Was That Painful or Nonpainful? The Sensation and Pain Rating Scale Performs Well in the Experimental Context

Do we perceive sensations inside and outside of our body differently? Perceptual, emotional, and behavioral differences between visceral and somatic sensation, discomfort, and pain

BACKGROUND

Experimental research evaluating differences between the visceral and somatic stimulation is limited to pain and typically uses different induction methods for visceral and somatic stimulation (e.g., rectal balloon distention vs. tactile hand stimulation). Our study aimed to compare differences in response time, intensity, unpleasantness, and threat between identical electrical visceral and somatic stimulations at both painful and non-painful perceptual thresholds.

METHODS

Electrical stimulation was applied to the wrist and distal esophagus in 20 healthy participants. A double pseudorandom staircase determined perceptual thresholds of Sensation, Discomfort, and Pain for the somatic and visceral stimulations, separately. Stimulus reaction time (ms, via button press), and intensity, unpleasantness, and threat ratings were recorded after each stimulus. General linear mixed models compared differences in the four outcomes by stimulation type, threshold, and the stimulation type-by-threshold interaction. Sigmoidal maximum effect models evaluated differences in outcomes across all delivered stimulation intensities.

KEY RESULTS

Overall, visceral stimulations were perceived as more intense, threatening, and unpleasant compared to somatic stimulations, but participants responded faster to somatic stimulations. There was no significant interaction effect, but planned contrasts demonstrated differences at individual thresholds. Across all delivered intensities, higher intensity stimulations were needed to reach the half-maximum effect of self-reported intensity, unpleasantness, and threat ratings in the visceral domain.

CONCLUSIONS AND INFERENCES

Differences exist between modalities for both non-painful and painful sensations. These findings may have implications for translating paradigms and behavioral treatments from the somatic domain to the visceral domain, though future research in larger clinical samples is needed.

Influence of skin type and laser wavelength on laser-evoked potentials

BACKGROUND

Infrared laser stimulation is a valuable tool in pain research, its primary application being the recording of laser-evoked brain potentials (LEPs). Different types of laser stimulators, varying in their skin penetrance, are likely to have a large influence on the LEPs, when stimulating different skin types. The aim of this study was to investigate how LEPs depend on laser type and skin location.

METHODS

Two different laser stimulators (CO2 and Nd:YAP) were used to compare LEPs in healthy subjects. Stimuli were delivered to the hand dorsum and palm to investigate the effects of skin type on the evoked responses. Stimulus-evoked brain responses were recorded using EEG and perceived intensity ratings were recorded. Computational modelling was used to investigate the observed differences.

RESULTS

LEPs evoked by stimulation of the hairy skin were similar between CO2 and Nd:YAP stimulation. In contrast, LEPs elicited from the palm were markedly different and barely present for CO2 stimulation. There was a significant interaction between laser type and skin type (RM-ANOVA, p < 0.05) likely due to smaller CO2 LEPs in the palm. CO2 stimuli to the palm also elicited significantly lower perceived intensities. The computational model showed that the observed differences were explainable by the laser absorption characteristics and skin thickness affecting the temperature profile at the dermo-epidermal junction (DEJ).

CONCLUSIONS

This study shows that LEP elicitation depends on the combination of laser penetrance and skin type. Low penetrance stimuli, from a CO2 laser, elicited significantly lower LEPs and perceived intensities in the palm.

SIGNIFICANCE

This study showed that the elicitation of laser-evoked potentials in healthy humans greatly depends on the combination of laser stimulator type and skin type. It was shown that high penetrance laser stimuli are capable of eliciting responses in both hairy and glabrous skin, whereas low penetrance stimuli barely elicited responses from the glabrous skin. Computational modelling was used to demonstrate that the results could be fully explained by the combination of laser type and skin thickness.

A Systematic Review and Meta-analysis of Non-pharmacological Methods to Manipulate Experimentally Induced Secondary Hypersensitivity

This systematic review and meta-analysis investigated the effects of non-pharmacological manipulations on experimentally induced secondary hypersensitivity in pain-free humans. We investigated the magnitude (change/difference in follow-up ratings from pre-manipulation ratings) of secondary hypersensitivity (primary outcome), and surface area of secondary hypersensitivity (secondary outcome), in 27 studies representing 847 participants. Risk of bias assessment concluded most studies (23 of 27) had an unclear or high risk of performance and detection bias. Further, 2 (of 27) studies had a high risk of measurement bias. Datasets were pooled by the method of manipulation and outcome. The magnitude of secondary hypersensitivity was decreased by diverting attention, anodal transcranial direct current stimulation, or emotional disclosure; increased by directing attention toward the induction site, nicotine deprivation, or negative suggestion; and unaffected by cathodal transcranial direct current stimulation or thermal change. Area of secondary hypersensitivity was decreased by anodal transcranial direct current stimulation, emotional disclosure, cognitive behavioral therapy, hyperbaric oxygen therapy, placebo analgesia, or spinal manipulation; increased by directing attention to the induction site, nicotine deprivation, or sleep disruption (in males only); and unaffected by cathodal transcranial direct current stimulation, thermal change, acupuncture, or electroacupuncture. Meta-analytical pooling was only appropriate for studies that used transcranial direct current stimulation or hyperbaric oxygen therapy, given the high clinical heterogeneity among the studies and unavailability of data. The evidence base for this question remains small. We discuss opportunities to improve methodological rigor including manipulation checks, structured blinding strategies, control conditions or time points, and public sharing of raw data. PERSPECTIVE: We described the effects of several non-pharmacological manipulations on experimentally induced secondary hypersensitivity in humans. By shedding light on the potential for non-pharmacological therapies to influence secondary hypersensitivity, it provides a foundation for the development and testing of targeted therapies for secondary hypersensitivity.

The influence of a manipulation of threat on experimentally-induced secondary hyperalgesia

Pain is thought to be influenced by the threat value of the particular context in which it occurs. However, the mechanisms by which a threat achieves this influence on pain are unclear. Here, we explore how threat influences experimentally-induced secondary hyperalgesia, which is thought to be a manifestation of central sensitization. We developed an experimental study to investigate the effect of a manipulation of threat on experimentally-induced secondary hyperalgesia in 26 healthy human adults (16 identifying as female; 10 as male). We induced secondary hyperalgesia at both forearms using high-frequency electrical stimulation. Prior to the induction, we used a previously successful method to manipulate threat of tissue damage at one forearm (threat site). The effect of the threat manipulation was determined by comparing participant-rated anxiety, perceived threat, and pain during the experimental induction of secondary hyperalgesia, between the threat and control sites. We hypothesized that the threat site would show greater secondary hyperalgesia (primary outcome) and greater surface area (secondary outcome) of induced secondary hyperalgesia than the control site. Despite a thorough piloting procedure to test the threat manipulation, our data showed no main effect of site on pain, anxiety, or threat ratings during high-frequency electrical stimulation. In the light of no difference in threat between sites, the primary and secondary hypotheses cannot be tested. We discuss reasons why we were unable to replicate the efficacy of this established threat manipulation in our sample, including: (1) competition between threats, (2) generalization of learned threat value, (3) safety cues, (4) trust, and requirements for participant safety, (5) sampling bias, (6) sample-specific habituation to threat, and (7) implausibility of (sham) skin examination and report. Better strategies to manipulate threat are required for further research on the mechanisms by which threat influences pain.

Immediate comfort perception of 3D-printed foot orthoses in individuals with unilateral heel pain

BACKGROUND

Custom-made foot orthoses (FOs) play an integral part in managing foot disorders. Traditional FO fabrication is time-consuming and labor intensive. Three-dimensional (3D) printed FOs save time and cost compared with the traditional manufacturing process. To date, the differences in dimensions and comfort perception of these orthoses have not been compared in a pathological population.

OBJECTIVE

Compare the dimensions between 3D-printed and traditionally made FOs and comfort perception between 3D-printed, traditionally made, and no FOs in individuals with flatfeet and unilateral heel pain.

STUDY DESIGN

Within-subject single-blinded randomized crossover study design.

METHODS

Thirteen participants had custom-made FOs using 3D-printing and traditional processes. Orthotic lengths, widths, arch heights, and heel cup heights were compared. Participants performed walking trials under three conditions: (1) no orthoses, (2) 3D-printed orthoses, and (3) traditionally made orthoses. Comfort perception was recorded. Orthotic dimensions were compared using paired t tests, and comfort perception were compared using one-way multiple analysis of variance and Bonferroni post hoc tests.

RESULTS

Three-dimensional-printed orthoses were wider, have higher arch heights, and heel cup heights compared with traditionally made FOs (medium to large effect sizes). There was a difference in comfort perception between the three orthotic conditions, F(12,62) = 1.99, P = 0.04; Wilk Λ = 0.521, ηp2= 0.279. Post hoc tests show that there is no difference in comfort perception between the 3D-printed and traditionally made FOs. Both FOs were significantly more comfortable than no orthoses.

CONCLUSIONS

Three-dimensional printing seems to be a viable alternative orthotic fabrication option. Future studies should compare the biomechanical effects of 3D-printed and traditionally made FOs.

Effects of transcranial direct current stimulation on experimental pain perception: A systematic review and meta-analysis

OBJECTIVE

Many studies have examined the effectiveness of transcranial direct current stimulation (tDCS) on human pain perception in both healthy populations and pain patients. Nevertheless, studies have yielded conflicting results, likely due to differences in stimulation parameters, experimental paradigms, and outcome measures. Human experimental pain models that utilize indices of pain in response to well-controlled noxious stimuli can avoid many confounds present in clinical data. This study aimed to assess the robustness of tDCS effects on experimental pain perception among healthy populations.

METHODS

We conducted three meta-analyses that analyzed tDCS effects on ratings of perceived pain intensity to suprathreshold noxious stimuli, pain threshold and tolerance.

RESULTS

The meta-analyses showed a statically significant tDCS effect on attenuating pain-intensity ratings to suprathreshold noxious stimuli. In contrast, tDCS effects on pain threshold and pain tolerance were statistically non-significant. Moderator analysis further suggested that stimulation parameters (active electrode size and current density) and experimental pain modality moderated the effectiveness of tDCS in attenuating pain-intensity ratings.

CONCLUSION

The effectiveness of tDCS on attenuating experimental pain perception depends on both stimulation parameters of tDCS and the modality of experimental pain.

SIGNIFICANCE

This study provides some theoretical basis for the application of tDCS in pain management.

Imprecise Visual Feedback About Hand Location Increases a Classically Conditioned Pain Expectancy Effect

We tested the hypotheses that rendering sensory input about hand location imprecise increases a classically conditioned pain expectancy effect, increases generalization of the effect to novel locations and reduces extinction of the effect. Forty healthy volunteers performed movements with their right hand along predefined paths. Each path passed through 2 locations that were defined as either i) the conditioned stimulus (CS+; paired with a painful unconditioned stimulus), or ii) unpaired (CS-). During acquisition phase, participants watched their hand as they moved it. Participants were randomly allocated to an Imprecise group, for whom visual feedback of the hand was offset 30 to 50 mm from its true location, or a Precise group, for whom vision was not disrupted. In the test phase, participants moved their hands to 5 locations-the CS+, CS-, and 3 locations that lay between the 2 ("generalization stimuli"). Our primary hypothesis was supported-pain expectancy was greater at the CS+ location in the Imprecise group than in the Precise group (6.9 [SD = 1.9] vs 5.4 [SD = 2.5], P= .02). Pain expectancies generalized to novel locations similarly in both groups and there was no difference in extinction between groups. Our primary hypothesis was supported but our subsequent hypotheses were not. PERSPECTIVE: We conditioned pain expectancy at a certain location of one hand, even though most participants were unaware of the contingency. Conditioned pain expectancy was greater when sensory information about location was less precise. This adds support to the possibility that associative learning may play a role in the progression of an acute pain episode to a more generalized pain disorder.

Sensory Processing in People With and Without Tendinopathy: A Systematic Review With Meta-analysis of Local, Regional, and Remote Sites in Upper- and Lower-Limb Conditions

OBJECTIVE

To synthesize results of somatosensory processing tests in people with upper- and lower-limb tendinopathy, compared to controls.

DESIGN

Systematic review with meta-analysis.

LITERATURE SEARCH

Four electronic databases (MEDLINE, CINAHL Plus, SPORTDiscus, and Embase) were searched.

STUDY SELECTION CRITERIA

Included studies measured a domain of sensory processing and compared a tendinopathy group to a healthy control group.

DATA SYNTHESIS

Meta-analysis was conducted for outcomes with homogeneous data from at least 2 studies. Upper- and lower-limb conditions were compared and outcomes were examined by measurement site (local, regional, or remote to location of pain).

RESULTS

Of the 30 studies included, 18 investigated lateral elbow tendinopathy. The most commonly assessed outcome measures were pressure pain threshold (PPT) and thermal pain threshold. There was moderate evidence for local and regional reduction of PPT in upper-limb tendinopathies, but not at remote sites. In lower-limb tendinopathies, there was conflicting evidence regarding reduced PPT at local sites and limited evidence of normal PPT at remote sites. There was moderate evidence of sensitization of thermal pain threshold at local sites in upper-limb tendinopathies and limited evidence of no difference in thermal pain threshold in lower-limb tendinopathies. Findings across other domains were variable.

CONCLUSION

Sensory processing was different between upper-limb tendinopathy and lower-limb tendinopathy. Upper-limb tendinopathies showed signs consistent with primary and secondary hyperalgesia, but lower-limb tendinopathies did not. There was mixed evidence for primary hyperalgesia and limited evidence against secondary hyperalgesia. J Orthop Sports Phys Ther 2021;51(1):12-26. doi:10.2519/jospt.2021.9417.

Shared Sensitivity to Physical Pain and Social Evaluation

Many studies have demonstrated a link between experiences of physical pain and those of social rejection, both of which can trigger cognitive processes involved in detecting, orienting toward, or reacting to potentially threatening events. This study tested the hypothesis that healthy individuals who are more sensitive to physical pain are also more sensitive to social rejection. We recruited participants with high or low pain-sensitivity (HPS and LPS), as assessed by scores on a pain-sensitivity questionnaire and confirmed by experimental pain-sensitivity assessment. A modified social-judgment task was adopted in which participants first provided expectations about being liked/disliked by "peers", and then received "peers" feedback indicating acceptance or rejection. While both groups rated rejection as more unpleasant than acceptance, this difference was greater in the HPS group. Electroencephalographic results showed that only participants in the HPS group exhibited greater early delta/theta-oscillations (δ/θ-oscillations) in response to rejection than to acceptance, regardless of whether the feedback was expected or unexpected. However, both groups consistently exhibited greater late δ/θ-oscillations in response to rejection when the feedback was unexpected. These results suggest that participants in the HPS group were more sensitive to social cues signaling acceptance or rejection at early stages of information processing. Furthermore, neither early nor late δ/θ-oscillations following nonsocial feedback (correct or incorrect time-estimation) differed between groups. Altogether, these results supported the idea of shared sensitivity in detecting potentially physical and social threats in the environment. PERSPECTIVES: This study showed the greater emotional reactions and early-latency δ/θ-oscillations in response to social evaluation among healthy individuals with high pain sensitivity. It supports the idea of shared sensitivity to physical pain and social evaluation, which could be governed by a common system for detecting and monitoring potentially environmental threats.

Modulating pain thresholds through classical conditioning

Background

Classical conditioning has frequently been shown to be capable of evoking fear of pain and avoidance behavior in the context of chronic pain. However, whether pain itself can be conditioned has rarely been investigated and remains a matter of debate. Therefore, the present study investigated whether pain threshold ratings can be modified by the presence of conditioned non-nociceptive sensory stimuli in healthy participant.

Methods

In 51 healthy volunteers, pain threshold to electrocutaneous stimuli was determined prior to participation in a simultaneous conditioning paradigm. Participants underwent an acquisition phase in which one non-painful vibrotactile stimulus (CS⁺) was repeatedly paired with a painful electrocutaneous stimulus, whereas a second vibrotactile stimulus of the same quality and intensity (CS⁻) was paired with a non-painful electrocutaneous stimulus. Stimulation was provided on the lower back with close proximity between the conditioned stimulus and the unconditioned stimulus. In the test phase, electrocutaneous stimuli at the individually-set threshold intensity were simultaneously delivered together with either a CS⁺ or CS⁻. Pain intensity ratings were obtained after each trial; expectancy ratings were obtained after each block. The primary outcome was the percentage of test stimuli that were rated as painful.

Results

Test stimuli were more likely to be rated as painful when they were paired with the CS⁺ than when they were paired with the CS⁻. This effect was not influenced by contingency awareness, nor by expectancies or mood states.

Discussion

The findings support the notion that the judgement of an event being painful or non-painful can be influenced by classical conditioning and corroborate the possible role of associative learning in the development and maintenance of chronic pain.