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

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