Effects of repeated exposures to experimental cold pain stimulus on pain perception in healthy young Indian men

Abstract

Background: The influence of repeated exposures to cold pain stimulus, a surrogate of clinical pain, has remained largely unexplored. The study was planned to test the effect of repeated exposures to cold pain through cold pressor task on pain sensitivity and vascular reactivity.

Methods: Single-group experimental study. Thirty-seven healthy male volunteers (18-25 years) were exposed to cold pressor tasks for seven consecutive days and on the 14th day on the nondominant hand. The same was repeated on dominant hand on first and seventh days; 31 completed the protocol.

Results: Pain threshold and tolerance in the nondominant hand increased from day 1 to day 7 (p < 0.001) and were positively correlated on day 1 ( $\rho$ =0.45, p = 0.011) and day 7 ( $\rho$ =0.38, p = 0.036). Diastolic blood pressure response was found to increase by day 7 (p < 0.0024) and positively correlated with tolerance. On the dominant hand, the threshold reduced from day 1 to day 7, while tolerance increased. Both threshold and tolerance remained lower than that of nondominant hand. Day 14 values of threshold and tolerance were in between day 1 and day 7 values but not significantly different from both.

Conclusion: Habituation in pain threshold and tolerance was observed on repeated exposure to cold pain, which was not significantly retained till the 14th day. The same was not observed with subjective feeling of pain perception. The increased diastolic blood pressure response is suggestive of peripheral vasoconstriction. Increased tolerance in the dominant hand by day 7 demonstrates a systemic effect in habituation.

Keywords: Cold pain; Pain perception; Threshold; Tolerance; Vascular reactivity.

Fig. 1

Study protocol. CPT: Cold Pressor Task.

Fig. 2

Pre-intervention parameters during the experimental protocol. SBP: Systolic BP; DBP: Diastolic BP; PR: Pulse rate. Values presented are mean and 95% confidence interval.

Fig. 3

Change in pain threshold and tolerance across seven days. Data in the table are Median (Interquartile range) of absolute values of Tolerance and Threshold. Data represented in a graph (Y-axis) are mean in logscale + 1SD % of day 1 nondominant hand value for each subject (values in the graph need not correspond exactly to absolute summary values). Comparisons have been made only using the % values. ∗ Threshold significantly different compared to Day 7 (p < 0.0024). # Threshold significantly different compared to Day 6 (p < 0.0024). ϯ Overall change from Day 1–7 significant, p < 0.0001.

Fig. 4

Correlation between Threshold and Tolerance on Days 1 and 7.

Fig. 5

Comparison of pain unpleasantness (Visual analog scale scores) across days. Nondominant hand Visual Analog Scale scores across days were significantly different by Friedman test (p < 0.001). Pairwise comparison of each day score with Day 1 not different (p > 0.05) by Wilcoxon sign rank test after Bonferroni adjustment for multiple comparisons across days.

Fig. 6

Change in systolic and diastolic blood pressures and pulse rate across seven days. # Significantly different from Day 7 value (p < 0.0024).