A novel radiant heat test for assessing pain threshold in human subjects: Measurement stability

Abstract

New methodologies to assess analgesic response in humans are needed to better integrate preclini­ cal and clinical data. In the present study we examined the test-retest stability of an innovative radi­ ant heat methodology compared with an electrical stimulation methodology. For the radiant heat task, a modified rodent tail flick apparatus was used, The latency for fmger withdrawal was recorded. For the electrical stimulation tasks, subjects placed two fingers on two electrodes from which they received a brief series of increasingly intense electrical stimulations. Maximum stimulus intensity (in milliamps) delivered was recorded. On each of 4 test days, the subjects received five test trials with a lO-min in­ terval between trials. Allthe subjects were tested twice on eachapparatus in a counterbalanced design. Finger withdrawal latencies for the radiant heat task did not differ significantly across test trials or test days. Finger withdrawal scores for electrical stimulation increased significantly across test trials as well as test days. These data show that the radiant heat method generates consistent latencies across trials and days, whereas shock produces trends over time. The radiant heat task, which is convenient to operate and inexpensive to build, appears promising as a reliable test of pain threshold in humans. Many attempts have been made to develop a simple and reliable laboratory method to assess pain thresholds in human volunteers. Some ofthese methods include ice water hand immersion, thermal heat, electrical stimula­ tion, and skin pressure tests (Chery-Croze, 1983; Collier, Mehta, & Spear, 1990; Harris & Rollman, 1983). Among these different stimuli, thermal heat and electrical stim­ ulation have been chiefly used to induce experimental pain (Chery-Croze, 1983). Additionally, these methods have been frequently used in nonclinical studies in which repeated exposure to the stimulus is often required. How­ ever, considerable variation in pain threshold values have been reported with electrical stimulation. For instance, Collier et a1. reported significant variation in threshold responses following repeated exposure to electrical stim­ ulation. This may be due to a developed tolerance or adaptation that may occur over time to the sensations pro­ duced by electrical stimulation. Additionally, Harris and Rollman reported that electrical stimulation is often ac­ companied by subjective reports of anxiety and stress. Hence, the psychological components associated with electrical stimulation may lead to variability in responding. Thermal heat tests have been successfully used to de­ termine pain threshold in human volunteers for the last three decades (Hardy, Goodell, & Wolff, 1952). The two most commonly employed methods of delivering ther­ mal heat are radiant heat (use ofa beam of light or laser pulse) and conducted heat (use of Peltier thermodes on