MYOFASCIAL TRIGGER POINTS have been defined as

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1 325 ORIGINAL ARTICLE Attenuated Skin Blood Flow Response to Nociceptive Stimulation of Latent Myofascial Trigger Points Yang Zhang, PhD, Hong-You Ge, MD, PhD, Shou-Wei Yue, MD, PhD, Yoshiyuki Kimura, PhD, Lars Arendt-Nielsen, DrMedSci, PhD ABSTRACT. Zhang Y, Ge H-Y, Yue S-W, Kimura Y, Arendt-Nielsen L. Attenuated skin blood flow response to nociceptive stimulation of latent myofascial trigger points. Arch Phys Med Rehabil 2009;90: Objectives: To investigate the effect of painful stimulation of latent myofascial trigger points (MTrPs) on skin blood flow and to evaluate the relative sensitivity of laser Doppler flowmetry (LDF) and thermography in the measurement of skin blood flow. Design: Painful stimulation was obtained by a bolus injection of glutamate (0.1mL, 0.5M) into a latent MTrP located in the right or left brachioradialis muscles. A bolus of glutamate injection into a non-mtrp served as control. Pain intensity (visual analog scale [VAS]) was assessed after glutamate injection. Pressure pain threshold (PPT) was recorded bilaterally in the brachioradialis muscle before and after glutamate-induced pain. Skin blood flow and surface skin temperature were measured bilaterally in the forearms before, during, and after glutamate-induced pain with LDF and thermography. Setting: A biomedical research facility. Participants: Fifteen healthy volunteer subjects. Interventions: Not applicable. Main Outcome Measures: VAS, PPT, skin blood flow, and surface skin temperature. Results: Glutamate injection into latent MTrPs induced higher pain intensity (F 7.16; P.05) and lower PPT (F 11.41, P.005) than into non-mtrps. Glutamate injection into non-mtrps increased skin blood flow bilaterally in the forearms, but skin blood flow after glutamate injection into latent MTrPs was significantly less increased at the local injection area or decreased at distant areas compared with non- MTrPs (all P.05). Skin temperature was not affected after glutamate injection into either latent MTrPs or non-mtrps (all P.05). Conclusions: The present study demonstrated an attenuated skin blood flow response after painful stimulation of latent MTrPs compared with non-mtrps, suggesting increased sympathetic vasoconstriction activity at latent MTrPs. Additionally, LDF was more sensitive than thermography in the detection of the changes in skin blood flow after intramuscular nociceptive stimulation. From the Laboratory for Experimental Pain Research, Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Aalborg, Denmark (Zhang, Ge, Kimura, Arendt-Nielsen) and Department of Physical Medicine and Rehabilitation, Qilu Hospital, Medical School of Shandong University, Jinan, China (Zhang, Yue). No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated. Correspondence to Hong-You Ge, MD, PhD, Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7 D-3, DK-9220, Aalborg, Denmark, ghy@hst.aau.dk. Reprints are not available from the author /09/ $36.00/0 doi: /j.apmr Key Words: Rehabilitation; Sympathetic nervous system; Myofascial pain syndromes; Vasoconstriction by the American Congress of Rehabilitation Medicine MYOFASCIAL TRIGGER POINTS have been defined as localized, hyperirritable spots in a palpable taut band of skeletal muscle fibers. 1 MTrPs may be active or latent. Active MTrPs can be found in patients with myofascial pain syndrome, and latent MTrPs can also be found in healthy subjects. 2 An active MTrP is one that produces pain spontaneously, is associated with tenderness in a taut band, reproduces a familiar pain, produces a local twitch response when stimulated manually or with a needle, and refers pain. A latent MTrP does not cause pain spontaneously, but is painful when stimulated manually or by a needle. 1,2 However, both active and latent MTrPs show spontaneous needle EMG activity when the muscle is at rest. 2,3 In addition to local pain, referred pain, and related motor dysfunction, active MTrPs can give rise to autonomic phenomena, such as vasoconstriction (blanching), coldness, sweating, and a pilomotor response. 1 There is experimental evidence to suggest that maneuvers that increase sympathetic outflow can increase motor unit activity 4-6 and facilitate muscle pain at MTrPs. 7 However, current experimental evidence is insufficient to explain the aforementioned autonomic phenomena (vasoconstriction and so forth) associated with MTrP activation. Active MTrPs produce clinical symptoms and usually produce more pronounced findings. Concurrent with the more pronounced findings at active and latent MTrPs, higher levels of algesic substances have been reported at active MTrPs compared with latent MTrPs. 8 Thus, an algogenic substance such as glutamate injected into latent MTrPs in healthy subjects may increase pain sensitivity at latent MTrPs 9 to mimic the clinical characteristics of active MTrPs. Noxious stimulation of the peripheral terminals of nociceptors in the normative tissues causes local release of vasodilative substances, such as calcitonin gene related neuropeptide and substance P, and the release of these vasodilative substances may lead to dilatation of small vessels and increase skin blood flow at the periphery. 10,11 Intramuscular acupuncture (not targeted at MTrPs) has been shown to induce vasodilatation and increase skin blood flow in healthy subjects and patients with fibromyalgia. 12,13 In view of the sympathetic DRR EMG LDF MTrP PPT RMANOVA VAS List of Abbreviations dorsal root reflex electromyograph laser Doppler flowmetry myofascial trigger point pressure pain threshold repeated-measures analysis of variance visual analog scale

2 326 SKIN BLOOD FLOW AT MYOFASCIAL TRIGGER POINTS, Zhang hyperactivity at MTrPs, 7 we therefore propose that painful stimulation of latent MTrPs could decrease skin blood flow compared with non-mtrps. Skin blood flow is usually measured with LDF and thermography. 14 LDF provides a semiquantitative assessment of microvascular blood perfusion, which is expressed in arbitrary units. LDF measurements from the skin reflect blood flow in capillaries, arterioles, venules, and dermal vascular plexuses. LDF measurements also reflect a large thermoregulatory aspect of perfusion, 15 whereas thermography reflects a local warming reaction depending mainly on increased blood flow in subcutaneous tissues. 16 The role of LDF compared with thermography in the measurement of skin blood flow is still controversial. The changes in skin temperature and skin blood flow have been reported to be inconsistent after cold stimulation, transcutaneous electrical nerve stimulation, and intramuscular acupuncture The sensitivity of LDF compared with thermography in the measurement of skin blood flow after intramuscular nociceptive stimulation needs further evaluation. The aims of this human study were to determine the effect of intramuscular glutamate injection into latent MTrPs and non- MTrPs on skin blood flow and to evaluate the relative sensitivity of LDF and thermography in the measurement of skin blood flow after glutamate injection into latent MTrPs. METHODS Subjects Fifteen healthy subjects (11 men and 4 women; mean age, y) without signs or symptoms of musculoskeletal pain in any body regions were recruited in this study. The study was conducted in accordance with the Declaration of Helsinki, approved by the local Ethics Committee (N ). Written informed consent was obtained from all participants prior to inclusion. The experiments were performed in a comfortable, quiet room with a temperature of 22 C to 23 C. Experimental Protocol This study consisted of 2 sessions with an interval of 1 week in between. A bolus of glutamate was injected into a latent MTrP or a non-mtrp in the left or right brachioradialis muscles in a randomized order in separate sessions. The subjects were asked to sit in front of a table with both forearms resting on the cushion on the table. Subjects were then asked to relax both forearms with both hands kept in a neutral position (fig 1). In each session, a tender point on digital palpation within a taut muscle band (a latent MTrP) or a nontender point on digital palpation out of a taut muscle band (a non-mtrp) in the middle region of the muscle was marked on the skin. Surface and intramuscular EMG activity were recorded before glutamate injection. A bolus of glutamate was injected into the muscle via the EMG-guided injection needle in order to induce muscle pain. The injection duration was seconds. After injection, the subjects were instructed to report the pain intensity on a VAS every 1 minute for 8 minutes. PPTs were measured at the injection point on the ipsilateral side and at the mirror point on the contralateral side before and 10 minutes after pain. Skin blood flow and skin temperature in both forearms were measured respectively with LDF and thermography before, during, and after pain. Electromyograph-Guided Intramuscular Injection The method of EMG-guided intramuscular injection was used in the current study based on the fact that the area of an MTrP is limited to a small area of 1 to 2 mm 2. 2 Thus, a small Fig 1. The position of the forearms and hands and the points extracted for analysis. The hands were kept in the neutral position. Skin blood flow and temperature at the local area, midarea, and presumed referred pain area (first web) were extracted for analysis. dose (0.1mL) of glutamate a was chosen to be injected into a latent MTrP and a non-mtrp to limit the amount of the solution within a targeted area. The EMG-guided injection needle b ( mm) was preconnected with an extension tube c (15cm, filling volume 0.2mL) and again connected to a syringe c (1mL volume) filled with the solution (0.3mL) to be injected. The volume finally injected was 0.1mL. After identification and marking of a latent MTrP or a non-mtrp, the skin was prepared. One pair of bipolar surface EMG electrodes b (intraelectrode distance, 2cm) was mounted 2cm rostral to the marked point. The surface electrodes were used to monitor for the relaxation of the muscle before injection. A reference surface electrode to the EMG-guided injection needle was placed 2cm lateral to the marked point. A ground electrode was wrapped around the ankle. The extension tube and the syringe were fixed to the cushion in order to avoid the needle displacement during the process of manual injection. The subjects were asked to relax both arms and fingers. Then the needle was advanced slowly into the muscle through the marked point. When there was a spontaneous electric activity 2,3 from the EMG-guided injection needle (fig 2, upper trace) and no EMG activity from the surface electrodes (fig 2, lower trace) as displayed on the monitor, d this tender point was defined as a latent MTrP. A nontender point was defined as a non-mtrp when there was neither spontaneous electric activity from EMG-guided injection needle recording nor from the surface electrode recording. The resting spontaneous electric activity was recorded for around 5 to 10 seconds before each injection. The EMG signals were amplified at a gain of 100 V/div and filtered (5Hz 5kHz) and sampled at 2kHz. Assessment of Muscle Pain Intensity and Pressure Pain Threshold Muscle pain intensity was scored on a 10cm VAS where 0 indicated no pain and 10 maximal pain. An electronic pressure algometer e with a 1-cm 2 rubber-tipped plunger mounted on a force transducer was used to measure the PPT. PPT was calculated as the mean of 3 trials with a 40-second interval between repetitions. The pressure was increased at a rate of 30 kpa/s until the subject detected the pain threshold or until the pressure applied reached an upper limit of 800 kpa. PPT was

3 SKIN BLOOD FLOW AT MYOFASCIAL TRIGGER POINTS, Zhang 327 RESULTS Fig 2. Resting intramuscular (upper trace) and surface (lower trace) EMG activity from latent myofascial trigger points. measured at the injection point and the mirror point on the contralateral side before and 10 minutes after pain. As a result of the intraindividual variability between right and left sides in prepain PPTs, postpain PPTs were normalized to the prepain PPTs, and the normalized values were used for statistical analysis. Assessment of Skin Blood Flow Skin surface over both forearms between the elbow and the metacarpophalangeal joint was scanned with LDF f to measure skin blood flow. The measurement was performed prepain; after the preparation of the skin; at 1, 3, 5, and 7 minutes during pain; and 10 minutes postpain. The image resolution was obtained at pixels with a speed of 4ms/pixel. Each scan lasted 2 minutes. Bandwidth was set at 250Hz to 15kHz. Skin blood flow (perfusion unit) values from the local area (2mm medial to the injection point), midarea (middle point of the forearm), and presumed referred pain area (the first web) were extracted for later analysis. Skin blood flow values from 3 mirror points on the contralateral side of the forearm were also extracted. The measurement area was 0.5cm 2. Skin blood flow was normalized to the prepain values, and the normalized values were used for statistical analysis. Assessment of Skin Temperature The skin temperature from the level of the elbow to metacarpophalangeal joint in both forearms was assessed with thermography. g The measurement of skin temperature was performed simultaneously with the measurement of skin blood flow prepain; after the preparation of the skin; at 1, 3, 5, and 7 minutes during pain; and 10 minutes postpain. The temperature resolution was 0.1 C. Skin temperature ( C) was measured at the same areas as those used in the measurement of skin blood flow in both forearms. Skin temperature was normalized to prepain values, and the normalized values were used for statistical analysis. Statistical Analysis A 2-way RMANOVA was applied to compare the differences in VAS, PPT, blood flow, and skin temperature. The 2 factors in RMANOVA were time (different time after injection) and point (latent MTrPs and non-mtrps). The Tukey test was used for post hoc comparison. The chi-square test was used to compare differences in the incidence of referred pain after injections into latent MTrPs and non-mtrps. Values in the text and figures are expressed as means SDs. The significance level was set at P.05. Pain Intensity, Referred Pain, and Subjective Responses After Intramuscular Glutamate Injection Visual analog scale scores were significantly increased over time after glutamate injection (F 9.52; P.005). VAS scores after glutamate injection into latent MTrPs were significantly higher than into non-mtrps (fig 3;F 7.16; P.05). There was a significant interaction (F 3.40; P.005) between these 2 factors (point and time). Post hoc analysis showed that glutamate injection induced higher VAS at latent MTrPs than at non-mtrps at 1, 3, 5, and 7 minutes during pain, and 10 minutes postpain (Tukey test, all P.05, respectively). Referred pain was observed at 13 of 15 latent MTrPs after glutamate injections, whereas there was no referred pain after glutamate injections into non-mtrps ( ; P.01). Referred pain was reported to the ipsilateral dorsum of hand and the first web in 7 of 15 latent MTrPs, to ipsilateral palm in 2 of 15 latent MTrPs, to ipsilateral fingers in 2 of 15 latent MTrPs, and to ipsilateral forearm in 2 of 15 latent MTrPs. Of the 15 subjects, both dizziness and sweating were observed in 2 subjects and dizziness only in 1 subject after glutamate injection into latent MTrPs. No such responses were observed after glutamate injection into non-mtrps. Pressure Pain Threshold After Glutamate Injection Into Latent Myofascial Trigger Points and Nonmyofascial Trigger Points On the ipsilateral side, PPT was significantly decreased postpain compared with prepain (F 9.33; P.005). PPT was significantly lower after glutamate injection into latent MTrPs than into non-mtrps (fig 4A; F 11.41; P.005). There was a significant interaction (F 11.41; P.005) between these 2 factors (point and time). Post hoc analysis revealed that PPT was decreased significantly after glutamate injection into latent MTrPs (q 6.42; P.005), but not into non-mtrps (q 0.18; P.897). However, no significant difference in PPT was found at the mirror point on the contralateral side after glutamate injection into either latent MTrPs or non-mtrps (for point, F 0.427, P.523; for time, F 0.403, P.535; fig 4B). Fig 3. Pain intensity measured with VAS after EMG-guided intramuscular injection of glutamate into latent MTrPs and non-mtrps. *P<.05 compared with non-mtrps.

4 328 SKIN BLOOD FLOW AT MYOFASCIAL TRIGGER POINTS, Zhang Fig 4. PPT before and after pain induced by glutamate injection into latent MTrPs and non-mtrps on the (A) ipsilateral and (B) contralateral sides. *P<.05 compared with PPT prepain. Skin Blood Flow After Glutamate Injection Into Latent Myofascial Trigger Points and Nonmyofascial Trigger Points At the local area on the ipsilateral side, skin blood flow was increased significantly after glutamate injection (F ; P.005) and was higher at non-mtrps than at latent MTrPs (fig 5A; F 5.980; P.05). There was a significant interaction (F 3.644; P.005) between these 2 factors (point and time). At the contralateral mirror area, skin blood flow was also significantly higher after injection into non-mtrps than into latent MTrPs (fig 5B; F 4.705; P.05). Post hoc comparisons showed that skin blood flow was increased significantly over time after glutamate injection into latent MTrPs and non- MTrPs at 1, 3, 5, and 7 minutes during pain and 10 minutes postpain at the local injection area (all P.05). The increase in skin blood flow at contralateral mirror area was observed only after glutamate injection into non-mtrps at 1, 3, 5, and 7 minutes during pain (all P.05). Skin blood flow was significantly higher at non-mtrps than at latent MTrPs at 1, 3, 5, and 7 minutes during pain and 10 minutes postpain on both ipsilateral and contralateral sides (all P.05). At the middle point of the forearm on the ipsilateral side, skin blood flow was significantly higher after injection into non-mtrps than into latent MTrPs (F ; P.05). Post hoc analysis revealed that skin blood flow was significantly increased after glutamate injection into non-mtrps (all P.05), but was decreased after glutamate injection into latent MTrPs at 1, 3, 5, and 7 minutes during pain and 10 minutes postpain (all P.05). Furthermore, skin blood flow was higher at non-mtrps than at latent MTrPs at 1, 3, 5, and 7 minutes during pain and 10 minutes postpain on the ipsilateral side and at 5 and 7 minutes during pain and 10 minutes postpain on the contralateral side (fig 5C and D; all P.05). At the presumed referred pain area, no significant difference in skin blood flow was found after glutamate injection into either latent MTrPs or non-mtrps (for point, F 1.378, P.260; for time, F 0.850, P.535) on both the ipsilateral and contralateral sides (for point, F 2.712, P.122; for time, F 1.411, P.22). However, there was a significant interaction between the 2 factors on both the ipsilateral side (fig 5E; F 4.057; P.005) and contralateral side (fig 5F; F 3.292; P.01). Post hoc comparisons showed that skin blood flow was increased significantly after injection into non-mtrps at 3, 5, and 7 minutes during pain and 10 minutes postpain on the ipsilateral side, and at 3, 5, and 7 minutes during pain on the contralateral side compared with prepain (all P.05). Skin blood flow was also significantly higher at non-mtrps than at latent MTrPs at 3, 5, and 7 minutes during pain and 10 minutes postpain on both the ipsilateral and contralateral sides (all P.05). Typical pictures of LDF of the forearm after glutamate injection into latent MTrPs and non-mtrps were shown in figures 5G and H. Skin Temperature After Glutamate Injection Into Latent Myofascial Trigger Points and Nonmyofascial Trigger Points At all measured areas on both the ipsilateral and contralateral sides, there was neither significant difference in skin temperature after glutamate injection compared with prepain nor significant difference between glutamate injection into latent MTrPs and non-mtrps. No interaction was found between the 2 factors (point and time) at all measured areas on both sides (fig 6; all P.05). DISCUSSION The present study demonstrated that glutamate injection into latent MTrPs evoked higher pain response and lower PPT than injection into non-mtrps (both P.05). Furthermore, glutamate injection into non-mtrps increased skin blood flow at the local area, midarea, and presumed referred pain area in both forearms (all P.05). An attenuated or decreased skin blood flow was observed after glutamate injection into latent MTrPs compared with non-mtrps (all P.05). Glutamate injection induced pronounced changes in skin blood flow measured with LDF, but no concurrent changes in skin temperature were detected with thermography. Nociceptive Hypersensitivity at Latent Myofascial Trigger Points In this current study, higher pain intensity and lower PPT after glutamate injection into latent MTrPs than into non- MTrPs indicated that nociceptors at latent MTrPs could be

5 SKIN BLOOD FLOW AT MYOFASCIAL TRIGGER POINTS, Zhang 329 Fig 5. Skin blood flow at the local area (A and B), midarea (C and D), and presumed referred pain area (E and F) on the ipsilateral and the contralateral side before, during, and after pain induced by glutamate injection into latent MTrPs and non-mtrps. Typical laser scanning flowmetry of the forearm after glutamate injection into latent MTrPs (G) and non-mtrps (H). *P<.05 compared with latent MTrPs. P<.05 compared with prepain. Abbreviations: SP, skin preparation;, Injection site.

6 330 SKIN BLOOD FLOW AT MYOFASCIAL TRIGGER POINTS, Zhang Fig 6. Skin temperature at the local area (A and B), midarea (C and D), and presumed referred pain area (E and F) on the ipsilateral and the contralateral side before, during, and after pain after glutamate injection into latent MTrPs and non-mtrps. Abbreviation: SP, skin preparation.

7 SKIN BLOOD FLOW AT MYOFASCIAL TRIGGER POINTS, Zhang 331 hypersensitive. This result is concurrent with 1 recent study. 9 This notion is also supported by other studies showing higher mechanical pain sensitivity at latent MTrPs than at a control point. 7,8,21,22 The higher pain scores and increased mechanical pain sensitivity after glutamate injection into latent MTrPs may be a result of the higher concentrations of algogenic substances at the MTrPs. 8,22 Skin Blood Flow Response to Nociceptive Stimulation of Nonmyofascial Trigger Points Skin blood flow on both sides of the forearms was significantly increased after glutamate injection into non-mtrps in the brachioradialis muscle. Glutamate is an important excitatory neurotransmitter of the primary afferents fibers and can serve as a noxious stimulus to induce hyperalgesia and allodynia. 23,24 Vasoactive neuropeptides, such as calcitonin gene related peptide and substance P, are released from peripheral terminals after noxious stimulation and evoke local vasodilatation. 10,11 The increased blood flow at the local area after glutamate injection may rely on the local release of vasoactive neuropeptides from peripheral terminals of nociceptors. Consistent with our results, intramuscular needle stimulation in healthy subjects and patients with fibromyalgia induces vasodilatation and an increase in skin blood flow. 12,13,25 In the present study, the glutamate injection into non-mtrps also increased skin blood flow at the middle point of the forearm and presumed referred pain areas on both forearms. In addition to vasodilation induced by local vasoactive neuropeptides released after noxious stimulation, DRRs triggered by excessive depolarization of the central terminals of primary afferent in the spinal dorsal horn play a critical role in inflammatory peptide release. 26 Thus, skin blood flow response to noxious stimulation at the middle point of the forearm and presumed referred pain areas may rely on the antidromic axon reflexes and/or DRRs. 10,26,27 Electric stimulation of C fiber can activate C fiber on the contralateral side. 28 Blocking of peripheral nerve, 28,29 but not the venous system of the ipsilateral or contralateral side, 28 blocks the contralateral response, indicating that the contralateral response is probably mediated by a neural mechanism rather than a systemic or circulatory effect. However, it is noteworthy that the occurrence of contralateral response may be related to the strength of stimulus. 30 This may explain the occurrence of contralateral response in the present study (needle insertion and chemical glutamate stimulation) and the lack of such response after acupuncture alone in a previous report. 13 Attenuated Skin Blood Flow Response to Nociceptive Stimulation of Latent Myofascial Trigger Points In the present study, skin blood flow after glutamate injection into latent MTrPs was significantly less increased or decreased than into non-mtrps. This may indicate that the sympathetic vasoconstriction mechanism was activated after glutamate injection into latent MTrPs. Our study provides further evidence that MTrPs might be associated with an increased sympathetic vasoconstriction activity in the skin. The blood vessels in the skin have been shown to be under control by 2 antagonistic systems: sympathetically maintained vasoconstriction and antidromic vasodilation mediated by the unmyelinated and thinly myelinated primary afferents. 10,31 The activation of sympathetic nervous system induces vasoconstriction. For example, the section of lumbar sympathetic trunk results in pronounced increase in cutaneous blood flow, 32 and maneuvers that increase the sympathetic outflow to the skin diminish electrically evoked antidromic vasodilatation. 33 Recent evidence shows that sympathetic efferents might participate in the modulation of the sensitivity of primary afferent nociceptors in the periphery, which in turn affect the induction and development of neurogenic inflammation. 34 However, sympathetic vasoconstriction overrides antidromic vasodilatation during simultaneous stimulation of both afferents and sympathetic fibers. 35,36 In the current study, we observed a significantly attenuated increase in skin blood flow at the local area on the ipsilateral side and a trend toward decrease in skin blood flow at other areas of the forearms after the glutamate injection into latent MTrPs. These results suggest that nociceptive stimulation of latent MTrPs could activate the sympathetic vasoconstriction mechanism and antagonize the primary afferent nerve-induced antidromic vasodilation. Subjective responses, such as dizziness and sweating, after the glutamate injections into latent MTrPs may suggest a generalized activation of the sympathetic nervous system. This attenuated skin blood flow response associated with latent MTrPs may provide 1 of the explanations of the autonomic phenomena observed in patients with myofascial pain, such as vasoconstriction (blanching), coldness, sweating, and so forth. Changes in Skin Temperature After Nociceptive Stimulation of Latent Myofascial Trigger Points and Nonmyofascial Trigger Points Glutamate injection into latent and non-mtrps did not induce significant changes in skin temperature in the current study. This could indicate that the thermography is less sensitive than LDF in the assessment of skin blood flow. A previous study has shown that increased blood flow and skin temperature were reported over all tender points after acupuncture treatment. 17 However, lowfrequency transcutaneous electrical nerve stimulation in healthy volunteers increased blood perfusion measured with LDF but not skin temperature. 19 Acupuncture and moxibustion in healthy volunteers were reported to decrease the blood flow, without effect on the skin temperature. 18 Concurrent with these findings, our results suggested that LDF was more sensitive than thermography to reflect the skin microcirculation after intramuscular nociceptive stimulation. Study Limitations The present study is limited by its small sample size and the lack of blinding. As shown in figure 5, the SDs of the average values for skin blood flow and skin temperature are large; thus the statistical power is relatively weak (30.7%) in the current study. A sample size of at least 28 subjects in each group might be necessary to have greater statistical power ( 50%) for a definitive study to evaluate further the changes in skin blood flow associated with latent MTrPs. Another limitation of the current study is that the processes of data acquisition and data analysis were not blind, which might influence the accuracy of the results. Additionally, current results might not adequately reflect the actual changes in skin blood flow in patients with active MTrPs. Further studies on skin blood flow in patients with active MTrPs are needed. CONCLUSIONS Skin blood flow response was attenuated after intramuscular nociceptive stimulation of latent MTrPs compared with non- MTrPs, suggesting increased sympathetic vasoconstriction activity at latent MTrPs. Additionally, LDF was more sensitive than thermography in the detection of the changes in skin blood flow after intramuscular nociceptive stimulation.

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