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Immediate effects of spinal manipulation on thermal pain sensitivity: an experimental study

Immediate effects of spinal manipulation on thermal pain sensitivity: an experimental study Background: The underlying causes of spinal manipulation hypoalgesia are largely unknown. The beneficial clinical effects were originally theorized to be due to biomechanical changes, but recent research has suggested spinal manipulation may have a direct neurophysiological effect on pain perception through dorsal horn inhibition. This study added to this literature by investigating whether spinal manipulation hypoalgesia was: a) local to anatomical areas innervated by the lumbar spine; b) correlated with psychological variables; c) greater than hypoalgesia from physical activity; and d) different for A-delta and C-fiber mediated pain perception. Methods: Asymptomatic subjects (n = 60) completed baseline psychological questionnaires and underwent thermal quantitative sensory testing for A-delta and C-fiber mediated pain perception. Subjects were then randomized to ride a stationary bicycle, perform lumbar extension exercise, or receive spinal manipulation. Quantitative sensory testing was repeated 5 minutes after the intervention period. Data were analyzed with repeated measures ANOVA and post-hoc testing was performed with Bonferroni correction, as appropriate. Results: Subjects in the three intervention groups did not differ on baseline characteristics. Hypoalgesia from spinal manipulation was observed in lumbar innervated areas, but not control (cervical innervated) areas. Hypoalgesic response was not strongly correlated with psychological variables. Spinal manipulation hypoalgesia for A-delta fiber mediated pain perception did not differ from stationary bicycle and lumbar extension (p > 0.05). Spinal manipulation hypoalgesia for C-fiber mediated pain perception was greater than stationary bicycle riding (p = 0.040), but not for lumbar extension (p = 0.105). Conclusion: Local dorsal horn mediated inhibition of C-fiber input is a potential hypoalgesic mechanism of spinal manipulation for asymptomatic subjects, but further study is required to replicate this finding in subjects with low back pain. Page 1 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 peripheral pain perception, instead of distinguishing Background There is considerable evidence suggesting that spinal SMT's separate effect on A-delta and C-fiber mediated manipulative therapy (SMT) is an effective treatment for pain perception [28]. Last, the previously cited literature subgroups of patients with low back pain (LBP) [1-4]. Fur- included comparisons with a true control group [26], thermore, a clinical prediction rule indicative of optimal detuned short-wave therapy [25], and oscillatory mobili- treatment outcomes from SMT has been proposed [5] and zation [27]. Subjects performing general physical activity validated [6]. While the clinical literature provides strong or specific back exercises are clinically relevant compari- support for the use of SMT for certain patients, its under- son groups missing from the current SMT hypoalgesia lit- lying effects and mechanisms are not widely understood. erature. SMT is frequently theorized to correct mechanical lesions, Consequently, the present study investigated the immedi- such as "subluxation" or "segmental dysfunction", despite ate hypoalgesic effects of lumbar SMT on thermal pain the lack of empirical support for such theories [7,8]. SMT sensitivity in asymptomatic subjects. We selected thermal does appear to have demonstrable mechanical (i.e. peak stimuli for pain induction because unlike other experi- forces and displacement) effects on spinal segments [9- mental pain methods, it offered the sensitivity to test dif- 17]. However, there is skepticism as to whether therapeu- ferent anatomical areas and separate testing of A-delta tic benefits of SMT can be solely attributed to these effects. fiber and C-fiber mediated pain perception [28-30]. Our First, peak forces generated from SMT vary greatly by prac- first purpose was to determine if lumbar SMT hypoalgesia titioner, suggesting this factor is not related to clinical was a locally observed phenomenon by demonstrating a) improvement [18,19]. Second, lasting positional changes hypoalgesia in lumbar innervated sites, but not in the con- following SMT have not been observed [20], suggesting trol (cervical innervated) sites and b) hypoalgesia had a this factor is also not related to clinical improvement. low correlation with relevant pain-related cognitions. Our second purpose was to determine if lumbar SMT Neurophysiological processes have also been used to hypoalgesic effects were a) greater than hypoalgesia from explain the underlying effect of SMT [13,21-23]. Specific physical activity and b) different for A-delta fiber or C- to the purposes of this study, SMT has been theorized to fiber mediated pain responses. affect spinal joint and muscle spindle mechanoreceptors, activating low (A-beta) and high (A-delta, C) threshold Methods afferents [7]. This afferent input converges on the spinal Subjects cord with the potential to inhibit dorsal horn cells This sample was comprised of undergraduate and gradu- involved with transmission or amplification of nocicep- ate students who responded to study advertisements tive input. In this scenario, SMT's underlying effect would placed in health science classrooms of a large research uni- be as a "counter-irritant" stimulus to peripheral nocicep- versity. Subjects read and signed a consent form that had tive input received by dorsal horn cells [7]. been approved by University's Institutional Review Board before participating in any study-related procedures. Sub- If these neurophysiological processes occurred, SMT jects were verbally screened for history of LBP and current would have a measurable hypoalgesic effect on pain per- use of pain relievers. Subjects not currently experiencing ception. This topic was reviewed by Vernon [24], with LBP and not using pain relievers were included in this SMT hypoalgesia observed by decreased cutaneous recep- study. tive field from pin-prick [25], tolerance from electrical current [26], and mechanical pressure [27], Collectively, Procedure these results demonstrate SMT's potential for dorsal horn Demographic information, previous pain experience, psy- mediated pain inhibition. chological questionnaires, and thermal pain sensitivity measures were collected before intervention was ran- There are, however, several important, unresolved issues domly assigned. Each of the randomly assigned interven- regarding SMT hypoalgesia. The previously cited studies tions was applied for a standard 5-minute period to investigated hypoalgesia in anatomical areas with the minimize variation in hypoalgesic effect due to differ- same or overlapping dermatomes as those affected by ences related to re-assessment time and treatment dosage. SMT [25-27], For example, assessing hypoalgesic response The thermal pain sensitivity measures were collected to cervical manipulation only in anatomical areas inner- again 5 minutes after intervention was administered. Our vated by cervical nerve roots [27]. As a result, these studies rationale for only measuring immediate effects was two- were unable to determine if the observed hypoalgesia was fold. First, this was a preliminary study and we wanted to a large, general effect or a specific effect local to the spinal confirm that we could detect hypoalgesic effects on ther- levels involved with the manipulation [24]. Previous stud- mal sensitivity under ideal circumstances. Second, this ies utilized pain induction protocols assessing general study involved asymptomatic subjects and we did not Page 2 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 expect them to have a long-term response to these inter- Thermal stimuli were delivered via contact thermode and ventions because of a lack of appropriate disease process. a computer-controlled Medoc Neurosensory Analyzer (TSA-2001, Ramat Yishai, Israel) with a hand-held, pel- Measures tier-element-based stimulator. In our pilot testing of this Psychological questionnaires protocol (n = 10), we included a stimulation site involv- We selected psychological variables with previously ing lumbar paraspinal musculature. However, subjects reported influence on quantitative sensory testing [31- were unable to distinguish between A-delta fiber and C- 34]. fiber mediated pain perception, in comparison to testing in the extremities. We attributed this difference to the rel- The Fear of Pain Questionnaire (FPQ-III) uses a 30-item, 5- atively short distance the thermal stimuli had to travel to point rating scale to measure fear about specific situations the dorsal horn from the lumbar musculature. This short that would normally produce pain [35]. The FPQ-III is a distance did not allow subjects to differentiate input commonly used and well-validated instrument that is based on fiber type. Therefore, we limited pain perception appropriate for use in non-clinical and clinical popula- testing to extremity areas innervated by lumbar and cervi- tions [35-37]. cal dermatomes in the present study. The Coping Strategies Questionnaire (CSQ) uses a 27-item, Stimuli were applied to the subjects' non-dominant sides 7-point rating scale to measures the frequency of use for and stimulus sites included areas innervated by lumbar common pain coping strategies [38]. The CSQ is com- dermatomes (the plantar surface of the foot and the pos- monly used in pain studies and is appropriate for use in terior calf). Control sites included areas innervated by cer- non-clinical and clinical populations. We utilized the cat- vical dermatomes (the volar surface of the hand and astrophizing subscale that measures helplessness and pes- forearm). Order of stimulation sites was counter-balanced simistic cognitions related to pain perception. The validity to prevent ordering effects and exact stimulation sites were of this particular subscale has been supported [38-41] and varied to prevent carryover effects due to spatial summa- the currently recommended scoring system was used in tion, local sensitization, or suppression of nociceptors. this study [40]. The interval between stimuli was at least 60 seconds to avoid carryover effects for the preceding thermal stimulus. The State-Trait Anxiety Questionnaire (STAI) uses a 40-item, Subject response to thermal stimuli was determined with 4-point rating scale to assess dispositional (trait) and situ- a numerical rating scale (NRS) for evoked pain intensity. ational (state) anxiety symptoms [42]. The STAI is com- The NRS for evoked pain intensity ranged from "0" (No monly used to assess anxiety and is appropriate for use in pain) to "100" (Worst pain intensity imaginable). non-clinical and clinical populations. We reported the state portion of the STAI as this construct better matched Subjects were familiarized to the thermal stimuli with a the purposes of this study. practice session. In the practice session, a continuous heat stimulus was delivered to the subjects' dominant arm. The The Anxiety Sensitivity Index (ASI) uses a 16-item, 4-point stimulus started at 35°C and was increased at a rate of rating scale to assess anxiety sensitivity, which is the per- 0.5°C with subjects terminating the stimulus when the ception of whether experiencing symptoms of anxiety temperature reached pain threshold. This was repeated causes harm. The ASI is commonly used in pain studies three times and the average threshold was calculated. In and is appropriate for use in non-clinical and clinical pop- addition to familiarizing the subjects to thermal stimuli, ulations. The ASI has been validated in community sam- the pain threshold data allowed us to investigate if the ples [43] and has demonstrated factor invariance across intervention groups were confounded by general pain different sex and age groups [44]. sensitivity. We then assessed specific components of ther- mal pain sensitivity from previously reported protocols Thermal pain sensitivity [29,30,45]. Subjects underwent quantitative sensory testing as per First pain response previously established protocols involving thermal stim- uli [29,30,45,46]. We selected this protocol because Heat stimuli of 3 seconds duration were applied to the unlike other methods of experimental pain induction subjects' skin. The temperature rose rapidly (10°C/sec) thermal stimuli is sensitive to A-delta fiber and C-fiber from a baseline of 35°C to a randomly determined peak mediated pain perception. We used a protocol with of 45, 47, 49, or 50°C. The research assistant recorded parameters that parallel those from basic studies [28] and NRS ratings of pain intensity. Subjects were asked to rate was successful in detecting hypoalgesic response for their "first" pain intensity felt. These ratings are believed healthy controls taking fentanyl [29]. to be primarily mediated by input from A-delta fibers [28,29]. Page 3 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 Temporal summation A train of 10 consecutive heat pulses of <1 second dura- tion at an inter-stimulus interval of .33 Hz was delivered to the subjects. A frequency of .33 Hz was selected to ensure the development of temporal summation [28]. The temperature of the heat pulses rapidly fluctuated (10°C/sec) from a low of 35°C to a peak of 47°C. Tem- perature levels were monitored by a contactor-contained thermistor, and returned to a preset baseline of 35°C by active cooling. The research assistant recorded NRS ratings of pain intensity. Subjects were asked to rate their delayed (second) pain intensity associated with the first, third, and fifth heat pulses. These ratings are believed to be primarily mediated by C-fiber input [28,29]. Spinal manipulation technique ut 56) Figure 1 ilized in this study(5;47;54– Intervention Spinal manipulation technique utilized in this Subjects were given a standard instructional set that each study(5;47;54–56). (Reprinted with permission of the intervention was commonly used as part of LBP treat- American Physical Therapy Association from Cibulka MT. ment. Subjects were then randomly assigned to receive The treatment of the sacroiliac joint component to low back one of the following interventions. All interventions were pain: a case report. Phys Ther. 1992;72:917–922.) performed under the supervision of research staff to ensure compliance with the described parameters. this specific technique and no adverse events have been Stationary bicycle. Subjects rode a stationary bicycle for 5 reported [5,54-56]. The SMT was performed four times minutes at 60–70 rpm and 1 KP. This intervention served within a 5-minute period, alternating thrusts between as a non-specific, active comparison group with which to right and left ASIS's. Specifically, the researcher applying compare specific active and passive interventions used to the manipulation was trained to pace the repositioning treat spine pain. Our rationale for not including a control process to take 1 minute, allowing 10–15 seconds to per- group is that we wanted the comparison group for this form the thrust. study to account for non-specific effects related to per- Data analysis forming general physical activity. All data analyses were performed using SPSS for Windows Lumbar extension exercise subjects performed a prone (SPSS Inc, 233 S. Wacker Drive, 11th floor, Chicago, Illi- extension exercise previously described in the literature nois 60606), Version 13.0 at a Type I error rate of 0.05. for treatment of LBP [47,48]. This exercise involves the Descriptive statistics were generated for the demographic, patient lying flat in a prone position. Then, the patient psychological, and pain threshold measures. Randomiza- used his/her arms to press his/her chest of the treatment tion effect was investigated by comparing treatment table, and extending the lumbar spine. Subjects were groups with one-way ANOVA. Any observed group differ- given verbal cues to maintain their hips in contact with ences were considered as covariates in the subsequent the treatment table to prevent substitution from other analyses. anatomical areas. Several studies support the effectiveness of this exercise and no adverse events have been reported Our purposes were investigated by testing for group × [49-53]. Subjects performed 3 sets of 15 repetitions within time interactions for either first pain response or temporal a 5-minute period. summation in the lumbar and cervical innervated testing sites. First pain response for 47 and 49°C was tested with SMT. Subjects received a lumbar SMT previously described repeated measures ANOVA. Data for 45 and 50°C were in the literature for treatment of LBP (Figure 1) [47]. This not presented because these data represented sub-thresh- SMT technique is performed with the patient supine, and old (i.e. floor effect for hypoalgesia) and tolerance (i.e. the researcher standing on the opposite side of the table. ceiling effect for hypoalgesia) values for a majority of The researcher passively side bent the patient toward the patients, respectively. Treatment group [3] and pre and side to be manipulated and asked the subject to interlock post NRS first pain ratings [2] were the model factors. hands behind his/her head. The researcher then passively Temporal summation was tested with repeated measures rotated the subject away from the side to be manipulated ANOVA, with treatment group [3] and pre and post NRS and delivered a posterior and inferior thrust to the oppo- temporal summation ratings [2] as the model factors. The site ASIS. Several randomized trials support the efficacy of 3 primary analyses involving repeated measures ANOVA Page 4 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 models were performed without correction of the Type I sites (F = 3.7, partial η = 0.12, p = 0.030). Temporal 1,57 error rate. This strategy was selected because this was a summation hypoalgesic responses are included in Figure preliminary study and we wanted to utilize a liberal defi- 2 to allow descriptive comparisons of the cervical and nition of statistical significance to avoid potential misin- lumbar innervated responses. Post-hoc testing revealed terpretation of the data. However, any post-hoc testing that SMT had a larger hypoalgesic effect in the lumbar was performed with Bonferroni correction of the 0.05 innervated sites than stationary bicycle (p = 0.040), but Type I error rate. We also calculated Pearson product cor- similar as lumbar extension exercise (p = 0.105). relations between psychological variables, pain threshold measures, and SMT hypoalgesia. The Pearson correlations among the psychological, pain threshold, and SMT hypoalgesic response variables were Results generally low, ranging from -0.31 to 0.25, and none The 3 treatment groups did not significantly differ on the reached statistical significance (Table 3). demographic, previous pain experience, psychological, and pain threshold measures (Table 1). There were no sig- Discussion This study investigated the immediate hypoalgesic effect nificant group × time interactions for first pain hypoalge- sia in the cervical innervated sites at either 47°C (F = of lumbar SMT on thermal pain sensitivity in asympto- 1,57 0.4, partial η = 0.02, p = 0.645) or at 49°C (F = 0.3, matic subjects. The first purpose was to investigate 1,57 partial η = 0.01, p = 0.720). In addition, there was no sig- whether SMT hypoalgesia was a local phenomenon. This nificant hypoalgesia (i.e. treatment effect) for the cervical purpose adds to the existing literature because previous innervated sites at either temperature (Table 2). Similarly, studies have demonstrated SMT hypoalgesia by testing there was no significant group × time interaction for tem- anatomical sites primarily affected by the manipulative poral summation hypoalgesia in the cervical innervated technique [25-27]. As a result, there is a question whether sites (F = 0.5, partial η = 0.02, p = 0.620) and there was SMT hypoalgesia was the result of a general or local nerv- 1,57 no general temporal summation hypoalgesic effect in the ous system response [24]. Our results support SMT cervical innervated sites (F = 0.7, p = 0.405). hypoalgesia as primarily a local phenomenon. First, there 1,57 were no hypoalgesic effects observed in cervical inner- There were no significant group × time interactions for vated sites, but there were hypoalgesic effects observed in first pain response hypoalgesia in lumbar innervated sites the lumbar innervated sites. The implication of this find- at either 47°C (F = 2.4, partial η = 0.08, p = 0.101) or ing is that the dorsal horn inhibition from SMT did not 1,57 at 49°C (F = 1.3, partial η = 0.05, p = 0.268). However, have a wide-ranging effect on peripheral input received 1,57 there was a significant hypoalgesia (i.e. treatment effect) from lumbar and cervical dermatomes. Second, there on the lumbar innervated sites at both temperatures. All were no statistically significant or large correlations interventions were associated with first pain hypoalgesia, between pain-related cognitions, pain threshold, and the but only SMT had a consistent association (Table 2). SMT hypoalgesic response. For example, the largest corre- There was a significant group × time interaction for tem- lation was with state anxiety (r = -0.31), suggesting this poral summation hypoalgesia in the lumbar innervated cognition accounted for only 9.6% variance in the Table 1: Descriptive statistics for sample Variable Stationary Bicycle (n = 20) Lumbar Extension (n = 20) Spinal Manipulation (n = 20) p-value Age (years) 23.9 (3.4) 24.1 (2.6) 24.1 (3.6) 0.975 Sex (# female, %) 12 (60%) 14 (70%) 14 (70%) 0.741 Worst pain experienced (NRS) 68.9 (18.5) 64.0 (21.8) 59.7 (25.9) 0.436 Fear of pain (FPQ) 82.6 (16.7) 75.1 (13.3) 77.5 (22.6) 0.406 Pain catastrophizing (CSQ-R) 7.6 (3.1) 7.2 (3.7) 7.5 (3.8) 0.955 Anxiety (STAI) 45.3 (10.4) 45.5 (11.6) 45.2 (10.7) 0.996 Anxiety sensitivity (ASI) 19.8 (7.6) 16.0 (7.1) 16.0 (7.2) 0.230 Pain threshold (degrees Celsius) 44.7 (2.4) 45.4 (2.2) 44.8 (2.5) 0.589 Pain threshold rating (NRS) 25.0 (21.0) 28.8 (19.0) 21.3 (15.1) 0.443 Key All data are reported as mean (standard deviation) ratings, unless otherwise indicated. NRS = Numerical rating scale FPQ = Fear of Pain Questionnaire CSQ-R = Coping Strategies Questionnaire-Revised STAI = State Trait Anxiety Inventory ASI = Anxiety Sensitivity Index Page 5 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 Table 2: First pain hypoalgesia for stationary bicycle, lumbar extension, and spinal manipulation. Variable Stationary Bicycle Lumbar Extension Spinal Manipulation Partial Eta-square# p-value# (n = 20) (n = 20) (n = 20) Lumbar Innervated* $ $ $ NRS Change @ 47°C 13.2 (17.2) 12.9 (17.9) 23.5 (17.3) 0.08 0.101 NRS Change @ 49°C 1.2 (20.2) 6.3 (22.4) 12.1 (19.7) 0.05 0.268 Cervical Innervated^ NRS Change @ 47°C -3.0 (13.7) 0.3 (11.6) 0.3 (10.2) 0.02 0.645 NRS Change @ 49°C 1.9 (9.0) -0.4 (10.1) 1.7 (10.8) 0.01 0.720 Key NRS = Numerical rating scale All data are reported as mean (standard deviation) ratings. Negative numbers indicate increased pain following treatment. # – Significance and partial eta-square estimate are for the interaction between type of treatment and first pain hypoalgesia * – Significant overall main effect for lower extremity hypoalgesia at 47°C (F = 53.8, p < 0.001) and at 49°C (F = 5.9, p = 0.018) 1,57 1,57 – Significant within group effect for hypoalgesia (p < 0.05) ^ – No significant main effect for upper extremity hypoalgesia at 47°C (F = 0.4, p = 0.525) and at 49°C (F = 0.6, p = 0.424) 1,57 1,57 hypoalgesic response. The implication of this finding is posed that SMT hypoalgesia is a result of the activation of that psychological influences on SMT hypoalgesic endogenous descending pain inhibitory systems medi- response were likely not present, or only a minor influ- ated through the periaqueductal gray region of the mid- ence. brain [60]. In a human study this central mechanism was supported by Vincenzino et al[61] who reported Our findings suggests SMT hypoalgesia is potentially a hypoalgesia from cervical manipulation was significantly local neurophysiological phenomenon in asymptomatic correlated (r = 0.82) with sympathoexcitation. In an ani- subjects, corroborating with other studies demonstrating mal study this central mechanism was supported by Sykba local SMT effects for EMG activity [57,58] and inflamma- et al [62] who reported hypoalgesia from knee manipula- tion control [59]. However it must also be considered that tion was not affected by local spinal blockade of GABA or the literature supports the potential of a central mecha- opioid receptors. Therefore, the current literature provides nism for SMT hypoalgesia. Specifically, it has been pro- available evidence suggesting SMT hypoalgesia may be resultant of local and/or central mechanisms. Our second purpose was to investigate whether SMT 25 hypoalgesia differed from physical activity for first pain Lumbar Innervated Area response or temporal summation. This purpose adds to Cervical Innervated Area the existing literature because previous studies of SMT hypoalgesia have not included these clinically relevant comparisons and have not used protocols that differenti- ated between A-delta and C-fiber mediated pain percep- tion [24]. Our results provided information supporting SMT as a "counter-irritant" to inhibit peripheral noxious stimuli at the dorsal horn [7]. SMT appeared to have a -5 general counter-irritant effect on A-delta fiber mediated -10 pain perception (first pain response). SMT had a consist- Stationary Bicyle* Lumbar Extension Spinal Manipulation* ent hypoalgesic effect on A-delta fiber mediated hypoalge- sia, while stationary bicycle riding and lumbar extension exercise hypoalgesic effects were noted only at 47°C. SMT appeared to have a specific counter-irritant effect on C- Temporal summation b Figure 2 ar extension, and spinal manipulation hypoalgesia for stationary bicycle, lum- fiber mediated pain perception (temporal summation), as Temporal summation hypoalgesia for stationary SMT hypoalgesia was greater than bicycle riding and bicycle, lumbar extension, and spinal manipulation. trended toward being greater than lumbar extension exer- Figure 2 Key • Positive numbers indicate hypoalgesia • Error cise. bars are 1 standard error • * – indicates statistically signifi- cant (p < 0.05) difference in intervention for pain sensitivity in lower extremity area. The specific hypoalgesic effect on C-fiber mediated pain perception is an intriguing finding and could provide par- Page 6 of 10 (page number not for citation purposes) C h ang e in te m p oral su m m a tion (C -fib e r m e d iated ) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 Table 3: Associations among pain threshold, psychological variables, and lumbar innervated hypoalgesic responses for subjects receiving spinal manipulation. Temporal Summation First Pain @ 47°C First Pain @ 49°C Fear of Pain (FPQ) -0.20 (p = 0.401) 0.01 (p = 0.972) -0.08 (p = 0.732) Pain catastrophizing (CSQ-R) -0.08 (p = 0.750) 0.18 (p = 0.442) 0.10 (p = 0.686) Anxiety (STAI) -0.16 (p = 0.499) -0.19 (p = 0.422) -0.31 (p = 0.178) Anxiety sensitivity (ASI) 0.21 (p = 0.370) 0.25 (p = 0.292) -0.10 (p = 0.674) Pain threshold (degrees Celsius) -0.26 (p = 0.273) 0.11 (p = 0.656) 0.14 (p = 0.545) Pain threshold rating (NRS) 0.05 (p = 0.843) -0.19 (p = 0.436) -0.14 (p = 0.549) Key NRS = Numerical rating scale FPQ = Fear of Pain Questionnaire CSQ-R = Coping Strategies Questionnaire-Revised STAI = State Trait Anxiety Inventory ASI = Anxiety Sensitivity Index tial explanation for the clinical effectiveness of SMT. unrelated factors, such as non-specific effects related to Numerous basic studies have suggested that central sensi- differences in active and passive interventions. tization of pain is a specific neurophysiological mecha- nism associated with the development and maintenance This experimental model offers several advantages in the of chronic pain syndromes [63-68]. Wind-up results from study of SMT hypoalgesia. The use of thermal stimuli tonic, peripheral nociceptive C-fiber input and is an exam- allowed us to precisely control levels of nociceptive input ple of central sensitization that occurs within dorsal horn and differentiate this input based on fiber type. The use of cells. This input activates NMDA and substance P recep- asymptomatic subjects eliminated confounding of the tors in wide dynamic range and nociceptive specific cells. hypoalgesic response from clinical conditions and pain Then, the tonic activation of these cells induces a central medications. However, there are also several limitations hyperalgesia mediated at the spinal cord level, such that to consider when interpreting this study. First, although subsequent evoked pain stimuli are relayed from the dor- use of asymptomatic subjects offers advantages, these sal horn as increasing in intensity, despite their being of findings cannot be directly generalized to patients with standard amplitude. In basic models, this temporal LBP. In patients with LBP a wider range of psychological parameter (increasing frequency of nociceptive input) is a scores would be expected, potentially making them more primary factor in eliciting wind-up [65]. robustly related to hypoalgesia from SMT. Also, patients with LBP experience ongoing, nociceptive input that is Direct measurement of wind-up is not feasible in human likely to result in enhanced temporal summation in com- subjects, but temporal summation of thermal stimuli is an parison to asymptomatic controls, thereby interacting accepted behavioral measure of wind-up [28]. The use of with the proposed mechanisms of SMT. Second, this study temporal summation as a proxy measure of wind-up is only tested the immediate hypoalgesic effects of SMT and supported by human studies that demonstrate an increase utilized standard treatment parameters that did not in the frequency of standard nociceptive input increases mimic clinical settings. This methodology was necessary the report of pain perception [29,30,45]. Specifically, to provide the internal validity to detect a short-term thermal input at .33 Hz or less tends to induce temporal hypoalgesic response, however this methodology also summation in humans, while input at .20 Hz or greater means that no assumptions can be made about longer- does not [28]. The results of the present study indicated term hypoalgesic effects or the effect of these particular that SMT reduced temporal summation, suggesting a interventions applied under different parameters. Third, potential underlying effect of SMT is the inhibition of dor- we did not include sham SMT in this study, so we were sal horn wind-up [7]. Inhibition of dorsal horn wind-up unable to account for hypoalgesic effects associated with would mean the individual was less likely to develop the specific expectation of SMT being a successful inter- chronic LBP, at least chronic LBP caused by this particular vention for pain relief in lumbar innervated areas. Fourth, pain mechanism. It should be noted that this explanation we did not report joint cavitation in this study because is speculative at this time, as only one study directly links previous work demonstrated considerable variability in temporal summation with chronic LBP [69]. One inter- the location of cavitation for lumbar SMT [70], and expe- pretation of these data is that SMT has the potential of riencing cavitation does not appear to affect EMG activity inhibiting dorsal horn windup from peripheral noxious [13] or pain outcomes [71,72]. Last, although we did stimuli. While this an intriguing explanation, we acknowl- implement a comparison group (bicycle), we did not uti- edge that these findings may also be explained by other lize a control group in our current design. This methodo- Page 7 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 JEB received support from the National Institutes of Health T-32 Neural logical selection means that non-specific effects related to Plasticity Research Training Fellowship (T32HD043730). differences in active (bicycle and prone press ups) and passive (SMT) interventions are a viable alternate explana- Kelli Eisenbrown assisted with data collection and data entry. tion to our findings. Megan Hurlburt and Julia Villa assisted with data collection. Conclusion The finding that SMT may have a local hypoalgesic effect Gabrielle Shumrak assisted with data entry. specific to C-fiber mediated input for asymptomatic sub- References jects adds to the previous literature and provides direction 1. Koes BW, Assendelft WJ, van der Heijden GJ, Bouter LM: Spinal for future study. First, this experiment should be repro- manipulation for low back pain. 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Behav Res Ther 1999, 37:313-323. mechanical force, manually assisted spinal manipulative 35. McNeil DW, Rainwater AJ: Development of the Fear of Pain therapy. Spine 2001, 26:1117-1124. Questionnaire--III. J Behav Med 1998, 21:389-410. 58. Lehman GJ, Vernon H, McGill SM: Effects of a mechanical pain 36. Albaret MC, Munoz Sastre MT, Cottencin A, Mullet E: The Fear of stimulus on erector spinae activity before and after a spinal Pain questionnaire: factor structure in samples of young, manipulation in patients with back pain: a preliminary inves- middle-aged and elderly European people. Eur J Pain 2004, tigation. J Manipulative Physiol Ther 2001, 24:402-406. 8:273-281. 59. Song XJ, Gan Q, Cao JL, Wang ZB, Rupert RL: Spinal manipulation 37. Osman A, Breitenstein JL, Barrios FX, Gutierrez PM, Kopper BA: reduces pain and hyperalgesia after lumbar intervertebral The Fear of Pain Questionnaire-III: further reliability and foramen inflammation in the rat. 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Price DD, Mao J, Frenk H, Mayer DJ: The N-methyl-D-aspartate Index psychometrics and factor structure in a community receptor antagonist dextromethorphan selectively reduces sample. J Anxiety Disord 2002, 16:33-49. temporal summation of second pain in man. Pain 1994, 44. Dehon C, Weems CF, Stickle TR, Costa NM, Berman SL: A cross- 59:165-174. sectional evaluation of the factorial invariance of anxiety 65. Price DD, Hayes RL, Ruda M, Dubner R: Spatial and temporal transformations of input to spinothalamic tract neurons and Page 9 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 their relation to somatic sensations. J Neurophysiol 1978, 41:933-947. 66. Dickenson AH, Sullivan AF: Evidence for a role of the NMDA receptor in the frequency dependent potentiation of deep rat dorsal horn nociceptive neurones following C fibre stim- ulation. Neuropharmacology 1987, 26:1235-1238. 67. 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J Manipulative Physiol Ther 2006, 29:40-45. 72. Flynn TW, Fritz JM, Wainner RS, Whitman JM: The audible pop is not necessary for successful spinal high-velocity thrust manipulation in individuals with low back pain. Arch Phys Med Rehabil 2003, 84:1057-1060. Pre-publication history The pre-publication history for this paper can be accessed here: http://www.biomedcentral.com/1471-2474/7/68/prepub Publish with Bio Med Central and every scientist can read your work free of charge "BioMed Central will be the most significant development for disseminating the results of biomedical researc h in our lifetime." 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Immediate effects of spinal manipulation on thermal pain sensitivity: an experimental study

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Springer Journals
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Copyright © 2006 by George et al; licensee BioMed Central Ltd.
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Medicine & Public Health; Orthopedics; Rehabilitation; Rheumatology; Sports Medicine; Internal Medicine; Epidemiology
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1471-2474
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10.1186/1471-2474-7-68
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16911795
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Abstract

Background: The underlying causes of spinal manipulation hypoalgesia are largely unknown. The beneficial clinical effects were originally theorized to be due to biomechanical changes, but recent research has suggested spinal manipulation may have a direct neurophysiological effect on pain perception through dorsal horn inhibition. This study added to this literature by investigating whether spinal manipulation hypoalgesia was: a) local to anatomical areas innervated by the lumbar spine; b) correlated with psychological variables; c) greater than hypoalgesia from physical activity; and d) different for A-delta and C-fiber mediated pain perception. Methods: Asymptomatic subjects (n = 60) completed baseline psychological questionnaires and underwent thermal quantitative sensory testing for A-delta and C-fiber mediated pain perception. Subjects were then randomized to ride a stationary bicycle, perform lumbar extension exercise, or receive spinal manipulation. Quantitative sensory testing was repeated 5 minutes after the intervention period. Data were analyzed with repeated measures ANOVA and post-hoc testing was performed with Bonferroni correction, as appropriate. Results: Subjects in the three intervention groups did not differ on baseline characteristics. Hypoalgesia from spinal manipulation was observed in lumbar innervated areas, but not control (cervical innervated) areas. Hypoalgesic response was not strongly correlated with psychological variables. Spinal manipulation hypoalgesia for A-delta fiber mediated pain perception did not differ from stationary bicycle and lumbar extension (p > 0.05). Spinal manipulation hypoalgesia for C-fiber mediated pain perception was greater than stationary bicycle riding (p = 0.040), but not for lumbar extension (p = 0.105). Conclusion: Local dorsal horn mediated inhibition of C-fiber input is a potential hypoalgesic mechanism of spinal manipulation for asymptomatic subjects, but further study is required to replicate this finding in subjects with low back pain. Page 1 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 peripheral pain perception, instead of distinguishing Background There is considerable evidence suggesting that spinal SMT's separate effect on A-delta and C-fiber mediated manipulative therapy (SMT) is an effective treatment for pain perception [28]. Last, the previously cited literature subgroups of patients with low back pain (LBP) [1-4]. Fur- included comparisons with a true control group [26], thermore, a clinical prediction rule indicative of optimal detuned short-wave therapy [25], and oscillatory mobili- treatment outcomes from SMT has been proposed [5] and zation [27]. Subjects performing general physical activity validated [6]. While the clinical literature provides strong or specific back exercises are clinically relevant compari- support for the use of SMT for certain patients, its under- son groups missing from the current SMT hypoalgesia lit- lying effects and mechanisms are not widely understood. erature. SMT is frequently theorized to correct mechanical lesions, Consequently, the present study investigated the immedi- such as "subluxation" or "segmental dysfunction", despite ate hypoalgesic effects of lumbar SMT on thermal pain the lack of empirical support for such theories [7,8]. SMT sensitivity in asymptomatic subjects. We selected thermal does appear to have demonstrable mechanical (i.e. peak stimuli for pain induction because unlike other experi- forces and displacement) effects on spinal segments [9- mental pain methods, it offered the sensitivity to test dif- 17]. However, there is skepticism as to whether therapeu- ferent anatomical areas and separate testing of A-delta tic benefits of SMT can be solely attributed to these effects. fiber and C-fiber mediated pain perception [28-30]. Our First, peak forces generated from SMT vary greatly by prac- first purpose was to determine if lumbar SMT hypoalgesia titioner, suggesting this factor is not related to clinical was a locally observed phenomenon by demonstrating a) improvement [18,19]. Second, lasting positional changes hypoalgesia in lumbar innervated sites, but not in the con- following SMT have not been observed [20], suggesting trol (cervical innervated) sites and b) hypoalgesia had a this factor is also not related to clinical improvement. low correlation with relevant pain-related cognitions. Our second purpose was to determine if lumbar SMT Neurophysiological processes have also been used to hypoalgesic effects were a) greater than hypoalgesia from explain the underlying effect of SMT [13,21-23]. Specific physical activity and b) different for A-delta fiber or C- to the purposes of this study, SMT has been theorized to fiber mediated pain responses. affect spinal joint and muscle spindle mechanoreceptors, activating low (A-beta) and high (A-delta, C) threshold Methods afferents [7]. This afferent input converges on the spinal Subjects cord with the potential to inhibit dorsal horn cells This sample was comprised of undergraduate and gradu- involved with transmission or amplification of nocicep- ate students who responded to study advertisements tive input. In this scenario, SMT's underlying effect would placed in health science classrooms of a large research uni- be as a "counter-irritant" stimulus to peripheral nocicep- versity. Subjects read and signed a consent form that had tive input received by dorsal horn cells [7]. been approved by University's Institutional Review Board before participating in any study-related procedures. Sub- If these neurophysiological processes occurred, SMT jects were verbally screened for history of LBP and current would have a measurable hypoalgesic effect on pain per- use of pain relievers. Subjects not currently experiencing ception. This topic was reviewed by Vernon [24], with LBP and not using pain relievers were included in this SMT hypoalgesia observed by decreased cutaneous recep- study. tive field from pin-prick [25], tolerance from electrical current [26], and mechanical pressure [27], Collectively, Procedure these results demonstrate SMT's potential for dorsal horn Demographic information, previous pain experience, psy- mediated pain inhibition. chological questionnaires, and thermal pain sensitivity measures were collected before intervention was ran- There are, however, several important, unresolved issues domly assigned. Each of the randomly assigned interven- regarding SMT hypoalgesia. The previously cited studies tions was applied for a standard 5-minute period to investigated hypoalgesia in anatomical areas with the minimize variation in hypoalgesic effect due to differ- same or overlapping dermatomes as those affected by ences related to re-assessment time and treatment dosage. SMT [25-27], For example, assessing hypoalgesic response The thermal pain sensitivity measures were collected to cervical manipulation only in anatomical areas inner- again 5 minutes after intervention was administered. Our vated by cervical nerve roots [27]. As a result, these studies rationale for only measuring immediate effects was two- were unable to determine if the observed hypoalgesia was fold. First, this was a preliminary study and we wanted to a large, general effect or a specific effect local to the spinal confirm that we could detect hypoalgesic effects on ther- levels involved with the manipulation [24]. Previous stud- mal sensitivity under ideal circumstances. Second, this ies utilized pain induction protocols assessing general study involved asymptomatic subjects and we did not Page 2 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 expect them to have a long-term response to these inter- Thermal stimuli were delivered via contact thermode and ventions because of a lack of appropriate disease process. a computer-controlled Medoc Neurosensory Analyzer (TSA-2001, Ramat Yishai, Israel) with a hand-held, pel- Measures tier-element-based stimulator. In our pilot testing of this Psychological questionnaires protocol (n = 10), we included a stimulation site involv- We selected psychological variables with previously ing lumbar paraspinal musculature. However, subjects reported influence on quantitative sensory testing [31- were unable to distinguish between A-delta fiber and C- 34]. fiber mediated pain perception, in comparison to testing in the extremities. We attributed this difference to the rel- The Fear of Pain Questionnaire (FPQ-III) uses a 30-item, 5- atively short distance the thermal stimuli had to travel to point rating scale to measure fear about specific situations the dorsal horn from the lumbar musculature. This short that would normally produce pain [35]. The FPQ-III is a distance did not allow subjects to differentiate input commonly used and well-validated instrument that is based on fiber type. Therefore, we limited pain perception appropriate for use in non-clinical and clinical popula- testing to extremity areas innervated by lumbar and cervi- tions [35-37]. cal dermatomes in the present study. The Coping Strategies Questionnaire (CSQ) uses a 27-item, Stimuli were applied to the subjects' non-dominant sides 7-point rating scale to measures the frequency of use for and stimulus sites included areas innervated by lumbar common pain coping strategies [38]. The CSQ is com- dermatomes (the plantar surface of the foot and the pos- monly used in pain studies and is appropriate for use in terior calf). Control sites included areas innervated by cer- non-clinical and clinical populations. We utilized the cat- vical dermatomes (the volar surface of the hand and astrophizing subscale that measures helplessness and pes- forearm). Order of stimulation sites was counter-balanced simistic cognitions related to pain perception. The validity to prevent ordering effects and exact stimulation sites were of this particular subscale has been supported [38-41] and varied to prevent carryover effects due to spatial summa- the currently recommended scoring system was used in tion, local sensitization, or suppression of nociceptors. this study [40]. The interval between stimuli was at least 60 seconds to avoid carryover effects for the preceding thermal stimulus. The State-Trait Anxiety Questionnaire (STAI) uses a 40-item, Subject response to thermal stimuli was determined with 4-point rating scale to assess dispositional (trait) and situ- a numerical rating scale (NRS) for evoked pain intensity. ational (state) anxiety symptoms [42]. The STAI is com- The NRS for evoked pain intensity ranged from "0" (No monly used to assess anxiety and is appropriate for use in pain) to "100" (Worst pain intensity imaginable). non-clinical and clinical populations. We reported the state portion of the STAI as this construct better matched Subjects were familiarized to the thermal stimuli with a the purposes of this study. practice session. In the practice session, a continuous heat stimulus was delivered to the subjects' dominant arm. The The Anxiety Sensitivity Index (ASI) uses a 16-item, 4-point stimulus started at 35°C and was increased at a rate of rating scale to assess anxiety sensitivity, which is the per- 0.5°C with subjects terminating the stimulus when the ception of whether experiencing symptoms of anxiety temperature reached pain threshold. This was repeated causes harm. The ASI is commonly used in pain studies three times and the average threshold was calculated. In and is appropriate for use in non-clinical and clinical pop- addition to familiarizing the subjects to thermal stimuli, ulations. The ASI has been validated in community sam- the pain threshold data allowed us to investigate if the ples [43] and has demonstrated factor invariance across intervention groups were confounded by general pain different sex and age groups [44]. sensitivity. We then assessed specific components of ther- mal pain sensitivity from previously reported protocols Thermal pain sensitivity [29,30,45]. Subjects underwent quantitative sensory testing as per First pain response previously established protocols involving thermal stim- uli [29,30,45,46]. We selected this protocol because Heat stimuli of 3 seconds duration were applied to the unlike other methods of experimental pain induction subjects' skin. The temperature rose rapidly (10°C/sec) thermal stimuli is sensitive to A-delta fiber and C-fiber from a baseline of 35°C to a randomly determined peak mediated pain perception. We used a protocol with of 45, 47, 49, or 50°C. The research assistant recorded parameters that parallel those from basic studies [28] and NRS ratings of pain intensity. Subjects were asked to rate was successful in detecting hypoalgesic response for their "first" pain intensity felt. These ratings are believed healthy controls taking fentanyl [29]. to be primarily mediated by input from A-delta fibers [28,29]. Page 3 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 Temporal summation A train of 10 consecutive heat pulses of <1 second dura- tion at an inter-stimulus interval of .33 Hz was delivered to the subjects. A frequency of .33 Hz was selected to ensure the development of temporal summation [28]. The temperature of the heat pulses rapidly fluctuated (10°C/sec) from a low of 35°C to a peak of 47°C. Tem- perature levels were monitored by a contactor-contained thermistor, and returned to a preset baseline of 35°C by active cooling. The research assistant recorded NRS ratings of pain intensity. Subjects were asked to rate their delayed (second) pain intensity associated with the first, third, and fifth heat pulses. These ratings are believed to be primarily mediated by C-fiber input [28,29]. Spinal manipulation technique ut 56) Figure 1 ilized in this study(5;47;54– Intervention Spinal manipulation technique utilized in this Subjects were given a standard instructional set that each study(5;47;54–56). (Reprinted with permission of the intervention was commonly used as part of LBP treat- American Physical Therapy Association from Cibulka MT. ment. Subjects were then randomly assigned to receive The treatment of the sacroiliac joint component to low back one of the following interventions. All interventions were pain: a case report. Phys Ther. 1992;72:917–922.) performed under the supervision of research staff to ensure compliance with the described parameters. this specific technique and no adverse events have been Stationary bicycle. Subjects rode a stationary bicycle for 5 reported [5,54-56]. The SMT was performed four times minutes at 60–70 rpm and 1 KP. This intervention served within a 5-minute period, alternating thrusts between as a non-specific, active comparison group with which to right and left ASIS's. Specifically, the researcher applying compare specific active and passive interventions used to the manipulation was trained to pace the repositioning treat spine pain. Our rationale for not including a control process to take 1 minute, allowing 10–15 seconds to per- group is that we wanted the comparison group for this form the thrust. study to account for non-specific effects related to per- Data analysis forming general physical activity. All data analyses were performed using SPSS for Windows Lumbar extension exercise subjects performed a prone (SPSS Inc, 233 S. Wacker Drive, 11th floor, Chicago, Illi- extension exercise previously described in the literature nois 60606), Version 13.0 at a Type I error rate of 0.05. for treatment of LBP [47,48]. This exercise involves the Descriptive statistics were generated for the demographic, patient lying flat in a prone position. Then, the patient psychological, and pain threshold measures. Randomiza- used his/her arms to press his/her chest of the treatment tion effect was investigated by comparing treatment table, and extending the lumbar spine. Subjects were groups with one-way ANOVA. Any observed group differ- given verbal cues to maintain their hips in contact with ences were considered as covariates in the subsequent the treatment table to prevent substitution from other analyses. anatomical areas. Several studies support the effectiveness of this exercise and no adverse events have been reported Our purposes were investigated by testing for group × [49-53]. Subjects performed 3 sets of 15 repetitions within time interactions for either first pain response or temporal a 5-minute period. summation in the lumbar and cervical innervated testing sites. First pain response for 47 and 49°C was tested with SMT. Subjects received a lumbar SMT previously described repeated measures ANOVA. Data for 45 and 50°C were in the literature for treatment of LBP (Figure 1) [47]. This not presented because these data represented sub-thresh- SMT technique is performed with the patient supine, and old (i.e. floor effect for hypoalgesia) and tolerance (i.e. the researcher standing on the opposite side of the table. ceiling effect for hypoalgesia) values for a majority of The researcher passively side bent the patient toward the patients, respectively. Treatment group [3] and pre and side to be manipulated and asked the subject to interlock post NRS first pain ratings [2] were the model factors. hands behind his/her head. The researcher then passively Temporal summation was tested with repeated measures rotated the subject away from the side to be manipulated ANOVA, with treatment group [3] and pre and post NRS and delivered a posterior and inferior thrust to the oppo- temporal summation ratings [2] as the model factors. The site ASIS. Several randomized trials support the efficacy of 3 primary analyses involving repeated measures ANOVA Page 4 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 models were performed without correction of the Type I sites (F = 3.7, partial η = 0.12, p = 0.030). Temporal 1,57 error rate. This strategy was selected because this was a summation hypoalgesic responses are included in Figure preliminary study and we wanted to utilize a liberal defi- 2 to allow descriptive comparisons of the cervical and nition of statistical significance to avoid potential misin- lumbar innervated responses. Post-hoc testing revealed terpretation of the data. However, any post-hoc testing that SMT had a larger hypoalgesic effect in the lumbar was performed with Bonferroni correction of the 0.05 innervated sites than stationary bicycle (p = 0.040), but Type I error rate. We also calculated Pearson product cor- similar as lumbar extension exercise (p = 0.105). relations between psychological variables, pain threshold measures, and SMT hypoalgesia. The Pearson correlations among the psychological, pain threshold, and SMT hypoalgesic response variables were Results generally low, ranging from -0.31 to 0.25, and none The 3 treatment groups did not significantly differ on the reached statistical significance (Table 3). demographic, previous pain experience, psychological, and pain threshold measures (Table 1). There were no sig- Discussion This study investigated the immediate hypoalgesic effect nificant group × time interactions for first pain hypoalge- sia in the cervical innervated sites at either 47°C (F = of lumbar SMT on thermal pain sensitivity in asympto- 1,57 0.4, partial η = 0.02, p = 0.645) or at 49°C (F = 0.3, matic subjects. The first purpose was to investigate 1,57 partial η = 0.01, p = 0.720). In addition, there was no sig- whether SMT hypoalgesia was a local phenomenon. This nificant hypoalgesia (i.e. treatment effect) for the cervical purpose adds to the existing literature because previous innervated sites at either temperature (Table 2). Similarly, studies have demonstrated SMT hypoalgesia by testing there was no significant group × time interaction for tem- anatomical sites primarily affected by the manipulative poral summation hypoalgesia in the cervical innervated technique [25-27]. As a result, there is a question whether sites (F = 0.5, partial η = 0.02, p = 0.620) and there was SMT hypoalgesia was the result of a general or local nerv- 1,57 no general temporal summation hypoalgesic effect in the ous system response [24]. Our results support SMT cervical innervated sites (F = 0.7, p = 0.405). hypoalgesia as primarily a local phenomenon. First, there 1,57 were no hypoalgesic effects observed in cervical inner- There were no significant group × time interactions for vated sites, but there were hypoalgesic effects observed in first pain response hypoalgesia in lumbar innervated sites the lumbar innervated sites. The implication of this find- at either 47°C (F = 2.4, partial η = 0.08, p = 0.101) or ing is that the dorsal horn inhibition from SMT did not 1,57 at 49°C (F = 1.3, partial η = 0.05, p = 0.268). However, have a wide-ranging effect on peripheral input received 1,57 there was a significant hypoalgesia (i.e. treatment effect) from lumbar and cervical dermatomes. Second, there on the lumbar innervated sites at both temperatures. All were no statistically significant or large correlations interventions were associated with first pain hypoalgesia, between pain-related cognitions, pain threshold, and the but only SMT had a consistent association (Table 2). SMT hypoalgesic response. For example, the largest corre- There was a significant group × time interaction for tem- lation was with state anxiety (r = -0.31), suggesting this poral summation hypoalgesia in the lumbar innervated cognition accounted for only 9.6% variance in the Table 1: Descriptive statistics for sample Variable Stationary Bicycle (n = 20) Lumbar Extension (n = 20) Spinal Manipulation (n = 20) p-value Age (years) 23.9 (3.4) 24.1 (2.6) 24.1 (3.6) 0.975 Sex (# female, %) 12 (60%) 14 (70%) 14 (70%) 0.741 Worst pain experienced (NRS) 68.9 (18.5) 64.0 (21.8) 59.7 (25.9) 0.436 Fear of pain (FPQ) 82.6 (16.7) 75.1 (13.3) 77.5 (22.6) 0.406 Pain catastrophizing (CSQ-R) 7.6 (3.1) 7.2 (3.7) 7.5 (3.8) 0.955 Anxiety (STAI) 45.3 (10.4) 45.5 (11.6) 45.2 (10.7) 0.996 Anxiety sensitivity (ASI) 19.8 (7.6) 16.0 (7.1) 16.0 (7.2) 0.230 Pain threshold (degrees Celsius) 44.7 (2.4) 45.4 (2.2) 44.8 (2.5) 0.589 Pain threshold rating (NRS) 25.0 (21.0) 28.8 (19.0) 21.3 (15.1) 0.443 Key All data are reported as mean (standard deviation) ratings, unless otherwise indicated. NRS = Numerical rating scale FPQ = Fear of Pain Questionnaire CSQ-R = Coping Strategies Questionnaire-Revised STAI = State Trait Anxiety Inventory ASI = Anxiety Sensitivity Index Page 5 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 Table 2: First pain hypoalgesia for stationary bicycle, lumbar extension, and spinal manipulation. Variable Stationary Bicycle Lumbar Extension Spinal Manipulation Partial Eta-square# p-value# (n = 20) (n = 20) (n = 20) Lumbar Innervated* $ $ $ NRS Change @ 47°C 13.2 (17.2) 12.9 (17.9) 23.5 (17.3) 0.08 0.101 NRS Change @ 49°C 1.2 (20.2) 6.3 (22.4) 12.1 (19.7) 0.05 0.268 Cervical Innervated^ NRS Change @ 47°C -3.0 (13.7) 0.3 (11.6) 0.3 (10.2) 0.02 0.645 NRS Change @ 49°C 1.9 (9.0) -0.4 (10.1) 1.7 (10.8) 0.01 0.720 Key NRS = Numerical rating scale All data are reported as mean (standard deviation) ratings. Negative numbers indicate increased pain following treatment. # – Significance and partial eta-square estimate are for the interaction between type of treatment and first pain hypoalgesia * – Significant overall main effect for lower extremity hypoalgesia at 47°C (F = 53.8, p < 0.001) and at 49°C (F = 5.9, p = 0.018) 1,57 1,57 – Significant within group effect for hypoalgesia (p < 0.05) ^ – No significant main effect for upper extremity hypoalgesia at 47°C (F = 0.4, p = 0.525) and at 49°C (F = 0.6, p = 0.424) 1,57 1,57 hypoalgesic response. The implication of this finding is posed that SMT hypoalgesia is a result of the activation of that psychological influences on SMT hypoalgesic endogenous descending pain inhibitory systems medi- response were likely not present, or only a minor influ- ated through the periaqueductal gray region of the mid- ence. brain [60]. In a human study this central mechanism was supported by Vincenzino et al[61] who reported Our findings suggests SMT hypoalgesia is potentially a hypoalgesia from cervical manipulation was significantly local neurophysiological phenomenon in asymptomatic correlated (r = 0.82) with sympathoexcitation. In an ani- subjects, corroborating with other studies demonstrating mal study this central mechanism was supported by Sykba local SMT effects for EMG activity [57,58] and inflamma- et al [62] who reported hypoalgesia from knee manipula- tion control [59]. However it must also be considered that tion was not affected by local spinal blockade of GABA or the literature supports the potential of a central mecha- opioid receptors. Therefore, the current literature provides nism for SMT hypoalgesia. Specifically, it has been pro- available evidence suggesting SMT hypoalgesia may be resultant of local and/or central mechanisms. Our second purpose was to investigate whether SMT 25 hypoalgesia differed from physical activity for first pain Lumbar Innervated Area response or temporal summation. This purpose adds to Cervical Innervated Area the existing literature because previous studies of SMT hypoalgesia have not included these clinically relevant comparisons and have not used protocols that differenti- ated between A-delta and C-fiber mediated pain percep- tion [24]. Our results provided information supporting SMT as a "counter-irritant" to inhibit peripheral noxious stimuli at the dorsal horn [7]. SMT appeared to have a -5 general counter-irritant effect on A-delta fiber mediated -10 pain perception (first pain response). SMT had a consist- Stationary Bicyle* Lumbar Extension Spinal Manipulation* ent hypoalgesic effect on A-delta fiber mediated hypoalge- sia, while stationary bicycle riding and lumbar extension exercise hypoalgesic effects were noted only at 47°C. SMT appeared to have a specific counter-irritant effect on C- Temporal summation b Figure 2 ar extension, and spinal manipulation hypoalgesia for stationary bicycle, lum- fiber mediated pain perception (temporal summation), as Temporal summation hypoalgesia for stationary SMT hypoalgesia was greater than bicycle riding and bicycle, lumbar extension, and spinal manipulation. trended toward being greater than lumbar extension exer- Figure 2 Key • Positive numbers indicate hypoalgesia • Error cise. bars are 1 standard error • * – indicates statistically signifi- cant (p < 0.05) difference in intervention for pain sensitivity in lower extremity area. The specific hypoalgesic effect on C-fiber mediated pain perception is an intriguing finding and could provide par- Page 6 of 10 (page number not for citation purposes) C h ang e in te m p oral su m m a tion (C -fib e r m e d iated ) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 Table 3: Associations among pain threshold, psychological variables, and lumbar innervated hypoalgesic responses for subjects receiving spinal manipulation. Temporal Summation First Pain @ 47°C First Pain @ 49°C Fear of Pain (FPQ) -0.20 (p = 0.401) 0.01 (p = 0.972) -0.08 (p = 0.732) Pain catastrophizing (CSQ-R) -0.08 (p = 0.750) 0.18 (p = 0.442) 0.10 (p = 0.686) Anxiety (STAI) -0.16 (p = 0.499) -0.19 (p = 0.422) -0.31 (p = 0.178) Anxiety sensitivity (ASI) 0.21 (p = 0.370) 0.25 (p = 0.292) -0.10 (p = 0.674) Pain threshold (degrees Celsius) -0.26 (p = 0.273) 0.11 (p = 0.656) 0.14 (p = 0.545) Pain threshold rating (NRS) 0.05 (p = 0.843) -0.19 (p = 0.436) -0.14 (p = 0.549) Key NRS = Numerical rating scale FPQ = Fear of Pain Questionnaire CSQ-R = Coping Strategies Questionnaire-Revised STAI = State Trait Anxiety Inventory ASI = Anxiety Sensitivity Index tial explanation for the clinical effectiveness of SMT. unrelated factors, such as non-specific effects related to Numerous basic studies have suggested that central sensi- differences in active and passive interventions. tization of pain is a specific neurophysiological mecha- nism associated with the development and maintenance This experimental model offers several advantages in the of chronic pain syndromes [63-68]. Wind-up results from study of SMT hypoalgesia. The use of thermal stimuli tonic, peripheral nociceptive C-fiber input and is an exam- allowed us to precisely control levels of nociceptive input ple of central sensitization that occurs within dorsal horn and differentiate this input based on fiber type. The use of cells. This input activates NMDA and substance P recep- asymptomatic subjects eliminated confounding of the tors in wide dynamic range and nociceptive specific cells. hypoalgesic response from clinical conditions and pain Then, the tonic activation of these cells induces a central medications. However, there are also several limitations hyperalgesia mediated at the spinal cord level, such that to consider when interpreting this study. First, although subsequent evoked pain stimuli are relayed from the dor- use of asymptomatic subjects offers advantages, these sal horn as increasing in intensity, despite their being of findings cannot be directly generalized to patients with standard amplitude. In basic models, this temporal LBP. In patients with LBP a wider range of psychological parameter (increasing frequency of nociceptive input) is a scores would be expected, potentially making them more primary factor in eliciting wind-up [65]. robustly related to hypoalgesia from SMT. Also, patients with LBP experience ongoing, nociceptive input that is Direct measurement of wind-up is not feasible in human likely to result in enhanced temporal summation in com- subjects, but temporal summation of thermal stimuli is an parison to asymptomatic controls, thereby interacting accepted behavioral measure of wind-up [28]. The use of with the proposed mechanisms of SMT. Second, this study temporal summation as a proxy measure of wind-up is only tested the immediate hypoalgesic effects of SMT and supported by human studies that demonstrate an increase utilized standard treatment parameters that did not in the frequency of standard nociceptive input increases mimic clinical settings. This methodology was necessary the report of pain perception [29,30,45]. Specifically, to provide the internal validity to detect a short-term thermal input at .33 Hz or less tends to induce temporal hypoalgesic response, however this methodology also summation in humans, while input at .20 Hz or greater means that no assumptions can be made about longer- does not [28]. The results of the present study indicated term hypoalgesic effects or the effect of these particular that SMT reduced temporal summation, suggesting a interventions applied under different parameters. Third, potential underlying effect of SMT is the inhibition of dor- we did not include sham SMT in this study, so we were sal horn wind-up [7]. Inhibition of dorsal horn wind-up unable to account for hypoalgesic effects associated with would mean the individual was less likely to develop the specific expectation of SMT being a successful inter- chronic LBP, at least chronic LBP caused by this particular vention for pain relief in lumbar innervated areas. Fourth, pain mechanism. It should be noted that this explanation we did not report joint cavitation in this study because is speculative at this time, as only one study directly links previous work demonstrated considerable variability in temporal summation with chronic LBP [69]. One inter- the location of cavitation for lumbar SMT [70], and expe- pretation of these data is that SMT has the potential of riencing cavitation does not appear to affect EMG activity inhibiting dorsal horn windup from peripheral noxious [13] or pain outcomes [71,72]. Last, although we did stimuli. While this an intriguing explanation, we acknowl- implement a comparison group (bicycle), we did not uti- edge that these findings may also be explained by other lize a control group in our current design. This methodo- Page 7 of 10 (page number not for citation purposes) BMC Musculoskeletal Disorders 2006, 7:68 http://www.biomedcentral.com/1471-2474/7/68 JEB received support from the National Institutes of Health T-32 Neural logical selection means that non-specific effects related to Plasticity Research Training Fellowship (T32HD043730). differences in active (bicycle and prone press ups) and passive (SMT) interventions are a viable alternate explana- Kelli Eisenbrown assisted with data collection and data entry. tion to our findings. Megan Hurlburt and Julia Villa assisted with data collection. Conclusion The finding that SMT may have a local hypoalgesic effect Gabrielle Shumrak assisted with data entry. specific to C-fiber mediated input for asymptomatic sub- References jects adds to the previous literature and provides direction 1. Koes BW, Assendelft WJ, van der Heijden GJ, Bouter LM: Spinal for future study. First, this experiment should be repro- manipulation for low back pain. 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BMC Musculoskeletal DisordersSpringer Journals

Published: Aug 15, 2006

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