Differences in spinal and supraspinal responses to surface electrical stimulation applied through patch and pin electrodes

J.A. BIURRUN MANRESA; F.G. ARGUISSAIN; S. BERGMANN; L.G. DAL; L.B. HENNINGSEN; C.V. JENSEN; L. KRISTENSEN; C.F. LINDSTRØM; O.K. ANDERSEN

Buenos Aires

Congreso; 15th World Congress on Pain; 2014

Aim of Investigation: The aim of this study was to characterize and quantify differences in spinal and supraspinal responses to surface electrical stimulation applied via patch and pin electrodes.Methods: Ten healthy volunteers (5 men and 5 women, age 20-28 years) participated in the experiment. Electrical stimulation was used to simultaneously evoke nociceptive withdrawal reflexes (NWR, spinal response) and somatosensory evoked potentials (SEP, supraspinal response). The anode (50 × 90 mm) was placed at the dorsum of the foot, whereas the cathode was placed on the arch of the foot. The cathode was either a standard patch electrode (15 × 15 mm) or a circular array of 16 blunt stainless steel pins (1 mm pin length, 10 mm diameter). Each stimulus consisted of a train of five constant-current pulses, 1 ms pulse width, delivered at 200 Hz by a computer-controlled constant-current stimulator. The stimulation was repeated with a random inter-stimulus interval ranging from 8 to 12 s, at two different stimulation intensities, calculated as 1.0 and 1.2 times the NWR threshold (NWR-T, i.e. the minimum intensity required to elicit the NWR). After the NWR-T was determined for each electrode, 6 blocks of 30 stimulations were performed (3 blocks per electrode type, 15 stimuli per stimulation intensity in each block), with a 5 min break between blocks. EMG was recorded from the tibialis anterior muscle, sampled at 2048 Hz and filtered (bandpass 5-500 Hz). The NWR responses were quantified by root-mean square (RMS) amplitudes in the 60-180 ms post-stimulation interval. Continuous EEG data was recorded by a 32-channel system, sampled at 256 Hz per channel, filtered (bandpass 0.5-30 Hz, notch 50 Hz), re-referenced to the linked mastoids, divided into epochs of 2000 ms and averaged to obtain SEP responses. Only SEPs from the electrode at the vertex were subsequently analyzed. SEPs were characterized by two negative peaks (N1 and N2) around 70-140 ms, and a positive peak (P2) around 250-300 ms. Pain intensity ratings to electrical stimulation were recorded using a visual analog scale. Differences in NWR thresholds for patch and pin electrodes were assessed using a paired t test. For each stimulation intensity, a two-way repeated measures analysis of variance (RM ANOVA) was used to detect differences in the NWR RMS amplitudes, whereas a point by point RM ANOVA was used to analyze the time course of the SEP responses. Block (1st, 2nd or 3rd) and electrode (patch or pin) were used as fixed factors. Post hoc analysis was performed using the Student-Newman-Keuls test. Values are presented as mean ± standard deviation, and p values < 0.05 were regarded as significant.Results: The NWR-T was higher for the patch electrode (6.8 ± 3.6 mA) compared to the pin electrode (3.7 ± 2.0 mA, p = 0.003). When the lower stimulation intensity (1.0 x NWR-T) was used, the patch electrode resulted in larger NWR RMS amplitudes and higher pain intensity ratings compared to the pin electrode (p = 0.018 and p = 0.006, respectively). When the higher stimulation intensity (1.2 x NWR-T) was used, a significant interaction between block and electrode type was found in NWR RMS amplitudes (p = 0.025). Post hoc analysis showed that NWR RMS amplitudes were significantly higher in the 1st block, compared to the 2nd (p = 0.022) and 3rd blocks (p = 0.044) when the patch electrode was used, whereas there were no significant differences between blocks when the pin electrode was used. Moreover, pain intensity ratings were also higher for the patch electrode compared to the pin electrode (p = 0.010). Regarding SEP responses, N1 and N2 peaks were generally larger in the 1st block compared to the other blocks regardless of the electrode type. The P2 peak was also larger in the 1st block, but only after stimulation with the patch electrode; no significant difference were found in the P2 peak after stimulation with the pin electrode.Conclusions: Electrical stimulation through the pin electrode resulted in smaller pain ratings, required less current intensity to evoke the NWR, and elicited more stable NWR and SEP responses over time compared to the patch electrode.