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Manipulation and Pain Sensitivity Nov/Dec 2012
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Nov/Dec 2012

Manipulation and Pain Sensitivity

Summary provided by Dean L. Smith, D.C., Ph.D.

Coronado RA, Gay CW, Bialosky JE, Carnaby GD, Bishop MD, George SZ. Changes in pain sensitivity following spinal manipulation: A systematic review and meta-analysis. J Electromyogr Kinesiol. 2012 Jan 30. [Epub ahead of print] PubMed PMID: 22296867
Spinal manipulation (SMT) is commonly used for treatment of individuals with pain. Despite the clinical effectiveness of SMT, the mechanisms by which it reduces pain/disability remain largely unknown. Studies using pain sensitivity testing for measuring the response to SMT are appropriate for considering potential mechanisms of action.
Reductions in pain sensitivity or hypoalgesia following SMT may be indicative of a mechanism related to pain processing. Examining the hypoalgesic mechanisms may provide insight into how SMT works. For instance Bialosky (2009) reported an immediate hypoalgesic response in the lower extremity to a specific noxious thermal stimulus following lumbar SMT in LBP (low back pain) patients. This effect is likely at the spinal cord level due to: 1) effect localized to innervated area; 2) findings specific to temporal summation of pain.
Characteristics of pain sensitivity include: 
1) Sensory modality – thermal, mechanical, electrical etc.
2) Psychophysical response – assessed by minimal amount of stimulus to generate pain (threshold) or change in pain sensitivity to repeated stimulation (temporal summation) or multiple locations of stimulation (spatial summation).
3) Location of stimulus – local or remote to injured or intervention area.
In studies of pain sensitivity related to SMT, pain sensitivity is usually checked before and after single or multiple SMT’s.
The purpose of this paper was to review and analyze the growing literature on SMT and its hypoalgesic effects and effects on pain sensitivity.
Specific interests of the authors of this paper were to: 1) quantify pain sensitivity after SMT; 2) characterize SMT based on sample (ie chronic pain vs healthy folks) and location of assessment (local or remote to SMT site).
If remote changes in pain sensitivity exist, it may indicate a generalized effect such as modulation through nociceptive afferent processing in the CNS.
Studies that were examined in this paper included RCTs excluded case reports, case series, and single case studies. Studies included healthy subjects, or patients with musculoskeletal disorders. Non-musculoskeletal conditions were excluded (ie cardiac).
SMTs in the studies included high velocity, and low-amplitude manipulations that did not have to generate an audible release. Both manual manipulations and instrument manipulations were included for analysis. Studies were excluded if the sole effect of SMT could not be established.
The outcome measure that was assessed in this systematic review was pain sensitivity (including characteristics such as sensory modality, response, and location). Quality assessment of the articles used the 12 item criteria from Cochrane Back Review Group (studies having met 6 or more of the 12 items are low risk of bias (higher quality). Only the immediate effect of SMT on pressure pain threshold (PPT) had an adequate number of studies to analyze.
The authors found 20 studies (n=974) to include in their analysis. Nine studies had symptomatic subjects (symptoms including: epicondylalgia, LBP, neck pain, and SI joint pain). Differences in manipulation site and type of manipulations were employed in the studies. No two studies used the same thoracic SMT technique. Studies investigated responses to chemical, electrical, thermal and mechanical stimuli. The included responses were threshold and temporal summation of pain. All studies using mechanical measures examined PPT, but there was variation in the region examined (neck, elbow, head, lumbar, trapezius, web space of fingers/toes). All but 3 studies only looked at the immediate effect of SMT.
Only 10 of the 20 studies met criteria for meta-analysis (all 10 investigated immediate effect of SMT).
Key findings of this study were:
•     A summary effect suggested a small but favorable effect of SMT on increasing PPT compared to other treatments.
•     Summary effect estimate demonstrated a small favorable, but non-significant effect of SMT on increasing PPT in both the clinical and healthy populations.
•     The summary effect estimate demonstrated a small favorable, but non-significant effect of SMT on increasing PPT at the local site.
•     For the remote site, the summary effect estimate demonstrated a small, but significant effect for SMT on increasing PPT.
•     SMT has a favorable effect on increasing PPT, or reducing pain sensitivity, when compared to other forms of intervention. This effect on PPT was largest when measured at a remote anatomical region.
•     When examining the effect of SMT based on population, there did not appear to be a different effect when studied in healthy versus clinical samples. Therefore, this difference in pain state does not seem to affect the response to SMT.
•     The findings lend support to a possible general effect of SMT beyond the effect expected at the local region of SMT application.
•     Mechanisms of SMT are theorized to result from both spinal cord mediated mechanisms and supraspinal mediated mechanisms.
So, what does this study tell us? It tells us that SMT has a pain reducing effect as measured by PPT. Additionally, the effect of SMT on pain reduction was statistically significant at remote locations. For example, adjusting the neck yielded reduction in pain at the elbow.
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