Evaluation Of The Entropy Of The Nociceptive Withdrawal Reflex Conditioned By High And Low Frequency Electrical Stimulation

The nociceptive withdrawal reflex (NWR) is evoked by applying a nociceptive stimulus to a limb. The NWR can be used as an objective electrophysiological measure for evaluating central nociceptive processing. In turn, in the phenomenon known as central sensitization, the central nervous system may alter the spinal nociceptive processing in intensity, duration and spatial extension in response to intense nociceptive input. This sensitization can be modeled in healthy subjects by high or low frequency electric stimulation (HFS or LFS, respectively). In this work, surface electromyography signals (sEMG) recorded after electrical stimulation to induce the NWR were evaluated using two kinds of entropies: approximate and sample, in order to obtain an estimator to characterize these signals. To do so, NWR signals from 15 healthy volunteers were analyzed. The experiment consisted in 3 sessions: HFS, LFS and a Control session without conditioning stimulation. The NWR was recorded twice before the conditioning stimulation (pre-conditioning) and seven times after (post-conditioning). The study showed that both entropies allowed to detect differences between sessions. In particular, the entropies in the LFS sessions were significantly lower compared to HFS and Control sessions. These results support the idea that entropies are able to characterize the NWR.