Cortical evoked responses to evaluate the effect of spinal cord stimulation on the pain pathways.

Objectives: The mechanisms of spinal cord stimulation (SCS) are insufficiently understood. Conditioned pain modulation (CPM) measures how a painful stimulus is affected by a second painful stimulus. We investigated whether cortical evoked response can be used to evaluate CPM in a patient treated with burst, tonic and sham SCS. Methods: A 40-year-old patient underwent 3 magnetoencephalography sessions (burst, tonic, sham SCS) with 1-week intervals. Painful electrical stimuli were applied to the tibial nerve before, during and after CPM (conditioning: icepack on forearm). Evoked responses were analysed in the primary somatosensory and anterior cingulate cortices. Results: Before CPM, the highest evoked response amplitude occurred under sham SCS, followed by tonic SCS. During CPM pain ratings remained unchanged. However, CPM reduced evoked response amplitudes in the primary somatosensory cortex under tonic and sham SCS and in the anterior cingulate cortex under all SCS paradigms. Conclusions: CPM reduced evoked response amplitudes, while pain ratings were unaffected, suggesting neurophysiological measures provide additional insights into CPM effects. Tonic and burst SCS both appeared to reduce cortical capacity to attend to stimuli, with burst showing the greatest effect. Significance: Cortical responses offer a valuable tool to assess pain pathways. Larger scale studies are needed to enhance our understanding of SCS mechanisms.