Structural and functional abnormalities of motor endplates in rat skeletal model of myofascial trigger spots

The key role of muscle spindles in the pathogenesis of myofascial trigger points according to ramp-and-hold stretch and drug intervention in a rat model

This study investigated whether abnormal peak inversion spontaneous potentials (PISPs) recorded at resting myofascial trigger points (MTrPs) stem from the discharge of muscle spindles. Forty-eight male Sprague-Dawley rats were randomly divided into six groups. Five groups underwent MTrP modeling intervention, whereas one group did not receive intervention and was duly designated as the blank control. After model construction, five rat models were randomly subjected to ramp-and-hold stretch tests, succinylcholine injection, eperisone hydrochloride injection, saline injection, and blank drug intervention. By contrast, the rats in the blank control group were subjected to ramp-and-hold stretch tests as a control. Frequencies and amplitudes of PISPs were recorded pre- and post-interventions and compared with those of the blank group. Stretch tests showed that the depolarization time and amplitude of PISPs ranged from 0.4 ms to 0.9 ms and from 80 uV to 140 μV, respectively. However, no PISPs were observed in the control rats. The frequency of PISPs in the ramp and hold phases and the first second after the hold phase was higher than that before stretching (p < 0.01). Succinylcholine and eperisone exerted excitatory and inhibitory effects on PISPs, respectively. In the group injected with 0.9% saline, no considerable differences of the PISPs were observed during the entire observation period. In conclusion, PISPs recorded at resting MTrPs are closely related to muscle spindles. The formation of MTrPs may be an important factor that regulate dysfunctional muscle spindles.

A New Unified Theory of Trigger Point Formation: Failure of Pre- and Post-Synaptic Feedback Control Mechanisms

The origin of the myofascial trigger point (TrP), an anomalous locus in muscle, has never been well-described. A new trigger point hypothesis (the new hypothesis) presented here addresses this lack. The new hypothesis is based on the concept that existing myoprotective feedback mechanisms that respond to muscle overactivity, low levels of adenosine triphosphate, (ATP) or a low pH, fail to protect muscle in certain circumstances, such as intense muscle activity, resulting in an abnormal accumulation of intracellular Ca²⁺, persistent actin-myosin cross bridging, and then activation of the nociceptive system, resulting in the formation of a trigger point. The relevant protective feedback mechanisms include pre- and postsynaptic sympathetic nervous system modulation, modulators of acetylcholine release at the neuromuscular junction, and mutations/variants or post-translational functional alterations in either of two ion channelopathies, the ryanodine receptor and the potassium-ATP ion channel, both of which exist in multiple mutation states that up- or downregulate ion channel function. The concepts that are central to the origin of at least some TrPs are the failure of protective feedback mechanisms and/or of certain ion channelopathies that are new concepts in relation to myofascial trigger points.

Creation of myofascial pain syndrome-like muscle by artificial electrical stimulation and stretching treatment

BACKGROUND

Myofascial pain syndrome（MPS）is a common chronic pain disorder characterized by muscle hardness, low extensibility, restriction of range of motion (ROM)　and pain with trigger point (TP). Eccentric contraction has been used in past animal studies of MPS. However, clinical experience suggests that concentric contraction is also involved in MPS formation.

NEW METHODS

In this study, we adopted artificial electrical stimulation to create artificial concentric contraction (ACC) in rat gastrocnemius muscle. After ACC, muscle hardness, torsion and range of motion (ROM) were compared between before ACC, after ACC, and stretching group. To clarify the association with pain, the expression of pERK in DRG were analyzed.

COMPARISON TO EXISTING METHODS

Previous animal studies have created MPS models by inducing eccentric contractions in muscles. In this study, full tetanus contraction of the gastrocnemius muscle was achieved via tibial nerve stimulation. This method substituted muscle contraction due to abnormal excitation.

RESULTS

We found that artificial abnormal contraction (ACC) induced muscle hardness and ROM restriction. The pERK expression in DRG was increased by ACC. Analysis of muscle tissue sections revealed a meandeling structure in muscle fibers. The stretching treatment improved these indicators. These results were similar to feature of the MPS muscles.

CONCLUSIONS

The ACC caused by artificial electrical stimulation leads to the characteristic of MPS in rat gastrocnemius muscle. This ACC model can be one of the useful options for MPS analysis.

Contracture Knots vs. Trigger Points. Comment on Ball et al. Ultrasound Confirmation of the Multiple Loci Hypothesis of the Myofascial Trigger Point and the Diagnostic Importance of Specificity in the Elicitation of the Local Twitch Response. Diagnostics 2022, 12, 321

A recent study published in Diagnostics attempted to visualize trigger points and contracture knots with high-definition ultrasound. Based on their findings, the authors reversed the commonly understood meaning of the two terms. However, they did so without providing any convincing evidence. The authors maintained that their sonography images represented trigger points within contracture knots, supporting the multiple loci hypothesis. On review of the paper, both conclusions seem premature and rather speculative.

Trigger Points and Contracture/Contraction Knots: What’s in a Name? Reply to Dommerholt, J.; Gerwin, R.D. Contracture Knots vs. Trigger Points. Comment on “Ball et al. Ultrasound Confirmation of the Multiple Loci Hypothesis of the Myofascial Trigger Point and the Diagnostic Importance of Specificity in the Elicitation of the Local Twitch Response. Diagnostics 2022, 12, 321”

We are responding to the comment by Dommerholt and Gerwin that we have reverse-defined “myofascial trigger point” (MTrP) and “contracture/contraction knot.” In attempting to maintain philosophical agreement with specific and implied aspects of their integrated hypothesis of trigger-point formation (namely a MTrP being ischemic and hypoxic), we referred to the MTrP as the small hyperechoic signal rather than the larger hypoechoic (and therefore hyperperfused) structure surrounding it. It was never our intent to re-define nor contribute to confusion. In making this concession with respect to Dommerholt and Gerwin’s preferred nomenclature, however, we must instead now reconcile what we image as a hypoechoic (and therefore hyperperfused) MTrP with it being concurrently hypoxic.

Efficacy of Dry Needling Under EMG Guidance for Myofascial Neck and Shoulder Pain: A Randomized Clinical Trial

Purpose

To determine the difference in maintenance of improvement of pain and disability for dry needling (DN) under needle electromyography (EMG) guidance technique in myofascial neck and shoulder pain patients, compared with DN alone.

Patients and Methods

In this randomized single-blind clinical trial, 30 participants with myofascial pain in the neck and shoulder were randomly allocated to two groups: myofascial trigger points (MTrPs) DN with EMG guided (DN-EMG) group and MTrPs DN without EMG (DN) group. Needling treatment lasted for 2 weeks, twice a week. The primary outcome was pain intensity as assessed by visual analogue scale (VAS) and neck disability index (NDI). A number of mappings referred to pain and spontaneous muscle activity (SEA) were considered secondary outcomes. VAS and NDI were measured before treatment, after 2 weeks of intervention and at 4-, 6- and 12-week follow-up periods after the intervention. Secondary outcomes were assessed before each treatment (T1–T4). Data were analysed using mixed-model analyses of variance (ANOVA) with time as a within-subject variable and groups as between-subject variables followed by Bonferroni’s post-hoc test.

Results

Mixed-model ANOVA revealed significant time-by-group interaction effects (F = 3.49, P = 0.01) for VAS. Post-hoc analysis showed a significant decrease in VAS and NDI after 2 weeks of intervention and at all follow-up periods compared with baseline in both groups (p < 0.01). The DN-EMG group exhibited higher improvements in VAS at 6- and 12-week follow-up period than the DN group (p < 0.05). In the SEA of MTrPs, we found positive sharp waves, fibrillation and fascicular potentials. DN-EMG group exhibited lower amplitudes at T2–T4 and frequencies at T2 and T3.

Conclusion

DN under needle EMG guidance technique exhibited greater improvements in maintenance of improvement of pain and lower SEA value than the DN group due to sufficient MTrPs inactivation.

A Dual-Modal Magnetic Resonance/Photoacoustic Imaging Tracer for Long-Term High-Precision Tracking and Facilitating Repair of Peripheral Nerve Injuries

Neuroanatomical tracing is considered a crucial technique to assess the axonal regeneration level after injury, but traditional tracers do not meet the needs of in vivo neural tracing in deep tissues. Magnetic resonance (MR) and photoacoustic (PA) imaging have high spatial resolution, great penetration depth, and rich contrast. Fe₃ O₄ nanoparticles may work well as a dual-modal diagnosis probe for neural tracers, with the potential to improve nerve regeneration. The present study combines antegrade neural tracing imaging therapy for the peripheral nervous system. Fe₃ O₄ @COOH nanoparticles are successfully conjugated with biotinylated dextran amine (BDA) to produce antegrade nano-neural tracers, which are encapsulated by microfluidic droplets to control leakage and allow sustained, slow release. They have many notable advantages over traditional tracers, including dual-modal real-time MR/PA imaging in vivo, long-duration release effect, and limitation of uncontrolled leakage. These multifunctional anterograde neural tracers have potential neurotherapeutic function, are reliable and may be used as a new platform for peripheral nerve injury imaging and treatment integration.

Ultrasound Confirmation of the Multiple Loci Hypothesis of the Myofascial Trigger Point and the Diagnostic Importance of Specificity in the Elicitation of the Local Twitch Response

The literature has hypothesized that a trigger point (TrP) area consists of a hyperperfused contracture knot with smaller hypoperfused TrPs within the contracture knot. By contrast, the only published ultrasound image of a TrP has it labeled hypoechoic (i.e., hyperperfused) with no commentary regarding smaller speckles of hypoperfusion within. Furthermore, the lack of clarity in objective definition of the terms associated with the TrP (namely, the palpable “contracture knot” and smaller nonpalpable “trigger point”) has led to unnecessary communication difficulties between and among clinicians and researchers. In this case series of three muscles across two patients, by using high-definition musculoskeletal ultrasound imaging technology, we present what we believe to be the first reliable capture of palpable hypoechoic (e.g., hypoperfused) contracture knots (previously mislabeled as a hypoechoic TrP), and a visual support of the multiple loci hypothesis first proposed by Hong and Simons—the first reliable confirmation of the hyperechoic (i.e., hypoperfused) TrP within. Initially proposed by a histological study and supported by microdialysis study, this case series lends further support for the multiple loci hypothesis through visual confirmation of palpable hypoechoic contracture knots, with smaller hypoechoic TrPs “speckles” within.

The anatomical, electrophysiological and histological observations of muscle contraction units in rabbits: a new perspective on nerve injury and regeneration

In the conventional view a muscle is composed of intermediate structures before its further division into microscopic muscle fibers. Our experiments in mice have confirmed this intermediate structure is composed of the lamella cluster formed by motor endplates, the innervating nerve branches and the corresponding muscle fibers, which can be viewed as an independent structural and functional unit. In this study, we verified the presence of these muscle construction units in rabbits. The results showed that the muscular branch of the femoral nerve sent out 4–6 nerve branches into the quadriceps and the tibial nerve sent out 4–7 nerve branches into the gastrocnemius. When each nerve branch of the femoral nerve was stimulated from the most lateral to the medial, the contraction of the lateral muscle, intermediate muscle and medial muscle of the quadriceps could be induced by electrically stimulating at least one nerve branch. When stimulating each nerve branch of the tibial nerve from the lateral to the medial, the muscle contraction of the lateral muscle 1, lateral muscle 2, lateral muscle 3 and medial muscle of the gastrocnemius could be induced by electrically stimulating at least one nerve branch. Electrical stimulation of each nerve branch resulted in different electromyographical waves recorded in different muscle subgroups. Hematoxylin-eosin staining showed most of the nerve branches around the neuromuscular junctions consisted of one individual neural tract, a few consisted of two or more neural tracts. The muscles of the lower limb in the rabbit can be subdivided into different muscle subgroups, each innervated by different nerve branches, thereby allowing much more complex muscle activities than traditionally stated. Together, the nerve branches and the innervated muscle subgroups can be viewed as an independent structural and functional unit. This study was approved by the Animal Ethics Committee of Peking University People’s Hospital (approval No. 2019PHE027) on October 20, 2019.

Pressing Intervention Promotes the Skeletal Muscle Repair of Traumatic Myofascial Trigger Points in Rats

Objective

To observe the effect of pressing intervention on the skeletal muscle repair of myofascial trigger points (MTrPs) in rats and explore the mechanism of pressing intervention on the deactivation of trigger points.

Methods

Thirty SPF rats were randomly divided into blank group, model group and press group, with 10 rats in each group. The MTrPs models were established by blunt striking plus eccentric exercise, and then evaluated. The press group was given a pressing intervention with a self-made device for 14 days, and the rats in the other two groups were fed normally. Soft tissue tension (STT) D_0.2 and pressure pain threshold (PPT) were measured before and after intervention. The skeletal muscle tissue at MTrPs was extracted and assessed by hematoxylin–eosin (HE) and Masson staining. The expression of collagen I, collagen III, α- smooth muscle actin (α-SMA), myosin heavy chain (MHC) and fibronectin (FN) were detected by Western Blotting. Enzyme linked immunosorbent assay (ELISA) was used to evaluate the expression of substance P (SP), 5-hydroxytryptamine (5-HT), cyclooxygenase 2 (COX-2) and prostaglandin E2 (PGE2).

Results

(1) Compared with the blank group, the PPT and D_0.2 reduced (P < 0.05) in the model group; while compared with the model group, the PPT and D_0.2 increased (P < 0.05) in the press group. (2) Compared with the blank group, the model group showed obvious spontaneous potentials with higher amplitude and frequency, which were also much higher than those of the press group (P < 0.05). (3) The HE and Masson staining results showed evident fibrosis in the muscle tissue of the model group, with a larger area of collagen fibers relative to that of the press group (P < 0.05). (4) The amount of collagen I, collagen III, FN, α- SMA, SP, 5-HT, COX-2 and PGE2 increased and the content of MHC decreased (P < 0.05) in the model group, as compared to the blank group; while all the substances (P < 0.05), instead of MHC which increased (P < 0.05), in the press group were decreased as compared to the model group.

Conclusion

Pressing intervention on the MTrPs in rats can alleviate chronic inflammation, inhibit fibrosis, promote skeletal muscle repair and relieve pain.

Sympathetic hyperinnervation in myofascial trigger points

OBJECTIVE

To evaluate the local distribution and activity of sympathetic nerves in myofascial trigger points (MTrPs) and explore the pathological mechanism of myofascial pain syndrome (MPS) using a rat model of disease.

METHODS

MPS was modeled in the model group (MG) by a combination of blunt trauma and eccentric exercise in Sprague-Dawley rats (n = 8). Eight rats were randomly assigned to the control group (CG). Histopathology was applied to detect the local conditions of the MTrPs. Tyrosine hydroxylase (TH), nerve growth factor (NGF), and norepinephrine (NE) were detected in the MTrPs to evaluate sympathetic remodeling.

RESULTS

Muscle fiber rupture, atrophy and shape irregularity were universally observed in the MTrPs. TH expression was significantly increased in the MG, as detected by immunofluorescence, and NGF and TH expression was also higher in MTrPs in the MG than in MTrPs in the CG, as determined by immunohistochemistry. Furthermore, the expression of NE was significantly increased in MTrPs, as determined by ELISA.

CONCLUSION

There was sympathetic hyperinnervation in MTrPs, which could partially explain the symptoms of MTrPs and is an interesting and useful new perspective regarding the diagnosis and treatment of MPS.

PERSPECTIVE

The sympathetic nerves in MTrPs are remodeled, leading to sympathetic hyperinnervation. Sympathetic hyperinnervation can partially explain the symptoms of MPS. The pathological mechanism of sympathetic hyperinnervation may be a new perspective for the diagnosis and treatment of MPS.