Article
Neurophysiological investigation of the cortex in patients with chronic refractory pain and SCS therapy – first insights of a pilot study
Neurophysiologische Untersuchung des Kortex bei Patienten mit chronischen refraktären Schmerzen und SCS-Therapie – erste Ergebnisse der Pilotstudie
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Published: | June 4, 2021 |
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Objective: Although spinal cord stimulation (SCS) is widely used in the treatment of refractory neuropathic pain, the underlying mechanisms of action are still not fully understood. The theory behind that mechanism is based on a simplified model, the gate control theory. It postulates that stimulation of the dorsal column leads to an activation of A-fibers, which closes the gate for nociceptive input via collaterals to the posterior horn and thus prevents the transmission of nociceptive information to the brain. Furthermore, activation of supraspinal centers also seems to play a role. Electroencephalography (EEG) allows the functional investigation of the nociceptive system by means of laser-evoked brain responses. In order to find non-invasive neurophysiological markers that objectify the clinical outcome of patients that underwent SCS implantation, we disentangled cortical generators and assessed frequency-specific oscillations using EEG.
Methods: We applied 40 noxious laser pulses (Nd:YAP, IR 1340 nm, 3 ms, 5mm, 2.00J) at the affected dermatome and its corresponding contralateral region in patients with neuropathic lower limb pain during on or off tonic electrical stimulation of the spinal cord. After each single pulse, patients were instructed to evaluate the perceived pain intensity on a numerical rating scale. EEG data were recorded after electrode implantation. For source analysis of the evoked cortical laser responses we used a distributed source model and evaluated nociceptive-induced oscillations using a wavelet analysis.
Results: In the cortical reconstruction, first results revealed pronounced activity in the on stimulation condition in the primary (S1) and secondary somatosensory cortex (S2) as well as in the posterior insula (pIC). While stimulation was switched off oscillatory low frequency activation could be observed in the contralateral pIC at the affected dermatome. Interestingly, in the tonic on stimulation condition enhanced low frequency activation was also additionally obvious in contralateral S1 as well as in S2.
Conclusion: Our results might elucidate the functional dynamics of tonic SCS at the supraspinal level. It appears that SCS might facilitate activation in the somatosensory cortices and thus help to engage feedforward inhibition processes for pain reduction.