Denying the Truth Does Not Change the Facts: A Systematic Analysis of Pseudoscientific Denial of Complex Regional Pain Syndrome.

PURPOSE: Several articles have claimed that complex regional pain syndrome (CRPS) does not exist. Although a minority view, it is important to understand the arguments presented in these articles. We conducted a systematic literature search to evaluate the methodological quality of articles that claim CRPS does not exist. We then examined and refuted the arguments supporting this claim using up-to-date scientific literature on CRPS. METHODS: A systematic search was conducted in MEDLINE, EMBASE and Cochrane CENTRAL databases. Inclusion criteria for articles were (a) a claim made that CRPS does not exist or that CRPS is not a distinct diagnostic entity and (b) support of these claims with subsequent argument(s). The methodological quality of articles was assessed if possible. RESULTS: Nine articles were included for analysis: 4 narrative reviews, 2 personal views, 1 letter, 1 editorial and 1 case report. Seven points of controversy were used in these articles to argue that CRPS does not exist: 1) disagreement with the label "CRPS"; 2) the "unclear" pathophysiology; 3) the validity of the diagnostic criteria; 4) CRPS as a normal consequence of immobilization; 5) the role of psychological factors; 6) other identifiable causes for CRPS symptoms; and 7) the methodological quality of CRPS research. CONCLUSION: The level of evidence for the claim that CRPS does not exist is very weak. Published accounts concluding that CRPS does not exist, in the absence of primary evidence to underpin them, can harm patients by encouraging dismissal of patients' signs and symptoms.

[Complex regional pain syndrome (CRPS) : An update]. / Das komplexe regionale Schmerzsyndrom (CRPS) : Ein Update.

The acute phase of complex regional pain syndrome (CRPS) is pathophysiologically characterized by an activation of the immune system and its associated inflammatory response. During the course of CRPS, central nervous symptoms like mechanical hyperalgesia, loss of sensation, and body perception disorders develop. Psychological factors such as pain-related anxiety and traumatic events might have a negative effect on the treatment outcome. While the visible inflammatory symptoms improve, the pain often persists. A stage adapted, targeted treatment could improve the prognosis. Effective multidisciplinary treatment includes the following: pharmacotherapy with steroids, bisphosphonates, or dimethylsulfoxide cream (acute phase), and antineuropathic analgesics (all phases); physiotherapy and behavioral therapy for pain-related anxiety and avoidance of movement; and interventional treatment like spinal cord or dorsal root ganglion stimulation if noninvasive options failed.

[Complex regional pain syndrome (CRPS) : An update]. / Das komplexe regionale Schmerzsyndrom (CRPS) : Ein Update.

The acute phase of complex regional pain syndrome (CRPS) is pathophysiologically characterized by an activation of the immune system and its associated inflammatory response. During the course of CRPS, central nervous symptoms like mechanical hyperalgesia, loss of sensation, and body perception disorders develop. Psychological factors such as pain-related anxiety and traumatic events might have a negative effect on the treatment outcome. While the visible inflammatory symptoms improve, the pain often persists. A stage adapted, targeted treatment could improve the prognosis. Effective multidisciplinary treatment includes the following: pharmacotherapy with steroids, bisphosphonates, or dimethylsulfoxide cream (acute phase), and antineuropathic analgesics (all phases); physiotherapy and behavioral therapy for pain-related anxiety and avoidance of movement; and interventional treatment like spinal cord or dorsal root ganglion stimulation if noninvasive options failed.

Mental load during cognitive performance in complex regional pain syndrome I.

BACKGROUND: Complex regional pain syndrome (CRPS) is associated with deficits in limb recognition. The purpose of our study was to determine whether mental load during this task affected performance, sympathetic nervous system activity or pain in CRPS patients. METHODS: We investigated twenty CRPS-I patients with pain in the upper extremity and twenty age- and sex-matched healthy controls. Each participant completed a limb recognition task. To experimentally manipulate mental load, the presentation time for each picture varied from 2 s (greatest mental load), 4, 6 to 10 s (least mental load). Before and after each run, pain intensity was assessed. Skin conductance was recorded continuously. RESULTS: Patients with CRPS did not differ from controls in terms of limb recognition and skin conductance reactivity. However, patients with CRPS reported an increase in pain during the task, particularly during high mental load and during the latter stages of the task. Interestingly, state anxiety and depressive symptoms were also associated with increases in pain intensity during high mental load. CONCLUSIONS: These findings indicate that high mental load intensifies pain in CRPS. The increase of pain in association with anxiety and depression indicates a detrimental effect of negative affective states in situations of high stress and mental load in CRPS. SIGNIFICANCE: The effects of mental load need to be considered when patients with CRPS-I are investigated for diagnostic or therapeutic reasons.

Are you better? A multi-centre study of patient-defined recovery from Complex Regional Pain Syndrome.

BACKGROUND: Complex Regional Pain Syndrome (CRPS) symptoms can significantly differ between patients, fluctuate over time, disappear or persist. This leads to problems in defining recovery and in evaluating the efficacy of therapeutic interventions. OBJECTIVES: To define recovery from the patients' perspective and better understand their priorities for treatment approaches. METHODS: Establishing an international consortium, we used a 2-Round Delphi-based study in eight countries across Europe and North America. Participants ≥18 years who met, or had met, Budapest clinical criteria were included. Round 1 participants completed the statement: 'I would/do consider myself recovered from CRPS if/because…' alongside demographic and health questionnaires. Data were thematically organised and represented as 62 statements, from which participants identified and ranked their recovery priorities in Round 2. RESULTS: Round 1 (N = 347, 80% female, 91% non-recovered) dominant ICF themes were: activities of daily living; bodily functions; external factors; participation and personal factors. The top five priority statements in Round 2 (N = 252) were: no longer having (1) CRPS-related pain, (2) generalised pain and discomfort, (3) restricted range of movement, (4) need for medication, (5) stiffness in the affected limb. With very few exceptions, priorities were consistent, irrespective of patient demographics/geography. Symptoms affecting daily activities were among those most frequently reported. CONCLUSIONS: Our data showed a small number of themes are of highest importance to CRPS patients' definition of recovery. Patients want their pain, movement restriction and reliance on medication to be addressed, above all other factors. These factors should therefore be foremost concerns for future treatment and rehabilitation programmes. SIGNIFICANCE: Those with longstanding CRPS may no longer meet diagnostic criteria but still be symptomatic. Defining recovery is therefore problematic in CRPS. Our study has identified patients' definition of recovery from CRPS, in order of priority, as relief from: their CRPS-related pain, generalised pain, movement restriction, reliance on medication, and stiffness.

Treatment of painful radiculopathies with capsaicin 8% cutaneous patch.

BACKGROUND AND OBJECTIVE: The treatment of neuropathic pain due to low-back (lumbosacral) radiculopathies, a common source of neuropathic pain, is challenging and often requires a multimodal therapeutic approach. The capsaicin 8% patch is the first topical analgesic licensed for peripheral neuropathic pain. To evaluate this treatment, a subset of patients with painful radiculopathy (lumbar and cervical, including ventral and dorsal rami) enrolled into the multicenter, non-interventional QUEPP study (Qutenza 2 - safety and effectiveness in peripheral neuropathic pain) was analyzed. METHODS: Of the 1044 study participants, 50 were diagnosed with painful radiculopathy as only peripheral neuropathic pain syndrome and were eligible for evaluation. Patients received a single treatment (visit 1) with follow-up visits 2-5 at weeks 1-2, 4, 8 and 12. Parameters assessed at all visits included pain intensity, neuropathy symptoms and side effects. Quality of life (SF-12) and painDETECT 1 questionnaires were completed at baseline and final visit. Data was analyzed by patch application site and duration of pain. RESULTS: Topical treatment led to a significant decrease of pain intensity between weeks 1/2 and week 12 versus baseline at the application sites representing dermatomes of ventral (N = 26) and dorsal rami (N = 13) of spinal nerves. A significant decline (p ≤ .001) of numeric pain rating scale scores was observed between weeks 1/2 following patch application and the end of observation (week 12) in the overall radiculopathy group (N = 50), and the groups with either 3 months to 2 years (N = 14) or >2 years (N = 23) duration of pain. Pain relief of at least 30% was observed in 50.0%, 71.4% and 39.1% of patients in the respective groups. Four patients experienced in total seven adverse drug reactions (application site pain or pruritus). CONCLUSION: Effective neuropathic pain relief was observed after patch application within the innervation territories of both dorsal and ventral branches of the spinal nerve. Further controlled randomized trials are indicated.

Differential effect of Incobotulinumtoxin A on pain, neurogenic flare and hyperalgesia in human surrogate models of neurogenic pain.

BACKGROUND: The effectiveness of Botulinum-neurotoxin A (BoNT/A) to treat pain in human pain models is very divergent. This study was conducted to clarify if the pain models or the route of BoNT/A application might be responsible for these divergent findings. METHODS: Sixteen healthy subjects (8 males, mean age 27 ± 5 years) were included in a first set of experiments consisting of three visits: (1) Visit: Quantitative sensory testing (QST) was performed before and after intradermal capsaicin injection (CAPS, 15 µg) on one thigh and electrical current stimulation (ES, 1 Hz) on the contralateral thigh. During stimulation pain and the neurogenic flare response (laser-Doppler imaging) were assessed. (2) Four weeks later, BoNT/A (Xeomin® , 25 MU) was injected intracutaneously on both sides. (3) Seven days later, the area of BoNT/A application was determined by the iodine-starch staining and the procedure of the (1) visit was exactly repeated. In consequence of these results, 8 healthy subjects (4 males, mean age 26 ± 3 years) were included into a second set of experiments. The experimental setting was exactly the same with the exception that stimulation frequency of ES was increased to 4 Hz and BoNT/A was injected subcutaneously into the thigh, which was stimulated by capsaicin. RESULTS: BoNT/A reduced the 1 Hz ES flare size (p < 0.001) and pain ratings (p < 0.01), but had no effect on 4 Hz ES and capsaicin-induced pain, hyperalgesia, or flare size, regardless of the depth of BoNT/A injection (i.c./s.c). Moreover, i.c. BoNT/A injection significantly increased warm detection and heat pain thresholds in naive skin (WDT, Δ 2.2 °C, p < 0.001; HPT Δ 1.8 °C, p < 0.005). CONCLUSION: BoNT/A has a moderate inhibitory effect on peptidergic and thermal C-fibers in healthy human skin. SIGNIFICANCE: The study demonstrates that BoNT/A (Incobotulinumtoxin A) has differential effects in human pain models: It reduces the neurogenic flare and had a moderate analgesic effects in low frequency but not high frequency current stimulation of cutaneous afferent fibers at C-fiber strength; BoNT/A had no effect in capsaicin-induced (CAPS) neurogenic flare or pain, or on hyperalgesia to mechanical or heat stimuli in both pain models. Intracutaneous BoNT/A increases warm and heat pain thresholds on naïve skin.

Increased prevalence of posttraumatic stress disorder in CRPS.

BACKGROUND: Although specific psychological disorders in complex regional pain syndrome (CRPS) have not been identified, studies suggest that CRPS patients may have increased rates of traumatic life events. Because these events do not always lead to apparent psychological symptoms, we systematically screened CRPS patients for posttraumatic stress disorder (PTSD) to determine if PTSD could be a risk factor for CRPS. METHODS: Consecutive CRPS patients referred to two university hospital centres (University of Erlangen, UMC Mainz) between December 2011 and April 2013 were prospectively examined using a diagnostic PTSD instrument (Post-traumatic Stress Diagnostic Scale (PDS). We also tested maladaptive coping strategies (brief-COPE inventory) and the PDS severity score as predictors for CRPS. Patients with non-CRPS extremity pain and healthy individuals were used as control groups. RESULTS: We collected data from 152 patients with CRPS, 55 control patients and 55 age- and sex-matched healthy individuals. Fifty-eight CRPS patients (38%), six non-CRPS pain patients (10%) and two healthy individuals (4%) met diagnostic criteria for PTSD. Initial PTSD symptom onset was prior to CRPS in 50 CRPS patients (86%) and during the course of CRPS in eight patients. Results of a logistic regression revealed that the PTSD severity score was associated with CRPS (p < 0.0001). Maladaptive coping strategies (p < 0.0001) were related to PTSD. CONCLUSIONS: posttraumatic stress disorder (PTSD) is more frequent in patients with CRPS than it is in the general population. SIGNIFICANCE: Research has not yet provided support for specific psychological predictors for CRPS.

[Pain - a neglected neurological issue]. / Schmerz - ein vernachlässigtes neurologisches Thema.

Chronic pain represents a great challenge; according to epidemiological data increasing numbers of patients should be expected. Based on recent advances, a better understanding of the pathophysiology of chronic pain has been achieved and neurologists have made a major contribution to this understanding. Chronic pain is accompanied by substantial maladaptive plastic alterations in both the peripheral and central nervous systems; therefore, neurological knowledge is of paramount importance for pain therapists but this contrasts with the current treatment situation of pain patients in Germany. There are basically too few departments and practices undertaking treatment, and neurologists are an exception in most pain centers. Furthermore, due to economic reasons neurological hospitals are currently experiencing a dearth of inpatients suffering from chronic pain. Diagnostic and/or treatment procedures for neurological pain entities (e.g. headaches or neuropathic pain) are insufficiently represented in the German diagnosis-related groups (DRG) reimbursement system and the obstacles for an efficient pain therapy in neurological practices are too high. Finally, there are too few academic positions for pain medicine in neurological hospitals; therefore, career opportunities for motivated young neurologists with an interest in pain are lacking. In order to address the unmet therapeutic needs of patients with chronic pain there is a high demand for (i) establishment of departments for neurological pain medicine, (ii) modification of the German DRG system and (iii) education of young neurologists with expertise in pain. Pain medicine in particular should be especially appealing to neurologists .

Zolmitriptan inhibits neurogenic inflammation and pain during electrical stimulation in human skin.

BACKGROUND: Triptans are agonists to 5-HT 1B/D/F receptors, which are present on nociceptive neurons not only within but also beyond the trigeminal system. The aim of this study was to investigate whether zolmitriptan interacts with peptidergic nociceptive afferents in human skin. METHODS: Twenty participants (13 women, median age: 25; interquartile range: 23-26 years) entered the randomized, double-blind, cross-over study. Electrically induced neurogenic flare and pain was assessed after either placebo or zolmitriptan on the ventral thigh. Mechanical pain thresholds were investigated at baseline and after electrical stimulation at the stimulation site. RESULTS: The size of the neurogenic flare (F = 10.9; p = 0.002) as well as electrically induced pain were significantly reduced by zolmitriptan (F = 4.46; p = 0.041). Moreover, electrically induced pinprick hyperalgesia was significantly decreased by zolmitriptan compared with placebo (F = 6.243; p = 0.017). CONCLUSIONS: Triptans may have effects outside of the trigeminal system and reduce electrically evoked neurogenic inflammation and pain in human skin.

Association between pain, central sensitization and anxiety in postherpetic neuralgia.

BACKGROUND: In postherpetic neuralgia (PHN), dorsal root ganglia neurons are damaged. According to the proposed models, PHN pain might be associated with nociceptive deafferentation, and peripheral (heat hyperalgesia) or central sensitization (allodynia). METHODS: In 36 PHN patients, afferent nerve fibre function was characterized using quantitative sensory testing and histamine-induced flare analysis. Psychological factors were evaluated with the Hospital Anxiety and Depression Scale (HADS), disease-related quality of life (QoL) with SF-36 and pain with the McGill questionnaire [pain rating index (PRI)]. The patients were also divided into subgroups according to the presence or absence of brush-evoked allodynia as a sign of central sensitization. RESULTS: For all patients, warm, cold and mechanical detection was impaired (p < 0.001 each) and the size of the histamine flare was diminished on the affected side (p < 0.05); pain thresholds with the exception of brush-evoked allodynia (p < 0.05) were unaltered. Correlation analysis revealed allodynia, anxiety, depression, QoL and age as relevant factors associated with pain severity (PRI). Allodynia was present in 23 patients (64%). In these patients, heat pain perception was preserved; the histamine flare was larger; the pinprick pain was increased as were McGill PRI sensory subscore, actual pain rating and almost significantly pain (McGill PRI) over the last 4 weeks. CONCLUSIONS: PHN is associated with damage of afferent fibres. Central sensitization (i.e., allodynia) might contribute to PHN pain. There was a striking association between anxiety, depression and age, and the magnitude of PHN pain.

[Pain-relieving effect of CGRP antagonism on inflammatory pain]. / Lindernde Wirkung des CGRP-Antagonismus auf Entzündungsschmerz.

The neuropeptide calcitonin gene-related peptide (CGRP) is known to play a major role in the pathogenesis of pain syndromes, in particular migraine pain; however, its implication in inflammatory processes is not well known. The CGRP receptor antagonist BIBN4096BS was shown to reduce migraine pain and trigeminal neuronal activity. An analgesic action of this compound can also be found in rats with induced acute inflammation by injection of complete Freund's adjuvant (CFA) in one hindpaw. In this model the compound reduced inflammatory pain and spinal neuronal activity. Behavioral experiments (Randall-Selitto test) revealed a reversal of the CFA-induced mechanical hyperalgesia in rats after systemic drug administration. In vivo electrophysiological studies performed in rats injected with CFA using recordings of wide dynamic range neurons in deep dorsal horn layers of the lumbar spinal cord, confirmed a reduction of neuronal activity after systemic drug administration. The same considerable amount of reduction occurred after topical administration onto the paw with resulting systemic plasma concentrations in the low nanomolar range. Spinal administration of BIBN4096BS did not modify the neuronal activity in the CFA model which suggests that peripheral blockade of CGRP receptors by BIBN4096BS significantly alleviates inflammatory pain.

Insula and sensory insular cortex and somatosensory control in patients with insular stroke.

BACKGROUND: In functional imaging studies, the insular cortex (IC) has been identified as an essential part of the processing of a whole spectrum of multimodal sensory input. However, there are no lesion studies including a sufficient number of patients, which would reinforce the functional imaging data obtained from healthy subjects. Such lesion studies should examine how damage to the IC affects sensory perception. We chose acute stroke patients with lesions affecting the IC in order to fill this gap. METHODS: A comprehensive sensory profiling by applying a quantitative sensory testing protocol was performed and a voxel-lesion behaviour mapping analysis in 24 patients with acute unilateral cortical damage was applied. RESULTS: Our data demonstrate that patients with lesions of the posterior IC have deficits in temperature perception, but did not show other sensory deficits such as hot or cold pain perception associated with specific lesion locations. CONCLUSION: Our data allow the conclusion that the posterior IC may represent the major region responsible for encoding warm and cold perception in the brain. To what extent focal IC lesions may also impair pain processing or induce post-stroke pain has to be addressed in future studies including more patients.

[Current aspects of the therapy of complex regional pain syndrome]. / Aktuelles zur Therapie des komplex-regionalen Schmerzsyndroms.

BACKGROUND: Complex regional pain syndrome (CRPS) constitutes an enigmatic post-traumatic pain disorder. AIM OF THE STUDY: The paper provides state of the art knowledge about CRPS. RESULTS: The typical constellation of symptoms of CRPS includes pain, sensory disturbances, motor symptoms, disturbances of the autonomic control of the limbs and trophic changes. These symptoms generalize distally and go beyond single nerve innervation territories. Diagnosis is made based on clinical findings. Three-phase bone scintigraphy may be the best supporting technical investigation. Symptoms typically change during the course of CRPS. In the acute stage inflammatory symptoms prevail and during chronic stages the most expressed findings are related to central neuroplasticity. These findings include hyperalgesia, sensory loss, CRPS movement disorder, body perception disturbances and autonomic symptoms. Medical treatment with anti-inflammatory agents (steroids) or bisphosphonates is most effective in the early stages and DMSO cream might also be beneficial. Administration of i.v. ketamine has been proven effective against CRPS pain and physical therapy with behavioral components, such as pain exposure helps to overcome central reorganization and functional impairment. Psychotherapy should be offered if there are significant comorbidities. All other forms of treatment are more or less empirical. Invasive treatment should be restricted to selected cases and should only be offered in specialized centers. DISCUSSION: If these recommendations are followed the prognosis for CRPS is not as poor as commonly assumed. Whether this means a return to the previous quality of life is unclear and often depends on very personal factors.

Mechanism- and experience-based strategies to optimize treatment response to the capsaicin 8% cutaneous patch in patients with localized neuropathic pain.

The capsaicin 8% cutaneous patch is an emergent new treatment option for patients with peripheral neuropathic pain. In randomized controlled clinical studies relevant pain relief for 12 weeks was achieved in about one third of patients following a single application. The first part of this paper is a review of the pathophysiology, pharmacology, and published clinical trials with the capsaicin 8% cutaneous patch. The second part reports on outcomes of an interdisciplinary expert workshop, where new treatment results of three major German pain centers were presented and reviewed with the objectives of obtaining responder rates for different pain syndromes, assessing maintenance of effect under real-life conditions, and giving recommendations for practical care. The 12 week responder rates with pain relief of ≥ 30% were comparable in patients with mononeuropathies (37.9%) and postherpetic neuralgia (38.8%). Similar responder rates were seen in a subgroup of patients with cervical spine radiculopathy and back pain (46.7%). In HIV-associated neuropathy the responder rates were high (47.8%) but lower in patients with other polyneuropathies (17.6%). Response rates were nearly identical after 1 week (46.6%) and 4 weeks (43.3) and dropped only slightly at 12 weeks (37.4%). In a subgroup of 54 patients who underwent a second treatment, efficacy was maintained. Response rates in patients with or without lidocaine pretreatment were comparable. Treatment with the capsaicin 8% cutaneous patch was generally safe and well tolerated. The workshop panel recommended further investigation of opportunities to improve the application procedure and to perform studies on the skin penetration and distribution of capsaicin. A modified quantitative sensory testing (QST) should be developed for clinical practice in order to better understand the correlation of sensory profiles and response to capsaicin treatment.

[Non-drug therapies for CRPS]. / Nicht medikamentöse Therapien beim CRPS.

State of the art CRPS therapy comprises medication, interventional therapies and non-pharmaceutical treatments like physiotherapy (PT), occupational therapy, PT with cognitive behavioural elements (mirror therapy, 'motor imagery', and 'graded exposure'), psychotherapeutic methods, local therapies and neurostimulation. These treatments are mostly as successful as medical or interventional treatment. These effects have been demonstrated in small but randomised controlled studies. Adjuvant therapies were shown to reduce pain and the severity of dysfunction in CRPS. Therefore, these non-drug therapies should be an essential part of any multimodal CRPS treatment.

Skin innervation at different depths correlates with small fibre function but not with pain in neuropathic pain patients.

BACKGROUND: Neuropathy can lead not only to impaired function but also to sensory sensitization. We aimed to link reduced skin nerve fibre density in different levels to layer-specific functional impairment in neuropathic pain patients and tried to identify pain-specific functional and structural markers. METHODS: In 12 healthy controls and 36 patients with neuropathic pain, we assessed clinical characteristics, thermal thresholds (quantitative sensory testing) and electrically induced pain and axon reflex erythema. At the most painful sites and at intra-individual control sites, skin biopsies were taken and innervation densities in the different skin layers were assessed. Moreover, neuronal calcitonin gene-related peptide staining was quantified. RESULTS: Perception of warm, cold and heat pain and nerve fibre density were reduced in the painful areas compared with the control sites and with healthy controls. Warm and cold detection thresholds correlated best with epidermal innervation density, whereas heat and cold pain thresholds and axon reflex flare correlated best with dermal innervation density. Clinical pain ratings correlated only with epidermal nerve fibre density (r = 0.38, p < 0.05) and better preserved cold detection thresholds (r = 0.39, p < 0.05), but not with other assessed functional and structural parameters. CONCLUSIONS: Thermal thresholds, axon reflex measurements and assessment of skin innervation density are valuable tools to characterize and quantify peripheral neuropathy and link neuronal function to different layers of the skin. The severity of small fibre neuropathy, however, did not correspond to clinical pain intensity and a specific parameter or pattern that would predict pain intensity in peripheral neuropathy could not be identified.

Sensory and sympathetic correlates of heat pain sensitization and habituation in men and women.

BACKGROUND: Habituation and sensitization are important behavioural responses to repeated exposure to painful stimuli, but little is known about the factors determining sensory, affective and sympathetic habituation to repeated pain stimulation in men and women. METHODS: Thirty volunteers (15 women) underwent a standardized heat pain paradigm spread over 8 consecutive days. At the beginning of the experiment, personality dimensions, coping strategies and pain catastrophizing thoughts were determined. Receiving a series of 10 blocks of six painful heat stimuli a day, participants rated pain intensity and unpleasantness. Skin conductance was recorded throughout the sessions. RESULTS AND CONCLUSION: The results show similar habituation of both the sensory and affective dimensions of pain in men and women, although skin conductance did not undergo a significant decrease across the eight days. When focusing on single daily sessions, women showed pain sensitization but sympathetic habituation, while men showed pain sensitization but stable sympathetic activation. Our findings therefore indicate that the process of long-term habituation to painful heat stimuli is a common feature in both genders, whereas men and women might differently recruit their sympathetic nervous system for short-term pain processing. This study could potentially help to better evaluate gender-specific mechanisms in pain perception.

Within-session sensitization and between-session habituation: a robust physiological response to repetitive painful heat stimulation.

Habituation and sensitization are important behavioural responses to repeated exposure of painful stimuli. Whereas within-session response dynamics to nociceptive stimuli is well characterized, little is known about long-term behaviour due to repetitive nociceptive stimulation. We used a standardized longitudinal heat pain paradigm in 66 healthy participants, 21 patients with chronic low back pain and 22 patients with depression who received daily sessions of 60 suprathreshold heat stimuli (48 °C each) for eight consecutive days. All three groups showed the same response: Repeated painful stimulation over several days resulted in substantially decreased pain ratings to identical painful stimuli. The decreased perception of pain over time was associated with a very robust increase in pain ratings in each single pain session, i.e., all participants sensitized within sessions and habituated between sessions. This uniform pattern was equally present in all examined groups. Chronic pain and depression do not seem to interfere with short-term sensitization and long-term habituation in this model of repetitive phasic heat pain.