In clinical practice, myofascial pain syndrome is a common muscular pain syndrome resulting from myofascial trigger points(MTrPs).1-10 MTrP has been defined as a hyperirritable spot within a taut band of skeletal muscle fibers that is painful on compression and that can give rise to characteristic referred pain, tenderness, tightness, and sometimes, local twitch response and autonomic phenomena.3-10
The major goal of MTrP therapy is to relieve pain and tightness of the involved muscles. The procedures of treatment include intermittent cold and stretch, postisometric relaxation, thermotherapy, massage therapy, trigger point injections, and the elimination of causative or perpetuating factors, if any.4-12
Electrotherapy has been considered as an effective therapeutic modality to relieve pain from myofascial trigger points.6, 13, 14 Two major types of electrical stimulation therapy on soft tissue lesions are electrical nerve stimulation (ENS) and electrical muscle stimulation (EMS). ENS is the application of electrical current on the peripheral nerve by applying lower intensity of electrical current, and EMS is the application of electrical current with stronger intensity directly to the involved muscle. In general, ENS is used for pain relief, such as transcutaneous nerve stimulation (TENS),15 and EMS can be used to enhance muscle circulation, reduce muscle spasm, eliminate muscle pain, and increase muscle strength.11, 14, 16-18 Electrotherapy alone has been used to treat MTrP, although it is reported not as effective as thermotherapy or intermittent cold with stretching.6 Unfortunately, controlled blind studies on the effectiveness of electrotherapy specifically for MTrP treatment have been rare in the literature. In a study of TENS on MTrP treatment, Graff-Radford, Reeves, Baker, et al.19 found that low frequency TENS was not effective in pain relief, but high frequency TENS could effectively relieve MTrP pain although no change in local MTrP sensitivity (threshold) could be found. Lee, Lin, and Hong14 found that the range of stretch of upper trapezius muscle was significantly increased immediately after the application of combined electrotherapy (EMS) and ultrasound thermotherapy to the MTrP, but was not significantly changed by EMS only. In clinical practice, it seems that EMS (such as interferential current or functional electrical stimulation) is more effective for long-term treatment of MTrPs than ENS.11 This study is designed to investigate the effectiveness of electrical nerve stimulation and electrical muscle stimulation on the immediate relief of pain and tightness in myofascial trigger point.
MATERIALS AND METHODS
Sixty patients (25 males and 35 females) with MTrPs in one side of the upper trapezius muscles were involved in this study. The mean age was 44.4± 13.9 yr. They were recruited consecutively from the outpatient clinic of a university hospital in a period of 1 yr. Exclusions for this study included patients with (1) age of less than 18 yr or more than 80 yr, (2) acute or serious illness, (3) mental retardation, (4) neurologic deficits involving the investigated upper limb, and (5) advanced osteopathic or arthropathic disorder of the cervical spine or the shoulder of the investigated side. Additionally, the patients should have had no therapy, such as physical therapy or injection therapy, within the last 2 mo on MTrPs selected for this study. After the patients were informed of the procedure, they signed consent forms.
Diagnosis of a MTrP was based on the following criteria: (1) a most sensitive (tender) spot (MTrP) in a palpable taut band of the upper trapezius muscle could be identified; (2) compression of this MTrP could reproduce the patient's usual complaint (recognized pain); (3) a typical referred pain pattern to ipsilateral and posterolateral cervical, and posterior upper thoracic paraspinal areas could be elicited by compression of this MTrP; and(4) a local twitch response could be elicited by snapping palpation of this MTrP. Examination for active MTrPs of upper trapezius muscles was performed and described by one of the authors (C.-Z.H.) who had more than 10 years experience in the diagnosis and treatment of MTrP.
The 60 subjects were divided randomly into three groups. Eighteen patients(8 males and 10 females, mean age 41.4 ± 13.0 yr) in group A were treated with placebo. Group B consisted of 20 patients (9 males and 11 females, mean age 42.7 ± 13.8 yr) who were treated with ENS therapy. The remaining 27 patients (8 males and 14 females, mean age 44.4 ± 14.5 yr) in group C were treated with EMS therapy. The patients were not informed about the type of treatment to be given. Measurements of subjective pain intensity, pain threshold, and range of motion of lateral bending to the opposite side were performed before and immediately after treatment by another author (T.-C.H.) who was blind to the assignment of groups.
Measurement of Effectiveness
Pain Intensity (PI) of MTrP
Pain intensity was assessed by the Visual Analog Pain Scale, which was a card with an uncalibrated scale ranging from zero to ten on one side (with zero representing no pain and ten representing the worst pain in life) and a corresponding 10-cm ruler on the other (with each centimeter representing one pain level). The patient subjectively estimated his or her pain level by moving the pointing device along the uncalibrated scale between zero and ten. Then the exact value of pain intensity could be obtained by referring the uncalibrated scale to the ruler on the back side.
Pressure Pain Threshold (PT) of MTrP
The procedure of pressure pain threshold measurement recommended by Fischer20-24 was applied in this study. First, the procedure was explained clearly to the patient. Then the patient, in the sitting position, was made comfortable and encouraged to maintain complete relaxation. When a MTrP of the upper trapezius muscle was identified, it was marked with an indelible marker so that the next measurement would be performed over the same area. A pressure algometer, the pressure threshold meter, was applied on this marked area with the metal rod perpendicular to the surface of the skin. The pressure of compression was increased gradually at a speed of 1 kg/s approximately. The patient was asked to say “yes” when he or she began to feel pain or any discomfort. The compression stopped as soon as the patient said “yes.” The patient was asked to remember this level of pain discomfort and to apply the same criterion for the next measurement. Three repetitive measurements at an interval of 20-60 s were performed at each site. The average values of the three readings were used for data analysis of the pain threshold measurement.
Range of Cervical Motion (ROM)
The range of motion of the cervical spine was measured as the amount of lateral bending to the opposite side of the involved upper trapezius muscle by a large scale goniometer.
Group A patients were given placebo “sham electrotherapy.” The patient was told that a certain type of therapy would be given to treat MTrPs, but was not told what treatment was to be given. The electrode pads were applied on the upper trapezius muscle as in groups B or C. Then the patient was treated with the current intensity set at 0.0 mAmp. The electrode pads were removed 20 min later for “after therapy” measurement.
Using a TENS unit, the negative electrode was placed on the MTrP of the upper trapezius muscle and the positive one was on its acromial tendon insertional site for group B patients. The current frequency was set at 60 Hz and the intensity was at a level that the patient could feel but was not strong enough to induce muscle contraction. The total duration of application was 20 min.
A functional electrical muscle stimulation machine was used to treat the patients in group C. The negative electrode was placed on the MTrP of the upper trapezius muscle and the positive one was on the acromial tendon insertional site. The current frequency was set at 10 Hz and the intensity was at a level that allowed the contraction of the upper trapezius muscle to be visible. The total duration of application was also 20 min.
Subgroups According to Initial Pain Intensity
Patients in each group were subdivided into two additional subgroups based on the initial pain intensity before therapy. Groups A1, B1, and C1 included 20 patients who had severe pain (before therapy) with initial pain intensity≥ 7, and groups A2, B2, and C2 consisted of 40 patients who had mild to moderate pain with initial pain intensity < 7. This criteria of division was based on our clinical observation that most patients reported PI of ≥ 7 when they complained of severe pain.
The measured values for the assessment of therapeutic effectiveness were standardized as the percentage of improvement calculated by the followingformula: The statistical analysis was based on analysis of variance compared with the differences among groups. A P value < 0.05 was considered statistically significant.
Comparison Among Groups
The immediate effectiveness of therapy in each groups is shown inTable 1. The improvement in PI or PT was significantly greater (P < 0.01) in group B than group A or C. On the other hand, the improvement in ROM was significantly more (P < 0.01) in group C than in group A or B.
Comparison Among Subgroups with Severe Pain (PI ≥ 7)
There was a total of 20 patients with severe MTrP pain with the pain intensity of ≥ 7. Four of five patients in group A1 and all seven patents in group B1 with severe pain had reduced pain intensity after therapy, and the remaining patient in group A had no change in pain intensity after therapy. In group C1, six of eight patients had increased pain intensity after treatment with EMS, and the remaining two patients had unchanged pain intensity after therapy.
The percentages of changes in PI, PT, and ROM after therapy in each subgroup are shown in Table 2. The results of statistical analysis were similar to that overall in patients shown inTable 1.
Comparison Among Subgroups with Mild-Moderate Pain (PI< 7)
Forty patients with initial pain intensity < 7 were considered as having mild to moderate pain. Four of 13 patients in group A2, all patients in group B2, and 13 of 14 patients in group C2 had reduced pain intensity after therapy. The remaining patient in group A2 and two patients in group C2 had no change in pain intensity after therapy.
Table 3 shows the percentages of changes in PT, PT, and ROM after therapy in each subgroup. In groups B2 and C2, PI was significantly more reduced (P < 0.05) than in group A2, and the improvement of PI in group B2 was greater than in group C2. Only patients in group B2 had significant improvement of PT (P < 0.05), compared with group A2. Patients in group C2 had significantly greater improvement in ROM (P< 0.05) than group A2 or B2. There was no significant difference(P > 0.05) in ROM changes between group A2 and group B2.
The important findings in this study included: (1) ENS was effective in reducing pain intensity and increasing the pain threshold of MTrP (no matter how severe the initial pain), but was not effective in improving the range of stretch in the muscles with MTrPs; (2) EMS was effective in reducing the tightness of the muscles with MTrPs (no matter how severe the initial pain), and was effective in reducing the pain intensity of MTrPs only for patients with mild to moderate pain, but could not change the pain threshold of MTrPs; and (3) for immediate pain relief, ENS was more effective than EMS; but for improvement of range of motion, EMS was more effective than ENS.
The pathophysiology of MTrP is still unknown. Simons5, 25 has proposed a hypothesis of“energy crisis” in the muscle fibers of a MTrP region to explain the mechanism of taut band formation. Recent studies suggested that the mechanism of MTrP is closely related to dysfunctional end plates25, 26 with the interaction of the spinal cord mechanism.27-31 Based on the observation of spontaneous electrical activity recorded from the active locus of a MTrP, it was suggested that excessive acetylcholine release in the dysfunctional end plate might be responsible for taut band formation.25 With the perpetuation of an energy crisis, the taut band may exist persistently unless the vicious cycle is broken by certain treatment.
With low intensity current stimulation in ENS therapy, the low-threshold large afferent fibers can be stimulated selectively. No pain sensation would be elicited by ENS therapy because the small pain fibers are not stimulated. The selective stimulation of large afferent fibers could suppress the pain impulses from small nociceptive fibers through the “gate control” mechanism.32-34 Apparently ENS is primarily a“pain control” therapy. The effect of ENS to relieve muscle tightness, if any, is probably a secondary response to pain relief.
In our study, we found that EMS could reduce muscle tightness in all cases, but could not relieve muscle pain in severe cases. For MTrP therapy, EMS involves currents that cause visible muscle fiber contraction. The rhythmic muscle activity may improve local circulation to increase the oxygen and energy supply and to break the vicious cycle of the energy crisis in MTrP, and, subsequently, may cause the relief of the pain and the tightness of muscles with MTrPs.14 However, because of the strong current stimulation by EMS, not only the large fibers but also the small nociceptive fibers are stimulated. When a patient had severe MTrP pain, the strong electrical current used during EMS treatment might aggravate nociceptive impulses to increase pain sensation. The effect of the “gate control” mechanism would be masked and the counterirritation mechanism(hyperstimulation analgesia)35 for apparent pain relief might be unremarkable in cases of severe MTrP pain.
By comparing the immediate effectiveness of three different therapeutic modalities on MTrP, Lee, Lin, and Hong14 found that electrical muscle stimulation was not effective in reducing pain intensity, increasing pain threshold, or increasing range of motion of the involved muscles. The electrotherapy technique used in their study was a combination of alternating and direct currents and only mild muscle twitching was noticed during therapy, instead of the vigorous muscle contraction observed in our study. It seems that vigorous muscle contraction is a critical factor in the determination the effectiveness of EMS.
Inasmuch as the ENS is more effective for pain relief and EMS is better for relief of muscle tightness, a combination of these two modalities may be a better approach to MTrP treatment. In a recent controlled, double-blind study by Moor and Shurman,17 it was found that a combination of TENS and neuromuscular electrical stimulation (NMES; i.e., EMS) could produce greater pain reduction than TENS or NMES alone for patients with chronic back pain.
Unfortunately, the long-term effectiveness of electrotherapy on MTrP was not investigated in this study. To our knowledge, no well controlled, double-blinded study of the long-term effect of a physical medicine modality on MTrPs has been reported in the English literature. In our clinical experience, most MTrP treatment usually can provide only a short-term effect of pain relief. The duration of effectiveness is varied from case to case. This is probably because most MTrPs are caused by or associated with certain underlying pathologic lesions, such as ligament sprain, tendinitis, arthritis, etc.36, 37 In case of a severe pathologic process, the “induced” or associated MTrPs may be inactivated for only a short period after MTrP treatment. On the other hand, the pain of MTrPs caused by or associated with a mild pathologic lesion may be relieved for a longer period after treatment. To obtain long-term relief of MTrP pain, the underlying pathologic lesion should be treated appropriately.37 Therefore, it is very difficult to investigate the long-term effectiveness of MTrP treatment without an appropriate control of the variables related to the underlying pathology. It is necessary to address this issue in a future study.
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