Abstract
Thoracic spinal manipulation (SM) is a routinely used intervention in clinical practice. While the clinical benefits of SM have been well established, the exact mechanism through which SM results in clinical benefits has been of research interest. A narrative review of literature indicated that SM has an influence at the peripheral, spinal and supraspinal levels, including the autonomic nervous system (ANS), especially the sympathetic nervous system (SNS) and the endocrine system. Hence, a hypothesis was proposed that a thoracic SM would result in neuro-endocrinal changes. Keeping in mind the breadth of the literature, two systematic reviews were undertaken to determine the effects of SM on the (1) ANS and (2) biochemical markers. The first systematic review (5 studies) found support for a differential ANS response based on the region manipulated and moderate level evidence was established that a thoracic SM results in SNS response. The second review with meta-analysis (8 studies) established moderate level evidence in favour of SM in influencing various biochemical markers such as substance-P, neurotensin, cortisol and oxytocin.
Based on the review (narrative and systematic) findings, a parallel group randomised controlled trial (RCT) was designed to investigate the neuroendocrine changes following a thoracic SM in healthy men. Near infrared spectroscopy (NIRS) parameters such as oxy-haemoglobin and tissue oxygenation index (TOI) were used as an index of SNS activity, heart rate variability (HRV) was used as an index of ANS activity, and salivary testosterone to cortisol (T/C) ratio was used as an index of HP axis activity. The findings from the study indicated that a thoracic SM has an effect on HPA axis activity as indicated by changes in salivary cortisol immediately and T/C ratio many hours following SM. A pattern of sympathetic response in the thoracic SM group was also noted. These findings provided important support to the hypothesis as noted in changes in the T/C ratio and NIRS parameters. A major strength was that there were no missing data or attrition of participants.
A logical next step was to follow-up with a trial in symptomatic population. Achilles tendinopathy (AT) is a chronic condition in which the involvement of the neuroendocrine system has been established in its pathogenesis. Further, the anatomical location of the Achilles tendon and the prevalence of AT made it a suitable model to investigate the neuroendocrine effects of thoracic SM. Though the methodology of the pilot RCT was successful, a few methodological changes were necessary for Trial-2, such as (1) study design (cross-over study design); (2) Sample population (inclusion of both males and females); and (3) additional NIRS probe on Achilles tendon (measurement of tissue oxygenation of the Achilles tendon as well as calf muscle). The findings support the hypothesis that a thoracic SM results in changes in the neuro-endocrine system in people with AT. These changes were evident in the T/C ratio, salivary testosterone and TOI of the calf muscle. However, no changes were evident for salivary cortisol or HRV values or TOI of a tissue field that incorporated the tendon. This thesis adds to the knowledge around the mechanisms of SM. This may have important implications for manual therapy practice. The findings from the thesis have generated numerous questions which may be of interest of future studies.