Relationships between the paradoxical painful and nonpainful sensations induced by a thermal grill

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.

Thermal illusions for thermal displays: a review

Thermal illusions, a subset of haptic illusions, have historically faced technical challenges and limited exploration. They have been underutilized in prior studies related to thermal displays. This review paper primarily aims to comprehensively categorize thermal illusions, offering insights for diverse applications in thermal display design. Recent advancements in the field have spurred a fresh perspective on thermal and pain perception, specifically through the lens of thermal illusions.

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.

It is one or the other: No overlap between healthy individuals perceiving thermal grill illusion or paradoxical heat sensation

Paradoxical heat sensation (PHS) and the thermal grill illusion (TGI) are both related to the perception of warmth or heat from innocuous cold stimuli. Despite being described as similar perceptual phenomena, recent findings suggested that PHS is common in neuropathy and related to sensory loss, while TGI is more frequently observed in healthy individuals. To clarify the relationship between these two phenomena, we conducted a study in a cohort of healthy individuals to investigate the association between PHS and TGI. We examined the somatosensory profiles of 60 healthy participants (34 females, median age 25 years) using the quantitative sensory testing (QST) protocol from the German Research Network on Neuropathic Pain. The number of PHS was measured using a modified thermal sensory limen (TSL) procedure where the skin was transiently pre-warmed, or pre-cooled before the PHS measure. This procedure also included a control condition with a pre-temperature of 32 °C. The number of TGI responses was quantified during simultaneous application of warm and cold innocuous stimuli. All participants had normal thermal and mechanical thresholds compared to the reference values from the QST protocol. Only two participants experienced PHS during the QST procedure. In the modified TSL procedure, we found no statistically significant differences in the number of participants reporting PHS in the control condition (N = 6) vs. pre-warming (N = 3; min = 35.7 °C, max = 43.5 °C) and pre-cooling (N = 4, min = 15.0 °C, max = 28.8 °C) conditions. Fourteen participants experienced TGI, and only one participant reported both TGI and PHS. Individuals with TGI had normal or even increased thermal sensation compared to individuals without TGI. Our findings demonstrate a clear distinction between individuals experiencing PHS or TGI, as there was no overlap observed when using identical warm and cold temperatures that were alternated either temporally or spatially. While PHS was previously related to sensory loss, our study revealed that TGI is associated with normal thermal sensitivity. This suggests that an efficient thermal sensory function is essential in generating the illusory sensation of pain of the TGI.

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

ABSTRACT

The thermal grill illusion of pain (TGIP) is a paradoxical burning pain sensation elicited by the simultaneous application of innocuous cutaneous warm and cold stimuli with a thermode ("thermal grill") consisting of interlaced heated and cooled bars. Its neurophysiological mechanisms are unclear, but TGIP may have some mechanisms in common with pathological pain, including central sensitization in particular, through the involvement of N-methyl- d -aspartate receptors. However, few studies have investigated TGIP in patients with chronic pain and its clinical relevance is uncertain. We hypothesized that the TGIP would be increased in comparison with controls in patients with fibromyalgia or irritable bowel syndrome, which are regarded as typical "nociplastic" primary pain syndromes related to changes in central pain processing. We compared the sensations elicited by a large range of combinations of temperature differentials between the warm and cold bars of a thermal grill applied to the hand between patients with fibromyalgia (n = 30) or irritable bowel syndrome (n= 30) and controls (n = 30). The percentage of TGIP responses and the intensity and unpleasantness of TGIP were significantly greater in patients than controls. Furthermore, positive correlations were found between TGIP intensity and clinical pain intensity and between TGIP intensity and the cold pain threshold measured on the hand. These results are consistent with our working hypothesis of shared mechanisms between TGIP and clinical pain mechanisms in patients with nociplastic chronic pain syndromes and suggest that TGIP might represent a clinical marker of central sensitization in these patients.

Cortical oscillatory changes during thermal grill illusion

OBJECTIVE

The thermal grill illusion (TGI) can cause a burning pain sensation when the skin is subjected to simultaneously harmless hot and cold stimuli, and the pain is reported to be similar to central neuropathic pain. Although electroencephalography (EEG) is commonly used in pain research, no reports have revealed EEG activity during TGI.

METHODS

One healthy subject was enrolled, and EEG activity was recorded during the experience of the TGI and a warm sensation. Independent component analysis (ICA) was applied to preprocessed EEG data, which was divided into several clusters.

RESULTS

Theta and alpha bands in the insular cortex and parietal operculum clusters were significantly more desynchronized under the TGI condition than under the warm condition ( P  < 0.05). Additionally, theta, alpha and beta bands in the frontal (middle and inferior frontal gyrus) cluster showed significantly more desynchronization under the TGI condition than under the warm condition ( P  < 0.05).

CONCLUSION

EEG oscillations in these brain areas could be useful markers of central neuropathic pain.

A novel thermoelectric device integrated with a psychophysical paradigm to study pain processing in human subjects

INTRODUCTION

Cerebral projections of nociceptive stimuli are of great interest as targets for neuromodulation in chronic pain. To study cerebral networks involved in processing noxious stimuli, researchers often rely on thermo-nociception to induce pain. However, various limitations exist in many pain-inducing techniques, such as not accounting for individual variations in pain and trial structure predictability.

METHODS

We propose an improved and reliable psychometric experimental method to evaluate human nociceptive processing to overcome some of these limitations. The developed testing paradigm leverages a custom-built, open-source, thermoelectric device (TED). The device construction and hardware are described. A maximum-likelihood adaptive algorithm is integrated into the TED software, facilitating individual psychometric functions representative of both hot and cold pain perception. In addition to testing only hot or cold thresholds, the TED may also be used to induce the thermal grill illusion (TGI), where the bars are set to alternating warm and cool temperatures.

RESULTS

Here, we validated the TED's capability to adjust between different temperatures and showed that the device quickly and automatically changes temperature without any experimenter input. We also validated the device and integrated psychometric pain task in 21 healthy human subjects. Hot and cold pain thresholds (HPT, CPT) were determined in human subjects with <1 °C of variation. Thresholds were anticorrelated, meaning a volunteer with a low CPT likely had a high HPT. We also showed how the TED can be used to induce the TGI.

CONCLUSION

The TED can induce thermo-nociception and provide probabilistic measures of hot and cold pain thresholds. Based on the findings presented, we discuss how the TED could be used to study thermo-nociceptive cerebral projections if paired with intracranial electrode monitoring.

Pain quality of thermal grill illusion is similar to that of central neuropathic pain rather than peripheral neuropathic pain

OBJECTIVES

Application of spatially interlaced innocuous warm and cool stimuli to the skin elicits illusory pain, known as the thermal grill illusion (TGI). This study aimed to discriminate the underlying mechanisms of central and peripheral neuropathic pain focusing on pain quality, which is considered to indicate the underlying mechanism(s) of pain. We compared pain qualities in central and peripheral neuropathic pain with reference to pain qualities of TGI-induced pain.

METHODS

Experiment 1:137 healthy participants placed their hand on eight custom-built copper bars for 60 s and their pain quality was assessed by the McGill Pain Questionnaire. Experiment 2: Pain quality was evaluated in patients suffering from central and peripheral neuropathic pain (42 patients with spinal cord injury, 31 patients with stroke, 83 patients with trigeminal neuralgia and 131 patients with postherpetic neuralgia).

RESULTS

Experiment 1: Two components of TGI-induced pain were found using principal component analysis: component 1 included aching, throbbing, heavy and burning pain, component 2 included itching, electrical-shock, numbness, and cold-freezing. Experiment 2: Multiple correspondence analysis (MCA) and cross tabulation analysis revealed specific pain qualities including aching, hot-burning, heavy, cold-freezing, numbness, and electrical-shock pain were associated with central neuropathic pain rather than peripheral neuropathic pain.

CONCLUSIONS

We found similar qualities between TGI-induced pain in healthy participants and central neuropathic pain rather than peripheral neuropathic pain. The mechanism of TGI is more similar to the mechanism of central neuropathic pain than that of neuropathic pain.

Effects of a Thermoelectric Element Band on Venipuncture-associated Pain and Anxiety: A Randomized Controlled Trial

PURPOSE

Venipuncture is an invasive procedure for diagnosis and treatment, which is often attributed to pain and anxiety. In this study, a thermoelectric element (TEE) band was developed to apply heat therapy (40∼45°C), cold therapy (0∼10°C), or thermal grill illusion (TGI) therapy (40∼45°C, 0∼10°C) to cause an illusion of pain by simultaneously applying heat and cold. This band was subsequently used to investigate its effect on patient pain, anxiety, and satisfaction.

METHODS

This was a randomized controlled study. Participants, who were to undergo venipuncture, were randomly assigned to the heat therapy, cold therapy, TGI therapy, or control groups. Each group had 30 participants. The interventions were employed for 10 seconds during venipuncture, and the pain, anxiety, and satisfaction were measured before and after the procedure.

RESULTS

Subjective pain, anxiety, and physiological responses after TEE band intervention were not significantly different between the four groups. However, there was a significant difference in satisfaction (F = 4.21, p = .007) between the four groups, and the cold therapy group showed the highest satisfaction.

CONCLUSION

In this study, when heat, cold, and TGI therapy were applied with a TEE band, pain and anxiety relief effects were not confirmed, but satisfaction was high. TEE band is a newly developed product that can easily apply hot and cold treatments without using ice packs or hot water packs. Further studies with various individual characteristics of chronic pain or repeated venipuncture are warranted to evaluate the effect of TEE.

The cornucopia of central disinhibition pain - An evaluation of past and novel concepts

Central disinhibition (CD), as applied to pain, decreases thresholds of endogenous systems. This provokes onset of spontaneous or evoked pain in an individual beyond the ability of the nervous system to inhibit pain resulting from a disease or tissue damage. The original CD concept as proposed by Craig entails a shift from the lateral pain pathway (i.e. discriminative pain processing) towards the medial pain pathway (i.e. emotional pain processing), within an otherwise neurophysiological intact environment. In this review, the original CD concept as proposed by Craig is extended by the primary "nociceptive pathway damage - CD" concept and the secondary "central pathway set point - CD". Thereby, the original concept may be transferred into anatomical and psychological non-functional conditions. We provide examples for either primary or secondary CD concepts within different clinical etiologies as well as present surrogate models, which directly mimic the underlying pathophysiology (A-fiber block) or modulate the CD pathway excitability (thermal grill). The thermal grill has especially shown promising advancements, which may be useful to examine CD pathway activation in the future. Therefore, within this topical review, a systematic review on the thermal grill illusion is intended to stimulate future research. Finally, the authors review different mechanism-based treatment approaches to combat CD pain.

The Cerebral Localization of Pain: Anatomical and Functional Considerations for Targeted Electrical Therapies

Millions of people in the United States are affected by chronic pain, and the financial cost of pain treatment is weighing on the healthcare system. In some cases, current pharmacological treatments may do more harm than good, as with the United States opioid crisis. Direct electrical stimulation of the brain is one potential non-pharmacological treatment with a long history of investigation. Yet brain stimulation has been far less successful than peripheral or spinal cord stimulation, perhaps because of our limited understanding of the neural circuits involved in pain perception. In this paper, we review the history of using electrical stimulation of the brain to treat pain, as well as contemporary studies identifying the structures involved in pain networks, such as the thalamus, insula, and anterior cingulate. We propose that the thermal grill illusion, an experimental pain model, can facilitate further investigation of these structures. Pairing this model with intracranial recording will provide insight toward disentangling the neural correlates from the described anatomic areas. Finally, the possibility of altering pain perception with brain stimulation in these regions could be highly informative for the development of novel brain stimulation therapies for chronic pain.

Organization of the Thermal Grill Illusion by Spinal Segments

Objective

A common symptom of neuropathy is the misperception of heat and pain from cold stimuli. Similar cold allodynic sensations can be experimentally induced using the thermal grill illusion (TGI) in humans. It is currently unclear whether this interaction between thermosensory and nociceptive signals depends on spinal or supraspinal integration mechanisms. To address this issue, we developed a noninvasive protocol to assess thermosensory integration across spinal segments.

Methods

We leveraged anatomical knowledge regarding dermatomes and their spinal projections to investigate potential contributions of spinal integration to the TGI. We simultaneously stimulated a pair of skin locations on the arm or lower back using 1 cold (∼20°C) and 1 warm thermode (∼40°C). The 2 thermodes were always separated by a fixed physical distance on the skin, but elicited neural activity across a varying number of spinal segments, depending on which dermatomal boundaries the 2 stimuli spanned.

Results

Participants consistently overestimated the actual cold temperature on the skin during combined cold and warm stimulation, confirming the TGI effect. The TGI was present when cold and warm stimuli were delivered within the same dermatome, or across dermatomes corresponding to adjacent spinal segments. In striking contrast, no TGI effect was found when cold and warm stimuli projected to nonadjacent spinal segments.

Interpretation

These results demonstrate that the strength of the illusion is modulated by the segmental distance between cold and warm afferents. This suggests that both temperature perception and thermal–nociceptive interactions depend upon low‐level convergence mechanisms operating within a single spinal segment and its immediate neighbors. Ann Neurol 2018;84:463–472

Ineffectiveness of tactile gating shows cortical basis of nociceptive signaling in the Thermal Grill Illusion

Painful burning sensations can be elicited by a spatially-alternating pattern of warm and cold stimuli applied on the skin, the so called "Thermal Grill Illusion" (TGI). Here we investigated whether the TGI percept originates spinally or centrally. Since the inhibition of nociceptive input by concomitant non-nociceptive somatosensory input has a strong spinal component, we reasoned that, if the afferent input underlying the TGI originates at spinal level, then the TGI should be inhibited by a concomitant non-nociceptive somatosensory input. Conversely, if TGI is the result of supraspinal processing, then no effect of touch on TGI would be expected. We elicited TGI sensations in a purely thermal condition without tactile input, and found no evidence that tactile input affected the TGI. These results provide further evidence against a spinal mechanism generating the afferent input producing the TGI, and indicate that the peculiar burning sensation of the TGI results from supraspinal interactions between thermoceptive and nociceptive systems.

Sex differences in thermal detection and thermal pain threshold and the thermal grill illusion: a psychophysical study in young volunteers

BACKGROUND

Sex-related differences in human thermal and pain sensitivity are the subject of controversial discussion. The goal of this study in a large number of subjects was to investigate sex differences in thermal and thermal pain perception and the thermal grill illusion (TGI) as a phenomenon reflecting crosstalk between the thermoreceptive and nociceptive systems. The thermal grill illusion is a sensation of strong, but not necessarily painful, heat often preceded by transient cold upon skin contact with spatially interlaced innocuous warm and cool stimuli.

METHODS

The TGI was studied in a group of 78 female and 58 male undergraduate students and was evoked by placing the palm of the right hand on the thermal grill (20/40 °C interleaved stimulus). Sex-related thermal perception was investigated by a retrospective analysis of thermal detection and thermal pain threshold data that had been measured in student laboratory courses over 5 years (776 female and 476 male undergraduate students) using the method of quantitative sensory testing (QST). To analyse correlations between thermal pain sensitivity and the TGI, thermal pain threshold and the TGI were determined in a group of 20 female and 20 male undergraduate students.

RESULTS

The TGI was more pronounced in females than males. Females were more sensitive with respect to thermal detection and thermal pain thresholds. Independent of sex, thermal detection thresholds were dependent on the baseline temperature with a specific progression of an optimum curve for cold detection threshold versus baseline temperature. The distribution of cold pain thresholds was multi-modal and sex-dependent. The more pronounced TGI in females correlated with higher cold sensitivity and cold pain sensitivity in females than in males.

CONCLUSIONS

Our finding that thermal detection threshold not only differs between the sexes but is also dependent on the baseline temperature reveals a complex processing of "cold" and "warm" inputs in thermal perception. The results of the TGI experiment support the assumption that sex differences in cold-related thermoreception are responsible for sex differences in the TGI.

The use of a battery of pain models to detect analgesic properties of compounds: a two-part four-way crossover study

AIM

The aim was to investigate the ability of a battery of pain models to detect analgesic properties of commonly used analgesics in healthy subjects.

METHODS

The battery consisted of tests eliciting electrical, mechanical and thermal (contact heat and cold pressor)-pain and included a UVB model, the thermal grill illusion and a paradigm of conditioned pain modulation. Subjects were administered fentanyl 3 μg kg^-1 , phenytoin 300 mg, (S)-ketamine 10 mg and placebo (part I), or imipramine 100 mg, pregabalin 300 mg, ibuprofen 600 mg and placebo (part II). Pain measurements were performed at baseline and up to 10 h post-dose. Endpoints were analysed using a mixed model analysis of variance.

RESULTS

Sixteen subjects (8 female) completed each part. The pain tolerance threshold (PTT) for electrical stimulation was increased (all P < 0.05) compared to placebo for (S)-ketamine (+10.1%), phenytoin (+8.5%) and pregabalin (+10.8%). The PTT for mechanical pain was increased by pregabalin (+14.1%). The cold pressor PTT was increased by fentanyl (+17.1%) and pregabalin (+46.4%). Normal skin heat pain detection threshold was increased by (S)-ketamine (+3.3%), fentanyl (+2.8%) and pregabalin (+4.1%). UVB treated skin pain detection threshold was increased by fentanyl (+2.6%) and ibuprofen (+4.0%). No differences in conditioned pain modulation were observed.

CONCLUSION

This study shows that these pain models are able to detect changes in pain thresholds after administration of different classes of analgesics in healthy subjects. The analgesic compounds all showed a unique profile in their effects on the pain tasks administered.