1431. Coderre, T.J., Katz, J., Vaccarino, A.L., Melzack, R., 1993. Contribution of central neuroplasticity to pathological pain: review of clinical and experimental evidence. Pain 52, 259]285. Elbert, T., Flor, H., Birbaumer, N. et al., 1994. Extensive reorganisation of the somatosensory cortex in adult humans after nervous system injury. NeuroReport 5, 2593]2595. Flor, H., Birbaumer, N., Furst, M., Lutzenberger, W., Elbert, T., Braun, C., 1993. Evidence of enhanced peripheral and cortical responses to painful stimulation in states of chronic pain. Psychophysiology 30, 9. Flor, H., Elbert, T., Knecht, S. et al., 1995. Phantom-limb pain as a perceptual correlate of cortical reorganization following arm amputation. Nature 375, 482]484. Fruhstorfer, H., Jarvilehto, T., Soveri, P., 1969. Short-term habituation and dishabituation of the sensory evoked response in man. Acta Physiol. Scand. 76, 14A]15A. Handwerker, H., Kobal, G., 1993. Psychophysiology of experimentally induced pain. Physiol. Rev. 73, 639]671. Hummel, T., Kobal, G., 1998. Chemosensory event-related potentials to trigeminal stimuli change in relation to the interval between repetitive stimulation of the nasal mucosa. Eur. Arch. Otorhinolaryngol. 256, 16]21. Hummel, T., Gruber, M., Pauli, E., Kobal, G., 1994a. Eventrelated potentials in response to repetitive painful stimulation. Electroencephalogr. Clin. Neurophysiol. 92, 426]432. Hummel, T., Hummel, C., Friedel, I., Pauli, E., Kobal, G., 1994b. Assessment of antinociceptive effects in man: comparison of imipramine, tramadol, and anpirtoline. Br. J. Clin. Pharmacol. 37, 325]333. Hummel, T., Roscher, S., Pauli, E. et al., 1996a. Assessment of analgesia in man: tramadol controlled release formula vs. tramadol standard formulation. Eur. J. Clin. Pharmacol. 51, 31]38. Hummel, T., Schiessl, C., Wendler, J., Kobal, G., 1996b. Peripheral electrophysiological responses decrease in response to repetitive painful stimulation of the human nasal mucosa. Neurosci. Lett. 212, 37]40.
183
Hummel, T., Kraetsch, H.-G., Pauli, E., Kobal, G., 1998. Responses to nasal irritation obtained from the human nasal mucosa. Rhinology 36, 168]172. Kobal, G., 1985. Pain-related electrical potentials of the human nasal mucosa elicited by chemical stimulation. Pain 22, 151]163. Kobal, G., Hummel, T., 1989. Brain responses to chemical stimulation of the trigeminal nerve in man. In: Green, B.G., Mason, J.R., Kare, M.R. ŽEds.., Chemical Senses, 2: Irritation. Marcel-Dekker, New York, pp. 123]139. Kobal, G., Hummel, T., Hoesl, M., 1990. Pain-related electrical evoked potentials by chemical stimuli: effects of fentanyl. In: Lipton, S., Tunks, E., Zoppi, M. ŽEds.., Advances in Pain Research and Therapy. Raven Press, New York, pp. 95]98. Lotsch, J., Geisslinger, G., Mohammadian, P., Brune, K., ¨ Kobal, G., 1995. Effects of flurbiprofen enantiomers on pain-related chemo-somatosensory evoked potentials in human subjects. Br. J. Clin. Pharmacol. 40, 339]346. Lotsch, J., Mohammadian, P., Hummel, T. et al., 1996. Effects ¨ of azapropazone on pain-related brain activity in human subjects. Br. J. Clin. Pharmacol. 40, 545]552. Lotsch, J., Hummel, T., Kraetsch, H.G., Kobal, G., 1997a. The ¨ negative mucosal potential: separating central and peripheral effects of NSAIDs in man. Eur. J. Clin. Pharmacol. 52, 359]364. Lotsch, J., Kobal, G., Stockmann, A., Brune, K., Geisslinger, ¨ G., 1997b. Lack of analgesic activity of morphine-6glucuronide after short-term intravenous administration in healthy volunteers. Anesthesiology 87, 1348]1358. Melzack, R., Katz, J., 1999. Pain measurement in persons in pain. In: Wall, P.D., Melzack, R. ŽEds.., Textbook of Pain, 4th ed Churchill Livingstone, Edinburgh, pp. 409]426. Schaible, H.G., Grubb, B.D., 1993. Afferent and spinal mechanisms of joint pain. Pain 55, 5]54. Steen, K.H., Reeh, P.W., Anton, F., Handwerker, H.O., 1992. Protons selectively induce lasting excitation and sensitization of nociceptors in rat skin. J. Neurosci. 12, 86]95. Thurauf, N., Friedel, I., Hummel, C., Kobal, G., 1991. The ¨ mucosal potential elicited by noxious chemical stimuli: is it a peripheral nociceptive even. Neurosci. Lett. 128, 297]300. Thurauf, N., Hummel, T., Kettenmann, B., Kobal, G., 1993. ¨ Nociceptive and reflexive responses recorded from the human nasal mucosa. Brain Res. 629, 293]299. Thurauf, N., Ditterich, W., Kobal, G., 1994. Different sensitiv¨ ity of pain-related chemosensory potentials evoked by stimulation with CO 2 , tooth pulp event-related potentials, and acoustic event-related potentials to the tranquilizer diazepam. Br. J. Clin. Pharmacol. 38, 545]555. Wendler, J., Hummel, T., Reissinger, M., et al., 2000. Patients with rheumatoid arthritis adapt differently to repetitive painful stimuli compared to healthy controls. J. Clin. Neurosci. in press.