Advanced Osteopathic and Chiropractic Techniques for Manual Therapists. Giles Gyer
spinal reflex pathways that project onto the target muscle, bypassing the muscle spindle. It reveals an estimate of changes to the alpha motor neuron excitability following spinal manipulation (Burke 2016). In contrast, the TMS technique uses changing magnetic fields to measure the corticospinal tract excitability between the motor cortex and targeted muscle, evoking motor-evoked potential (MEP). It reveals the alterations in the motor cortex excitability after manipulation (Klomjai, Katz and Lackmy-Vallée 2015).
A series of studies conducted by Dishman and colleagues (Dishman and Bulbulian 2000, 2001; Dishman and Burke 2003; Dishman, Cunningham and Burke 2002; Dishman, Dougherty and Burke 2005) has consistently reported a significant but temporary attenuation of alpha motor neuron excitability after spinal manipulation using H-reflexes. One major shortcoming of these studies was that they lacked a no intervention control group. These findings, however, were contrasted by Suter, McMorland and Herzog (2005) who, after observing no alteration in H-reflexes in a subgroup, argued that the decreases in the H-reflex could be due to movement artifact during manipulation. In contrast, with a randomised controlled crossover design, Fryer and Pearce (2012) supported the findings of Dishman and colleagues but opposed Suter et al.’s (2005) conclusion. They reported that inhibition of H-reflexes was not associated with movement artifact as the control group showed no significant changes undergoing the same repositioning of the intervention group. More recently, in a cross-sectional study that included both asymptomatic healthy volunteers and subacute LBP patients, Dishman, Burke and Dougherty (2018) reported suppression of the Ia afferent-alpha motor neuron pathway and a valid and reliable attenuation of the Hmax/Mmax ratio1 following spinal manipulation, which was beyond movement or position artifacts. This finding presents a completely meaningful physiological difference and provides convincing evidence in support of the long-held notion that HVLAT manipulation inhibits the excitability of the alpha motor neuron pool.
Since 2000, changes in MEPs following spinal manipulation have been examined by only a few researchers, and they reported conflicting results. While Dishman et al. (Dishman, Ball and Burke 2002; Dishman, Greco and Burke 2008) reported a transient but significant increase in MEPs compared with baseline after manipulation, Clark et al. (2011) found a slight decrease but no significant alteration in the erector spinae MEP amplitude. In contrast, Fryer and Pearce (2012) observed a significant reduction in MEP amplitudes following manipulation. However, it is to be noted that Fryer and Pearce followed an established protocol to measure MEPs and recorded amplitudes roughly 10 minutes after the intervention, and thus speculated that a transient facilitation of MEPs might have occurred at the beginning. On the other hand, Dishman et al. observed that changes in MEPs returned to baseline 30–60 seconds following manipulation. Nevertheless, such conflicting data do not evidently indicate that spinal manipulation alters the corticospinal tract excitability.
Taken together, although spinal manipulation has been reported to result in a significant decrease in H-reflexes and EMG amplitudes, the clinical relevance of such short-lived changes on the motor neuron pool in the mechanisms that underlie the effectiveness of spinal manipulation is still speculative and needs to be determined.
Autonomic responses
The autonomic nervous system (ANS) acts largely unconsciously and controls the involuntary responses to maintain the internal body environment. It regulates several body processes (e.g., heart rate, respiratory rate, sweat and salivary secretion, blood pressure and pupillary response) and supplies various internal organs that have smooth muscle (e.g., heart, lungs, pupils, salivary glands, liver, kidneys, bladder and digestive glands). The system is regulated from the hypothalamus portion of the brain and is also in control of the underlying mechanisms during a fight-or-flight response (Cannon 1915). The ANS also has potential interaction with the nociceptive (pain) system on multiple levels, which include the brain stem, fore brain, periphery and dorsal horn (Benarroch 2006). Hence, any intervention that influences the functions of the ANS may have significant implications, as this may provide important mechanistic information and even shed some light on the possible neurophysiological mechanisms of that intervention.
In the manual therapy literature, autonomically mediated responses following spinal manipulation have been well established. A variety of outcome measures have been used to determine ANS activity after manipulation, including skin blood flow (SBF) indexes, blood pressure changes, pupillary reflex and heart rate variability (HRV). Studies performed to assess short-term changes in SBF following manipulation suggested a sympathoexcitatory effect, although this effect might be challenged because of overlooked local endothelial mechanisms regulating SBF (Zegarra-Parodi et al. 2015). Comparison of blood pressure changes pre and post manipulation has demonstrated ANS involvement (Welch and Boone 2008; Win et al. 2015). Pupillary reflex is also reported as an indicator of ANS activity (Sillevis et al. 2010). HRV is another well-established marker of cardiac autonomic neural activity, and reflects whether the sympathetic or parasympathetic branches of the ANS are influenced (Win et al. 2015). Therefore, it has been presumed that the effects of spinal manipulation on the ANS might lead to opioid-independent analgesia, influencing the reflex neural outputs on the segmental and extrasegmental levels (Sampath et al. 2015).
Significance of ANS changes following manipulation
Anatomically, the two complementary parts of the ANS include the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The interaction between both these systems is known to influence the stress response of tissues (Cramer and Darby 2013). The SNS plays an active role in mediating the fight-or-flight response and serves as a medium for the efferent communication between the immune system and the central nervous system. It releases catecholamine as an end product, which modulates several immune parameters during acute and chronic inflammation (Elenkov et al. 2000; Pongratz and Straub 2014). The mediating role of SNS between somatic and supportive processes has been demonstrated in Korr’s pioneering work (Korr 2012). In addition, it has also been found that musculoskeletal abnormalities are associated with alterations in cutaneous patterns of sympathetic activity (Korr, Wright and Thomas 1962). In the manual therapy literature, this modulatory effect of the SNS on inflammation has been of special interest, as it may explain some of the neurophysiological effects observed after spinal manipulation. Hence, in the proposed physiological mechanisms of spinal manipulation, a prominent role of the peripheral sympathetic nervous system (PSNS) in the modulation of pain and inflammation has been theorised by both Pickar (2002) and Bialosky et al. (2009).
A number of studies since 2000 have investigated the effects of spinal manipulation on SNS changes. While some studies have reported immediate activation of the SNS following spinal manipulation (Budgell and Polus 2006; Welch and Boone 2008; Win et al. 2015; Zegarra-Parodi et al. 2015), others reported no change in sympathetic activity (Giles et al. 2013; Sillevis et al. 2010; Ward et al. 2013; Younes et al. 2017; Zhang et al. 2006). Welch and Boone (2008) suggested that the autonomic responses observed after manipulation might vary based on the specific segment(s) of the spine manipulated. The authors concluded that sympathetic responses are likely to be elicited from thoracic/lumbar manipulation while parasympathetic responses might result from cervical spine manipulation. Several studies have supported this hypothesis to some extent (Budgell and Polus 2006; Giles et al. 2013; Win et al. 2015). However, contrary findings have also been reported. After measuring the HRV in healthy asymptomatic subjects at two separate time points, Zhang et al. (2006) reported a dominance of the PNS following thoracic manipulation. Recently, using both HRV and baroreflex sensitivity, another study (Ward et al. 2013) conducted on acute back pain patients has also demonstrated increased parasympathetic autonomic control after lumbar manipulation.
However, there were methodological differences between these studies, and no gold-standard technique was used to measure the SNS changes. In addition, the differences in findings were also somewhat dependent on the type of outcome measure used. It appears that the conflicting results mostly came from studies (Budgell and Polus 2006; Giles et al. 2013; Welch and Boone 2008; Ward et al. 2013; Win et al. 2015; Younes et al. 2017; Zhang et al. 2006) that used HRV analysis as a means to determine the nature of autonomic responses after manipulation. The findings of these studies were in favour of either the SNS or PNS. On the other hand, a recent systematic review on post-manipulation