Abstract
Background and aim
Previous studies have shown that patients with chronic pain had significantly lower nociceptive withdrawal reflex thresholds (NWR-T) and electrical pain thresholds (EP-T) than healthy controls. Patients with chronic pain and implanted spinal cord stimulator (SCS) system presents an opportunity to study dynamic pain sensitivity changes in a situation where patients just have been stimulated (i.e., pain-free or greatly reduced pain) and compare with the situation where patients are not stimulated (i.e., experiencing severe pain).
Methods
Seventeen volunteers with chronic neuropathic pain and a SCS implanted for pain relief participated in the experiment. Volunteers were asked to turn off the SCS and refrain from medication at least 8 h before the experiment. Electrical stimulation (train-of-five 1-ms pulses delivered at 200 Hz) was applied to the arc of the foot in order to elicit the NWR and electromyographic responses were recorded from tibialis anterior muscle. A staircase procedure was used to assess the NWR-T and a Visual-Analog-Scale (VAS) scale (range 0–10) was used to assess the EP-T. After the initial testing, the SCS was turned on, and thresholds were reassessed after 1 h. Paired t-tests were used to compare the thresholds before and after the SCS was turned on. All values are presented as mean ± SD.
Results
The NWR-T were significantly higher after the SCS was turned on (before: 16.3 ±8.1 mA; after: 19.0 ± 10.9 mA; p = 0.028). EP-T did not show any significant differences (before: 3.2 ± 1.5 mA; after: 2.9 ± 2.0 mA; p = 0.324).
Conclusion
Results showed a higher NWR-T after the SCS was turned on, which indicates a depression of spinal nociception. Moreover, the NWR-T was able to detect ongoing and relatively quick changes in pain sensitivity.