Investigation of neural markers in chronic pain in spinal cord injury: a TMS and EEG preliminary study and a brief systematic review

Objectives: We aim to understand further neural mechanisms in spinal cord injury (SCI) pain as indexed by transcranial magnetic stimulation (TMS) and quantitative electroencephalography (qEEG). Design: Observational study and a brief systematic review. Methods: Six SCI pain and 10 healthy subjects were assessed by TMS, qEEG and clinical measures involving Visual Analog Scale (VAS) for pain perception, Pressure-Pain threshold (PPT), and Diffuse Noxious Inhibitory Control (DNIC). We also conducted a review of the literature to compare our TMS and EEG results with similar studies in chronic neuropathic pain associated with SCI, chronic neuropathic pain alone and SCI without pain. Results : Our findings in SCI pain group were similar to the literature involving decreased intracortical inhibition (ICI), decreased peak frequency and decreased alpha power. Additionally we showed that EEG power ratios (alpha/theta ratio and alpha/low-beta) positively correlated with changes in DNIC. Conclusion: The similarities between our findings and the literature support the idea that SCI pain has a similar neural signature when compared to other deafferentation syndromes.  The correlation between alpha ratios and response to DNIC indicates the existence of oscillations that can be used as a guide for future studies investigating neurophysiologic predictors of treatment response.