Low threshold unmyelinated mechanoafferents can modulate pain

Affective touch reduces histamine evoked itch experience

Itch is a commonly experienced symptom of skin diseases such as eczema. Topical corticosteroid medications are widely used in chronic itch conditions but can lead to skin thinning, and in certain cases, topical corticosteroid withdrawal. As such, non-pharmaceutical alternatives are being researched. The present research explored affective touch (slow stroking, gentle touch signalled by C-tactile afferents) as a strategy to reduce histamine induced itch. Whilst experiencing histamine induced itch on the volar side of the forearms/wrist, participants (n = 60) were subjected to 3 experimental conditions of modulatory somatosensation applied to the volar aspect of the same forearm relative to the site of itch induction (18 trials of each); 1) affective touch (stroking the forearm with a soft brush at 3 cm/s), 2) non-affective touch (stroking the forearm with a soft brush at 18 cm/s) and 3) active control (static brush tapping on the forearm at 1Hz). Participants were asked to rate the severity of itch, and pleasantness of touch, after each trial. We also investigated whether changes in itch severity scores during the affective touch condition were moderated by individual differences in somatosensory experiences and attitudes as measured on the Touch Experiences and Attitudes Questionnaire (TEAQ), and the Pain Vigilance and Awareness Questionnaire (PVAQ). A linear mixed effects model indicated a main effect of condition on itch severity, whereby affective touch significantly reduced itch severity compared to non-affective touch (p < .001) and active control (p < .001). The TEAQ and PVAQ scores did not correlate significantly with itch scores in the affective touch condition. These results suggest that affective touch has a relieving effect on histamine-induced itch. Our findings lend further credibility to the idea that affective touch might be able to serve as an effective non-pharmaceutical treatment of itch conditions complementing established approaches.

Neural response associated with the modulation of temporal summation of second pain by affective touch

Temporal summation of second pain (TSSP) is a phenomenon that has clinical relevance but insights into its functioning are limited. Lately, 'affective touch' (AT) has been shown to have pain relieving properties but only one study has investigated its effects on TSSP and the neural underpinnings of such interaction are unknown. In the present EEG study, thirty-six healthy participants went through three conditions where a TSSP protocol was applied in concomitance with no touch (NoT), discriminative touch (DT) and AT. A fourth no-pain no-touch condition acted as a baseline. Measures of attention during the four conditions and of pleasantness during the touch conditions were also recorded. Pain ratings were significantly lower only during the AT condition. The neural response during NoT, compared to the baseline, brought about a temporal decrease in power at delta and theta frequencies and a fronto-central increase mainly in the alpha rhythm. Adding AT to TSSP yielded, compared to NoT, a decrease in delta, theta and beta bands in midline regions at both central (Cz) and parietal (Pz) and also of gamma at Pz. Notably, DT was not associated with significant changes compared to pain alone (NoT), but a specific marked difference was found between AT and DT with the former showing a significant decrease in beta frequencies localized at Pz. While TSSP seems to be characterized by a modulation mostly of the lower frequencies, adding AT to TSSP brings a clear depression of all the major frequency bands. Additionally, the parietal beta reduction may be a biomarker of AT. Future studies can examine if such brain response can help finding a suitable intervention for TSSP-related chronic pain conditions. PERSPECTIVE: This study consolidates the idea that AT can lower pain in a TSSP paradigm and shows what are the brain (EEG) responses associated with both TSSP and TSSP modulation by AT. Given that TSSP is linked to central sensitization and that it can be used as an experimental model for chronic pain, our results pave the way for further studies into the neural mechanisms of AT-led analgesia, which can lead to future effective treatments.

Slow depolarizing electrical stimuli reveal differential time courses of nociceptor recovery after prolonged topical capsaicin in human skin

BACKGROUND

We examined de-functionalization and temporal functional recovery of C-nociceptor evoked pain after topical 8% capsaicin applied for 4 consecutive days.

METHODS

Capsaicin and placebo patches were applied to human forearm skin (n = 14). Cold, warmth and heat pain thresholds, pain NRS to electrical and thermal (48°C, 5 s) stimuli and axon reflex flare were recorded weekly for 49 days. Mechanical and heat sensitive ('polymodal') nociceptors were activated by single electrical half-period sinusoidal pulses (0.5 s, 1 Hz). Mechanical and heat insensitive ('silent') nociceptors were activated by 4 Hz sinusoidal stimuli.

RESULTS

Capsaicin abolished heat pain. Sensation to electrical sinusoidal stimulation was reduced but never abolished during the treatment. Pain to electrical 1 Hz 'polymodal' nociceptor stimulation took longer to recover than pain ratings to 4 Hz 2.5 s sinusoidal stimulation activating 'polymodal' and 'silent' nociceptors (35 vs. 21 days). Heat pain was indifferent to placebo from day 21-49. Axon reflex flare was abolished during capsaicin and only recovered to ~50% even after 49 days.

CONCLUSIONS

Capsaicin abolishes heat transduction at terminal nociceptive endings, whereas small-diameter axons sensitive to sinusoidal electrical stimulation can still be activated. 1 Hz depolarizing stimuli evoke burst discharges, as demonstrated before, and recover slower after capsaicin than single pulses induced by 4 Hz. The difference in recovery suggests differential time course of functional regeneration for C-nociceptor sub-types after capsaicin. All sensations recovered completely within 7 weeks in healthy subjects. Our findings contrast analgesia lasting for months in spontaneous neuropathic pain patients treated with 8% capsaicin.

SIGNIFICANCE

Sinusoidal electrical stimulation can still activate small diameter axons desensitized to heat after 4 consecutive days of topical 8% capsaicin application and reveals differential temporal functional regeneration of C-nociceptor sub-types. Electrical sinusoidal stimulation may detect such axons that no longer respond to heat stimuli in neuropathic skin.

Different ways to reach the same goal: Are the analgesic effects of different types of tactile stimuli similar?

BACKGROUND

Tactile-induced analgesia (TIA) is a phenomenon in which different types of tactile stimulation alleviate pain via different mechanisms including empathy. As TIA plays an essential role in therapeutic situations and clinical conditions, it is crucial to determine whether specific tactile stimulations confer distinct benefits.

METHODS

Fifty-two subjects (31 females; 21-47 years) were exposed to four distinct experimental conditions involving three types of touch provided within a simulated supportive therapeutic setting. First, a contact heat (70 s) at a pain intensity of 60/100 Numerical Pain Scale (NPS) was applied to the forearm. The pain stimulus was then given simultaneously with vibration, gentle stroking, or handholding in random order. Pain ratings were reported at 5 and 65 s of each stimulation. Given the role of empathy in TIA, we also assessed the levels of empathy experienced by the subjects and the experimenter.

RESULTS

Handholding and vibration conditions were associated with a more rapid decrease in pain ratings compared to pain-alone (B values: handholding = -150.94 vs. pain-alone = -99.38, p = 0.01; vibration = -163.54 vs. pain-alone = -99.38, p < 0.001). Higher levels of the experimenter's empathy toward the subjects were associated with greater pain alleviation during vibration vs. pain-alone condition (B values: vibration = -56.42 vs. B pain-alone = -9.57, p = 0.04).

CONCLUSIONS

Vibration's potent analgesic effects may be attributed to its multi-channel analgesic mechanisms, including the therapist's empathy toward the participant. On the other hand, for handholding establishing an empathic interaction should be considered in a therapeutic setting to enhance its analgesic efficacy.

SIGNIFICANCE

This article explores the effectiveness of touch-based pain relief methods and their association with empathetic therapeutic interactions. The study emphasizes the significance of positive therapeutic interactions in facilitating tactile-induced analgesia.

The Neurophysiological Impact of Touch-Based Therapy: Insights and Clinical Benefits

The evidence on how touch-based therapy acts on the brain activity opens novel cues for the treatment of chronic pain conditions for which no definitive treatment exists. Touch-based therapies, particularly those involving C-tactile (CT)-optimal touch, have gained increasing attention for their potential in modulating pain perception and improving psychological well-being. While previous studies have focused on the biomechanical effects of manual therapy, recent research has shifted towards understanding the neurophysiological mechanisms underlying these interventions. CT-optimal touch, characterized by gentle stroking that activates CT afferents, may be used to reduce pain perception in chronic pain conditions and to enhance psychological well-being. Further research is needed to fully elucidate the neurophysiological mechanisms involved and to establish the therapeutic efficacy of CT-optimal touch in various clinical populations.

The effects of C-tactile stimulation on temporal summation of second pain: A study of the central and peripheral neural correlates

Affective touch is mediated by specialized receptors sensitive to gentle and slow touch called C-tactile afferents (CT). The activation of these receptors has shown promise in reducing subjective pain ratings, however, how this type of touch can affect central sensitization processes is poorly studied. This work aimed to investigate if affective touch is able to modulate pain sensitization and its electrophysiological correlates during Temporal Summation of Second Pain (TSSP), a phenomenon characterized by an increase in pain perception due to repeated noxious stimuli. Thirty-seven participants underwent a TSSP protocol involving three conditions: TSSP alone, TSSP during vibrotactile stimulation, and TSSP during CT stimulation (administered with a brush mounted in a robot arm). We measured subjective pain ratings, electroencephalographic (N2-P2 complex) and electrocardiographic activity during these conditions. Participants reported a significantly lower increase of pain during CT stimulation compared to vibrotactile stimulation, but not to TSSP alone. In addition, TSSP was reduced when administered in the ipsilateral arm compared to the other somatosensory stimulation. Subjective reports of attention towards painful stimuli, amplitude of the N2-P2 complex, and heart rate were also reduced during CT stimulation. Conclusion: Our results indicated that the activation of CT receptors may reduce sensitization compared to other types of somatosensory stimulation, which is possibly related to the reduction of the attention devoted to nociceptive stimulation. Our results suggest that activation of CT receptors may alleviate the occurrence of central pain sensitization.

A narrative review of research linking non-sexual social touch to sleep quality

This narrative review describes the current state of the literature that has examined associations between non-sexual social touch (i.e., affectionate touch, touch therapies, touch with animals and inanimate objects that mimic social touch) and sleep quality. It also highlights areas for future research to clarify the links and to identify underlying mechanisms. Most existing studies have focussed on and shown positive effects of touch therapies (e.g., massage, therapeutic touch) on sleep quality in clinical populations. Although there are fewer studies examining how other forms of social touch are linked with sleep quality, the existing research provides preliminary evidence supporting affectionate touch (e.g., hugging, skin-to-skin contact) and tactile contact with animals (e.g., dogs) and objects that mimic social touch (e.g., robots, weighted blankets) as predictors of better sleep quality, while touch deprivation and touch aversion are associated with worse sleep quality. Informed by the existing literature, we additionally reviewed potential relational-cognitive (e.g., felt-security) and neurobiological (e.g., oxytocin) mechanisms likely to underlie associations between social touch and sleep quality. Overall, current research supports associations between non-sexual social touch and sleep quality. However, future research is needed to establish these links for specific forms of social touch (and in various populations), to test explanatory mechanisms, and to identify boundary conditions. Understanding associations between non-sexual social touch and sleep quality can inform the development of touch-based interventions to improve sleep quality and health.

Insecure Attachment, Oxytocinergic System and C-Tactile Fibers: An Integrative and Translational Pathophysiological Model of Fibromyalgia and Central Sensitivity Syndromes

Although the pathophysiology of fibromyalgia syndrome has been better understood in recent decades, a unified model of its pathogenesis and an effective therapeutic approach are still far from being realized. The main aim of this article will be to delve into the fundamental mechanisms of the pathophysiology of fibromyalgia conceptualized as stress intolerance syndrome. Using the biopsychosocial model of chronic pain syndromes, we will describe the potential role of the attachment system, C-tactile fibers, and oxytocinergic system dysfunction in the pathophysiology of fibromyalgia syndrome and other central sensitivity syndromes. At the end of the article, the therapeutic implications of this new global and translational pathophysiological model will be briefly discussed.

The mutual effect of progesterone and vitamin D in an animal model of peripheral nerve injury

Background and purpose

Experimental and clinical studies have shown the potential role of progesterone in relieving neural injury. In addition, emerging data on vitamin D, a steroid hormone, have shown its neuroprotective properties. This study was designed to evaluate the mutual effect of vitamin D and progesterone on neuropathic pain (NP) in male rats.

Experimental approach

Chronic constriction injury (CCI) was induced by inserting four ligatures around the sciatic nerve. Hyperalgesia and allodynia (cold and mechanical) were considered positive behavioral scores of NP. After surgery, Sprague Dawley male rats (weighing 200-250 g) were assigned into 7 groups. Vitamin D (250 and 500 units/kg/day, i.p.) and progesterone (4 and 6 mg/kg/day, i.p.) were injected from the 1st day after CCI which continued for 21 days. Moreover, one group received the co-administration of vitamin D (500 units/kg/day, i.p.) and progesterone (6 mg/kg/day, i.p.) from the 1st day until the 21st post-CCI day. Behavioral tests were performed on the 7th, 14th, and 21st days.

Findings/Results

Daily supplementation with vitamin D (250 and 500 units/kg) did not alter nociception. Progesterone (4 and 6 mg/kg/day) was ineffective on thermal hyperalgesia. In the allodynia test, progesterone significantly decreased pain-related behaviors. The co-administration of vitamin D (500 units/kg/day) with progesterone (6 mg/kg/day) significantly relieved thermal hyperalgesia. Finally, the combination significantly decreased cold and mechanical allodynia.

Conclusion and implications

This study showed the mutual effect of progesterone and vitamin D on NP for the first time. Hyperalgesia and allodynia were significantly relieved following co-administration of vitamin D and progesterone.

Age-dependent small fiber neuropathy: Mechanistic insights from animal models

Small fiber neuropathy (SFN) is a common and debilitating disease in which the terminals of small diameter sensory axons degenerate, producing sensory loss, and in many patients neuropathic pain. While a substantial number of cases are attributable to diabetes, almost 50% are idiopathic. An underappreciated aspect of the disease is its late onset in most patients. Animal models of human genetic mutations that produce SFN also display age-dependent phenotypes suggesting that aging is an important contributor to the risk of development of the disease. In this review we define how particular sensory neurons are affected in SFN and discuss how aging may drive the disease. We also evaluate how animal models of SFN can define disease mechanisms that will provide insight into early risk detection and suggest novel therapeutic interventions.

To touch or to be touched? comparing appraisal of vicarious execution and reception of interpersonal touch

Unmyelinated C-Tactile (CT) fibres are activated by caress-like touch, eliciting a pleasant feeling that decreases for static and faster stroking. Previous studies documented this effect also for vicarious touch, hypothesising simulation mechanisms driving the perception and appreciation of observed interpersonal touch. Notably, less is known about appreciation of vicarious execution of touch, that is as referred to the one giving gentle touch. To address this issue, 53 healthy participants were asked to view and rate a series of videoclips displaying an individual being touched by another on hairy (i.e., hand dorsum) or glabrous (i.e., palm) skin sites, with touch being delivered at CT-optimal (5 cm/s) or non-CT optimal velocities (0 cm/s or 30 cm/s). Following the observation of each clip, participants were asked to rate self-referred desirability and model-referred pleasantness of vicarious touch for both executer (toucher-referred) and receiver (touchee-referred). Consistent with the CT fibres properties, for both self-referred desirability and model-referred pleasantness judgements of vicarious touch execution and reception, participants provided higher ratings for vicarious touch delivered at CT-optimal than other velocities, and when observed CT-optimal touch was delivered to the hand-dorsum compared to the palm. However, higher ratings were attributed to vicarious reception compared to execution of CT-optimal touch. Notably, individual differences in interoceptive trusting and attitude to interpersonal touch were positively correlated with, respectively, toucher- and touchee-related overall appraisal ratings of touch. These findings suggest that the appreciation of both toucher- and touchee-referred vicarious touch is specifically attuned to CT-optimal touch, even though they might rely on different neurocognitive mechanisms to understand affective information conveyed by interpersonal tactile interactions.

Effect of parental touch on relieving acute procedural pain in neonates and parental anxiety (Petal): a multicentre, randomised controlled trial in the UK

BACKGROUND

Touch interventions such as massage and skin-to-skin contact relieve neonatal pain. The Parental touch trial (Petal) aimed to assess whether parental stroking of their baby before a clinically required heel lance, at a speed of approximately 3 cm/s to optimally activate C-tactile nerve fibres, provides effective pain relief.

METHODS

Petal is a multicentre, randomised, parallel-group interventional superiority trial conducted in the John Radcliffe Hospital (Oxford University Hospitals NHS Foundation Trust, Oxford, UK) and the Royal Devon and Exeter Hospital (Royal Devon University Healthcare NHS Foundation Trust, Exeter, UK). Neonates without neurological abnormalities who were born at 35 weeks gestational age or more and required a blood test via a heel lance in the first week of life were randomly assigned (1:1) to receive parental touch for 10 s either before (intervention group) or after (control group) the clinically required heel lance. Randomisation was managed at the Oxford site using a web-based minimisation algorithm with allocation concealment. The primary outcome measure was the magnitude of noxious-evoked brain activity in response to the heel lance measured with electroencephalography (EEG). Secondary outcome measures were Premature Infant Pain Profile-Revised (PIPP-R) score, development of tachycardia, and parental anxiety score. For all outcomes, the per-protocol effect was estimated via complier average causal effect analysis on the full analysis set. The trial is registered on ISRCTN (ISRCTN14135962) and ClinicalTrials.gov (NCT04901611).

FINDINGS

Between Sept 1, 2021, and Feb 7, 2023, 159 parents were approached to participate in the study, and 112 neonates were included. 56 neonates were randomly assigned to the intervention group of parental stroking before the heel lance and 56 to the control group of parental stroking after the heel lance. The mean of the magnitude of the heel lance-evoked brain activity was 0·85 arbitrary units (a.u.; SD 0·70; n=39; a scaled magnitude of 1 a.u. represents the expected mean response to a heel lance in term-aged neonates) in the intervention group and 0·91 a.u. (SD 0·76; n=43) in the control group. Therefore, the primary outcome did not differ significantly between groups, with a mean difference of -0·11 a.u. (lower in intervention group; SD 0·77; 95% CI -0·42 to 0·20; p=0·38; n=82). No significant difference was observed across secondary outcomes. The PIPP-R difference in means was 1·10 (higher in intervention group, 95% CI -0·42 to 2·61; p=0·15; n=100); the odds ratio of becoming tachycardic was 2·08 (95% CI 0·46 to 9·46; p=0·34, n=105) in the intervention group with reference to the control group; and the difference in parental State-Trait Anxiety Inventory-State score was -0·44 (higher in control group; SD 6·85; 95% CI -2·91 to 2·02; p=0·72; n=106). One serious adverse event (desaturation) occurred in a neonate randomly assigned to the control group, which was not considered to be related to the study.

INTERPRETATION

Parental stroking delivered at an optimal speed to activate C-tactile fibres for a duration of 10 s before the painful procedure did not significantly change neonates' magnitude of pain-related brain activity, PIPP-R score, or development of tachycardia. The trial highlighted the challenge of translating an experimental researcher-led tactile intervention into a parent-led approach, and the value of involving parents in their baby's pain management.

FUNDING

Wellcome Trust and Bliss.

CT-optimal touch and chronic pain experience in Parkinson's Disease; An intervention study

One of the most underdiagnosed and undertreated non-motor symptoms of Parkinson's Disease is chronic pain. This is generally treated with analgesics which is not always effective and can cause several side-effects. Therefore, new ways to reduce chronic pain are needed. Several experimental studies show that CT-optimal touch can reduce acute pain. However, little is known about the effect of CT-optimal touch on chronic pain. The aim of the current study is to investigate whether CT-optimal touch can reduce the chronic pain experience in Parkinson patients. In this intervention study, 17 Parkinson patients underwent three conditions; no touch, CT-optimal touch and CT non-optimal touch with a duration of one week each. During each touch week, participants received touch from their partners twice a day for 15 minutes. Results show that both types of touch ameliorate the chronic pain experience. Furthermore, it appears that it is slightly more beneficial to apply CT-optimal touch also because it is perceived as more pleasant. Therefore, we argue that CT-optimal touch might be used when immediate pain relief is needed. Importantly, this study shows that CT-optimal touch can reduce chronic pain in Parkinson's Disease and can be administered by a partner which makes it feasible to implement CT-optimal touch as daily routine.

Spatial factors influencing the pain-ameliorating effect of CT-optimal touch: a comparative study for modulating temporal summation of second pain

Recent studies show that CT-optimal touch, gentle slow stroking of the skin, can reduce pain. However, much is unknown regarding the factors influencing its pain-ameliorating effect, such as tactile attention and touch application site. The current study investigates in 36 healthy individuals, whether CT-optimal touch can reduce temporal summation of second pain (TSSP) compared to CT non-optimal touch and tapping the skin. TSSP refers to activation of the C-nociceptors; by stimulating these fibers a burning and/or tingling sensation can be elicited. All participants underwent three conditions on both the contralateral and ipsilateral side of pain induction. The results show that tapping the skin did not reduce TSSP, meaning that pain reduction through touch cannot be explained by tactile attention effects. CT non-optimal touch only reduced TSSP when applied on the ipsilateral side. Importantly, CT-optimal touch effectively reduced TSSP when applied on the contralateral or ipsilateral side. Furthermore, CT-optimal touch was more effective in reducing TSSP compared to CT non-optimal touch and Tapping. This study shows that that CT-optimal touch can reduce TSSP and this effect appears to be independent of touch application site, which is highly relevant for implementing CT-optimal touch as a treatment.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

The perception of pain, discriminative touch and affective touch in patients suffering from Borderline Personality Disorder

Borderline Personality Disorder (BPD) is often characterized by self-injurious behaviors, with one-half to two-third of these patients reporting hypalgesic or analgesic phenomena during self-harming. Research on pain perception in BPD suggested abnormal processing of nociception either within the sensory-discriminative and/or motivational-affective systems of pain. Nevertheless, it is still unclear whether pain insensitivity could be generalized to other somatosensory submodalities. To investigate this question, 30 BPD patients and 30 matched healthy controls were enrolled in the current study and underwent a somatosensory battery composed of well-established psychophysical test assessing all the principal submodalities of somatosensation, namely pain perception (i.e., warm, cold and mechanical), discriminative touch (i.e., tactile acuity and tactile sensitivity) as well as affective touch. Results showed abnormal warm detection threshold, warm pain threshold, mechanical pain perception, and tactile sensitivity in BPD patients, but no differences emerged neither for tactile acuity nor for cold pain thresholds, cold tolerance, or for affective touch perception. Findings point to a deficit in nociception, as well as in tactile sensitivity in BPD individuals, and were discussed in relation to BPD clinical features including self-injurious behaviors.

Slaughtering of Water Buffalo (Bubalus bubalis) with and without Stunning: A Focus on the Neurobiology of Pain, Hyperalgesia, and Sensitization

The slaughter process in livestock is considered a stressor where the transport and handling of animals, as well as the selected stunning and bleeding methods, can cause acute pain, distress, and suffering. In water buffaloes, although stunning is known to be performed before bleeding to induce unconsciousness, no emphasis is made on the nociceptive events during this process. Particularly, current mechanical stunning methods applied to cattle are unsuitable for water buffaloes due to anatomical differences in the skull from other large ruminants. Furthermore, although very high-pressure pneumatic (200-220 psi) may be effective in the frontal position for lighter-weight water buffalos, for heavier animals, it is less likely to be effective. The present review aims: (1) to analyze the anatomical particularities of water buffaloes to discuss the importance of selecting a stunning method suitable for buffaloes, and (2) to revise the potential pain-related consequences, such as hyperalgesia and sensitization, and the signs to assess the stun quality and death to comprehend the relevance of a proper technique according to the species.

Social Touch Reduces Pain Perception—An fMRI Study of Cortical Mechanisms

Unmyelinated low-threshold mechanoreceptors (C-tactile, CT) in the human skin are important for signaling information about hedonic aspects of touch. We have previously reported that CT-targeted brush stroking by means of a robot reduces experimental mechanical pain. To improve the ecological validity of the stimulation, we developed standardized human–human touch gestures for signaling attention and calming. The attention gesture is characterized by tapping of the skin and is perceived as neither pleasant nor unpleasant, i.e., neutral. The calming gesture is characterized by slow stroking of the skin and is perceived as moderately to very pleasant. Furthermore, the attention (tapping) gesture is ineffective, whereas the calming (stroking) gesture is effective in activating CT-afferents. We conducted an fMRI study (n = 32) and capitalized on the previous development of touch gestures. We also developed an MR compatible stimulator for high-precision mechanical pain stimulation of the thenar region of the hand. Skin-to-skin touching (stroking or tapping) was applied and was followed by low and high pain. When the stroking gesture preceded pain, the pain was rated as less intense. When the tapping gesture preceded the pain, the pain was rated as more intense. Individual pain perception related to insula activation, but the activation was not higher for stroking than for tapping in any brain area during the stimulation period. However, during the evaluation period, stronger activation in the periaqueductal gray matter was observed after calming touch compared to after tapping touch. This finding invites speculation that human–human gentle skin stroking, effective in activating CT-afferents, reduced pain through neural processes involving CT-afferents and the descending pain pathway.

Effect of low-frequency dorsal root ganglion stimulation in the treatment of chronic pain

BACKGROUND

The role of stimulation parameters, especially stimulation frequency is not well understood in dorsal root ganglion stimulation. Previous studies documented higher effectiveness for frequencies as low as 20 Hz, but there is evidence that even lower values could lead to better outcomes. In this study, we investigate the influence of low-frequency DRG-S.

METHOD

This is a randomized double-blind clinical trial with a crossover design. Patients with an already implanted DRG-S system were included and randomly tested with 4 Hz, 20 Hz, 60 Hz, and sham stimulation. Amplitude was adjusted to subthreshold values for each frequency. Each frequency was tested for 5 days, followed by a 2-day washout period. Patients were assessed using VAS, McGill Pain Questionnaire, EQ-5D-5L, and Beck Depression Inventory.

RESULTS

Seventeen patients were in included. Time between inclusion in this study and primary implant was 32.8 months. Baseline stimulation frequency was 20 Hz in all patients. Mean baseline pain intensity was VAS 3.2 (SD 2.2). With 4-Hz stimulation, VAS was 3.8 (SD 1.9), with 20 Hz VAS 4.2 (SD 2.0) and with 60 Hz VAS 4.6 (SD 2.7). Worst pain control was seen with sham stimulation with a VAS of 5.3 (SD 3.0). Stimulation with 4 Hz achieved lower VAS scores, but this was only statistically significant when compared to sham (p = 0.001). A similar trend favoring 4-Hz stimulation was seen using the Beck Depression Inventory, but in this case no statistical significance was found. Outcomes of McGill Pain Questionnaire and EQ-5D-5L favored 20 Hz stimulation, but again without statistical significance.

CONCLUSIONS

Low-frequency stimulation was not significantly better than classic 20-Hz stimulation in relieving pain intensity; the study might however be underpowered. Longer washout and observational periods might also be necessary to show clear differences in frequency response.

Processing of sensory, painful and vestibular stimuli in the thalamus

OBJECTIVES

The thalamus plays an important role in the mediation and integration of various stimuli (e.g., somatosensory, pain, and vestibular). Whether a stimulus-specific and topographic organization of the thalamic nuclei exists is still unknown. The aim of our study was to define a functional, in vivo map of multimodal sensory processing within the human thalamus.

METHODS

Twenty healthy individuals (10 women, 21-34 years old) participated. Defined sensory stimuli were applied to both hands (innocuous touch, mechanical pain, and heat pain) and the vestibular organ (galvanic stimulation) during 3 T functional MRI.

RESULTS

Bilateral thalamic activations could be detected for touch, mechanical pain, and vestibular stimulation within the left medio-dorsal and right anterior thalamus. Heat pain did not lead to thalamic activation at all. Stimuli applied to the left body side resulted in stronger activation patterns. Comparing an early with a late stimulation interval, the mentioned activation patterns were far more pronounced within the early stimulation interval.

CONCLUSIONS

The right anterior and ventral-anterior nucleus and the left medio-dorsal nucleus appear to be important for the processing of multimodal sensory information. In addition, galvanic stimulation is processed more laterally compared to mechanical pain. The observed changes in activity within the thalamic nuclei depending on the stimulation interval suggest that the stimuli are processed in a thalamic network rather than a distinct nucleus. In particular, the vestibular network within the thalamus recruits bilateral nuclei, rendering the thalamus an important integrative structure for vestibular function.

The analgesic power of pleasant touch in individuals with chronic pain: Recent findings and new insights

This mini-review covers recent works on the study of pleasant touch in patients with chronic pain (CP) and its potential use as a treatment. While experiments have demonstrated that pleasant touch, through the activation of CT-afferents and the brain regions involved in its affective value, might reduce the unpleasantness and intensity of induced pain, the interaction between pleasant touch and CP remains under-examined. Some experiments show that CP might disrupt the positive aspects of receiving pleasant touch, while in other studies the perception of pleasantness is preserved. Moreover, only a few attempts have been made to test whether touch can have a modulatory effect on CP, but these results also remain inconclusive. Indeed, while one recent study demonstrated that CT-touch can diminish CP after a short stimulation, another study suggested that pleasant touch might not be sufficient. Future studies should further investigate the psychological and neural interplay between pleasant touch and CP. In the conclusion of this mini-review, we propose a new tool we have recently developed using immersive virtual reality (IVR).

Gentle Touch Therapy, Pain Relief and Neuroplasticity at Baseline in Fibromyalgia Syndrome: A Randomized, Multicenter Trial with Six-Month Follow-Up

Background: Fibromyalgia (FM) is considered a stress-related disorder characterized mainly by chronic widespread pain. Its pathogenesis is unknown, but cumulative evidence points at dysfunctional transmitter systems and inflammatory biomarkers that may underlie the major symptoms of the condition. This study aimed to evaluate pain scores (primary outcome), quality of life, inflammatory biomarkers and neurotransmitter systems in women with FM (secondary outcomes) subjected to gentle touch therapy (GTT) or placebo. Methods: A total of 64 female patients with FM were randomly assigned to two groups, namely GTT (n = 32) or Placebo (n = 32). Clinical assessments were conducted at baseline and post-intervention with six-month follow-up. We measured serum catecholamines (dopamine), indolamines and intermediary metabolites (serotonin or 5-hydroxyindolacetic acid (5-HIAA)), as well as tetrahydrobiopterin (BH4), which is a cofactor for the synthesis of neurotransmitters and inflammatory biomarkers in women with FM. A group of healthy individuals with no intervention (control group) was used to compare biochemical measurements. Intervention effects were analyzed using repeated measures (RM) two-way ANOVA followed by Bonferroni post hoc test and mixed ANCOVA model with intention to treat. Results: Compared to placebo, the GTT group presented lower pain scores and brain-derived neurotrophic factor (BDNF) levels without altering the quality of life of women with FM. Changes in BDNF had a mediating role in pain. Higher baseline serum BDNF and 5-HIAA or those with a history of anxiety disorder showed a higher reduction in pain scores across time. However, women with higher serum dopamine levels at baseline showed a lower effect of the intervention across the observation period revealed by an ANCOVA mixed model. Conclusions: In conclusion, lower pain scores were observed in the GTT group compared to the placebo group without altering the quality of life in women with FM. Reductions in BDNF levels could be a mechanism of FM pain status improvement. In this sense, the present study encourages the use of these GTT techniques as an integrative and complementary treatment of FM.

Cyclic changes of sensory parameters in migraine patients

BACKGROUND

Migraine shows a cyclic pattern with an inter-ictal-, a pre-ictal, an ictal- and a post-ictal phase. We aimed to examine changes in psychophysical parameters during the migraine cycle.

METHODS

The perception of nociceptive and non-nociceptive stimuli and an electrically induced axon-reflex-erythema were assessed in 20 healthy controls and 14 migraine patients on five consecutive days according to different phases of the migraine cycle. Pain was rated three times during a 10-second electrical stimulus. The size of the axon-reflex-erythema was determined using laser-Doppler-imaging. Intensity and hedonic estimates of odours presented by Sniffin' Sticks were rated.

RESULTS

In healthy controls, no significant changes over the test days were observed. In migraine patients pain thresholds at the head decreased with an ictal minimum. Less habituation after five seconds of stimulation at the head was found pre-ictally, whereas reduced habituation to 10-second electrical stimulation was present in all phases. The axon-reflex-erythema size showed an inter-ictal-specific minimum at the head. odours were perceived ictally as more unpleasant and intense.

CONCLUSIONS

Somatosensory functions, pain thresholds and habituation as predominantly central parameters, axon-reflex-erythema as a peripheral function of trigeminal neurons and odour perception as a predominantly extra-thalamic sensation change specifically over the migraine cycle indicating complex variations of neuronal signal processing.

Frequency dependency of therapeutic efficacy in dorsal root ganglion stimulation for neuropathic pain

Background

The influence of the stimulation frequency on the outcomes of dorsal root ganglion stimulation (DRG-S) to treat pain is not well understood. It is assumed that specific neural components dedicated to different tasks in the DRG can be preferably influenced at specific frequencies. The identification of frequencies designed for the type of pain and the ratio of neuropathic versus nociceptive pain might improve overall pain control and open new indications in DRG-S.

Method

We report on a randomized double-blind clinical trial with a crossover design. Patients with a permanent DRG-S system underwent phases of stimulation with 20 Hz, 40 Hz, 60 Hz, 80 Hz, and sham in a randomized order. Each phase lasted for 4 days and was followed by a 2-day washout period. Pain intensity and quality of life were assessed with visual analog scale (VAS), McGill Pain Questionnaire (MPQ), EQ-5D, and Beck Depression Inventory (BDI). Analgesics intake was assessed.

Results

Overall 19 patients were included in the study. CRPS was the most frequent pain etiology (7). Five patients had a PainDetect score of 12 or lower at baseline. The mean VAS before the system was implanted was 8.6 and 3.9 at the baseline. Pain intensity was reduced to 3.7 by the stimulation with 20 Hz but increased with higher frequencies reaching 5.8 at 80 Hz. A significant difference among the groups was shown over all variables examined (VAS, MPQ, EQ-5D, BDI). The best results were seen at 20 Hz for all variables, including the smallest increase in pain medication consumption.

Conclusions

The choice of the stimulation frequency shows a clear influence on pain reduction and quality of life. Lower stimulation frequencies seem to be most effective in neuropathic pain. Further studies are required to determine whether specific frequencies should be preferred based on the condition treated.

Application areas and effects of aquatic therapy WATSU - A survey among practitioners

INTRODUCTION

WATSU (WaterShiatsu) is a treatment administered in warm water. The present study investigated if and how frequently scientifically studied application areas and effects of WATSU occur in practice, whether similar effectiveness of WATSU is observed in trials and practice, and whether practitioners can contribute additional application areas and effects of WATSU.

METHODS

Application areas and effects of WATSU reported in a recent systematic review were extracted verbatim to be assessed in a worldwide multilingual cross section online survey, generating quantitative and qualitative data. A pre-test and retest were conducted to ensure quality and evaluate the questionnaire's psychometric properties.

RESULTS

Answers of 191 respondents were processed. All proposed 26 application areas and 20 effects were confirmed, each with relatively high ratings of observed effectiveness of WATSU. WATSU was frequently applied in healthy individuals (including during pregnancy), and individuals in various pain- (e.g., low back pain, neck pain, myofascial pain, fibromyalgia) and stress-related (e.g., stress, depression, sleep disorders, fatigue, anxiety disorders) conditions. Frequently confirmed effects were physical relaxation, relief of physical tension, pain relief, increased mobility and flexibility, improved quality of life, spiritual experiences, and increased psychological health. Respondents contributed 73 additional application areas and effects (both, mental and physical) of WATSU.

CONCLUSIONS

Application areas and effects of WATSU are consistently employed practically and scientifically. Respondents' ratings of effectiveness of WATSU match tentative research efforts. WATSU is cautiously recommended for the use in pain- and stress-related conditions. Short- and long-term effectiveness of WATSU need to be evaluated in high level intervention studies.

Robotic Stroking on the Face and Forearm: Touch Satiety and Effects on Mechanical Pain

Background: Slow stroking touch is generally perceived as pleasant and reduces thermal pain. However, the tactile stimuli applied tend to be short-lasting and typically applied to the forearm. This study aimed to compare the effects of a long-lasting brushing stimulus applied to the facial region and the forearm on pressure pain thresholds (PPTs) taken on the hand. Outcome measurements were touch satiety and concurrent mechanical pain thresholds of the hand. Methods: A total of 24 participants were recruited and randomized to receive continuous stroking, utilizing a robotic stimulator, at C-tactile (CT) favorable (3 cm/s) and non-favorable (30 cm/s) velocities applied to the right face or forearm. Ratings of touch pleasantness and unpleasantness and PPTs from the hypothenar muscle of the right hand were collected at the start of stroking and once per minute for 5 min. Results: A reduction in PPTs (increased pain sensitivity) was observed over time (P < 0.001). However, the increase in pain sensitivity was less prominent when the face was stroked compared to the forearm (P = 0.001). Continuous stroking resulted in a significant interaction between region and time (P = 0.008) on pleasantness ratings, with a decline in ratings observed over time for the forearm, but not on the face. Unpleasantness ratings were generally low. Conclusion: We observed touch satiety for 5 min of continuous robotic brushing on the forearm confirming previous studies. However, we did not observe any touch satiety for brushing the face. Mechanical pain sensitivity, measured in the hand, increased over the 5-min period but less so when paired with brushing on the face than with brushing on the forearm. The differential effects of brushing on the face and forearm on touch satiety and pain modulation may be by the differences in the emotional relevance and neuronal pathways involved.

Widespread Pressure Delivered by a Weighted Blanket Reduces Chronic Pain: A Randomized Controlled Trial

Pleasant sensation is an underexplored avenue for modulation of chronic pain. Deeper pressure is perceived as pleasant and calming, and can improve sleep. Although pressure can reduce acute pain, its effect on chronic pain is poorly characterized. The current remote, double-blind, randomized controlled trial tested the hypothesis that wearing a heavy weighted blanket - providing widespread pressure to the body - relative to a light weighted blanket would reduce ratings of chronic pain, mediated by improvements in anxiety and sleep. Ninety-four adults with chronic pain were randomized to wear a 15-lb. (heavy) or 5-lb. (light) weighted blanket during a brief trial and overnight for one week. Measures of anxiety and chronic pain were collected pre- and post-intervention, and ratings of pain intensity, anxiety, and sleep were collected daily. After controlling for expectations and trait anxiety, the heavy weighted blanket produced significantly greater reductions in broad perceptions of chronic pain than the light weighted blanket (Cohen's f = .19, CI [-1.97, -.91]). This effect was stronger in individuals with high trait anxiety (P = .02). However, weighted blankets did not alter pain intensity ratings. Pain reductions were not mediated by anxiety or sleep. Given that the heavy weighted blanket was associated with greater modulation of affective versus sensory aspects of chronic pain, we propose that the observed reductions are due to interoceptive and social/affective effects of deeper pressure. Overall, we demonstrate that widespread pressure from a weighted blanket can reduce the severity of chronic pain, offering an accessible, home-based tool for chronic pain. The study purpose, targeted condition, study design, and primary and secondary outcomes were pre-registered in ClinicalTrials.gov (NCT04447885: "Weighted Blankets and Chronic Pain"). Perspective: This randomized-controlled trial showed that a 15-lb weighted blanket produced significantly greater reductions in broad perceptions of chronic pain relative to a 5-lb weighted blanket, particularly in highly anxious individuals. These findings are relevant to patients and providers seeking home-based, nondrug therapies for chronic pain relief.

Low mechano-afferent fibers reduce thermal pain but not pain intensity in CRPS

Background

Human hairy (not glabrous skin) is equipped with a subgroup of C-fibers, the C-tactile (CT) fibers. Those do not mediate pain but affective aspects of touch. CT-fiber-activation reduces experimental pain if they are intact. In this pilot study we investigated pain modulating capacities of CT-afferents in CRPS.

Methods

10 CRPS-patients (mean age 33 years, SEM 3.3) and 11 healthy controls (mean age 43.2 years, SEM 3.9) participated.

CT-targeted-touch (brush stroking, velocity: 3 cm/s) was applied on hairy and glabrous skin on the affected and contralateral limb. Patients rated pleasantness of CT-targeted-touch (anchors: 1 “not pleasant”—4 “very pleasant”) twice daily on 10 days. Pain intensity (NRS: 0 “no pain” – 10 “worst pain imaginable”) was assessed before, 0, 30, 60 and 120 min after each CT-stimulation. To assess sensory changes, quantitative-sensory-testing was performed at the beginning and the end of the trial period.

Results

CT-targeted-touch was felt more pleasant on the healthy compared to the affected limb on hairy (p < 0.001) and glabrous skin (p 0.002), independent of allodynia. In contrast to healthy controls patients felt no difference between stimulating glabrous and hairy skin on the affected limb. Thermal pain thresholds increased after CT-stimulation on the affected limb (cold-pain-threshold: p 0.016; heat-pain-threshold: p 0.033).

Conclusions

CT-stimulation normalizes thermal pain thresholds but has no effect on the overall pain in CRPS. Therefore, pain modulating properties of CT-fibers might be too weak to alter chronic pain in CRPS. Moreover, CT-fibers appear to lose their ability to mediate pleasant aspects of touch in CRPS.

Interoception and Emotion: A Potential Mechanism for Intervention With Manual Treatment

Interoception is considered a perception pathway as important as the exteroceptive pathways for determining responses to maintain homeostasis. There is evidence about the influence of the interoception on emotional responses as these expressions are considered to be a combination of physical, environmental and individual beliefs. A large percentage of afferent fibers in the body are related to free nerve endings which, when stimulated, reach the insular cortex that participates in the process of emotions. The viscera afferent fibers represent 5% to 15% of all these inputs. Evidence emerges that demonstrates the importance of visceral health as part of the treatment of patients with emotional imbalances. It can be postulated that manual treatment applied to visceral fasciae can assist in interoceptive balance and have a positive impact on emotions. Therefore, the objective of the present study is to discuss the concepts of interoception, central sensitization, emotional health and visceral manual treatment.

Neural basis of affective touch and pain: A novel model suggests possible targets for pain amelioration

Pain is one of the most common health problems and has a severe impact on quality of life. Yet, a suitable and efficient treatment is still not available for all patient populations suffering from pain. Interestingly, recent research shows that low threshold mechanosensory C‐tactile (CT) fibres have a modulatory influence on pain. CT‐fibres are activated by slow gentle stroking of the hairy skin, providing a pleasant sensation. Consequently, slow gentle stroking is known as affective touch. Currently, a clear overview of the way affective touch modulates pain, at a neural level, is missing. This review aims to present such an overview. To explain the interaction between affective touch and pain, first the neural basis of the affective touch system and the neural processing of pain will be described. To clarify these systems, a schematic illustration will be provided in every section. Hereafter, a novel model of interactions between affective touch and pain systems will be introduced. Finally, since affective touch might be suitable as a new treatment for chronic pain, possible clinical implications will be discussed.

Affective Touch Reduces Electrically Induced Itch Experience

Introduction: Itch is a common symptom in dermatologic and other diseases and can have a severe impact on quality of life and mental health. As a proportion of patients with itch-symptoms is resistant to commonly used anti-histamine treatments, development of new treatments is desirable. Past research on pain, itch and affective touch (i.e. slow, gentle stroking of the skin activating C-tactile fibers) revealed an inhibitory relationship between affective touch and pain and between pain and itch. Given the overlap in neural processing between these three sensory submodalities, a possible interaction between affective touch and itch might be expected. This study investigated whether there is a relationship between itch and affective touch, and if so, whether affective touch inhibits itch.

Methodology: Itch was electrically induced with the use of electrodes that were placed at the ventral side of the wrist of 61 participants. A within-subject design was conducted with two conditions. An experimental -affective touch- condition (stroking the forearm with a soft brush at 3 cm/s) and a control -non-affective touch- condition (stroking the forearm with a soft brush at 18 cm/s). Touch was applied on the dorsal side of the forearm, the same arm as were the electrodes were placed. For each condition itch was induced for 20 min, with every 2 min a VAS-scale measurement of the level of experienced itch.

Results: Both types of touch reduced the experienced itch compared to baseline (p < 0.01, partial η² = 0.67). However, affective touch had an additional significant relieving effect compared to non-affective touch (p = 0.03, partial η²= 0.08). The alleviation of itch started after 2 min of stroking and continued to increase up till 6 min, where after the relieving effect stabilized but still persisted.

Conclusion: This finding suggest that affective touch, as with acute pain, has a relieving effect on electrically induced itch.

A Better Touch: C-tactile Fibers Related Activity is Associated to Pain Reduction During Temporal Summation of Second Pain

C-tactile (CT) fibers, responsible for the so-called "affective" touch (AT), have drawn a fair amount of attention within the scientific community for their marked social dimension. However, while the pain-relieving potential of discriminative touch (DT) has been documented, proofs of the analgesic properties of AT are still scarce. Additionally, no study has so far tested its possible pain-relieving effects on a clinically-relevant model. Temporal summation of second pain (TSSP), otherwise referred to as "wind-up," relies on repetitive stimulation of C-nociceptors and it is thought to reflect central sensitization, a process linked to many chronic pain conditions. In the present experimental, within participants, design we induced TSSP through trains of ascending and descending repetitive heat stimulation. Forty-two healthy participants' pain was measured during 2 different tactile stimulations (stroking velocities AT: 10 cm/s; DT: 0.3 cm/s) or without concomitant tactile input. Since measures of pleasantness of the tactile stimulation have been found to strongly correlate with C-tactile fibers' firing rate, these, together with participants' body awareness, were also taken into account. Our results show that AT brought about a decrease of our participants' pain as opposed to both DT and no touch, while DT did not produce any significant pain reduction. Thus, only AT successfully modulated wind-up. As expected, AT was perceived as more pleasant than DT, while a clear relationship between body awareness and pain was found only during DT. Targeting CT fibers could pave the way to new treatments for chronic pain conditions whose aetiology depend on abnormal C-nociceptors' physiology. PERSPECTIVE: This study extends previous findings on the analgesic potential of affective touch, documenting a clear pain reduction during temporal summation of second pain (TSSP). Since TSSP is thought to reflect central sensitization, the psychophysiological mechanisms of affective touch could be exploited for new chronic pain treatments.

Mechanisms for the Clinical Utility of Low-Frequency Stimulation in Neuromodulation of the Dorsal Root Ganglion

BACKGROUND

Dorsal root ganglion stimulation (DRG-S) involves the electrical modulation of the somata of afferent neural fibers to treat chronic pain. DRG-S has demonstrated clinical efficacy at frequencies lower than typically used with spinal cord stimulation (SCS). In a clinical study, we found that the frequency of DRG-S can be tapered to a frequency as low as 4 Hz with no loss of efficacy. This review discusses possible mechanisms of action underlying effective pain relief with very low-frequency DRG-S.

MATERIALS AND METHODS

We performed a literature review to explore the role of frequency in neural transmission and the corresponding relevance of frequency settings with neuromodulation.

FINDINGS

Sensory neural transmission is a frequency-modulated system, with signal frequency determining which mechanisms are activated in the dorsal horn. In the dorsal horn, low-frequency signaling (<20 Hz) activates inhibitory processes while higher frequencies (>25 Hz) are excitatory. Physiologically, low-threshold mechanoreceptors (LTMRs) fibers transmit or modulate innocuous mechanical touch at frequencies as low as 0.5-5 Hz, while nociceptive fibers transmit pain at high frequencies. We postulate that very low-frequency DRG-S, at least partially, harnesses LTMRs and the native endogenous opioid system. Utilizing lower stimulation frequency decreases the total energy delivery used for DRG-S, extends battery life, and facilitates the development of devices with smaller generators.

Altered Interoceptive Perception and the Effects of Interoceptive Analgesia in Musculoskeletal, Primary, and Neuropathic Chronic Pain Conditions

Chronic pain (CP) severely disrupts the daily life of millions. Interoception (i.e., sensing the physiological condition of the body) plays a pivotal role in the aetiology and maintenance of CP. As pain is inherently an interoceptive signal, interoceptive frameworks provide important, but underutilized, approaches to this condition. Here we first investigated three facets of interoceptive perception in CP, compared with pain-free controls. We then introduce a novel interoceptive treatment and demonstrate its capacity to reduce pain severity in CP, potentially providing complementary analgesic treatments. Study 1 measured interoceptive accuracy, confidence and sensibility in patients (N = 60) with primary, secondary musculoskeletal, and neuropathic CP. Compared with matched controls, CP participants exhibited significantly lower interoceptive accuracy and interoceptive confidence. Pain severity was predicted positively by interoceptive accuracy, anxiety and depression, and negatively by interoceptive confidence. Study 2 tested a promising new interoceptive treatment for CP, in a single-blind between-subjects design (N = 51) with primary, secondary musculoskeletal, and neuropathic CP patients. The treatment specifically activates the C-Tactile system, by means of controlled stimulation of interoceptive unmyelinated afferents, at 3 cm/s with a force of 2.5 mN. This treatment led to significant pain reduction (mean 23%) in the CP treatment group after only 11 min, while CP controls who received comparable but non-interoceptive stimulation reported no change in pain intensity. These studies highlight the importance of interoceptive approaches to CP and demonstrate the potential of this novel method of C-Tactile stimulation to provide complementary analgesic treatments.

Low-Threshold Mechanosensitive VGLUT3-Lineage Sensory Neurons Mediate Spinal Inhibition of Itch by Touch

Innocuous mechanical stimuli, such as rubbing or stroking the skin, relieve itch through the activation of low-threshold mechanoreceptors. However, the mechanisms behind this inhibition remain unknown. We presently investigated whether stroking the skin reduces the responses of superficial dorsal horn neurons to pruritogens in male C57BL/6J mice. Single-unit recordings revealed that neuronal responses to chloroquine were enhanced during skin stroking, and this was followed by suppression of firing below baseline levels after the termination of stroking. Most of these neurons additionally responded to capsaicin. Stroking did not suppress neuronal responses to capsaicin, indicating state-dependent inhibition. Vesicular glutamate transporter 3 (VGLUT3)-lineage sensory nerves compose a subset of low-threshold mechanoreceptors. Stroking-related inhibition of neuronal responses to chloroquine was diminished by optogenetic inhibition of VGLUT3-lineage sensory nerves in male and female Vglut3-cre/NpHR-EYFP mice. Conversely, in male and female Vglut3-cre/ChR2-EYFP mice, optogenetic stimulation of VGLUT3-lineage sensory nerves inhibited firing responses of spinal neurons to pruritogens after the termination of stimulation. This inhibition was nearly abolished by spinal delivery of the κ-opioid receptor antagonist nor-binaltorphimine dihydrochloride, but not the neuropeptide Y receptor Y1 antagonist BMS193885. Optogenetic stimulation of VGLUT3-lineage sensory nerves inhibited pruritogen-evoked scratching without affecting mechanical and thermal pain behaviors. Therefore, VGLUT3-lineage sensory nerves appear to mediate inhibition of itch by tactile stimuli.SIGNIFICANCE STATEMENT Rubbing or stroking the skin is known to relieve itch. We investigated the mechanisms behind touch-evoked inhibition of itch in mice. Stroking the skin reduced the activity of itch-responsive spinal neurons. Optogenetic inhibition of VGLUT3-lineage sensory nerves diminished stroking-evoked inhibition, and optogenetic stimulation of VGLUT3-lineage nerves inhibited pruritogen-evoked firing. Together, our results provide a mechanistic understanding of touch-evoked inhibition of itch.

Epidermal expression of human TRPM8, but not of TRPA1 ion channels, is associated with sensory responses to local skin cooling

Human cold perception and nociception play an important role in persisting pain. However, species differences in the target temperature of thermosensitive ion channels expressed in peripheral nerve endings have fueled discussions about the mechanism of cold nociception in humans. Most frequently implicated thermosensors are members of the transient receptor potential (TRP) ion channel family TRPM8 and TRPA1. Regularly observed, distinct cold pain phenotype groups suggested the existence of interindividually differing molecular bases. In 28 subjects displaying either high or medium sensitivity to local cooling of the skin, the density at epidermal nerve fibers of TRPM8, but not that of TRPA1 expression, correlated significantly with the cold pain threshold. Moreover, reproducible grouping of the subjects, based on high or medium sensitivity to cooling, was reflected in an analogous grouping based on high or low TRPM8 expression at epidermal nerve fibers. The distribution of TRPM8 expression in epidermal nerve fibers provided an explanation for the previously observed (bi)modal distribution of human cold pain thresholds which was reproduced in this study. In the light of current controversies on the role of human TRPA1 ion channels in cold pain perception, the present observations demonstrating a lack of association of TRPA1 channel expression with cold sensitivity-related measures reinforce doubts about involvement of this channel in cold pain in humans. Since TRP inhibitors targeting TRPM8 and TRPA1 are currently entering clinical phases of drug development, the existence of known species differences, in particular in the function of TRPA1, emphasizes the increasing importance of new methods to directly approach the roles of TRPs in humans.

Modulation of experimental facial pain via somatosensory stimuli targeting sensations of different valence

BACKGROUND

Knowledge of pain modulation from oro-facial somatosensory stimuli with different valence (pleasant-unpleasant) is limited.

OBJECTIVES

To investigate (a) the modulatory effects of painful, pleasant and unpleasant somatosensory stimuli on two models of experimental facial pain, (b) whether modulation could be changed by blocking peripheral nerves via application of a local anaesthetic, EMLA, or blocking endogenous opioid receptors via naltrexone and (c) whether pain ratings were significantly correlated with participant psychological profiles.

METHODS

Thirty-eight healthy women received experimental facial skin burning pain or jaw myalgia for four randomised sessions on different days. The painful region was stimulated with mechanical or thermal painful, pleasant, unpleasant and control stimuli, with ratings recorded before and during stimulation. Sessions differed in pre-treatment: EMLA/naltrexone/placebo tablet/cream.

RESULTS

Significant effects of thermal or mechanical stimuli (P < .017), but not session (P > .102), were found on pain ratings for both models. In myalgia, painful cold resulted in a greater reduction in pain ratings than unpleasant cold, pleasant cold, control and pleasant warmth (P < .004). Decreases in pain ratings from painful, unpleasant and pleasant mechanical stimuli were greater than control (P < .002). In burning pain, painful cold resulted in a greater reduction in pain ratings than all but one of the other thermal stimuli (P < .033). The pleasant mechanical stimulus reduced pain ratings more than all other mechanical stimuli (P ≤ .003). There were no significant correlations between pain and psychometrics.

CONCLUSION

Valence-targeted thermal and mechanical stimuli modulated experimental myalgia and skin burning pain (P < .017). Partially blocking peripheral afferents or opioid receptors did not affect modulation.

Applications, indications, and effects of passive hydrotherapy WATSU (WaterShiatsu)-A systematic review and meta-analysis

Background

WATSU (portmanteau word: water and shiatsu) is a form of passive hydrotherapy in chest-deep thermoneutral water (35°C = 95°F = 308.15 K). It combines elements of myofascial stretching, joint mobilization, massage, and shiatsu and is reported to be used to address physical and mental issues. The objective of this systematic review (PROSPERO Registration No. CRD42016029347) and the meta-analyses was to assess the applications, indications, and the effects of WATSU to form a basis for further studies.

Methods

A search for “WATSU OR watershiatsu OR (water AND shiatsu)” was conducted without any restrictions in 32 databases. Peer reviewed original articles addressing WATSU as a stand-alone hydrotherapy were assessed for risk of bias. Quantitative data of effects on pain, physical function, and mental issues were processed in random model meta-analyses with subgroup analyses by study design. Effect sizes were expressed as Hedges's g (± 95% confidence intervals).

Results

Of 1,906 unique citations, 27 articles regardless of study design were assessed for risk of bias. WATSU has been applied to individuals of all ages. Indications covered acute (e.g. pregnancy related low back pain) and chronic conditions (e.g. cerebral palsy) with beneficial effects of WATSU regarding e.g. relaxation or sleep quality. Meta-analyses suggest beneficial effect sizes of WATSU on pain (overall Hedges’s g = -0.71, 95% CI = -0.91 to -0.51), physical function (overall Hedges’s g = -0.76, 95% CI = -1.08 to -0.44), and mental issues (overall Hedges’s g = -0.68, 95% CI = -1.02 to -0.35).

Conclusion

Various applications, indications and beneficial effects of WATSU were identified. The grade of this evidence is estimated to be low to moderate at the best. To strengthen the findings of this study, high-quality RCTs are needed.

C-tactile touch perception in migraineurs – a case-control study

Background

Migraine is characterized by sensory hypersensitivity and habituation deficits. Slow brushing over the skin activates C-tactile nerve fibers, which mediate pleasant touch and analgesic effects in healthy subjects. As this function is altered in painful conditions, we aimed to examine whether the C-tactile processing is disrupted in migraines.

Methods

To psychophysically assess C-tactile function, we applied optimal and suboptimal C-tactile stroking stimuli on the dorsal forearm (body reference area) and the trigeminally innervated skin of 52 interictal migraineurs and 52 matched healthy controls. For habituation testing, 60 repeated C-tactile optimal stimuli were presented in both test areas. The participants rated each stimulus on a visual analogue scale by intensity, pleasantness, and painfulness.

Results

Regarding C-tactile function, migraineurs showed unphysiological rating patterns but no significantly different pleasantness ratings than controls. During repeated stimulation, controls showed stable pleasantness ratings while migraineurs’ ratings decreased, especially in those experiencing tactile allodynia during headaches. Migraineurs taking triptans responded like controls.

Conclusion

The C-tactile function of migraineurs is subclinically altered. Repeated C-tactile stimulation leads to altered habituation but differs from previous work by the direction of the changes. Although the pathophysiology remains unknown, causative mechanisms could include central and peripheral neuronal sensitization, tactile allodynia and hedonic stimulus attributions.

Dynamic touch reduces physiological arousal in preterm infants: A role for c-tactile afferents?

Preterm birth is a significant risk factor for a range of long-term health problems and developmental disabilities. Though touch plays a central role in many perinatal care strategies, the neurobiological basis of these approaches is seldom considered. C-Tactile afferents (CTs) are a class of unmyelinated nerve fibre activated by low force, dynamic touch. Consistent with an interoceptive function, touch specifically targeted to activate CTs activates posterior insular cortex and has been reported to reduce autonomic arousal. The present study compared the effect of 5 min of CT optimal velocity stroking touch to 5 min of static touch on the heart-rate and oxygen saturation levels of preterm infants between 28- & 37-weeks gestational age. CT touch produced a significant decrease in infants' heart-rates and increase in their blood oxygenation levels, which sustained throughout a 5-min post-touch period. In contrast, there was no significant change in heart-rate or blood oxygenation levels of infants receiving static touch. These findings provide support for the hypothesis that CTs signal the affective quality of nurturing touch, providing a neurobiological substrate for the apparent beneficial effects of neonatal tactile interventions and offering insight for their optimisation.

Architecture of the Cutaneous Autonomic Nervous System

The human skin is a highly specialized organ for receiving sensory information but also to preserve the body's homeostasis. These functions are mediated by cutaneous small nerve fibers which display a complex anatomical architecture and are commonly classified into cutaneous A-beta, A-delta and C-fibers based on their diameter, myelinization, and velocity of conduction of action potentials. Knowledge on structure and function of these nerve fibers is relevant as they are selectively targeted by various autonomic neuropathies such as diabetic neuropathy or Parkinson's disease. Functional integrity of autonomic skin nerve fibers can be assessed by quantitative analysis of cutaneous responses to local pharmacological induction of axon reflex responses which result in dilation of cutaneous vessels, sweating, or piloerection depending on the agent used to stimulate this neurogenic response. Sensory fibers can be assessed using quantitative sensory test. Complementing these functional assessments, immunohistochemical staining of superficial skin biopsies allow analysis of structural integrity of cutaneous nerve fibers, a technique which has gained attention due to its capacity of detecting pathogenic depositions of alpha-synuclein in patients with Parkinson's disease. Here, we reviewed the current literature on the anatomy and functional pathways of the cutaneous autonomic nervous system as well as diagnostic techniques to assess its functional and structural integrity.

Nociception and the neonatal brain

Measuring brain activity in infants provides an objective surrogate approach with which to infer pain perception following noxious events. Here we discuss different approaches which can be used to measure noxious-evoked brain activity, and discuss how these measures can be used to assess the analgesic efficacy of pharmacological and non-pharmacological interventions. We review factors that can modulate noxious-evoked brain activity, which may impact infant pain experience, including gestational age, sex, prior pain, stress, and illness.

Interpersonal touch interventions for patients in intensive care: A design-oriented realist review

AIM

To develop a theoretical framework to inform the design of interpersonal touch interventions intended to reduce stress in adult intensive care unit patients.

DESIGN

Realist review with an intervention design-oriented approach.

METHODS

We searched CINAHL, MEDLINE, EMBASE, CENTRAL, Web of Science and grey literature sources without date restrictions. Subject experts suggested additional articles. Evidence synthesis drew on diverse sources of literature and was conducted iteratively with theory testing. We consulted stakeholders to focus the review. We performed systematic searches to corroborate our developing theoretical framework.

RESULTS

We present a theoretical framework based around six intervention construction principles. Theory testing provided some evidence in favour of treatment repetition, dynamic over static touch and lightening sedation. A lack of empirical evidence was identified for construction principles relating to intensity and positive/negative evaluation of emotional experience, moderate pressure touch for sedated patients and intervention delivery by relatives versus healthcare practitioners.