Effects of helium-neon laser irradiation on skin resistance and pain in patients with trigger points in the neck or back.

Effects of Helium-Neon Laser Irradiation on Skin Resistance and Pain in Patients with Trigger Points in the Neck or Back A trigger point is defined as a point that demonstrates referred pain upon deep palpation as well as decreased skin resistance when compared with adjacent tissues. [1-3] Skin resistance has been used since the 1930s to determine sites of pathological conditions and is defined as the resistance offered to the passage of an electrical current (direct current) through the skin. Skin resistance reflects the activity of the sympathetic nervous system. [4, 5] Areas of decreased skin resistance correlate well with a description of pain pattern. [4, 5] A return to normal skin resistance, therefore, may indicate the resolution of a pathological condition and may be accompanied by decreased pain. [4, 5] Riley and Richter reported that skin resistance could be used to localize the spinal level of nerve irritation, because low skin resistance patterns observed prior to diskectomy were absent postsurgically. [5]

The helium-neon (He-Ne) laser is monochromatic and emits a visible, red laser beam with a wavelength of 632.8 nm that penetrates the skin to a depth of 0.8 mm without divergence and offers some divergence up to 10 to 15 mm. A therapeutic dosage of 15 to 20 seconds at a maximum intensity of 0.95 mW produces a low-level output (14-29 mJ) that will not produce tissue heating. [6]

Known as "cold" or "soft" lasers because of their lack of heat-producing capabilities, He-Ne lasers have been used recently in Canada and Europe in the treatment of a large range of pathological conditions including enhancement of wound healing, reduction of pain in patients with arthritis, and management of orthopedic problems. [7-9] From 1970 until the early 1980s, cold laser manufacturers claimed all of the previously mentioned therapeutic capabilities including its use in the elimination of trigger points, but reported research consisted primarily of uncontrolled, unsubstantiated, and equivocal findings. [8-10]

Since the early 1980s, laser research has improved. [7-10] Investigators have discussed the nonthermal effects of He-Ne laser irradiation. Walker reported success with the laser in relief of chronic pain. [9] The relief of pain was accompanied by an increase in the urinary excretion of 5-hydroxy-indoleacetic acid, a by-product of serotonin. Subsequently, she concluded that laser irradiation may have an effect on serotonin metabolism, thereby serving as a mechanism of pain relief. Snyder-Mackler et al found that three short-duration HE-Ne laser treatments normalized skin resistance in patients with musculoskeletal trigger points. [11] Results of a double-blind investigation by Goldman et al examining the effects of neodymium laser irradiation on rheumatoid arthritis revealed a significant reduction in erythema and pain over a 13-session course of treatment. [8] Most recently, a double-blind study by Snyder-Mackler and Bork determined that He-Ne laser treatment increased the distal sensory latency (corresponding to a decrease in the sensory nerve conduction velocity) of the superficial radial nerve in humans. [12] They hypothesized that this increase in sensory latency could be a mechanism for pain relief, and thus the HE-Ne laser may be a beneficial analgesic modality.

The purpose of this study was to ascertain the effects of He-Ne laser irradiation on skin resistance and pain in patients with associated trigger points in the neck or back. This study entailed a partial replication of a previous study by Snyder-Mackler et al, which determined that the use of the He-Ne laser increased skin resistance overlying a trigger point. [11] We hypothesized that cold laser treatment would have the effect of increasing skin resistance and decreasing pain overlying a trigger point and that a negative correlation would exist between these two variables.

Method

Subjects

Twenty-four patients exhibiting neck or low back pain (14 patients with low back pain and 10 patients with neck pain, distributed approximately equally between experimental and placebo groups) with an associated musculoskeletal trigger point were treated in either the Boston University physical therapy clinic or the Underwood-Memorial Hospital. They were randomly assigned to either a Laser Group (n = 13) or a Placebo Group (n = 11) by drawing 1 of 30 envelopes labeled "A" (treatment) or "B" (placebo). The patients may or may not have been receiving physical therapy prior to inclusion in this study. Informed consent was obtained prior to treatment. The protocol was approved by the Sargent College of Allied Health Professions, the Temple University, and the Underwood-Memorial Hospital human research review committees. Data regarding age and sex were not recorded.

Instrumentation

The Dynatron 1120 (*1) was the cold laser unit used in our study (Fig. 1). It is a Class II laser product; that is, it is a visible laser, safe for momentary viewing. [6] Protective eye wear is recommended for continuous exposure. This unit is also classified as a Class III medical device and is therefore cleared for investigational use only. [13] The same unit was used for the placebo treatment, for which no laser beam was emitted.

Procedure

Assessment. Data were collected for the Laser and Placebo Groups over three consecutive sessions, with at least one but not more than three days between treatments. A visual analog scale was used to assess the level of pain. As described by Elton et al, the visual analog scale correlates significantly with other forms of assessment and is quantifiable. [14] These investigators discuss the rationale for pain assessment, the important implications being the following: It offers a means of recording improvement, it facilitates expression of pain, and it allows the physical therapist to direct treatment. Furthermore, investigations by Huskisson, [15] Scott and Huskisson, [16] and Ohnhaus and Adler [17] indicate that the visual analog scale, when compared with other pain rating methods, is the most sensitive tool for assessment of what the patient actually feels as well as the pain intensity level.

After appropriate positioning of the patient, a single trigger point was localized by palpation. The patients were asked to identify the painful area, and the therapist then palpated the area to locate points that were locally tender. If referred pain was also elicited from one of these points, it was identified as a trigger point. Skin resistance was measured using the dermometer on the Dynatron 1120. Each patient held the reference electrode in the contralateral hand. The stylus of the dermometer was then held on the surface of the skin overlying the trigger point, and the resistance was read from the digital display of the device. The dermometer measures direct current resistance inversely in arbitrary units of 0% to 100%. The 100% value represents a totally open circuit (the absence of resistance), and the device is linear to 3,000 [Omega]. Intrarater reliability of this device was calculated as .94 from data obtained during a previous study of this population [11] using the 1, 1 formula of the interclass correlation coefficient. [6]

A 1-cm area around the apex of the trigger point was circled with indelible ink to locate the point for subsequent measurements and treatments. The visual analog scale was then used to assess pain. This scale is a 10-cm horizontal line without numbers. The left end corresponds to no pain, and the right end indicates severe pain. The patients indicated their perceived level of pain on the continuum.

Treatment. The posttest procedure was identical to the pretest procedure and was performed immediately after treatment with the laser or the placebo. Although data were collected for pretreatment and posttreatment skin resistance and pain for all three treatment sessions, data analysis was performed on pretreatment 1 and posttreatment 3 data only. [11]

Duration of treatment in the literature ranges from seconds to 30 minutes. [9, 10, 18] A duration of 20 seconds per square centimeter of tissue is used most commonly [14, 19] and was therefore used in our investigation. The laser is set to deliver a continuous beam at a maximum intensity of 0.95 mW for each trigger point. The stylus aperture rested on the skin without pressure during the delivery of the treatment because the recommended distances between skin and aperture range from 0 to 1 mm. [6, 10] Duration of treatment and distance between aperture and skin were chosen to approximate the way this laser is used in clinical practice (Fig. 2).

To preserve the double-blind format of this study, the experimenters conducting the treatments (AJB, AIP, MDS) were unaware as to which switch on the Dynatron activated the laser and they wore goggles that prevented viewing of the light. The patients were positioned prone or sitting so as to obscure viewing of the laser.

Following the pretest procedure, the patients received a 20-second laser or placebo application to the marked trigger point. Posttreatment skin resistance was then measured, and the visual analog scale completed. Any additional physical therapy was administered at this time. Twelve patients (6 Laser Group, 6 Placebo Group) received treatment of hot packs and high voltage pulsed current (HVPC) to the painful area (neck or low back). Thirteen patients (7 Laser Group, 5 placebo Group) received no additional treatment. The additional treatment was required by the Institutional Review Board of Underwood-Memorial Hospital.

Data Analysis

An unpaired t test indicated that a significant difference existed between the pretreatment skin resistance of the Laser and Placebo Groups (p [is less than] .001). A one-way analysis of covariance (ANCOVA), therefore, was used to analyze the data using the pretreatment skin resistance as a covariate. For the purpose of data analysis, the points indicated on the visual analog scale were converted to millimeters from the zero point to objectify pain assessment. An unpaired t test revealed no significant difference in pretreatment values between the two groups for the pain level variable; therefore, an analysis of variance (ANOVA) was used to analyze the differences in pain. A probability level of .05 was considered an acceptable level of statistical significance. Simple correlation between posttreatment skin resistance and pain was performed. Lastly, we used a linear regression to examine the changes between pretreatment and posttreatment skin resistance and pain to determine the predictability of pain difference based on the difference in skin resistance for both Laser and Placebo Groups. The BMDP (Biomedical Statistical Package) subprograms 1V and 3D were used for the ANOVA and ANCOVA and the unpaired t test, respectively. Subsets 1 and 2 of the RASS conversational statistical package were used to perform the correlation and the linear regression.

Results

An unpaired t test revealed a significant difference in the pretreatment skin resistance measurements between the two groups at the outset of the study. The mean pretreatment skin resistance for the Laser Group was 57.8% (s = 12.1%) and for the Placebo Group was 48.0% (s = 10.1%) (Fig. 3). An ANCOVA was used to analyze skin resistance with the pretreatment skin resistance as the covariate. The results of the one-way ANCOVA are summarized in Table 1. The adjusted group means for posttreatment skin resistance are reported in Table 2. A statisically significant difference (p [is less than] .001) was revealed between the posttreatment skin resistances in the Laser and Placebo Groups.

Results of the ANOVA indicated a significant reduction in pain for the Laser Group (p [is less than] .005) (Tab. 3). Pretreatment and posttreatment means for pain are reported in Figure 4. A correlation of .318 demonstrates a weak association between skin resistance and pain across subjects. Lastly the linear regression of the differences between pretreatment and posttreatment skin resistance and pain revealed no significant correlation with coefficients of .116 and .044 for the Laser and Placebo Groups, respectively (Fig. 5).

To determine whether the additional physical therapy had an effect, these same statistical tests were run separately on the patients who received additional treatment and the patients who did not receive additional treatment. Results of the ANCOVA and the ANOVA were significant (p [is less than] .01] for differences between the Laser and Placebo Groups within these subgroups.

Discussion

Our hypothesis that cold laser irradiation of musculoskeletal trigger points would have the effect of increasing skin resistance and decreasing pain overlying a trigger point was supported by our data. As previously mentioned, increases in skin resistance as compared with normal levels may indicate the resolution of a pathological condition. [4, 5] The increase in skin resistance substantiated the previous findings of Snyder-Mackler et al, [11] and these data may have implications in advocating a short course of He-Ne laser irradiation as a therapeutic modality in the treatment of musculoskeletal trigger points. The brevity of treatment duration and its ease of application make it a feasible adjunct to conventional physical therapy methods.

Most significantly, this study demonstrates pain reduction with He-Ne laser treatment of trigger points. As previously mentioned, the mechanisms by which the laser acts to decrease pain have not been fully elucidated in the literature. [7, 8, 10, 18, 19] Additional investigation may provide insight in this area.

Although the results revealed a significant increase in skin resistance in all of the Laser Group patients and a reduction in pain with 12 Laser Group patients reporting pain relief, neither a strong negative association nor a predictability in trends was found between these two variables. (The coefficients reported were positive because the dermometer measures the inverse of resistance resulting in decreasing values with increasing skin resistance.) This finding may be attributable to a lack of linearity in pain assessment across subjects, secondary to the subjectivity of individual perception of pain. From these findings, we might conclude that the visual analog scale may not be the ideal tool to assess trends of predictability across subjects, although no such indication was found in the literature. [14-17] Other intervening variables such as time of day, activity level, and stress level were not controlled for and subsequently may have influenced these data.

The effects of the additional physical therapy did not alter the results of the study. In addition, because 7 patients in the Laser Group received no additional treatment whereas only 5 patients in the Placebo Group received no additional treatment, we are confident that any benefit derived from the additional treatment would have benefited the Placebo Group more than the Laser Group. This fact also serves to strengthen our results.

We did not analyze the differences in skin resistance or pain for each treatment session, as in the previous investigation by Snyder-Mackler et al. [11] Analysis of the entire regimen rather than each treatment session better reflects the effect of a three-session course of treatment. Because a minimum of three treatment sessions is suggested to determine efficacy and a 10-session course of treatment is recommended for those who benefit from the three-session course of treatment, implementing a 10-session course of treatment and measuring both skin resistance and pain may be indicated. These additional trials may be necessary to ascertain whether a proportional association exists between skin resistance and pain in individual patients, and their implementation is thus a suggestion for further investigation.

Conclusions

Irradiation of musculoskeletal trigger points with the He-Ne laser over three consecutive treatment sessions increased skin resistance and reduced pain in a sample of patients with neck and low back pain with associated trigger points. Neither a correlation nor a proportional association, however, was demonstrated between these two variables across patients. This finding may be attributed to the lack of linearity in pain assessment. Therefore, additional investigation is suggested in this area. The normalization of skin resistance to that of surrounding tissue and reduction in pain may indicate the resolution of a pathological condition. Because these data substantiate the previous findings of Snyder-Mackler et al, [11] as well as demonstrate a reduction in pain, we propose that the He-Ne laser may be a beneficial adjunct to other traditional therapeutic modalities.

(*1) Dynatronics Research Corp, 270 W Crossroads Square, Salt Lake City, UT 84115.

References

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[5] Riley LH, Richter CP: Use of electrical skin resistance method in the study of patients with neck and upper extremity pain. Johns Hopkins Med J 137:69-74, 1975

[6] Dynatron 1120 Operator's Manual. Salt Lake City, UT, Dynatronics Research Corp, 1983

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[12] Snyder-Mackler L, Bork CE: Effect of Helium-neon laser irradiation on peripheral sensory nerve latency. Phys Ther 68:223-225, 1988

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[18] Greathouse DG, Currier DP, Gilmore RL: Effects of clinical infrared laser on superficial radial nerve conduction. Phys Ther 65:1184-1187, 1985

[19] Seitz L, Kleinkort JA: Low-power laser: Its applications in physical therapy. In Michlovitz SL, Wolf SL (eds): Thermal Agents in Rehabilitation. Philadelphia, PA, F A Davis Co, 1986, pp 217-238

L Snyder-Mackler, MS, PT, is Assistant Professor, Department of Physical Therapy, Sargent College of Allied Health Professions, Boston University, University Rd, Boston, MA 02215 (USA).

A Barry, MSPT, OTR, is Staff Physical Therapist, Braintree Hospital, 250 Pond St, Braintree, MA 02184.

A Perkins, MSPT, is Staff Physical Therapist, Framingham Union Hospital, 115 Lincoln St, Framingham, MA 01701.

M Soucek, BS, PT, is Sen. or Physical Therapist and Coordinator of Clinical Education, Underwood-Memorial Hospital, 509 N Broad St, Woodbury, NJ 08096.

Ms Barry and Ms Perkins were students in the Master of Science in Physical Therapy Program, Sargent College of Allied Health Professions, Boston University, when this study was conducted. A portion of this research was completed in partial fulfillment of the requirements of their degree.

This article was submitted May 12, 1988; was with the authors for revision for four weeks; and was accepted December 15, 1988.