Inflammation of the thoracolumbar fascia excites and sensitizes rat dorsal horn neurons

Pain quality patterns in delayed onset muscle soreness of the lower back suggest sensitization of fascia rather than muscle afferents: a secondary analysis study

Delayed onset muscle soreness (DOMS) of the lower back is considered a surrogate for acute low back pain (aLBP) in experimental studies. Of note, it is often unquestioningly assumed to be muscle pain. To date, there has not been a study analyzing lumbar DOMS in terms of its pain origin, which was the aim of this study. Sixteen healthy individuals (L-DOMS) were enrolled for the present study and matched to participants from a previous study (n = 16, L-PAIN) who had undergone selective electrical stimulation of the thoracolumbar fascia and the multifidus muscle. DOMS was induced in the lower back of the L-DOMS group using eccentric trunk extensions performed until exhaustion. On subsequent days, pain on palpation (100-mm analogue scale), pressure pain threshold (PPT), and the Pain Sensation Scale (SES) were used to examine the sensory characteristics of DOMS. Pain on palpation showed a significant increase 24 and 48 h after eccentric training, whereas PPT was not affected (p > 0.05). Factor analysis of L-DOMS and L-PAIN sensory descriptors (SES) yielded a stable three-factor solution distinguishing superficial thermal ("heat pain ") from superficial mechanical pain ("sharp pain") and "deep pain." "Heat pain " and "deep pain" in L-DOMS were almost identical to sensory descriptors from electrical stimulation of fascial tissue (L-PAIN, all p > 0.679) but significantly different from muscle pain (all p < 0.029). The differences in sensory description patterns as well as in PPT and self-reported DOMS for palpation pain scores suggest that DOMS has a fascial rather than a muscular origin.

Sustained pain and macrophage infiltration in a mouse muscle contusion model

INTRODUCTION/AIMS

Prior studies have emphasized the role of inflammation in the response to injury and muscle regeneration, but little emphasis has been placed on characterizing the relationship between innate inflammation, pain, and functional impairment. The aim of our study was to determine the contribution of innate immunity to prolonged pain following muscle contusion.

METHODS

We developed a closed-impact mouse model of muscle contusion and a macrophage-targeted near-infrared fluorescent nanoemulsion. Closed-impact contusions were delivered to the lower left limb. Pain sensitivity, gait dysfunction, and inflammation were assessed in the days and weeks post-contusion. Macrophage accumulation was imaged in vivo by injecting i.v. near-infrared nanoemulsion.

RESULTS

Despite hindpaw hypersensitivity persisting for several weeks, disruptions to gait and grip strength typically resolved within 10 days of injury. Using non-invasive imaging and immunohistochemistry, we show that macrophage density peaks in and around the affected muscle 3 day post-injury and quickly subsides. However, macrophage density in the ipsilateral sciatic nerve and dorsal root ganglia (DRG) increases more gradually and persists for at least 14 days.

DISCUSSION

In this study, we demonstrate pain sensitivity is influenced by the degree of lower muscle contusion, without significant changes to gait and grip strength. This may be due to modulation of pain signaling by macrophage proliferation in the sciatic nerve, upstream from the site of injury. Our work suggests chronic pain developing from muscle contusion is driven by macrophage-derived neuroinflammation in the peripheral nervous system.

Thoracolumbar fascia mobility and chronic low back pain: Phase 1 of a pilot and feasibility study assessing repeated measures and the influence of paraspinal muscle contraction

BACKGROUND

Differential movement, or shear strain (SS), between layers of thoracolumbar fascia is reduced with chronic low back pain. To provide a foundation for clinical research involving SS, this study assessed temporal stability and the effect of paraspinal muscle contraction on SS in persons with chronic low back pain.

METHODS

We used ultrasound imaging to measure SS in adults self-reporting low back pain ≥1 year. Images were obtained by placing a transducer 2-3 cm lateral to L2-3 with participants lying prone and relaxed on a table moving the lower extremities downward 15°, for 5 cycles at 0.5 Hz. To assess paraspinal muscle contraction effects, participants raised the head slightly from the table. SS was calculated using 2 computational methods. Method 1 averaged the maximum SS from each side during the third cycle. Method 2 used the maximum SS from any cycle (2-4) on each side, prior to averaging. SS was also assessed after a 4-week no manual therapy period.

RESULTS

Of 30 participants (n = 14 female), mean age was 40 years; mean BMI 30.1. Mean (SE) SS in females with paraspinal muscle contraction was 66% (7.4) (method 1) and 78% (7.8) (method 2); 54% (6.9) (method 1) and 67% (7.3) (method 2) in males. With muscles relaxed, mean SS in females was 77% (7.6) (method 1) or 87% (6.8) (method 2); 63% (7.1) (method 1) and 78% (6.4) (method 2) in males. Mean SS decreased 8-13% in females and 7-13% in males after 4-weeks CONCLUSION: Mean SS in females was higher than males at each timepoint. Paraspinal muscle contraction temporarily reduced SS. Over a 4-week no-treatment period, mean SS (with paraspinal muscles relaxed) decreased. Methods less likely to induce muscle guarding and enabling assessment with broader populations are needed.

Spinal neurovascular coupling is preserved despite time-dependent alterations of spinal cord blood flow responses in a rat model of chronic back pain: implications for functional spinal cord imaging

ABSTRACT

Functional magnetic resonance imaging has been used to investigate nociceptive processes in patients with chronic pain. However, the results may be confounded with changes in neurovascular coupling induced by chronic pain. The objective of this study was to examine spinal neurovascular coupling in a rat model of chronic back pain induced by muscle inflammation. Rats received 150 µL intramuscular injections of either complete Freund adjuvant (CFA: n = 18) or saline (control [CTL]: n = 18) in L5-L6 paravertebral muscles. Under 1.2% isoflurane anesthesia, spinal cord blood flow (SCBF) and local field potentials evoked by electrical stimulation of the sciatic nerve were recorded simultaneously in the lumbar enlargement of the spinal cord, 14 or 28 days after the injections. Mechanical hypersensitivity was observed in CFA rats compared with CTL rats for the back ( P < 0.001) and hind paws ( P < 0.01). Spinal cord blood flow response amplitude and local field potential amplitude were not significantly different between groups (day 14: P > 0.5; day 28: P > 0.6). However, the time course of SCBF responses was different between groups on day 14 ( P < 0.001) and day 28 ( P < 0.001). Nevertheless, neurovascular coupling was comparable between groups on days 14 and 28, whether neurovascular coupling was calculated with the amplitude or the area under the curve of SCBF responses (all P > 0.2). These results indicate that spinal hemodynamic changes reflect neuronal activity in this animal model, although the time course of SCBF responses is affected by chronic inflammatory back pain. This warrants a careful use of spinal functional magnetic resonance imaging in animal models and patients with chronic back pain.

Molecular Mechanisms Underlying the Pain-Relieving Effects of Extracorporeal Shock Wave Therapy: A Focus on Fascia Nociceptors

In recent years, extracorporeal shock wave therapy (ESWT) has received increasing attention for its potential beneficial effects on various bone and soft-tissue pathologies, yielding promising outcomes for pain relief and functional recovery. In fact, ESWT has emerged as an alternative, non-invasive, and safe treatment for the management of numerous musculoskeletal disorders, including myofascial pain syndrome (MPS). In particular, MPS is a common chronic painful condition, accounting for the largest proportion of patients affected by musculoskeletal problems. Remarkably, sensory innervation and nociceptors of the fascial system are emerging to play a pivotal role as pain generators in MPS. At the same time, increasing evidence demonstrates that application of ESWT results in selective loss of sensory unmyelinated nerve fibers, thereby inducing long-lasting analgesia. The findings discussed in the present review are supposed to add novel viewpoints that may further enrich our knowledge on the complex interactions occurring between disorders of the deep fascia including changes in innervation, sensitization of fascial nociceptors, the pathophysiology of chronic musculoskeletal pain of MPS, and EWST-induced analgesia. Moreover, gaining mechanistic insights into the molecular mechanisms of pain-alleviating effects of ESWT may broaden the fields of shock waves clinical practice far beyond the musculoskeletal system or its original application for lithotripsy.

Dose-Dependent Pain and Pain Radiation after Chemical Stimulation of the Thoracolumbar Fascia and Multifidus Muscle: A Single-Blinded, Cross-Over Study Revealing a Higher Impact of Fascia Stimulation

Acute low back pain can be experimentally induced by injections of hypertonic saline into deep tissues of the back, such as fascia and muscle. The current study investigated the dose-dependency of peak-pain and spatial extent of concomitant radiating pain following 50, 200 and 800 μL bolus injections of hypertonic saline (5.8%) into the thoracolumbar fascia and multifidus muscle, since data on dose-dependency is lacking in humans. Sixteen healthy subjects rated (11 female, 5 male; 23.3 ± 3.1 years, mean ± SD) intensity and spatial extent of pain. Injections into the fascia resulted in significantly higher peak-pain (+86%, p < 0.001), longer pain durations (p < 0.05), and larger pain areas (+65%, p < 0.02) and were less variable than intramuscular injections. Peak-pain ratings and pain areas were 2−3-fold higher/larger for 200 μL vs. 50 μL. In contrast, peak pain increased only marginally at 800 μL by additional 20%, while pain areas did not increase further at all in both, fascia and muscle. Thus, higher injection volumes did also not compensate the lower sensitivity of muscle. Peak-pain ratings and pain areas correlated between fascia and muscle (r = 0.530, p < 0.001 and r = 0.337, p < 0.02, respectively). Peak-pain ratings and pain areas correlated overall (r = 0.490, p < 0.0001), but a weak correlation remained when the impact of between-tissue differences and different injection volumes were singled out (partial r = 0.261, p < 0.01). This study shows dose-dependent pain responses of deep tissues where an injection volume of 200 μL of hypertonic saline is deemed an adequate stimulus for tissue differentiation. We suggest that pain radiation is not simply an effect of increased peripheral input but may afford an individual disposition for the pain radiation response. Substantially higher pain-sensitivity and wider pain areas support fascia as an important contributor to non-specific low back pain.

Deep fascia as a potential source of pain: A narrative review

BACKGROUND

The fascial component of the myofascial pain syndrome and the contribution of the deep fascia to various painful conditions has not been well-described and is still less understood.

OBJECTIVES

The aims of this study were to evaluate the possible role of the deep fascia on musculoskeletal pain, focusing on findings from histological and experimental studies; and to assess the nociceptive and associated responses of the deep fascia to experimentally-induced irritation.

METHODS

Narrative review of the English scientific literature.

RESULTS AND CONCLUSIONS

Different components of the deep fascia, both in humans and animals are richly innervated, with some differences between body segments. These fascial components usually exhibit dense innervation, encompassing amongst others, nociceptive afferents. The application of different types of stimuli, i.e., electrical, mechanical, and chemical to these fascial components produces long-lasting pain responses. In some cases, the intensity and severity of pain produced by the stimulation of fascia were higher than ones produced by the stimulation of the related muscular tissue. These observations may denote that the deep fascia and its various components could be a source of pain in different pathologies and various pain syndromes.

Potential Nociceptive Role of the Thoracolumbar Fascia: A Scope Review Involving In Vivo and Ex Vivo Studies

Nociceptive innervation of the thoracolumbar fascia (TLF) has been investigated over the past few decades; however, these studies have not been compiled or collectively appraised. The purpose of this scoping review was to assess current knowledge regarding nociceptive innervation of the TLF to better inform future mechanistic and clinical TLF research targeting lower back pain (LBP) treatment. PubMed, ScienceDirect, Cochrane, and Embase databases were searched in January 2021 using relevant descriptors encompassing fascia and pain. Eligible studies satisfied the following: (a) published in English; (b) preclinical and clinical (in vivo and ex vivo) studies; (c) original data; (d) included quantification of at least one TLF nociceptive component. Two-phase screening procedures were conducted by a pair of independent reviewers, after which data were extracted and summarized from eligible studies. The search resulted in 257 articles of which 10 met the inclusion criteria. Studies showed histological evidence of nociceptive nerve fibers terminating in lower back fascia, suggesting a TLF contribution to LBP. Noxious chemical injection or electrical stimulation into fascia resulted in longer pain duration and higher pain intensities than injections into subcutaneous tissue or muscle. Pre-clinical and clinical research provides histological and functional evidence of nociceptive innervation of TLF. Additional knowledge of fascial neurological components could impact LBP treatment.

Is "Delayed Onset Muscle Soreness" a False Friend? The Potential Implication of the Fascial Connective Tissue in Post-Exercise Discomfort

Strenuous and unaccustomed exercise frequently lead to what has been coined “delayed onset muscle soreness” (DOMS). As implied by this term, it has been proposed that the associated pain and stiffness stem from micro-lesions, inflammation, or metabolite accumulation within the skeletal muscle. However, recent research points towards a strong involvement of the connective tissue. First, according to anatomical studies, the deep fascia displays an intimate structural relationship with the underlying skeletal muscle and may therefore be damaged during excessive loading. Second, histological and experimental studies suggest a rich supply of algogenic nociceptors whose stimulation evokes stronger pain responses than muscle irritation. Taken together, the findings support the hypothesis that DOMS originates in the muscle-associated connective tissue rather than in the muscle itself. Sports and fitness professionals designing exercise programs should hence consider fascia-oriented methods and techniques (e.g., foam rolling, collagen supplementation) when aiming to treat or prevent DOMS.

Soft Tissue Manipulation May Attenuate Inflammation, Modulate Pain, and Improve Gait in Conscious Rodents With Induced Low Back Pain

INTRODUCTION

Low back pain (LBP) is common in warfighters. Noninvasive interventions are necessary to expedite return-to-function. Soft tissue manipulation, for example, massage, is a method used to treat LBP. Instrument-assisted soft tissue manipulation (IASTM) uses a rigid device to mobilize the tissue. This study explored the effects of IASTM on pain, function, and biomarkers.

METHODS

Sprague-Dawley rats (n = 44) were randomized to groups (n = 6/grp): (A) cage control; (B) 3 days (3d) postinjury (inj), untreated; (C) 3d inj, < 30-minute post-IASTM treatment; (D) 3d inj, 2 hours (2h) post-IASTM; (E) 14 days (14d) inj, untreated; (F) 14d inj, < 30-minute post-IASTM; and (G) 14d inj, 2h post-IASTM. Researchers induced unilateral LBP in Sprague-Dawley rats using complete Freund's adjuvant injection. Conscious rodents received IASTM for 5 min/session once at 3 days or 3×/week × 2weeks (6× total) over 14 days. Biomarker plasma levels were determined in all groups, while behavioral outcomes were assessed in two groups, D and G, at three time points: before injury, pre-, and post-IASTM treatment. Circulating mesenchymal stem cell levels were assessed using flow cytometry and cytokine plasma levels assayed.

RESULTS

The back pressure pain threshold (PPT) lowered bilaterally at 3 days postinjury (P < .05), suggesting increased pain sensitivity. IASTM treatment lowered PPT more on the injured side (15.8%; P < 0.05). At 14 days, back PPT remained lower but similar side to side. At 3 days, paw PPT increased 34.6% in the contralateral rear limb following treatment (P < .01). Grip strength did not vary significantly. Gait coupling patterns improved significantly (P < .05). Circulating mesenchymal stem cell levels altered significantly postinjury but not with treatment. Neuropeptide Y plasma levels increased significantly at 3 days, 2h post-IASTM (53.2%) (P < .05). Interleukin-6 and tumor necrosis factor-alpha did not vary significantly. At 14 days, regulated on activation, normal T cell expressed and secreted decreased significantly <30-minute post-IASTM (96.1%, P < .002), while IL-10 trended upward at 2h (53.1%; P = .86).

CONCLUSIONS

LBP increased pain sensitivity and diminished function. IASTM treatment increased pain sensitization acutely in the back but significantly reduced pain sensitivity in the contralateral rear paw. Findings suggest IASTM may positively influence pain modulation and inflammation while improving gait patterns. Soft tissue manipulation may be beneficial as a conservative treatment option for LBP.

Rat dorsal horn neurons primed by stress develop a long-lasting manifest sensitization after a short-lasting nociceptive low back input

Background

A single injection of nerve growth factor (NGF) into a low back muscle induces a latent sensitization of rat dorsal horn neurons (DHNs) that primes for a manifest sensitization by a subsequent second NGF injection. Repeated restraint stress also causes a latent DHN sensitization.

Objective

In this study, we investigated whether repeated restraint stress followed by a single NGF injection causes a manifest sensitization of DHNs.

Methods

Rats were stressed repeatedly in a narrow plastic restrainer (1 hour on 12 consecutive days). Control animals were handled but not restrained. Two days after stress paradigm, behavioral tests and electrophysiological in vivo recordings from single DHNs were performed. Mild nociceptive low back input was induced by a single NGF injection into the lumbar multifidus muscle just before the recording started.

Results

Restraint stress slightly lowered the low back pressure pain threshold (Cohen d = 0.83). Subsequent NGF injection increased the proportion of neurons responsive to deep low back input (control + NGF: 14%, stress + NGF: 39%; P = 0.041), mostly for neurons with input from outside the low back (7% vs 26%; P = 0.081). There was an increased proportion of neurons with resting activity (28% vs 55%; P = 0.039), especially in neurons having deep input (0% vs 26%; P = 0.004).

Conclusions

The results indicate that stress followed by a short-lasting nociceptive input causes manifest sensitization of DHNs to deep input, mainly from tissue outside the low back associated with an increased resting activity. These findings on neuronal mechanisms in our rodent model suggest how stress might predispose to radiating pain in patients.

Reconnecting the Brain With the Rest of the Body in Musculoskeletal Pain Research

A challenge in understanding chronic musculoskeletal pain is that research is often siloed between neuroscience, physical therapy/rehabilitation, orthopedics, and rheumatology which focus respectively on 1) neurally mediated effects on pain processes, 2) behavior and muscle activity, 3) tissue structure, and 4) inflammatory processes. Although these disciplines individually study important aspects of pain, there is a need for more cross-disciplinary research that can bridge between them. Identifying the gaps in knowledge is important to understand the whole body, especially at the interfaces between the silos-between brain function and behavior, between behavior and tissue structure, between musculoskeletal and immune systems, and between peripheral tissues and the nervous system. Research on "mind and body" practices can bridge across these silos and encourage a "whole person" approach to better understand musculoskeletal pain by bringing together the brain and the rest of the body. PERSPECTIVE: Research on chronic musculoskeletal pain is limited by significant knowledge gaps. To be fully integrated, musculoskeletal pain research will need to bridge across tissues, anatomical areas, and body systems. Research on mind and body approaches encourages a "whole person" approach to better understand musculoskeletal pain.

Spinal Mobilization Prevents NGF-Induced Trunk Mechanical Hyperalgesia and Attenuates Expression of CGRP

Introduction

Low back pain (LBP) is a complex and growing global health problem in need of more effective pain management strategies. Spinal mobilization (SM) is a non-pharmacological approach recommended by most clinical guidelines for LBP, but greater utilization and treatment optimization are hampered by a lack of mechanistic knowledge underlying its hypoalgesic clinical effects.

Methods

Groups of female Sprague-Dawley rats received unilateral trunk (L5 vertebral level) injections (50 μl) of either vehicle (phosphate-buffer solution, PBS; VEH) or nerve growth factor (NGF; 0.8 μM) on Days 0 and 5 with or without daily L5 SM (VEH, NGF, VEH + SM, VEH + SM). Daily passive SM (10 min) was delivered by a feedback motor (1.2 Hz, 0.9N) from Days 1 to 12. Changes in pain assays were determined for mechanical and thermal reflexive behavior, exploratory behavior (open field events) and spontaneous pain behavior (rat grimace scale). On Day 12, lumbar (L1–L6) dorsal root ganglia (DRG) were harvested bilaterally and calcitonin gene-related peptide (CGRP) positive immunoreactive neurons were quantified from 3 animals (1 DRG tissue section per segmental level) per experimental group.

Results

NGF induced bilateral trunk (left P = 0.006, right P = 0.001) mechanical hyperalgesia and unilateral hindpaw allodynia (P = 0.006) compared to the vehicle group by Day 12. Additionally, we found for the first time that NGF animals demonstrated decreased exploratory behaviors (total distance traveled) and increased grimace scale scoring compared to the VEH group. Passive SM prevented this development of local (trunk) mechanical hyperalgesia and distant (hindpaw) allodynia, and normalized grimace scale scores. NGF increased CGRP positive immunoreactive neurons in ipsilateral lumbar DRGs compared to the VEH group ([L1]P = 0.02; [L2]P = 0.007) and SM effectively negated this increase in pain-related neuropeptide CGRP expression.

Conclusion

SM prevents the development of local (trunk) NGF-induced mechanical hyperalgesia and distant (hindpaw) allodynia, in part, through attenuation of CGRP expression in lumbar DRG sensory neurons. NGF decreases rat exploratory behavior and increases spontaneous pain for which passive SM acts to mitigate these pain-related behavioral changes. These initial study findings suggest that beginning daily SM soon after injury onset might act to minimize or prevent the development of LBP by reducing production of pain-related neuropeptides.

Myelopathy and Reactive Microgliosis and Astrogliosis in Equine Back Pain

Equine chronic back pain (CBP) has been linked to different pathologic processes, which directly or indirectly involve spinal structures. Thus, making diagnosis and management very challenging with most horses with the condition recommended for early retirement from athletic activity. This study described the spinal cord lesions and the development of reactive microgliosis and astrocytosis in the spinal cords of horse with CBP. Thoracolumbar spinal cord segments from three horses euthanized because of unresolved CBP were dissected and grossly and histopathologically examined. The expression of activated microglia and astrocytes were demonstrated immunohistochemically using polyclonal rabbit anti-Iba-1 and anti-glial fibrillary acidic protein antibodies, respectively. All horses had radiological evidence of varying degrees of kissing spine involving six to nine vertebrae with the majority of the lesions graded between 2 and 5. Grossly, there was myelomalacia with intramedullary hemorrhages. The gray matters of the spinal cords were characterized by hemorrhagic malacic lesions with medullary disintegration. Reactive microgliosis and astrocytosis were evident in the spinal dorsal horns. White matter lesions include axonal swollen and/or loss, satellitosis, and varying degrees of dilation of myelin sheaths with some containing macrophages. In conclusion, the presence of reactive microgliosis and astrogliosis in the spinal dorsal horn indicates that they are possible precipitating factors in the development of equine CBP.

Innervation of the thoracolumbar fascia

The aim of the study was to obtain information on the sensory functions of the thoracolumbar fascia (TLF). The types of nerve fibres present in the TLF were visualized with specific antibodies to neuropeptides and sympathetic fibres. Most data were obtained from the TLF in rats, but some findings from the human fascia are also included. The only receptive nerve ending found was the free nerve ending, i.e. no corpuscular receptors existed in our specimen. An exclusive innervation with free nerve endings speaks for a nociceptive function, but the TLF may also fulfill proprioceptive functions, since many of the free nerve endings have a low mechanical threshold. Most of the fibres could be visualized with antibodies to CGRP [calcitonin gene- related peptide (CGRP)] and SP [substance P (SP)]. The latter ones most likely were nociceptors. The TLF contained a great proportion of postganglionic sympathetic fibres, which may be vasoconstrictors. A comparison between an inflamed and intact fascia showed an increase of the CGRP- and SP-positive fibres in the inflamed TLF. This finding could be one explanation for the low back pain of many patients, since practically all lesions of the fascia are accompanied by a sterile inflammation.

Action potentials and subthreshold potentials of dorsal horn neurons in a rat model of myositis: a study employing intracellular recordings in vivo

Intracellular in vivo recordings from rat dorsal horn neurons were made to study the contribution of microglia to the central sensitization of spinal synapses induced by a chronic muscle inflammation. To block microglia activation, minocycline was continuously administered intrathecally during development of the inflammation. The aim was to test whether an inflammation-induced sensitization of dorsal horn neurons is mediated by changes in synaptic strength or other synaptic changes and how activated microglia influence these processes. Intracellular recordings were used to measure subthreshold excitatory postsynaptic potentials (EPSPs) and suprathreshold action potentials (APs). The muscle inflammation significantly increased the proportion of dorsal horn neurons responding with APs or EPSPs to electrical stimulation of the muscle nerve from 27 to 56% (P < 0.01) and to noxious muscle stimulation (3 vs. 44%, P < 0.01). Neurons showing spontaneous ongoing AP or EPSP activity increased from 28 to 74% (P < 0.01). Generally, the increases in suprathreshold AP responses did not occur at the expense of subthreshold EPSPs, because EPSP-only responses also increased. Intrathecal minocycline prevented the inflammation-induced increase in responsiveness to electrical (24%, P < 0.02) and mechanical stimulation (14%, P < 0.02); the effect was stronger on suprathreshold APs than on subthreshold EPSPs. The increase in ongoing activity was only partly suppressed. These data suggest that the myositis-induced hypersensitivity of the dorsal horn neurons to peripheral input and its prevention by intrathecal minocycline treatment were due to both an increase in the number of active synapses and an increased synaptic strength.NEW & NOTEWORTHY During a chronic muscle inflammation (myositis), activated microglia controls both the increase in the number of active synapses and the increase in synaptic strength.

H-reflex disinhibition by lumbar muscle inflammation in a mouse model of spinal cord injury

Inflammation is a common comorbidity in patients with traumatic spinal cord injury (SCI). Recent reports indicate that inflammation hinders functional recovery in animal models of SCI. However, the spinal mechanisms underlying this alteration are currently unknown. Considering that spinal plasticity is a therapeutic target in patients and animal models of SCI, these mechanisms remain to be clarified. Using injections of complete Freund's adjuvant (CFA) in lumbar muscles as a model of persistent inflammation, the objective of this study was to assess the impact of inflammation on spinal reflex excitability after a complete midthoracic spinal transection in mice. To this end, the excitability of spinal reflexes was examined by measuring H-reflex frequency-dependent depression (FDD) on days 7, 14 and 28 following a complete spinal transection. H-reflex parameters were compared between spinal mice with CFA and control spinal mice. On day 7, lumbar muscle inflammation disinhibited the H-reflex, reflected by an attenuation of H-reflex FDD (p < 0.01), although this effect did not persist later on, either on day 14 or day 28. These results indicate that lumbar muscle inflammation alters spinal reflex excitability transiently in spinal mice. Considering that changes in spinal reflex excitability are associated with poor functional recovery after SCI, this implies that inflammation should be treated effectively to promote optimal recovery following SCI.

Neural therapy of an athlete's chronic plantar fasciitis: a case report and review of the literature

Background

The focus of this case report is on the role of inflammation as a contributor to pain in plantar fasciitis and its cure by the injection of local anesthetics.

Case presentation

This is a case report on a 24-year-old white man, a middle-distance runner, with chronic unilateral plantar fasciitis and perceived heel pain for almost 1.5 years. He was treated with neural therapy (that is, injection of < 1 ml procaine 1% which is a local anesthetic with strong anti-inflammatory properties) of the surgical scar and along the surgical puncture channel. The follow-up period from the time of first presentation until publication was 2.5 years. At admission, pain intensity (visual analog scale) in the affected leg was severe (10 cm, visual analog scale; range 0–10 cm) when walking and moderate (5 cm, visual analog scale) when standing. After the first session of injections he could stand pain-free and pain when walking was markedly reduced (− 90%). After the third session, he reported no pain in the affected leg and could return to sports at his former level (no difference in training load compared to non-injured state). There was no recurrence of inflammatory signs or heel pain despite intense athletics training up to the date of publication.

Conclusions

In prolonged cases of plantar fasciitis, inflammation is an important component in the development of persistent pain. The results of our case describe the effects of three neural therapy sessions that abolished inflammation and associated heel pain. Neural therapy might be an effective and time-efficient approach in the treatment of plantar fasciitis, enabling an early return to sports.

Effects of supplemental heat therapy in multimodal treated chronic low back pain patients on strength and flexibility

BACKGROUND

The beneficial effects of thermotherapy on analgesia and relaxation are widely known for various diseases. To date, however, thermotherapy in chronic low back pain is not explicitly recommended in international guidelines. The effects of thermotherapy on biomechanical parameters within a multimodal back pain treatment concept are also unknown.

METHODS

Within a multimodal treatment concept, 176 patients with chronic low back pain were treated either with or without supplemental heat wrap therapy. The range of movement and strength parameters of the trunk in flexion, extension, lateral flexion and rotation were measured before and after 12 weeks of treatment.

FINDINGS

The range of movement as well as strength parameters of the trunk improved on average within the multimodal treatment. Patients receiving additional thermotherapy supplemental to basic multimodal treatment showed a further improvement of strength parameters regarding extension (P = 0.09, 1 - β = 0.41), rotation to the right (P = 0.09, 1 - β = 0.41) and rotation to the left (P = 0.08, 1 - β = 0.42) in comparison to those conducting only the multimodal treatment. No group differences were detected in flexibility.

INTERPRETATION

The implementation of thermotherapy for several hours a day (heat wrap therapy) in daily clinical practice additional to an individualized, evidence-based multimodal treatment concept can be recommended to enhance strength parameters. The potential causes of improved strength parameters as well as the meaning for the patients in activity of daily living are discussed.

Withdrawal-associated injury site pain (WISP): a descriptive case series of an opioid cessation phenomenon

Supplemental Digital Content is Available in the Text.

This descriptive case series among adults documents that pain can return temporarily at healed, previously pain-free injury sites during acute opioid withdrawal.

Withdrawal pain can be a barrier to opioid cessation. Yet, little is known about old injury site pain in this context. We conducted an exploratory mixed-methods descriptive case series using a web-based survey and in-person interviews with adults recruited from pain and addiction treatment and research settings. We included individuals who self-reported a past significant injury that was healed and pain-free before the initiation of opioids, which then became temporarily painful upon opioid cessation—a phenomenon we have named withdrawal-associated injury site pain (WISP). Screening identified WISP in 47 people, of whom 34 (72%) completed the descriptive survey, including 21 who completed qualitative interviews. Recalled pain severity scores for WISP were typically high (median: 8/10; interquartile range [IQR]: 2), emotionally and physically aversive, and took approximately 2 weeks to resolve (median: 14; IQR: 24 days). Withdrawal-associated injury site pain intensity was typically slightly less than participants' original injury pain (median: 10/10; IQR: 3), and more painful than other generalized withdrawal symptoms which also lasted approximately 2 weeks (median: 13; IQR: 25 days). Fifteen surveyed participants (44%) reported returning to opioid use because of WISP in the past. Participants developed theories about the etiology of WISP, including that the pain is the brain's way of communicating a desire for opioids. This research represents the first known documentation that previously healed, and pain-free injury sites can temporarily become painful again during opioid withdrawal, an experience which may be a barrier to opioid cessation, and a contributor to opioid reinitiation.

Prevention and reversal of latent sensitization of dorsal horn neurons by glial blockers in a model of low back pain in male rats

In an animal model of nonspecific low back pain, recordings from dorsal horn neurons were made to investigate the influence of glial cells in the central sensitization process. To induce a latent sensitization of the neurons, nerve growth factor (NGF) was injected into the multifidus muscle; the manifest sensitization to a second NGF injection 5 days later was used as a read-out. The sensitization manifested in increased resting activity and in an increased proportion of neurons responding to stimulation of deep somatic tissues. To block microglial activation, minocycline was continuously administered intrathecally starting 1 day before or 2 days after the first NGF injection. The glia inhibitor fluorocitrate that also blocks astrocyte activation was administrated 2 days after the first injection. Minocycline applied before the first NGF injection reduced the manifest sensitization after the second NGF injection to control values. The proportion of neurons responsive to stimulation of deep tissues was reduced from 50% to 17.7% (P < 0.01). No significant changes occurred when minocycline was applied after the first injection. In contrast, fluorocitrate administrated after the first NGF injection reduced significantly the proportion of neurons with deep input (15.8%, P < 0.01). A block of glia activation had no significant effect on the increased resting activity. The data suggest that blocking microglial activation prevented the NGF-induced latent spinal sensitization, whereas blocking astrocyte activation reversed it. The induction of spinal neuronal sensitization in this pain model appears to depend on microglia activation, whereas its maintenance is regulated by activated astrocytes.NEW & NOTEWORTHY Activated microglia and astrocytes mediate the latent sensitization induced by nerve growth factor in dorsal horn neurons that receive input from deep tissues of the low back. These processes may contribute to nonspecific low back pain.

Bone Trauma Causes Massive but Reversible Changes in Spinal Circuitry

Bone fracture with subsequent immobilization of the injured limb can cause complex regional pain syndrome (CRPS) in humans. Mechanisms of CRPS are still not completely understood but bone fracture with casting in mice leads to a similar post-traumatic inflammation as seen in humans and might therefore be an analog to human CRPS. In this article we report behavioral and spinal electrophysiological changes in mice that developed swelling of the paw, warming of the skin, and pain in the injured limb after bone fracture. The receptive field sizes of spinal neurons representing areas of the hind paws increased after trauma and recovered over time-as did the behavioral signs of inflammation and pain. Interestingly, both sides-the ipsi- and the contralateral limb-showed changes in mechanical sensitivity and neuronal network organization after the trauma. The characteristics of evoked neuronal responses recorded in the dorsal horn of the mice were similar between uninjured controls and fractured animals. However, we saw a caudal extension of the represented area of the hind paw in the spinal cord at the injured side and an occurrence of large receptive fields of wide dynamic range neurons. The findings in mice compare with human symptoms in CRPS with ipsi- and also contralateral allodynia and pain. In all mice tested, all signs subsided 12 weeks after trauma. Our data suggest a significant reorganization of spinal circuitry after limb trauma, in a degree more comprehensive than most models of neuropathies. This process seems to be reversible in the rodent.

PERSPECTIVE

The discovery of enlarged spinal neuronal receptive fields and caudal extension of the representation area of the injured body part, which subsides several weeks after a bone trauma in mice, might give hope to patients of CRPS if-in the future-we are able to translate the rodent recovery mechanisms to post-traumatic humans.

Evidence for the existence of nociceptors in rat thoracolumbar fascia

Recently, the existence of nociceptive fibers in fascia tissue has attracted much interest. Fascia can be a source of pain in several disorders such as fasciitis and non-specific low back pain. However, little is known about the properties of fascia nociceptors and possible changes of the fascia innervation by nociceptors under pathological circumstances. In this histologic study, the density of presumably nociceptive fibers and free nerve endings was determined in the three layers of the rat TLF: inner layer (IL, covering the multifidus muscle), middle layer (ML) and outer layer (OL). As markers for nociceptive fibers, antibodies to the neuropeptides CGRP and SP as well as to the transient receptor potential vanilloid 1 (TRPV1) were used. As a pathological state, inflammation of the TLF was induced with injection of complete Freund's adjuvant. The density of CGRP- and SP-positive fibers was significantly increased in the inner and outer layer of the inflamed fascia. In the thick middle layer, no inflammation-induced change occurred. In additional experiments, a neurogenic inflammation was induced in the fascia by electrical stimulation of dorsal roots. In these experiments, plasma extravasation was visible in the TLF, which is clear functional evidence for the existence of fascia nociceptors. The presence of nociceptors in the TLF and the increased density of presumably nociceptive fibers under chronic painful circumstances may explain the pain from a pathologically altered fascia. The fascia nociceptors probably contribute also to the pain in non-specific low back pain.

Innervation changes induced by inflammation of the rat thoracolumbar fascia

Recently, the fascia innervation has become an important issue, particularly the existence of nociceptive fibers. Fascia can be a source of pain in several disorders such as fasciitis and non-specific low back pain. However, nothing is known about possible changes of the fascia innervation under pathological circumstances. This question is important, because theoretically pain from the fascia cannot only be due to increased nociceptor discharges, but also to a denser innervation of the fascia by nociceptive endings. In this histological study, an inflammation was induced in the thoracolumbar fascia (TLF) of rats and the innervation by various fiber types compared between the inflamed and intact TLF. Although the TLF is generally considered to have proprioceptive functions, no corpuscular proprioceptors (Pacini and Ruffini corpuscles) were found. To obtain quantitative data, the length of fibers and free nerve endings were determined in the three layers of the rat TLF: inner layer (IL, adjacent to the multifidus muscle), middle layer (ML) and outer layer (OL). The main results were that the overall innervation density showed little change; however, there were significant changes in some of the layers. The innervation density was significantly decreased in the OL, but this change was partly compensated for by an increase in the IL. The density of substance P (SP)-positive - presumably nociceptive - fibers was significantly increased. In contrast, the postganglionic sympathetic fibers were significantly decreased. In conclusion, the inflamed TLF showed an increase of presumably nociceptive fibers, which may explain the pain from a pathologically altered fascia. The meaning of the decreased innervation by sympathetic fibers is obscure at present. The lack of proprioceptive corpuscular receptors within the TLF does not preclude its role as a proprioceptive structure, because some of the free nerve endings may function as proprioceptors.