Abstract
BackgroundThe underlying causes of spinal manipulation hypoalgesia are largely unknown. The beneficial clinical effects were originally theorized to be due to biomechanical changes, but recent research has suggested spinal manipulation may have a direct neurophysiological effect on pain perception through dorsal horn inhibition. This study added to this literature by investigating whether spinal manipulation hypoalgesia was: a) local to anatomical areas innervated by the lumbar spine; b) correlated with psychological variables; c) greater than hypoalgesia from physical activity; and d) different for A-delta and C-fiber mediated pain perception.MethodsAsymptomatic subjects (n = 60) completed baseline psychological questionnaires and underwent thermal quantitative sensory testing for A-delta and C-fiber mediated pain perception. Subjects were then randomized to ride a stationary bicycle, perform lumbar extension exercise, or receive spinal manipulation. Quantitative sensory testing was repeated 5 minutes after the intervention period. Data were analyzed with repeated measures ANOVA and post-hoc testing was performed with Bonferroni correction, as appropriate.ResultsSubjects in the three intervention groups did not differ on baseline characteristics. Hypoalgesia from spinal manipulation was observed in lumbar innervated areas, but not control (cervical innervated) areas. Hypoalgesic response was not strongly correlated with psychological variables. Spinal manipulation hypoalgesia for A-delta fiber mediated pain perception did not differ from stationary bicycle and lumbar extension (p > 0.05). Spinal manipulation hypoalgesia for C-fiber mediated pain perception was greater than stationary bicycle riding (p = 0.040), but not for lumbar extension (p = 0.105).ConclusionLocal dorsal horn mediated inhibition of C-fiber input is a potential hypoalgesic mechanism of spinal manipulation for asymptomatic subjects, but further study is required to replicate this finding in subjects with low back pain.
Highlights
The underlying causes of spinal manipulation hypoalgesia are largely unknown
spinal manipulative therapy (SMT)'s underlying effect would be as a "counter-irritant" stimulus to peripheral nociceptive input received by dorsal horn cells [7]. If these neurophysiological processes occurred, SMT would have a measurable hypoalgesic effect on pain perception. This topic was reviewed by Vernon [24], with SMT hypoalgesia observed by decreased cutaneous receptive field from pin-prick [25], tolerance from electrical current [26], and mechanical pressure [27], Collectively, these results demonstrate SMT's potential for dorsal horn mediated pain inhibition
All interventions were associated with first pain hypoalgesia, but only SMT had a consistent association (Table 2)
Summary
The underlying causes of spinal manipulation hypoalgesia are largely unknown. The beneficial clinical effects were originally theorized to be due to biomechanical changes, but recent research has suggested spinal manipulation may have a direct neurophysiological effect on pain perception through dorsal horn inhibition. There is considerable evidence suggesting that spinal manipulative therapy (SMT) is an effective treatment for subgroups of patients with low back pain (LBP) [1,2,3,4]. While the clinical literature provides strong support for the use of SMT for certain patients, its underlying effects and mechanisms are not widely understood. Subjects performing general physical activity or specific back exercises are clinically relevant comparison groups missing from the current SMT hypoalgesia literature. SMT does appear to have demonstrable mechanical (i.e. peak forces and displacement) effects on spinal segments [917]. Peak forces generated from SMT vary greatly by practitioner, suggesting this factor is not related to clinical improvement [18,19]. Lasting positional changes following SMT have not been observed [20], suggesting this factor is not related to clinical improvement
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