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

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(degrees)C and 50 degrees C, respectively), and peak duration (0.5 s). Both produced summation that peaked at moderate pain (similar to 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 degrees C baseline and 50(degrees)C peak temperatures, at >= 0.33 Hz stimulus frequency, and peak pulse durations of at least 0.5 seconds.