back-mechanical-sensitivity-assessment-rat-for-mechanistic

Back Mechanical Sensitivity Assessment in the Rat for Mechanistic Investigation of Chronic Back Pain

Low back pain is the leading cause of disability worldwide, with dramatic personal, economic, and social consequences. To develop novel therapeutics, ...

Heterotopic noxious counterstimulation (HNCS) by the application of a sustained noxious stimulus has been shown to inhibit nociceptive processes and decrease pain induced by a competing noxious stimulus. However, it is still not clear how attentional processes contribute to these effects. The main objective of this study was to compare the analgesic effects of HNCS in 2 sessions during which top-down attention was manipulated. Acute shock pain and the nociceptive flexion reflex were evoked by transcutaneous electrical stimulations of the right sural nerve in 4 blocks (15 stimuli/block): baseline, heterotopic innocuous counterstimulation (HICS), HNCS, and recovery. Counterstimulation was applied on the left upper limb with a thermode (HICS) or a cold pack (HNCS). Attention was manipulated between sessions by instructing participants to focus their attention on shock pain or counterstimulation. Shock pain ratings decreased significantly during counterstimulation (P<.001) with stronger effects of HNCS vs HICS in both sessions (P<.01). Furthermore, shock pain inhibition during HNCS relative to baseline was stronger with attention focusing on counterstimulation compared to attention focusing on shocks (P = .015). However, the relative decrease in pain ratings during HNCS vs HICS was not significantly affected by the direction of attention (P = .7). As for spinal nociceptive processes, nociceptive flexion reflex amplitude was significantly decreased during counterstimulation (P<.001) with larger reductions during HNCS compared to HICS (P = .03). However, these effects were not altered by attention (P = .35). Together, these results demonstrate that top-down attention and HNCS produce additive analgesic effects. However, attentional modulation of HNCS analgesia seems to depend on supraspinal processes.

Top-down attentional modulation of analgesia induced by heterotopic noxious counterstimulation.

Heterotopic noxious counter-stimulation (HNCS) inhibits pain and pain processes through cerebral and cerebrospinal mechanisms. However, it is unclear whether HNCS inhibits non-nociceptive processes, which needs to be clarified for a better understanding of HNCS analgesia. The aim of this study was to examine the effects of HNCS on perception and scalp somatosensory evoked potentials (SEPs). Seventeen healthy volunteers participated in two counter-balanced sessions, including non-nociceptive (selective Aβ-fibre activation) or nociceptive electrical stimulation, combined with HNCS. HNCS was produced by a 20-min cold pressor test (left hand) adjusted individually to produce moderate pain (mean ± SEM: 42.5 ± 5.3 on a 0-100 scale, where 0 is no pain and 100 the worst pain imaginable). Non-nociceptive electrical stimulation was adjusted individually at 80% of pain threshold and produced a tactile sensation in every subject. Nociceptive electrical stimulation was adjusted individually at 120% of RIII-reflex threshold and produced moderate pain (45.3 ± 4.5). Shock sensation was significantly decreased by HNCS compared with baseline for non-nociceptive (P < 0.001) and nociceptive (P < 0.001) stimulation. SEP peak-to-peak amplitude at Cz was significantly decreased by HNCS for non-nociceptive (P < 0.01) and nociceptive (P < 0.05) stimulation. These results indicate that perception and brain activity related to Aβ-fibre activation are inhibited by HNCS. The mechanisms of this effect remain to be investigated to clarify whether it involves inhibition of spinal wide-dynamic-range neurons by diffuse noxious inhibitory controls, supraspinal processes or both.

Inhibitory effects of heterotopic noxious counter-stimulation on perception and brain activity related to Aβ-fibre activation.

Together, neck pain and back pain are the first cause of disability worldwide, accounting for more than 10% of the total years lived with disability. In this context, chiropractic care provides a safe and effective option for the management of a large proportion of these patients. Chiropractic is a healthcare profession mainly focused on the spine and the treatment of spinal disorders, including spine pain. Basic studies have examined the influence of chiropractic spinal manipulation (SM) on a variety of peripheral, spinal and supraspinal mechanisms involved in spine pain. While spinal cord mechanisms of pain inhibition contribute at least partly to the pain-relieving effects of chiropractic treatments, the evidence is weaker regarding peripheral and supraspinal mechanisms, which are important components of acute and chronic pain. This narrative review highlights the most relevant mechanisms of pain relief by SM and provides a perspective for future research on SM and spine pain, including the validation of placebo interventions that control for placebo effects and other non-specific effects that may be induced by SM. SIGNIFICANCE: Spinal manipulation inhibits back and neck pain partly through spinal segmental mechanisms and potentially through peripheral mechanisms regulating inflammatory responses. Other mechanisms remain to be clarified. Controls and placebo interventions need to be improved in order to clarify the contribution of specific and non-specific effects to pain relief by spinal manipulative therapy.

Neurophysiological mechanisms of chiropractic spinal manipulation for spine pain.

The aim of this study was to examine the mechanisms underlying hypoalgesia induced by spinal manipulation (SM). Eighty-two healthy volunteers were assigned to one of the four intervention groups: no intervention, SM at T4 (homosegmental to pain), SM at T8 (heterosegmental to pain) or light mechanical stimulus at T4 (placebo). Eighty laser stimuli were applied on back skin at T4 to evoke pain and brain activity related to Aδ- and C-fibers activation. The intervention was performed after 40 stimuli. Laser pain was decreased by SM at T4 (p = 0.028) but not T8 (p = 0.13), compared with placebo. However, brain activity related to Aδ-fibers activation was not significantly modulated (all p > 0.05), while C-fiber activity could not be measured reliably. This indicates that SM produces segmental hypoalgesia through inhibition of nociceptive processes that are independent of Aδ fibers. It remains to be clarified whether the effect is mediated by the inhibition of C-fiber activity.

Effects of chiropractic spinal manipulation on laser-evoked pain and brain activity.

Spine pain is a highly prevalent condition affecting over 11% of the world's population. It is the single leading cause of activity limitation and ranks fourth in years lost to disability globally, representing a significant personal, social, and economic burden. For the vast majority of patients with back and neck pain, a specific pathology cannot be identified as the cause for their pain, which is then labeled as non-specific. In a growing proportion of these cases, pain persists beyond 3 months and is referred to as chronic primary back or neck pain. To decrease the global burden of spine pain, current data suggest that a conservative approach may be preferable. One of the conservative management options available is spinal manipulative therapy (SMT), the main intervention used by chiropractors and other manual therapists. The aim of this narrative review is to highlight the most relevant and up-to-date evidence on the effectiveness (as it compares to other interventions in more pragmatic settings) and efficacy (as it compares to inactive controls under highly controlled conditions) of SMT for the management of neck pain and low back pain. Additionally, a perspective on the current recommendations on SMT for spine pain and the needs for future research will be provided. In summary, SMT may be as effective as other recommended therapies for the management of non-specific and chronic primary spine pain, including standard medical care or physical therapy. Currently, SMT is recommended in combination with exercise for neck pain as part of a multimodal approach. It may also be recommended as a frontline intervention for low back pain. Despite some remaining discrepancies, current clinical practice guidelines almost universally recommend the use of SMT for spine pain. Due to the low quality of evidence, the efficacy of SMT compared with a placebo or no treatment remains uncertain. Therefore, future research is needed to clarify the specific effects of SMT to further validate this intervention. In addition, factors that predict these effects remain to be determined to target patients who are more likely to obtain positive outcomes from SMT.

Clinical Effectiveness and Efficacy of Chiropractic Spinal Manipulation for Spine Pain.

Musculoskeletal injuries lead to sensitization of nociceptors and primary hyperalgesia (hypersensitivity to painful stimuli). This occurs with back injuries, which are associated with acute pain and increased pain sensitivity at the site of injury. In some cases, back pain persists and leads to central sensitization and chronic pain. Thus, reducing primary hyperalgesia to prevent central sensitization may limit the transition from acute to chronic back pain. It has been shown that spinal manipulation (SM) reduces experimental and clinical pain, but the effect of SM on primary hyperalgesia and hypersensitivity to painful stimuli remains unclear. The goal of the present study was to investigate the effect of SM on pain hypersensitivity using a capsaicin-heat pain model. Laser stimulation was used to evoke heat pain and the associated brain activity, which were measured to assess their modulation by SM. Eighty healthy participants were recruited and randomly assigned to one of the four experimental groups: inert cream and no intervention; capsaicin cream and no intervention; capsaicin cream and SM at T7; capsaicin cream and placebo. Inert or capsaicin cream (1%) was applied to the T9 area. SM or placebo were performed 25 min after cream application. A series of laser stimuli were delivered on the area of cream application (1) before cream application, (2) after cream application but before SM or placebo, and (3) after SM or placebo. Capsaicin cream induced a significant increase in laser pain ( < 0.001) and laser-evoked potential amplitude ( < 0.001). However, SM did not decrease the amplification of laser pain or laser-evoked potentials by capsaicin. These results indicate that segmental SM does not reduce pain hypersensitivity and the associated pain-related brain activity in a capsaicin-heat pain model.

Segmental Chiropractic Spinal Manipulation Does not Reduce Pain Amplification and the Associated Pain-Related Brain Activity in a Capsaicin-Heat Pain Model.

 Spinal manipulation (SM) is currently recommended for the management of back pain. Experimental studies indicate that the hypoalgesic mechanisms of SM may rely on inhibition of segmental processes related to temporal summation of pain and, possibly, on central sensitization, although this remains unclear. The aim of this study was to determine whether experimental back pain, secondary hyperalgesia, and pain-related brain activity induced by capsaicin are decreased by segmental SM.  Seventy-three healthy volunteers were randomly allocated to one of four experimental groups: SM at T5 vertebral level (segmental), SM at T9 vertebral level (heterosegmental), placebo intervention at T5 vertebral level, or no intervention. Topical capsaicin was applied to the area of T5 vertebra for 40 min. After 20 min, the interventions were administered. Pressure pain thresholds (PPTs) were assessed outside the area of capsaicin application at 0 and 40 min to examine secondary hyperalgesia. Capsaicin pain intensity and unpleasantness were reported every 4 min. Frontal high-gamma oscillations were also measured with electroencephalography.  Pain ratings and brain activity were not significantly different between groups over time ( > 0.5). However, PPTs were significantly decreased in the placebo and control groups ( < 0.01), indicative of secondary hyperalgesia, while no hyperalgesia was observed for groups receiving SM ( = 1.0). This effect was independent of expectations and greater than placebo for segmental ( < 0.01) but not heterosegmental SM ( = 1.0).  These results indicate that segmental SM can prevent secondary hyperalgesia, independently of expectations. This has implications for the management of back pain, particularly when central sensitization is involved.

Article	Year
Top-down attentional modulation of analgesia induced by heterotopic noxious counterstimulation. Pain\| PubMed ID: 22717100	2012
Inhibitory effects of heterotopic noxious counter-stimulation on perception and brain activity related to Aβ-fibre activation. The European journal of neuroscience\| PubMed ID: 27086672	2016
Neurophysiological mechanisms of chiropractic spinal manipulation for spine pain. European journal of pain (London, England)\| PubMed ID: 33786932	2021
Effects of chiropractic spinal manipulation on laser-evoked pain and brain activity. The journal of physiological sciences : JPS\| PubMed ID: 34167458	2021
Clinical Effectiveness and Efficacy of Chiropractic Spinal Manipulation for Spine Pain. Frontiers in pain research (Lausanne, Switzerland)\| PubMed ID: 35295422	2021
Segmental Chiropractic Spinal Manipulation Does not Reduce Pain Amplification and the Associated Pain-Related Brain Activity in a Capsaicin-Heat Pain Model. Frontiers in pain research (Lausanne, Switzerland)\| PubMed ID: 35295444	2021
Chiropractic Spinal Manipulation Prevents Secondary Hyperalgesia Induced by Topical Capsaicin in Healthy Individuals. Frontiers in pain research (Lausanne, Switzerland)\| PubMed ID: 35295504	2021

Benjamin Provencher

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