Fig 1.
Characterization of pain and reflex responses evoked by intradermal electrical stimulation.
A. The first trace shows the absence of an EMG response at the nonpainful detection threshold (DETth). The second trace shows a pain rating at the pain threshold (PAINth), although no EMG response was detected. The third trace shows an EMG response at the NWR threshold (NWRth). B. DETth, PAINth, and NWRth were significantly different (DETth: 0.5 (0.2) mA, PAINth: 2.0 (1.5) mA, NWRth: 13.5 (12.0) mA, f(2) = 44.00, p < 0.0001, post hoc test: **P = 0.0027; ****P < 0.0001, n = 22, Friedman test). C. A total of 198 NWR responses were recorded with no instances of dual EMG bursts within our latency bandwidth (50–150 ms).
Fig 2.
Assessment of vibratory and thermal perception and NWR during nerve block.
A-C. Vibration intensity ratings for 200 Hz (A) and 20 Hz (B) declined significantly during the nerve block (200 Hz: baseline 9.9 (1.4), block 1.8 (4.4), recovery 9.9 (1.2), f(2) = 32.38, p < 0.0001; 20 Hz: baseline 5.1 (1.6), block 0.3 (1.6), recovery 5.1 (1.7), f(2) = 33.77, p < 0.0001, post hoc test: ****P < 0.0001, ns > 0.9999, n = 22, Friedman test). Vibration discrimination (6 trials per condition) was also significantly impaired during the block (3AFC: baseline 6.0 (0.0), block 3.0 (3.0), recovery 6.0 (0.0), f(2) = 40, p < 0.0001, post hoc test: ****P < 0.0001, ns > 0.9999, n = 21, Friedman test). D-E. Cold detection thresholds (CDTs) significantly changed during the block (baseline 28.8 (1.9°C, block 21.6 (5.6°C, recovery 28.0 (2.8°C, f(2) = 35.27, p < 0.0001, post hoc test: ****P < 0.0001, ns = 0.395, n = 22, Friedman test). The dotted line at 23°C represents the lower border of normal values for innocuous cold detection, with cold pain emerging ≤10–14°C. Warm detection thresholds (WDTs) remained unchanged during the block but were elevated in the recovery condition compared to baseline (baseline 35.8 (3.6°C, block 37.0 (3.3°C, recovery 38.4 (5.5°C, f(2) = 7.44, p = 0.024, post hoc test: *P = 0.031, ns (baseline vs block) > 0.9999, ns (baseline vs recovery) = 0.150, n = 22, Friedman test). F. The reflex responses were completely abolished during the nerve block (NWR latencies: baseline 90.0 (14.0) ms, block 0.0 (0.0) ms, recovery 92.5 (13.0) ms, p = 0.052, U = 4113, n = 198, Mann Whitney test).
Fig 3.
Effect of nerve block on pain intensity and quality.
A. Reduction in pain ratings at PAINth during nerve block. Pain ratings at the pre-block PAINth were significantly reduced during the block (baseline 0.9 (0.7), block 0.0 (0.7), recovery 1.0 (0.9), f(2) = 11.55, p = 0.003, post hoc test: *P = 0.013, **P = 0.008, ns > 0.999, n = 22, Friedman test). Pain was completely abolished in 14 participants, greatly reduced in another 4, and increased in the remaining 4 (highlighted in red). B. Pain ratings at the pre-block NWRth did not significantly change across conditions (baseline 2.8 (3.1), block 1.8 (3.2), recovery 3.0 (2.2), f(2) = 6.181, p = 0.0331, post hoc test: ns (baseline vs block and baseline vs recovery) = 0.071, ns (baseline vs block) > 0.9999, n = 22, Friedman test). The 4 participants who showed an increase in PAINth during nerve block in (a) are highlighted in red. C-D. Pain qualities at the pre-block PAINth is shown on the left, while pain qualities at the pre-block NWRth is shown on the right. E-F. Pain qualities at block PAINth is shown on the left and pain qualities at block NWRth is shown on the right. On each occasion, participants chose any number of descriptors and ranked their intensity as mild, moderate, or severe. Thus, the maximum number of “events” for each descriptor equals the number of participants (n = 22). The y-axis shows how many times a descriptor was chosen, and the x-axis shows the complete list of descriptors from the McGill short-form questionnaire.