Advanced Osteopathic and Chiropractic Techniques for Manual Therapists. Giles Gyer
cerebral responses
The relevance of non-specific variables such as expectation and psychosocial factors in the mechanisms of spinal manipulation cannot be totally dismissed (Bialosky et al. 2009). Expectation of good functional outcomes may decrease pain perception without spinal involvement. In addition, a systematic review indicated that spinal manipulation is associated with better psychological outcomes than verbal interventions (Williams et al. 2007). However, studies done to determine the influence of non-specific cerebral processes in manipulation-induced hypoalgesia have found that manipulation has greater and specific effects on pain sensitivity than expectations of receiving the intervention (Bialosky et al. 2008b, 2014). Nevertheless, additional work is needed to determine whether application of spinal manipulation with increased positive expectations could provide an additive effect on pain perception.
Temporal summation
The effects of spinal manipulation on temporal summation of pain constitutes another experimental model that can be used to explain the mechanisms of manipulation-induced hypoalgesia. Temporal summation refers to an increased perception of pain evoked by repetitive painful (noxious) stimulus of the same amplitude and frequency. It represents a psychophysical correlate of a frequency-dependent, progressively increasing excitability of dorsal horn neurons (i.e., wind-up) (Anderson et al. 2013). Wind-up is an interesting model to study for manual therapy researchers, as it is a central phenomenon and not mediated by peripheral mechanisms (Herrero, Laird and Lopez-Garcia 2000). The constant nociceptive input into dorsal horn neurons through temporal summation can trigger transcriptional and translational changes that are related to the short-lived aspect of central sensitisation (Anderson et al. 2013; Staud et al. 2007). Thus, temporal summation can be used to characterise mechanisms of central processing in chronic pain conditions.
Early experimental studies (Bialosky et al. 2008b; George et al. 2006) done with cutaneous heat application to examine the effects of lumbar spinal manipulation reported immediate reduction of temporal summation in the lower extremity regions but not in upper limb dermatomes. This finding suggested that the hypoalgesic effects observed following manipulation might be regionally specific or segmental in nature. To confirm this hypothesis, Bishop, Beneciuk and George (2011) conducted a study to test whether thoracic spinal manipulation reduces temporal summation of pain. In contrast to earlier findings, they found that temporal summation was reduced in both upper and lower extremities, which suggested an involvement of both segmental and descending inhibitory mechanisms in manipulation-induced hypoalgesia. Recently, Randoll et al. (2017), using repeated electrical stimulus, also found that temporal summation of pain was reduced by thoracic spinal manipulation. The authors supported an involvement of segmental mechanism and suggested that deep high-threshold mechanoreceptors might be responsible for HVLA-induced hypoalgesia. However, further research is needed to establish the clinical relevance of these findings.
Conclusion
In this study, we discussed various theories proposed to date to explain the neurophysiological effects of spinal manipulation, and reviewed the mechanistic studies that have been done to validate the relevance of these theories. So far, the exact mechanism(s) through which spinal manipulation works has not been established. Experimental models conducted on both animal and human subjects have indicated that mechanical stimulus applied during manipulation produces a barrage of input into the dorsal horn of the spinal cord, which initiates a cascade of neural responses involving complex interactions between the PNS and CNS. Observing neurophysiological responses following spinal manipulation, these models have suggested possible mechanisms underlying the neuromuscular, autonomic, neuroendocrine and hypoalgesic effects of manipulation. However, the relevance of these implications in relation to the observed clinical effects remains unclear. This is because a majority of the mechanistic studies published to date have mainly investigated short-latency changes or immediate effects of spinal manipulation using their experimental models. In addition, the dose–response relationship associated with the specific neural effect of manipulation has frequently been overlooked in the current literature. Therefore, future studies projected at understanding the possible neural mechanisms of spinal manipulation should carefully consider these two variables.
References
Alonso-Perez, J.L., Lopez-Lopez, A., La Touche, R., Lerma-Lara, S., Suarez, E., Rojas, J. et al. (2017) ‘Hypoalgesic effects of three different manual therapy techniques on cervical spine and psychological interaction: A randomized clinical trial.’ Journal of Bodywork and Movement Therapies 21(4), 798–803. Available at www.ncbi.nlm.nih.gov/pubmed/29037630
Anderson, R.J., Craggs, J.G., Bialosky, J.E., Bishop, M.D., George, S.Z., Staud, R. et al. (2013) ‘Temporal summation of second pain: Variability in responses to a fixed protocol.’ European Journal of Pain 17(1), 67–74. Available at www.ncbi.nlm.nih.gov/pubmed/22899549
Benarroch, E.E. (2006) ‘Pain-autonomic interactions.’ Neurological Sciences 27(Suppl. 2), S130–S133. Available at www.ncbi.nlm.nih.gov/pubmed/16688616
Bialosky, J.E., George, S.Z. and Bishop, M.D. (2008a) ‘How spinal manipulative therapy works: Why ask why?’ Journal of Orthopaedic & Sports Physical Therapy 38(6), 293–295. Available at www.ncbi.nlm.nih.gov/pubmed/18515964
Bialosky, J.E., Bishop, M.D., Price, D.D., Robinson, M.E. and George, S.Z. (2009) ‘The mechanisms of manual therapy in the treatment of musculoskeletal pain: A comprehensive model.’ Manual Therapy 14(5), 531–538. Available at www.ncbi.nlm.nih.gov/pubmed/19027342
Bialosky, J.E., Bishop, M.D., Robinson, M.E., Barabas, J.A. and George, S.Z. (2008b) ‘The influence of expectation on spinal manipulation induced hypoalgesia: An experimental study in normal subjects.’ BMC Musculoskeletal Disorders 9(1), 19. Available at www.ncbi.nlm.nih.gov/pubmed/18267029
Bialosky, J.E., George, S.Z., Horn, M.E., Price, D.D., Staud, R. and Robinson, M.E. (2014) ‘Spinal manipulative therapy – Specific changes in pain sensitivity in individuals with low back pain (NCT01168999).’ The Journal of Pain 15(2), 136–148. Available at www.ncbi.nlm.nih.gov/pubmed/24361109
Bicalho, E., Setti, J.A., Macagnan, J., Cano, J.L. and Manffra, E.F. (2010) ‘Immediate effects of a high-velocity spine manipulation in paraspinal muscles activity of nonspecific chronic low-back pain subjects.’ Manual Therapy 15(5), 469–475. Available at www.ncbi.nlm.nih.gov/pubmed/20447857
Billman, G.E. (2013) ‘The effect of heart rate on the heart rate variability response to autonomic interventions.’ Frontiers in Physiology 4, 222. Available at www.ncbi.nlm.nih.gov/pubmed/23986716
Birznieks, I., Burton, A.R. and Macefield, V.G. (2008) ‘The effects of experimental muscle and skin pain on the static stretch sensitivity of human muscle spindles in relaxed leg muscles.’ Journal of Physiology 586(11), 2713–2723. Available at www.ncbi.nlm.nih.gov/pubmed/18403422
Bishop, M.D., Beneciuk, J.M. and George, S.Z. (2011) ‘Immediate reduction in temporal sensory summation after thoracic spinal manipulation.’ The Spine Journal 11(5), 440–446. Available at www.ncbi.nlm.nih.gov/pubmed/21463970
Budgell, B. and Polus, B. (2006) ‘The effects of thoracic manipulation on heart rate variability: A controlled crossover trial.’ Journal of Manipulative and Physiological Therapeutics 29(8), 603–610.