Comparative EEG activation to skin pain and muscle pain induced by capsaicin injection

Effect of dry needling on pain and central sensitization in women with chronic pelvic pain: A randomized parallel-group controlled clinical trial

Chronic pelvic pain (CPP) is a debilitating problem in women with clear evidence of myofascial dysfunction. It seems that Myofascial trigger points (MTrPs) contribute to the development of central sensitization (CS). This study aimed to investigate the effect of dry needling on pain and CS in women with CPP. Thirty-six women with CPP participated in this randomized controlled clinical trial and randomly assigned into three groups: dry needling group (DNG), placebo needling group (PNG) and control group (CG). The DNG received five sessions of DN using the "static needling", the PNG received non-penetrating method, and the CG did not receive any intervention. Assessment of outcomes including central sensitization inventory (CSI), short-form McGill pain questionnaire (SF-MPQ), electroencephalography (EEG), conditioned pain modulation (CPM), salivary cortisol concentration, 7-item general anxiety disorder scale (GAD-7), pain catastrophizing scale (PCS), and SF-36 questionnaire was performed pre-intervention, post-intervention, and three months post-intervention by a blind examiner. The result showed a significant group-by-time interaction for CSI, SF-MPQ, and PCS. There was a significant decrease in CSI score in post-intervention and three-months post-intervention compare to pre-intervention in the DNG and PNG. SF-MPQ-PPI score in DNG significantly decreased post-intervention. PCS-Total score decreased significantly post-intervention in DNG and PNG. No significant group-by-time interactions were observed for other variables. EEG results showed regional changes in the activity of frequency bands in both eye closed and eye open conditions. It seems that DN can affect central pain processing by removing the source of peripheral nociception. Trial registration: Iranian Registry of Clinical Trials (IRCT20211114053057N1, registered on: December 03, 2021. https://irct.behdasht.gov.ir/search/result?query=IRCT20211114053057N1).

Chronic pain domains and their relationship to personality, abilities, and brain networks

Chronic pain is a multidimensional pathological state. Recent evidence suggests that specific brain properties and patients' psychological and physical traits are distorted in chronic pain patients. However, the relationship between these alterations and pain dimensions remains poorly understood. Here, we first evaluated multiple dimensions of chronic pain by assessing a broad battery of pain-related questionnaire scores (23 outcomes) of 107 chronic low back pain patients and identified 3 distinct chronic pain domains: magnitude, affect & disability, and quality. Second, we investigated the pain domains relationship with measures of personality, social interaction, psychological traits, and ability traits (77 biopsychosocial & ability [biopsy&ab] outcomes). Pain magnitude (out-of-sample [OOS] ) is associated with emotional control, attention, and working memory, with higher pain scores showing lower capacity to regulate and adapt behaviorally. Pain affect & disability (OOS associated with anxiety, catastrophizing and social relationships dysfunction. Pain quality did not relate significantly to biopsy&ab variables. Third, we mapped these 3 pain domains to brain functional connectivity. Pain magnitude mainly associated with the sensorimotor and the cingulo-opercular networks (OOS ). Pain affect & disability related to frontoparietal and default mode networks (OOS . Pain quality integrated sensorimotor, auditory, and cingulo-opercular networks (OOS ). Mediation analysis could link functional connectivity and biopsy&ab models to respective pain domains. Our results provide a global overview of the complexity of chronic pain, showing how underlying distinct domains of the experience map to different biopsy&ab correlates and underlie unique brain network signatures.

Neuroimaging Assessment of Pain

Pain is an unpleasant sensory and emotional experience. Understanding the neural mechanisms of acute and chronic pain and the brain changes affecting pain factors is important for finding pain treatment methods. The emergence and progress of non-invasive neuroimaging technology can help us better understand pain at the neural level. Recent developments in identifying brain-based biomarkers of pain through advances in advanced imaging can provide some foundations for predicting and detecting pain. For example, a neurologic pain signature (involving brain regions that receive nociceptive afferents) and a stimulus intensity-independent pain signature (involving brain regions that do not show increased activity in proportion to noxious stimulus intensity) were developed based on multivariate modeling to identify processes related to the pain experience. However, an accurate and comprehensive review of common neuroimaging techniques for evaluating pain is lacking. This paper reviews the mechanism, clinical application, reliability, strengths, and limitations of common neuroimaging techniques for assessing pain to promote our further understanding of pain.

Development and validation of a real-time method characterizing spontaneous pain in women with dysmenorrhea

AIM

Prior research has primarily focused on static pain assessment, largely ignoring the dynamic nature of pain over time. We used a novel assessment tool for characterizing pain duration, frequency, and amplitude in women with dysmenorrhea and evaluated how these metrics were affected by naproxen treatment.

METHODS

Dysmenorrheic women (n = 25) rated their menstrual pain by squeezing a pressure bulb proportional to the magnitude of their pain. To evaluate whether bulb squeezing was affected by naproxen, we compared parameters before and after naproxen. We also analyzed the correlation between pain relief on a numerical rating scale to changes in bulb squeezing parameters. Random bulb-squeezing activity in pain-free participants (n = 14) was used as a control for nonspecific effects or bias.

RESULTS

In dysmenorrheic women, naproxen reduced the duration of the squeezing during a painful bout, the number of painful bouts and bout intensity. Before naproxen, the correlation between these bulb squeeze parameters and self-reported pain on numeric rating scale was not significant (R² = 0.12, p = 0.304); however, there was a significant correlation between changes in bulb squeeze activity and self-reported pain relief after naproxen (R² = 0.55, p < 0.001).

CONCLUSION

Our study demonstrates a convenient technique for continuous pain assessment, capturing three different dimensions: duration, frequency, and magnitude. Naproxen may act by reducing the duration and frequency of episodic pain in addition to reducing the severity. After further validation, these methods could be used for other pain conditions for deeper phenotyping and assessing novel treatments.

Changes of EEG band oscillations to tonic cold pain and the behavioral inhibition and fight-flight-freeze systems

Using electroencephalography (EEG) power measures within conventional delta, theta, alpha, beta, and gamma bands, the aims of the current study were to highlight cortical correlates of subjective perception of cold pain (CP) and the associations of these measures with behavioral inhibition system (BIS), fight-flight-freeze system (FFFS), and behavioral approach system personality traits. EEG was recorded in 55 healthy right-handed women under (i) a white noise interruption detection condition (Baseline); (ii) enduring CP induced by the cold cup test. CP and Baseline EEG band power scores within conventional frequency bands served for covariance analyses. We found that: (1) higher Pain scorers had higher EEG beta power changes at left frontal, midline central, posterior temporal leads; (2) higher BIS was associated with greater EEG delta activity changes at parietal scalp regions; (3) higher FFFS was associated with higher EEG delta activity changes at temporal and left-parietal regions, and with lower EEG gamma activity changes at right parietal regions. High FFFS, compared to Low FFFS scorers, also showed a lower gamma power across the midline, posterior temporal, and parietal regions. Results suggest a functional role of higher EEG beta activity in the subjective perception of tonic pain. EEG delta activity underpins conflict resolution system responsible for passive avoidance control of pain, while higher EEG delta and lower EEG gamma activity changes, taken together, underpin active avoidance system responsible for pain escape behavior.

Quantitative Data for Transcutaneous Electrical Nerve Stimulation and Acupuncture Effectiveness in Treatment of Fibromyalgia Syndrome

Aim. To evaluate the effects of acupuncture and transcutaneous electric nerve stimulation (TENS) applications on the quantitative electroencephalography (qEEG) changes and to evaluate their therapeutic effects in patients with fibromyalgia syndrome (FMS). The study included 42 patients with FMS and 21 healthy volunteers. The patients were randomly assigned to two groups (n=21 in each) to undergo either TENS or acupuncture application. In both acupuncture and TENS groups, baseline electroencephalography (EEG) recording was performed for 10 min and, then, TENS or acupuncture was performed for 20 min, followed by another 10 min EEG recording. Baseline qEEG findings of FMS patients in the TENS and acupuncture groups were similar. Delta and theta powers over the frontal region of FMS patients were lower than controls. Theta powers of right posterior region were also lower than controls. In the TENS group, after the treatment, an increase was observed in the alpha power of the left anterior region as well as a decrease in pain scores. In the acupuncture group, an increase was determined in the alpha power of the right and left posterior regions as well as a decrease in pain score after the treatment. The power of low- and moderate-frequency waves on resting EEG was decreased in the patients with FMS. Decreased pain and increased inhibitor activity were found on qEEG after TENS and acupuncture applications. In conclusion, both TENS and acupuncture applications seem to be beneficial in FMS patients.

Exploration of the Pathophysiology of Chronic Pain Using Quantitative EEG Source Localization

Chronic pain affects more than 35% of the US adult population representing a major public health imperative. Currently, there are no objective means for identifying the presence of pain, nor for quantifying pain severity. Through a better understanding of the pathophysiology of pain, objective indicators of pain might be forthcoming. Brain mechanisms mediating the painful state were imaged in this study, using source localization of the EEG. In a population of 77 chronic pain patients, significant overactivation of the "Pain Matrix" or pain network, was found in brain regions including, the anterior cingulate, anterior and posterior insula, parietal lobule, thalamus, S1, and dorsolateral prefrontal cortex (DLPFC), consistent with those reported with conventional functional imaging, and extended to include the mid and posterior cingulate, suggesting that the increased temporal resolution of electrophysiological measures may allow a more precise identification of the pain network. Significant differences between those who self-report high and low pain were reported for some of the regions of interest (ROIs), maximally on left hemisphere in the DLPFC, suggesting encoding of pain intensity occurs in a subset of pain network ROIs. Furthermore, a preliminary multivariate logistic regression analysis was used to select quantitative-EEG features which demonstrated a highly significant predictive relationship of self-reported pain scores. Findings support the potential to derive a quantitative measure of the severity of pain using information extracted from a multivariate descriptor of the abnormal overactivation. Furthermore, the frequency specific (theta/low alpha band) overactivation in the regions reported, while not providing direct evidence, are consistent with a model of thalamocortical dysrhythmia as the potential mechanism of the neuropathic painful condition.

Brodmann area 10: Collating, integrating and high level processing of nociception and pain

Multiple frontal cortical brain regions have emerged as being important in pain processing, whether it be integrative, sensory, cognitive, or emotional. One such region, Brodmann Area 10 (BA 10), is the largest frontal brain region that has been shown to be involved in a wide variety of functions including risk and decision making, odor evaluation, reward and conflict, pain, and working memory. BA 10, also known as the anterior prefrontal cortex, frontopolar prefrontal cortex or rostral prefrontal cortex, is comprised of at least two cytoarchitectonic sub-regions, medial and lateral. To date, the explicit role of BA 10 in the processing of pain hasn't been fully elucidated. In this paper, we first review the anatomical pathways and functional connectivity of BA 10. Numerous functional imaging studies of experimental or clinical pain have also reported brain activations and/or deactivations in BA 10 in response to painful events. The evidence suggests that BA 10 may play a critical role in the collation, integration and high-level processing of nociception and pain, but also reveals possible functional distinctions between the subregions of BA 10 in this process.

Brain Rhythms of Pain

Pain is an integrative phenomenon that results from dynamic interactions between sensory and contextual (i.e., cognitive, emotional, and motivational) processes. In the brain the experience of pain is associated with neuronal oscillations and synchrony at different frequencies. However, an overarching framework for the significance of oscillations for pain remains lacking. Recent concepts relate oscillations at different frequencies to the routing of information flow in the brain and the signaling of predictions and prediction errors. The application of these concepts to pain promises insights into how flexible routing of information flow coordinates diverse processes that merge into the experience of pain. Such insights might have implications for the understanding and treatment of chronic pain.

Changes of gamma-band oscillatory activity to tonic muscle pain

It is well know that phasic pain could induce suppression of alpha oscillations and enhancement of gamma oscillations. However, the cortical responses to tonic pain, especially tonic pain originating from deep tissue, which was proposed to better resemble the clinical pain, are not well understood. Here we aimed to investigate electroencephalographic (EEG) responses to tonic muscle pain. EEG signals and pain perceptions of three order-counterbalanced conditions: innocuous condition (A, infusion of isotonic saline), noxious conditions with low (B) and medium (C) intensities (infusion of hypertonic saline) were recorded from 43 subjects. We observed the enhancement of gamma oscillations in frontal-central region in condition C, as compared to either condition A or B. Positive relationship between the amplitude of gamma oscillations and pain intensity was also observed in frontal-central region. Therefore, we provide novel evidence for the encoding of frontal-central gamma oscillations in tonic pain processing.

Spectral and spatial changes of brain rhythmic activity in response to the sustained thermal pain stimulation

The aim of this study was to investigate the neurophysiological correlates of pain caused by sustained thermal stimulation. A group of 21 healthy volunteers was studied. Sixty-four channel continuous electroencephalography (EEG) was recorded while the subject received tonic thermal stimulation. Spectral changes extracted from EEG were quantified and correlated with pain scales reported by subjects, the stimulation intensity, and the time course. Network connectivity was assessed to study the changes in connectivity patterns and strengths among brain regions that have been previously implicated in pain processing. Spectrally, a global reduction in power was observed in the lower spectral range, from delta to alpha, with the most marked changes in the alpha band. Spatially, the contralateral region of the somatosensory cortex, identified using source localization, was most responsive to stimulation status. Maximal desynchrony was observed when stimulation was present. The degree of alpha power reduction was linearly correlated to the pain rating reported by the subjects. Contralateral alpha power changes appeared to be a robust correlate of pain intensity experienced by the subjects. Granger causality analysis showed changes in network level connectivity among pain-related brain regions due to high intensity of pain stimulation versus innocuous warm stimulation. These results imply the possibility of using noninvasive EEG to predict pain intensity and to study the underlying pain processing mechanism in coping with prolonged painful experiences. Once validated in a broader population, the present EEG-based approach may provide an objective measure for better pain management in clinical applications. Hum Brain Mapp 37:2976-2991, 2016. © 2016 Wiley Periodicals, Inc.

Placebo Analgesia Changes Alpha Oscillations Induced by Tonic Muscle Pain: EEG Frequency Analysis Including Data during Pain Evaluation

Placebo exhibits beneficial effects on pain perception in human experimental studies. Most of these studies demonstrate that placebo significantly decreased neural activities in pain modulatory brain regions and pain-evoked potentials. This study examined placebo analgesia-related effects on spontaneous brain oscillations. We examined placebo effects on four order-fixed 20-min conditions in two sessions: isotonic saline-induced control conditions (with/without placebo) followed by hypertonic saline-induced tonic muscle pain conditions (with/without placebo) in 19 subjects using continuous electroencephalography (EEG) recording. Placebo treatment exerted significant analgesic effects in 14 placebo responders, as subjective intensity of pain perception decreased. Frequency analyses were performed on whole continuous EEG data, data during pain perception rating and data after rating. The results in the first two cases revealed that placebo induced significant increases and a trend toward significant increases in the amplitude of alpha oscillation during tonic muscle pain compared to control conditions in frontal-central regions of the brain, respectively. Placebo-induced decreases in the subjective intensity of pain perception significantly and positively correlated with the increases in the amplitude of alpha oscillations during pain conditions. In conclusion, the modulation effect of placebo treatment was captured when the pain perception evaluating period was included. The strong correlation between the placebo effect on reported pain perception and alpha amplitude suggest that alpha oscillations in frontal-central regions serve as a cortical oscillatory basis of the placebo effect on tonic muscle pain. These results provide important evidence for the investigation of objective indicators of the placebo effect.

The pain quality response profile of a corticosteroid injections and heated lidocaine/tetracaine patch in the treatment of shoulder impingement syndrome

OBJECTIVES

To describe the effects of 2 pain treatments for shoulder impingement syndrome (SIS), and illustrate how investigators can use pain quality information to understand treatment response differences.

MATERIALS AND METHODS

This study presents pain quality data from a randomized open-label study comparing the effects of an injection of triamcinolone and up to twice daily application of a heated lidocaine/tetracaine (Trilexis) patch in individuals with SIS. Study participants completed a measure of pain quality at baseline and again on study days 14, 28, and 42 following initiation of 2 treatments for SIS. Baseline and posttreatment pain quality scores were graphed to provide a visual representation of treatment-associated changes. Analyses of variance were used to examine the differences between treatment conditions in changes in pain quality with treatment.

RESULTS

Both treatments resulted in substantial (and similar) pretreatment to posttreatment improvements in many pain qualities. However, differences in the time course of treatment effects were observed for itchy and heavy qualities.

DISCUSSION

Although 2 different pain treatments appear to have the same effects when only pretreatment to posttreatment changes are examined, treatment differences emerged when the time course of treatment is examined. The findings support the importance of assessing both pain qualities and time course of treatment as outcome domains. The results illustrate how investigators can use data from clinical trials to provide a more fine-tuned description of treatment effects, providing knowledge that could be helpful in selecting treatment options at the individual patient level.

SUMMARY

Examination of the effects of pain treatments on pain qualities over time will help researchers and clinicians understand if certain pain quality domains respond faster to one treatment versus another, and may identify differences between treatments that would not be observed by measures of global pain intensity alone.

Unravelling the mystery of capsaicin: a tool to understand and treat pain

A large number of pharmacological studies have used capsaicin as a tool to activate many physiological systems, with an emphasis on pain research but also including functions such as the cardiovascular system, the respiratory system, and the urinary tract. Understanding the actions of capsaicin led to the discovery its receptor, transient receptor potential (TRP) vanilloid subfamily member 1 (TRPV1), part of the superfamily of TRP receptors, sensing external events. This receptor is found on key fine sensory afferents, and so the use of capsaicin to selectively activate pain afferents has been exploited in animal studies, human psychophysics, and imaging studies. Its effects depend on the dose and route of administration and may include sensitization, desensitization, withdrawal of afferent nerve terminals, or even overt death of afferent fibers. The ability of capsaicin to generate central hypersensitivity has been valuable in understanding the consequences and mechanisms behind enhanced central processing of pain. In addition, capsaicin has been used as a therapeutic agent when applied topically, and antagonists of the TRPV1 receptor have been developed. Overall, the numerous uses for capsaicin are clear; hence, the rationale of this review is to bring together and discuss the different types of studies that exploit these actions to shed light upon capsaicin working both as a tool to understand pain but also as a treatment for chronic pain. This review will discuss the various actions of capsaicin and how it lends itself to these different purposes.

Sensory pain qualities in neuropathic pain

The qualities of chronic neuropathic pain (NeP) may be informative about the different mechanisms of pain. We previously developed a 2-factor model of NeP that described an underlying structure among sensory descriptors on the Short-Form McGill Pain Questionnaire. The goal of this study was to confirm the correlated 2-factor model of NeP. Individual descriptive scores from the Short-Form McGill Pain Questionnaire were analyzed. Confirmatory factor analysis was used to test a correlated 2-factor model. Factor 1 (stabbing pain) was characterized by high loadings on stabbing, sharp, and shooting sensory items; factor 2 (heavy pain) was characterized by high loadings on heavy, gnawing, and aching items. Results of the confirmatory factor analysis strongly supported the correlated 2-factor model.

PERSPECTIVE

This article validates a model that describes the qualities of neuropathic pain associated with diabetic peripheral neuropathy and postherpetic neuralgia. These data suggest that specific pain qualities may be associated with pain mechanisms or may be useful for predicting treatment response.

Differential central pain processing following repetitive intramuscular proton/prostaglandin E₂ injections in female fibromyalgia patients and healthy controls

BACKGROUND

While the etiology of fibromyalgia syndrome (FMS) remains unclear, it is assumed that both peripheral and central components are involved.

AIMS/METHODS

To investigate central activation patterns following chemically-induced muscle pain we repetitively injected protons (low pH) and prostaglandin E(2) (PGE(2)) in isotonic solution into the left extensor carpi radialis brevis muscle of female FMS patients and female healthy control subjects (HC). The injection of protons/PGE(2) has the advantage that it is not prone to tachyphylaxis compared to capsaicin and hypotonic saline solution. During the repetitive injections continuous pain ratings were recorded and functional magnetic resonance imaging measurements were conducted.

RESULTS

Injection of protons/PGE(2) led to activation of the anterior and medial cingulate cortices, contralateral primary sensory cortex, bilateral insula and thalamus, left basal ganglia, left orbitofrontal cortex and the cerebellum in FMS patients. In HC, activations were found only in the anterior, medial, and posterior cingulate cortices, and the primary somatosensory cortex. The contrast between the groups revealed significantly stronger activation for FMS patients in the left anterior insula. Peak pain ratings were comparable between HC and FMS patients, but pain duration (sustained pain) was prolonged in FM.

CONCLUSION

Repetitive proton/PGE(2)-induced excitation of muscle tissue led to a more prolonged perception of pain and more wide-spread activation in pain-related brain areas in FMS, especially in the left (ipsilateral) insula, whereas acute protons/PGE(2)-induced pain processing was similar in the two groups. These data provide further evidence for enhanced central pain processing in FMS patients.

Neuropathic pain-like alterations in muscle nociceptor function associated with vibration-induced muscle pain

We recently developed a rodent model of the painful muscle disorders induced by occupational exposure to vibration. In the present study we used this model to evaluate the function of sensory neurons innervating the vibration-exposed gastrocnemius muscle. Activity of 74 vibration-exposed and 40 control nociceptors, with mechanical receptive fields in the gastrocnemius muscle, were recorded. In vibration-exposed rats ∼15% of nociceptors demonstrated an intense and long-lasting barrage of action potentials in response to sustained suprathreshold mechanical stimulation (average of 2635 action potentials with frequency of ∼44Hz during a 1min suprathreshold stimulus) much greater than that has been reported to be produced even by potent inflammatory mediators. While these high-firing nociceptors had lower mechanical thresholds than the remaining nociceptors, exposure to vibration had no effect on conduction velocity and did not induce spontaneous activity. Hyperactivity was not observed in any of 19 neurons from vibration-exposed rats pretreated with intrathecal antisense for the IL-6 receptor subunit gp130. Since vibration can injure peripheral nerves and IL-6 has been implicated in painful peripheral neuropathies, we suggest that the dramatic change in sensory neuron function and development of muscles pain, induced by exposure to vibration, reflects a neuropathic muscle pain syndrome.

Volunteers with high versus low alpha EEG have different pain-EEG relationship: a human experimental study

The alpha rhythm (7.5-12 Hz) is one of the fundamental features of the human EEG which usually has maximum amplitude over occipital regions. It is well recognized that individuals have highly different magnitudes of alpha EEG. This study examined occipital alpha EEG activity during different levels of experimental tonic cuff-pressure pain. The aim was to study the pain reactions and pain-EEG relationship in subjects with high alpha (H(alpha)) and low alpha (L(alpha)) EEG. Tonic experimental cuff-pressure induced pain, and high density EEG (124 channels) were used. The pain-EEG responses for the high alpha (above 600 microV(2) in total alpha power at baseline), and low alpha (below 600 microV(2) in alpha power at baseline) subjects were analyzed. Forty healthy volunteers were included and received tonic pain for 3 min at three intensities (VRS2) intense, but no pain, (VRS4) slight pain, and (VRS6) moderate pain. There were no differences in stimulus intensities to reach the three ratings between the H(alpha) and the L(alpha) groups. The H(alpha) and L(alpha) groups are highly different in alpha1(PO3), alpha1(PO4), alpha1(PO7) and alpha1(PO8) EEG powers. A positive correlation (P = 0.008) between alpha2(PO3) EEG and average subjective pain ratings was specific for the L(alpha) group. The H(alpha) group showed alpha1 desynchronization as pain increased, but no significant correlation between alpha1 EEG powers and average subjective pain ratings. The differences between the L(alpha) and the H(alpha) in alpha EEG powers and the different pain-EEG responses may be related to different degrees of attention, fear of pain and pain related coping strategies.

EEG responses to tonic heat pain

The aim of the present study was to characterize the EEG response pattern specific for tonic pain which is an experimental pain model resembling clinical pain more closely than phasic pain. Tonic experimental pain was produced by a series of heat pulses 1 degree C above pain threshold over 10 min. A series of heat pulses 0.3 degree C below pain threshold and a constant temperature of 37 degrees C served as non-painful heat control and as baseline condition, respectively. The level of attention was experimentally manipulated by instruction and by a distraction task. Twenty male, pain-free subjects had to rate the sensation intensity and sensation unpleasantness during thermal stimulation. Furthermore, a German version of the McGill Pain Questionnaire was to be filled out after tonic painful heat stimulation. The EEG was recorded via 10 leads according to 10/20 convention. Power density was calculated for the usual frequency bands. The ratings showed that tonic painful heat was experienced clearly distinct from tonic non-painful heat. An EEG response pattern emerged characterized by a rather generalized increased delta(2) activity, a left-biased fronto-temporally diminished theta activity, a fronto-temporal decrease in the alpha(1) activity and a left-sided temporal increase in the beta(1) activity. This observation agrees well with the findings of others. However, there was no evidence in our data that these EEG changes are specific to tonic heat pain as opposed to changes observed during tonic non-painful heat stimulation. Accordingly, the repeatedly reported EEG patterns are also likely to be produced by other forms of strong somatosensory stimuli and to be not specific for pain.

Using Pain Quality Assessment Measures for Selecting Analgesic Agents

Although those in the field of pain management have long recognized the multidimensional character of pain, global pain intensity remains the most frequently assessed domain measured in clinical trials of pain treatments. However, a number of specific pain qualities have been shown to be associated with neuropathic versus nociceptive pain and, as such, can be used for diagnostic purposes. In addition, preliminary evidence suggests that pain quality measures may be helpful for measuring outcomes in clinical trials. A number of such measures are currently available, each with its strengths and weaknesses. These measures are reviewed, and potential future uses of these tools are discussed.

Assessment of pain quality in chronic neuropathic and nociceptive pain clinical trials with the Neuropathic Pain Scale

Although a number of measures of pain qualities exist, little research has examined the potential for these measures to identify the unique effects of pain treatments on different pain qualities. We examined the utility of the Neuropathic Pain Scale (NPS) for assessing changes in pain qualities after open label lidocaine patch 5% in 3 samples of patients: patients with peripheral neuropathic pain, low back pain, and osteoarthritis. With one exception ("cold" pain in subjects with low back pain), each of the NPS items showed significant change after open label lidocaine patch. In addition, significantly larger changes were observed for the NPS items reflecting global pain intensity and pain unpleasantness and for items assessing sharp and deep pain than for items assessing cold, sensitive, and itchy pain. The pattern of changes in pain qualities did not differ across the 3 diagnostic groups, but it did differ from the patterns of changes in pain qualities associated with other analgesic treatments. The results support the potential utility of the NPS for assessing the patterns of changes in pain qualities that can be observed after pain treatment.

PERSPECTIVE

Pain clinical trials that include measures of pain qualities, such as the NPS, might identify distinct patterns of treatment effects on those pain qualities. This research might be used to help clinicians target analgesics to match the specific qualities associated with a patient's pain and to better understand the mechanisms of analgesic effects in drug development programs.

Psychophysical and EEG responses to repeated experimental muscle pain in humans: pain intensity encodes EEG activity

Clinical pain is often characterized by repetitive and persistent occurrence in deep structures, but few studies investigated repetitive tonic pain in humans. To determine cerebral responses to repetitive tonic pain, psychophysical responses, and electroencephalographic (EEG) activation to five trials of repeated tonic muscle pain induced by hypertonic saline were examined and analyzed in 13 male subjects. The study was composed of two experimental sessions performed in separate days. Five sequential injections of hypertonic saline (5.8%) were used to induce repeated muscle pain in the left forearm, and five sequential injections of isotonic saline (0.9%) acted as control. Visual analogue scales (VAS) for pain intensity and 32-channels EEG activities were recorded simultaneously. Five trials of relatively stable muscle pain were induced by intramuscular injections of hypertonic saline, but no evident pain was induced by the injections of isotonic saline. Significant decreases in alpha-1 and -2 activities in posterior part of the head were found during repeated muscle pain in comparison with non-pain. In comparison with baseline, alpha-1 and -2 activities reduced significantly during the first two trials, and gradually resumed in the following three trials of muscle pain. However, beta-2 activity increased consistently throughout the five trials of muscle pain compared to baseline. Alpha-1 activity was negatively, but beta-2 activity was positively correlated to the pain intensity and pain area on the skin. Throughout five injections, the reduction of alpha-1 activity was contrary to the changes of pain intensity. These results indicates that pain-related EEG activities were encoded by the pain intensity. The thalamo-cortical system and descending inhibitory neuronal networks may be involved in the regulation of pain intensity.