Optimizing Temporal Summation of Heat Pain Using a Constant Contact Heat Stimulator

Abstract

Purpose

Temporal summation (TS) of pain occurs when pain increases over repeated presentations of identical noxious stimuli. TS paradigms can model central sensitization, a state of hyperexcitability in nociceptive pathways that promotes chronic pain onset and maintenance. Many experimenters use painful heat stimuli to measure TS (TS-heat); yet, TS-heat research faces unresolved challenges, including difficulty evoking summation in up to 30–50% of participants. Moreover, substantial variability exists between laboratories regarding the methods for evoking and calculating TS-heat.

Patients and Methods

To address these limitations, this study sought to identify optimal parameters for evoking TS-heat in healthy participants with a commercially available constant contact heat stimulator, the Medoc TSA-II. Working within constraints of the TSA-II, stimulus trains with varying parameters (eg, stimulus frequency, baseline temp, peak temp, peak duration, testing site) were tested in a sample of 32 healthy, chronic pain-free participants to determine which combination best evoked TS-heat. To determine whether TS scoring method altered results, TS-heat was scored using three common methods.

Results

Across all methods, only two trains successfully evoked group-level TS-heat. These trains shared the following parameters: site (palmar hand), baseline and peak temperatures (44°C and 50°C, respectively), and peak duration (0.5 s). Both produced summation that peaked at moderate pain (~50 out of 100 rating).

Conclusion

Future TS-heat investigations using constant contact thermodes and fixed protocols may benefit from adopting stimulus parameters that include testing on the palmar hand, using 44°C baseline and 50°C peak temperatures, at ≥0.33 Hz stimulus frequency, and peak pulse durations of at least 0.5 seconds.

Keywords:

• temporal summation
• heat pain
• second pain
• wind-up
• Medoc TSA-II

Summary

This study sought to identify optimal stimulus parameters for evoking temporal summation of heat pain (TS-heat) using a commercially available heat stimulator, the Medoc TSA-II, to provide methodical recommendations for future investigations. Based on our findings, future protocols should deliver stimulations to the palmar hand (as opposed to the volar forearm), utilize the highest possible 44°C baseline and 50°C peak pulse temperatures, and use a peak pulse duration of 0.5 seconds. Pulse intervals of 0.33 and 0.40Hz were successful for evoking TS.

Disclosure

The authors have no conflicts of interest to report in this work.