Thermal grill illusion of pain in patients with chronic pain: a clinical marker of central sensitization?

Thermosensory predictive coding underpins an illusion of pain

The human brain has a remarkable ability to learn and update its beliefs about the world. Here, we investigate how thermosensory learning shapes our subjective experience of temperature and the misperception of pain in response to harmless thermal stimuli. Through computational modeling, we demonstrate that the brain uses a probabilistic predictive coding scheme to update beliefs about temperature changes based on their uncertainty. We find that these expectations directly modulate the perception of pain in the thermal grill illusion. Quantitative microstructural brain imaging further revealed that individual variability in computational parameters related to uncertainty-driven learning and decision-making is reflected in the microstructure of brain regions such as the precuneus, posterior cingulate gyrus, cerebellum, as well as basal ganglia and brainstem. These findings provide a framework to understand how the brain infers pain from innocuous thermal inputs, with important implications for the etiology of thermosensory symptoms under chronic pain conditions.

Association between C-reactive protein and chronic pain in US adults: A nationwide cross-sectional study

BACKGROUND

Chronic pain has emerged as a significant global public health concern. Hence, it is imperative to acquire a more comprehensive comprehension of these characteristics in the adult population of the United States in order to facilitate the development of effective interventions. The objective of this study is to ascertain the prevalence of chronic pain among people in the United States and investigate its association with C-reactive protein (CRP) levels.

METHODS

The present study employed a cross-sectional design and utilized data from three cycles of the National Health and Nutrition Examination Survey (NHANES). The study aimed to investigate the relationship between chronic pain status, CRP levels, and potential confounding factors. The study incorporated individuals who successfully fulfilled chronic questionnaires and had CRP assays. Weighted univariate and multivariate logistic regression analyses were performed to examine the correlation between chronic pain and CRP levels. To explore the non-linear relationship, weighted restricted cubic spline (RCS) with three knots coupled with a weighted logistic regression model to assess the dose-response relationship between CRP (continuous variables) and chronic pain.

RESULTS

A total of 10,680 (Weighted 250,814,660.8) adult participants with complete information were included in the analysis and 2612 (Weighted 67978784.88, 27.1%) subjects met the definition of chronic pain. Compared with participants without chronic pain, those with chronic pain had a higher CRP level (P < 0.001). The results of the multivariable adjusted logistic regression model suggested that the highest CRP quartile (CRP >  0.52 mg/dL) was associated with a 32% increase in the risk of chronic pain compared with the lowest CRP quartile (CRP ≤  0.09 mg/dL). The RCS result showed that the OR of chronic pain and CRP displayed a linear relationship (P = 0.027, Non-linear P = 0.541).

CONCLUSIONS

The study found a significant correlation between CRP levels and the presence of chronic pain among people in the United States. Individuals exhibiting elevated levels of CRP demonstrated a heightened propensity for experiencing chronic pain in comparison to individuals with lower CRP levels. Additional investigation is necessary to explore the presence of a causal association between the two variables, as well as the potential underlying mechanisms.

Assessing individual sensitivity to the Thermal Grill Illusion: A two-dimensional adaptive psychophysical approach

In the Thermal Grill Illusion (TGI), the spatial alternation of non-noxious warm and cool temperatures elicits burning sensations that resemble the presence of noxious stimuli. Previous research has largely relied on the use of specific temperature values (i.e., 20 °C and 40 °C) to study this phenomenon in both healthy individuals and patient populations. However, this methodology fails to account for inter-individual differences in thermal sensitivity, limiting the precision with which TGI responses can be evaluated across diverse populations. To address this gap, we created a Two-Dimensional Thermal Grill Calibration (2D-TGC) protocol, enabling an efficient and precise estimation of the combinations of warm and cool temperatures needed to elicit burning sensations tailored to each individual. By applying the 2D-TGC protocol in 43 healthy participants, we demonstrated that the TGI can be thresholded using an adaptive psychophysical method, and that multiple combinations of warm and cool temperatures can elicit this phenomenon. More specifically, the protocol facilitated the identification of temperature combinations that elicit TGI with varying levels of probability, intensity, and perceived quality ranging from freezing cold to burning hot. Finally, our results indicate that TGI responsivity can be quantified as a continuous variable, moving beyond the conventional classification of individuals as responders vs. non-responders based on arbitrary temperature values in the innocuous temperature range. The 2D-TGC offers a comprehensive approach to investigate the TGI across populations with altered thermal sensitivity, and can be integrated with other methods (e.g., neuroimaging) to elucidate the mechanisms responsible for perceptual illusions in the thermo-nociceptive system. PERSPECTIVE: This study reveals that the Thermal Grill Illusion can be accurately measured using psychophysical methods. The innovative Two-Dimensional Thermal Grill Calibration protocol allows for personalized temperature assessments, enhancing our understanding of thermal sensitivity variations and perceptual illusions in the thermo-nociceptive system across different populations.

Temporal Summation of the Thermal Grill Illusion is Comparable to That Observed Following Noxious Heat

The thermal grill illusion (TGI) describes a peculiar or even painful percept caused by non-noxious, interlaced warm and cold stimuli. It involves the glutamatergic system and is affected in putatively nociplastic syndromes such as fibromyalgia. The glutamatergic system is also involved in wind-up, that is, the increased activation of spinal neurons following repeated noxious stimulation leading to a temporal summation of perceived stimulus intensity. Here we combined both stimulation methods to further investigate whether non-noxious stimuli as employed in the TGI can lead to a similar summation of perceived stimulus intensity. In an experiment using a full crossover within-subjects design, 35 healthy volunteers received repeated stimuli, either in a thermal grill configuration or simply noxious heat. Both modalities were presented as sequences of 1 lead-in contact, followed by 11 consecutive contacts (each between 1.5 and 3 seconds), with either fast repetition ("wind-up" condition), or 2 slow-repeating control conditions. The main analyses concerned the relative pre-to-post sequence changes to quantify putatively wind-up-related effects. Pain ratings and skin conductance level (SCL) increased more strongly in "wind-up" than in control conditions. Interestingly, wind-up-related effects were of the same magnitude in TGI as compared to the pain control modality. Further, contact-by-contact SCL tracked how the effect emerged over time. These results indicate that although TGI does not involve noxious stimuli it is amenable to temporal summation and wind-up-like processes. Since both phenomena involve the glutamatergic system, the combination of wind-up with the TGI could yield a promising tool for the investigation of chronic pain conditions. PERSPECTIVE: Using thermal stimuli in an experimental protocol to combine 1) the TGI (painful or peculiar percept from simultaneous cold/warm stimulation) and 2) wind-up (increase in stimulus intensity after repeated exposure) holds promise to investigate pain and thermoceptive mechanisms, and chronic pain conditions.

Central neuropathic pain

Central neuropathic pain arises from a lesion or disease of the central somatosensory nervous system such as brain injury, spinal cord injury, stroke, multiple sclerosis or related neuroinflammatory conditions. The incidence of central neuropathic pain differs based on its underlying cause. Individuals with spinal cord injury are at the highest risk; however, central post-stroke pain is the most prevalent form of central neuropathic pain worldwide. The mechanisms that underlie central neuropathic pain are not fully understood, but the pathophysiology likely involves intricate interactions and maladaptive plasticity within spinal circuits and brain circuits associated with nociception and antinociception coupled with neuronal hyperexcitability. Modulation of neuronal activity, neuron-glia and neuro-immune interactions and targeting pain-related alterations in brain connectivity, represent potential therapeutic approaches. Current evidence-based pharmacological treatments include antidepressants and gabapentinoids as first-line options. Non-pharmacological pain management options include self-management strategies, exercise and neuromodulation. A comprehensive pain history and clinical examination form the foundation of central neuropathic pain classification, identification of potential risk factors and stratification of patients for clinical trials. Advanced neurophysiological and neuroimaging techniques hold promise to improve the understanding of mechanisms that underlie central neuropathic pain and as predictive biomarkers of treatment outcome.

A Predictive Model for the Risk of Recurrence of Cervical Spondylotic Radiculopathy After Surgery

INTRODUCTION

This study aimed to analyze the risk factors affecting the recurrence of cervical spondylotic radiculopathy after surgery, construct a nomogram predictive model, and validate the model's predictive performance using a calibration plot.

METHODS

In this study, 304 cervical spondylotic radiculopathy patients who underwent computed tomography (CT)-guided radiofrequency ablation (RFA) of cervical intervertebral discs or low-temperature plasma RFA for cervical radiculopathy were enrolled at the Pain Department of Jiaxing College Affiliated Hospital from January 2019 to March 2022. The patients were randomly divided into training (n = 213) and testing (n = 91) groups in a 7:3 ratio. Lasso regression analysis was used to screen for independent predictors of recurrence 1 year after surgery. A nomogram predictive model was established based on the selected factors using multiple logistic regression analysis.

RESULTS

One year after surgery, 250 of the 304 cervical spondylotic radiculopathy patients did not have recurrences, while 54 had recurrences. Lasso regression combined with multiple logistic regression analysis revealed that duration, numbness, and the Numeric Rating Scale (NRS) were significant predictors of recurrence 1 year after surgery (P < 0.05). A nomogram predictive model was established using these variables. The area under the curve (AUC) of the nomogram predictive model for predicting recurrence in the training group was 0.918 [95% confidence interval (CI) 0.866-0.970], and the AUC in the testing group was 0.892 (95% CI 0.806-0.978). The Hosmer-Lemeshow goodness-of-fit test exhibited a good model fit (P > 0.05). Decision curve analysis (DCA) indicated that the nomogram predictive model had a higher net benefit for predicting the risk of postoperative recurrence in cervical radiculopathy patients when the threshold probability was between 0 and 0.603.

CONCLUSION

This study successfully developed and validated a high-precision nomogram prediction model (predictive variables include duration, numbness, and NRS) for predicting the risk of postoperative recurrence in cervical radiculopathy patients. The model can help improve the early identification of high-risk patients and screening for postoperative recurrence.

Thermal Grill Illusion in Post-Stroke Patients: Analysis of Clinical Features and Lesion Areas

Purpose

In the thermal grill illusion, participants experience a feeling similar to burning pain. The illusion is induced by simultaneously touching warm and cool stimuli in alternating positions. In post-stroke pain, central sensitization is caused by a variety of factors, including damage to the spinothalamic tract and shoulder pain. Because the thermal grill illusion depends on central mechanisms, it has recently been suggested that it may be a useful indicator of central sensitization. Therefore, we hypothesized that post-stroke patients who are more likely to experience central sensitization may also be more likely to experience a thermal grill sensation of pain and discomfort than the likelihood among those who are less likely to experience central sensitization. However, the effects of the thermal grill illusion in post-stroke patients have not yet been reported. In this pilot study, we conducted the thermal grill illusion procedure in post-stroke patients and analyzed the relationship between clinical somatosensory functions and thermal grill sensations. We also conducted brain imaging analysis to identify brain lesion areas that were associated with thermal grill sensations.

Patients and Methods

Twenty patients (65.7 ± 11.9 years old) with post-stroke patients participated in this study. The thermal grill illusion procedure was performed as follows: patients simultaneously touched eight water-filled copper bars, with the water temperature adjusted to provide alternate warm (40°C) and cold (20°C) stimuli.

Results

Thermal grill sensation of pain and discomfort tended to be associated with the wind-up phenomenon in bedside quantitative sensory testing and thermal grill sensation of discomfort was also related to damage to the thalamic lateral nucleus.

Conclusion

These findings suggest that the thermal grill illusion might measure central sensitization, and that secondary brain hyperactivity might lead to increased thermal grill sensations.

Strong and aversive cold processing and pain facilitation in fibromyalgia patients relates to augmented thermal grill illusion

The thermal grill illusion (TGI) is assumed to result from crosstalk between the thermoreceptive and nociceptive pathways. To elucidate this further, we compared 40 female fibromyalgia patients to 20 healthy women in an exploratory cross-sectional study. Sensations (cold, warm/heat, unpleasantness, pain and burning) evoked by 20 °C, 40 °C and alternating 20 °C/40 °C (TGI) and somatosensory profiles according to standardized quantitative sensory testing (QST) were assessed on the palm of the dominant hand. Compared to healthy controls, fibromyalgia patients reported stronger thermal grill-evoked cold, warm, unpleasantness and pain as well as stronger and more aversive 20 °C- and 40 °C-evoked sensations. They showed a loss in warm, mechanical and vibration detection, a gain in thermal pain thresholds and higher temporal summation (TS). Among QST parameters higher TS in fibromyalgia patients was most consistently associated with an augmented TGI. Independently, an increased TGI was linked to cold (20 °C) but less to warm (40 °C) perception. In fibromyalgia patients all thermal grill-evoked sensations were positively related to a higher 20 °C-evoked cold sensation and/or 20 °C-evoked unpleasantness. In conclusion, the TGI appears to be driven mainly by the cold-input. Aversive cold processing and central pain facilitation in fibromyalgia patients seem to independently augment the activation of the pain pathway.

Diagnosing nociplastic pain in cancer survivors: a major step forward

Nociplastic pain syndromes include particular fibromyalgia, irritable bowel syndrome, headache, complex regional pain syndrome, and idiopathic orofacial pain. Several mechanisms have been proposed to account for nociplastic pain including central sensitisation, alterations of pain modulatory controls, epigenetic changes, and peripheral mechanisms. Importantly, nociplastic pain might also be present in patients with cancer pain, particularly those with pain related to complications of cancer treatment. Increased awareness of nociplastic pain associated with cancer should have important implications for monitoring and managing such patients.