Influence of surface anesthesia on the pressure pain threshold measured with different-sized probes

Effect of acupuncture depth on muscle pain

Background

While evidence supports efficacy of acupuncture and/or dry needling in treating musculoskeletal pain, it is unclear which needling method is most effective. This study aims to determine the effects of depth of needle penetration on muscle pain.

Methods

A total of 22 healthy volunteers performed repeated eccentric contractions to induce muscle soreness in their extensor digital muscle. Subjects were assigned randomly to four groups, namely control group, skin group (depth of 3 mm: the extensor digital muscle), muscle group (depth of 10 mm: the extensor digital muscle) and non-segmental group (depth of 10 mm: the anterior tibial muscle). Pressure pain threshold and electrical pain threshold of the skin, fascia and muscle were measured at a point 20 mm distal to the maximum tender point on the second day after the exercise.

Results

Pressure pain thresholds of skin group (depth of 3 mm: the extensor digital muscle) and muscle group (depth of 10 mm: the extensor digital muscle) were significantly higher than the control group, whereas the electrical pain threshold at fascia of muscle group (depth of 10 mm: the extensor digital muscle) was a significantly higher than control group; however, there was no significant difference between the control and other groups.

Conclusion

The present study shows that acupuncture stimulation of muscle increases the PPT and EPT of fascia. The depth of needle penetration is important for the relief of muscle pain.

Further validation of a model of fibromyalgia syndrome in the rat

We have recently developed an animal model of fibromyalgia syndrome in the rat. In this model, rats exposed to unpredictable sound stress develop a delayed onset enhancement and prolongation of cytokine-induced mechanical hyperalgesia in muscle and skin. In this study, we tested the hypothesis that our model also manifests symptoms of common comorbid diagnoses: irritable bowel syndrome, temporomandibular disorder, and anxiety. Both visceral sensitivity and cytokine hyperalgesia in masseter muscle were present in the stressed rats. Furthermore, in an established model of irritable bowel syndrome-water avoidance-we observed significant muscle hyperalgesia. Finally, using the elevated plus maze to assess for anxiety level, we observed a significantly higher anxiety level in sound stress-exposed rats. Thus, unpredictable sound stress produces a condition in the rat with several features-delayed onset visceral and temporomandibular hyperalgesia and increased anxiety, as well as cutaneous and muscle hyperalgesia-commonly found in patients with fibromyalgia syndrome.

PERSPECTIVE

A stress model-unpredictable sound-in the rat exhibits several features (cutaneous, musculoskeletal, and visceral hyperalgesia, as well as anxiety) that are found in patients with fibromyalgia syndrome. Thus, this model may be used to test hypotheses about the underlying mechanisms and response to therapy in patients with fibromyalgia.

Increased sensitivity of group III and group IV afferents from incised muscle in vitro

Understanding deep muscle pain is of increasing importance for evaluating clinical pathologic pain states. Previously, a central role of deep muscle tissue in the development of ongoing pain behavior after incision was demonstrated. The underlying mechanisms, however, remain unclear. Using a new in vitro plantar flexor digitorum brevis (FDB) muscle-nerve preparation, we investigated properties of mechanosensitive group III and IV afferents innervating incised and unincised muscle, and explored potential mediators of afferent excitation after incision. Afferents of uninjured muscle had a low incidence (14.3%) of ongoing activity. A high proportion (65.8%) of afferents responded to heat and a minority, 20.8%, were activated by pH 6.0 lactic acid. Incision increased the prevalence of afferents with ongoing activity to 54.7%. A greater proportion of group III and IV afferents responded to pH 6.0 lactic acid after incision compared to control (55.4% vs. 20.8%). Sensitization of afferents to heat and mechanical stimulation was prominent in group IV afferents after incision; both heat (38.0 vs. 40.5°C in control) and mechanical response threshold (median: 5.0 vs. 22.0 mN in control) were decreased. The finding hat incision increased ongoing activity of muscle afferents is consistent with our previous in vivo studies and supports the idea that deep muscle tissue has a prominent role in the genesis of ongoing pain after incision. The enhanced chemosensitivity of muscle afferents to lactic acid after incision suggests an increased response to an ischemic mediator that may contribute to pain and hyperalgesia caused by surgery in deep tissues.

Bradykinin and nerve growth factor play pivotal roles in muscular mechanical hyperalgesia after exercise (delayed-onset muscle soreness)

Unaccustomed strenuous exercise that includes lengthening contraction (LC) often causes delayed-onset muscle soreness (DOMS), a kind of muscular mechanical hyperalgesia. The substances that induce this phenomenon are largely unknown. Peculiarly, DOMS is not perceived during and shortly after exercise, but rather is first perceived after approximately 1 d. Using B(2) bradykinin receptor antagonist HOE 140, we show here that bradykinin released during exercise plays a pivotal role in triggering the process that leads to muscular mechanical hyperalgesia. HOE 140 completely suppressed the development of muscular mechanical hyperalgesia when injected before LC, but when injected 2 d after LC failed to reverse mechanical hyperalgesia that had already developed. B(1) antagonist was ineffective, regardless of the timing of its injection. Upregulation of nerve growth factor (NGF) mRNA and protein occurred in exercised muscle over a comparable time course (12 h to 2 d after LC) for muscle mechanical hyperalgesia. Antibodies to NGF injected intramuscularly 2 d after exercise reversed muscle mechanical hyperalgesia. HOE 140 inhibited the upregulation of NGF. In contrast, shortening contraction or stretching induced neither mechanical hyperalgesia nor NGF upregulation. Bradykinin together with shortening contraction, but not bradykinin alone, reproduced lasting mechanical hyperalgesia. We also showed that rat NGF sensitized thin-fiber afferents to mechanical stimulation in the periphery after 10-20 min. Thus, NGF upregulation through activation of B(2) bradykinin receptors is essential (though not satisfactory) to mechanical hyperalgesia after exercise. The present observations explain why DOMS occurs with a delay, and why lengthening contraction but not shortening contraction induces DOMS.

Eccentric muscle contraction and stretching evoke mechanical hyperalgesia and modulate CGRP and P2X(3) expression in a functionally relevant manner

Non-invasive, movement-based models were used to investigate muscle pain. In rats, the masseter muscle was rapidly stretched or electrically stimulated during forced lengthening to produce eccentric muscle contractions (EC). Both EC and stretching disrupted scattered myofibers and produced intramuscular plasma extravasation. Pro-inflammatory cytokines (IL-1beta, TNF-alpha, IL-6) and vascular endothelial growth factor (VEGF) were elevated in the masseter 24h following EC. At 48h, neutrophils increased and ED1 macrophages infiltrated myofibers while ED2 macrophages were abundant at 4d. Mechanical hyperalgesia was evident in the ipsilateral head 4h-4d after a single bout of EC and for 7d following multiple bouts (1 bout/d for 4d). Calcitonin gene-related peptide (CGRP) mRNA increased in the trigeminal ganglion 24h following EC while immunoreactive CGRP decreased. By 2d, CGRP-muscle afferent numbers equaled naive numbers implying that CGRP is released following EC and replenished within 2d. EC elevated P2X(3) mRNA and increased P2X(3) muscle afferent neuron number for 12d while electrical stimulation without muscle contraction altered neither CGRP nor P2X(3) mRNA levels. Muscle stretching produced hyperalgesia for 2d whereas contraction alone produced no hyperalgesia. Stretching increased CGRP mRNA at 24h but not CGRP-muscle afferent number at 2-12d. In contrast, stretching significantly increased the number of P2X(3) muscle afferent neurons for 12d. The sustained, elevated P2X(3) expression evoked by EC and stretching may enhance nociceptor responsiveness to ATP released during subsequent myofiber damage. Movement-based actions such as EC and muscle stretching produce unique tissue responses and modulate neuropeptide and nociceptive receptor expression in a manner particularly relevant to repeated muscle damage.

Persistent deep mechanical hyperalgesia induced by repeated cold stress in rats

Chronic muscle pain of the neck, shoulder and low back is quite common and often related to a stressed condition. In this study we tried to make a model of long-lasting muscle mechanical hyperalgesia based on one type of stress, repeated cold stress (RCS) (Kita T, Hata T, Yoneda R, Okage T. Stress state caused by alternation of rhythm in environmental temperature, and the functional disorders in mice and rats. Folia Pharmacol Jpn 1975;71:195-210). We first validated a method of measuring the muscle mechanical nociceptive threshold through skin, with surface anesthesia of the skin covering the muscle. We found that a pressure test using a Randall-Selitto analgesiometer equipped with a larger probe (varphi 2.6 mm) can measure the deep mechanical withdrawal threshold even under the presence of cutaneous punctuate hyperalgesia. RCS was performed by changing the temperature from 22 degrees C to either 4 degrees C (RCS at 4 degrees C) or -3 degrees C (RCS at -3 degrees C) every 30 min, and then maintained at 4 degrees C/-3 degrees C from 17:30 to 10:00 the next day. RCS at 4 degrees C for 5 days induced bilateral deep mechanical hyperalgesia lasting 2-3 weeks without cutaneous punctuate hyperalgesia. Deep mechanical hyperalgesia observed after RCS at -3 degrees C lasted longer ( approximately 6 weeks) and was severer than RCS at 4 degrees C. Bilateral cutaneous punctuate hyperalgesia was also observed with RCS at -3 degrees C. Intramuscular injection of lidocaine confirmed that the muscle was hyperalgesic. RCS might serve as a useful model for study of the mechanism of chronic muscle pain and its treatment.

Experimental occlusal interference induces long-term masticatory muscle hyperalgesia in rats

Temporomandibular joint or related masticatory muscle pain represents the most common chronic orofacial pain condition. Patients frequently report this kind of pain after dental alterations in occlusion. However, lack of understanding of the mechanisms of occlusion-related temporomandibular joint and muscle pain prevents treating this problem successfully. To explore the relationship between improper occlusion (occlusal interference) and masticatory muscle pain, we created an occlusal interference animal model by directly bonding a crown to a maxillary molar to raise the masticating surface of the tooth in rats. We raised the occlusal surface to three different heights (0.2, 0.4, and 0.6mm), and for one month we quantitatively measured mechanical nociceptive thresholds of the temporal and masseter muscles on both sides. Results showed a stimulus-response relationship between the height of occlusal interference and muscle hyperalgesia. Removal of the crown 6 days after occlusal interference showed that the removal at this time could not terminate the 1 month duration of mechanical hyperalgesia in the masticatory muscles. Lastly, we systemically administered NMDA antagonist MK801 (0.2, 0.1, and 0.05 mg/kg) to the treated rats and found that MK801 dose dependently attenuated the occlusal interference-induced hyperalgesia. These findings suggest that occlusal interference is directly related to masticatory muscle pain, and that central sensitization mechanisms are involved in the maintenance of the occlusal interference-induced mechanical hyperalgesia.

Effects of tender point acupuncture on delayed onset muscle soreness (DOMS)--a pragmatic trial

Background

Acupuncture is used to reduce inflammation and decrease pain in delayed onset muscle soreness (DOMS). This study investigates the efficacy of acupuncture on the symptoms of DOMS.

Methods

Thirty subjects were assigned randomly to there groups, namely the control, non-tender point and tender point groups. Measurement of pain with full elbow flexion was used as indices of efficacy. Measurements were taken before and after exercise, immediately after treatment and seven days after treatment.

Results

Significant differences in visual analog scores for pain were found between the control group and tender point group immediately after treatment and three days after exercise (P < 0.05, Dunnetts multiple test).

Conclusion

The results show that tender point acupuncture relieves muscle pain of DOMS.

Subcutaneous Botulinum toxin type A reduces capsaicin-induced trigeminal pain and vasomotor reactions in human skin

The present human study aimed at investigating the effect of subcutaneous administration of Botulinum toxin type A (BoNT/A) on capsaicin-induced trigeminal pain, neurogenic inflammation and experimentally induced cutaneous pain modalities. Fourteen healthy males (26.3+/-2.6 years) were included in this double-blind and placebo-controlled trial. The subjects received subcutaneous BoNT/A (22.5U) and isotonic saline in the mirror sides of their forehead. Pain and neurogenic inflammation was induced by four intradermal injections of capsaicin (100mug/muL) (before, and days 1, 3 and 7 after treatments). The capsaicin-induced pain intensity, pain area, the area of secondary hyperalgesia, the area of visible flare and vasomotor reactions were recorded together with cutaneous heat, electrical and pressure pain thresholds. BoNT/A reduced the capsaicin-induced trigeminal pain intensity compared to saline (F=37.9, P<0.001). The perceived pain area was smaller for the BoNT/A-treated side compared to saline (F=7.8, P<0.05). BoNT/A reduced the capsaicin-induced secondary hyperalgesia (F=5.3, P<0.05) and flare area (F=10.3, P<0.01) compared to saline. BoNT/A reduced blood flow (F(1,26)=109.5, P<0.001) and skin temperature (F(1,26)=63.1, P<0.001) at the capsaicin injection sites compared to saline and its suppressive effect was maximal at days 3 and 7 (P<0.05, post hoc test). BoNT/A elevated cutaneous heat pain thresholds (F=17.1, P<0.001) compared to saline; however, no alteration was recorded for electrical or pressure pain thresholds (P>0.05). Findings from the present study suggest that BoNT/A appears to preferentially target Cfibers and probably TRPV1-receptors, block neurotransmitter release and subsequently reduce pain, neurogenic inflammation and cutaneous heat pain threshold.

TRP channels and ASICs mediate mechanical hyperalgesia in models of inflammatory muscle pain and delayed onset muscle soreness

The roles of ion channels in sensory neurons were examined in experimental models of muscle pain in the rat. Rats were injected with 50 microl of 4% carrageenan or subjected to an eccentric exercise (ECC) of the gastrocnemius muscle (GM). The Randall-Selitto and von Frey tests were performed on the calves to evaluate mechanical hyperalgesia of the muscle. The changes in expression of four genes and proteins of ion channels in dorsal root ganglia were examined using quantitative PCR and immunohistochemistry, respectively. Effects of antagonists to transient receptor potential (TRP) channels and acid sensing ion channels (ASICs) on the mechanical hyperalgesia induced by carrageenan injection or ECC were evaluated. The mechanical hyperalgesia was observed 6-24h after carrageenan injection and 1-3 days after ECC in the Randall-Selitto test. Infiltrations of the inflammatory cells in the GM were seen in carrageenan-injected animals but not in those subjected to ECC. Expressions of genes and proteins in sensory neurons showed no changes. Intramuscular injection of antagonists to TRPV1 showed an almost complete suppressive effect on ECC-induced muscle hyperalgesia but not a carrageenan-induced one. Antagonists to TRP channels and ASICs showed suppressive effects for both carrageenan- and ECC-induced muscle hyperalgesia. The carrageenan injection and ECC models are useful models of acute inflammatory pain and delayed onset muscle soreness (DOMS), respectively, and the time course and underlying etiology might be different. TRP channels and ASICs are closely related to the development of muscle mechanical hyperalgesia, and TRPV1 is involved in ECC-induced DOMS.

Dorsal horn neurons having input from low back structures in rats

The mechanisms of nociception in the low back are poorly understood, partly because systematic recordings from dorsal horn neurons with input from the low back are largely missing. The purpose of this investigation was to (1) identify spinal segments and dorsal horn neurons receiving input from the low back, (2) test the effect of nerve growth factor (NGF) injected into the multifidus muscle (MF) on the neurons' responsiveness, and (3) study the influence of a chronic MF inflammation on the responses. In rats, microelectrode recordings were made in the segments L2, L3, and L5 to find dorsal horn neurons having input from the low back (LB neurons). In control animals, the proportion of LB neurons in L2 was larger than in L3 and L5. Most LB neurons had a convergent input from several tissues. Injections of NGF into MF increased the proportion of LB neurons significantly. A chronic MF inflammation likewise increased the proportion of LB neurons and the input convergence. The centers of the neurons' receptive fields (RFs) were consistently located 2-3 segments caudally relative to their recording site. The results show that (1) input convergence from various tissues is common for LB neurons, (2) the input from structures of the low back is processed 2-3 segments cranially relative to the vertebral level of the RFs, and (3) the responsiveness of LB neurons is increased during a pathologic alteration of the MF. The above findings may be relevant for some cases of chronic low back pain in patients.

Test-retest reliability of pressure pain threshold measurements of the upper limb and torso in young healthy women

The goal of this study was to determine the intra- and interday reliability of pressure pain thresholds (PPT) in the upper extremity and torso of asymptomatic women. Nineteen healthy women (20-39 years) with no underlying musculoskeletal problems had 3 PPT trials performed on 8 different locations in the upper extremity and torso over 4 consecutive days. The test-retest reliability of PPT values was robust and highly consistent over the 4 days. The PPT intraclass correlations (ICC) were highly consistent and repeatable over the 4 days of testing (day 1: ICC = 0.94; day 2: ICC = 0.96; day 3: ICC = 0.97 and day 4: ICC = 0.96). When compared with baseline measurements obtained on day 1, the PPT values were significantly lower (P < .05) on days 2, 3, and 4 at all 8 locations. Although the PPT test-retest reliability is robust and consistent throughout the 4 days, there appears to be a similar overall decline in the magnitude of the absolute PPT response at each of the 8 locations. A specific explanation for this greater overall sensitivity in PPTs at all 8 locations is lacking; however, a centrally mediated alteration in pressure/pain sensation could contribute to the overall trend observed in this study.

PERSPECTIVE

PPT measurements of the upper limb and torso will be significantly lower with repeated measures over a short period time. A standardized evaluation grid should be included in baseline so as to accurately evaluate the progression in shoulder rehabilitation in women with shoulder dysfunction.

Involvement of ATP and its receptors on nociception in rat model of masseter muscle pain

The exact mechanism of the masseter muscle pain recognized as a prominent symptom in temporomandibular disorders remains unclear, although it is clinically known that excessive muscular contraction causes tenderness in masseter muscles. It has been demonstrated that P2X3 receptors (P2X3Rs) in sensory neurons play a role in pain signaling from the periphery. We determined the role of P2X(3)R on pressure pain and mechanical hyperalgesia in a newly developed rat model of masseter muscle pain. The pain in the masseter muscle was assessed by the pressure pain threshold (PPT), which was defined as the amount of pressure required to induce head flinching. In naive animals, systemic treatment with morphine was associated with increase of PPTs. Changes in PPTs were examined after administration of P2XR agonists or antagonists into the masseter muscle. The masseter muscle injection of alpha,beta-meATP (P2X(1,3,2/3)R-specific agonist) induced a significantly greater behavioral response than its vehicle. This enhanced response was completely blocked by the co-application of alpha,beta-meATP with PPADS (P2X(1,2,3,5,1/5,2/3)R-specific antagonist). Excessive contraction in masseter muscle was produced by electrical stimulation. The exerted masseter muscles showed a significant reduction in PPTs indicating the induction of mechanical hyperalgesia of the muscle. Moreover, administration of PPADS to the exerted masseter muscles produced a complete recovery of reducing PPT. Immunohistochemically, the number of P2X3R-positive neurons innervating the masseter muscles increased in the excessively contracted condition in trigeminal ganglia. Our results suggested that P2X3R plays an important role in pressure pain and mechanical hyperalgesia in masseter muscle caused by excessive muscular contraction.

Effects of local anesthetics on somatosensory function in the temporomandibular joint area

There is a need for systematic studies regarding the pathophysiology and pain mechanisms of somatosensory function in the temporomandibular joint (TMJ). So far, the effects on somatosensory functions of local anesthetics (LA) applied to the auriculotemporal (AT) nerve or intraarticularly (IA) into the TMJ have not been studied systemically. This study aimed to examine in a double-blinded, placebo-controlled manner the effects of LA on mechanical and thermal sensitivity in the TMJ area. Twenty-eight healthy subjects (27.4 +/- 6.2 years) without temporomandibular disorders (TMD) participated. The subjects received an AT nerve block (n = 14) or an IA injection (n = 14) with LA (Bupivacaine, 2.5 mg/ml) on one side, and a placebo injection (isotonic saline) on the contralateral side. Mechanical (tactile and pin-prick) and thermal sensitivity (40 and 5 degrees C) were assessed at 11 standardized points in the TMJ area before injections (baseline) as well as 30 min, 1 and 2 h after injections. All stimuli were rated by the subjects on a 0-100 numerical rating scale (NRS). TMJ pressure pain threshold (PPT) and pressure pain tolerance (PPTOL) were assessed laterally over both TMJs using an algometer. IA injections with LA were not associated with any changes in sensitivity of the TMJ or surrounding area. In contrast, AT nerve blocks with LA caused a decrease over time in the pin-prick sensitivity (P = 0.016), which however, did not differ significantly from saline, and an increase of the PPTs 30 min (P = 0.010) and PPTOLs 30 min, 1 h and 2 h (P < 0.05) after LA injections in comparison to saline. No other measures showed a significant change after the injections. Our results showed that IA bupivacaine injection in healthy subjects has no effect on the sensitivity of the TMJ or surrounding area, while AT nerve block has a more pronounced effect on deep mechanical, but not on superficial mechanical or thermal sensitivity. Further research to investigate the effect of LA on somatosensory functions in TMJ patients in comparison with this study results will give valuable information about the sensitivity in the TMJ area.