Comparative evaluation of visual analogue scale and pupillary diameter for post-operative pain: An observational study

Background and Aims: An objective understanding of pain guides the effectiveness of various postoperative pain management strategies. The measurement of pain by the visual analogue scale (VAS) is dependent on the individual's cognitive and emotional ability. Pupillary dilatation has been successfully used to measure pain in quantitative terms. The present study was conducted to study the correlation between VAS and pupillary diameter in the evaluation of postoperative pain. Methods: This observational study was carried out on 85 patients 18-65 years of age, scheduled for lower abdominal surgical procedures under general anaesthesia. Postoperative pain assessment was done using the VAS at 6 h, 12 h, 18 h and 24 h. In addition, the pupillary diameter of both eyes was measured simultaneously. Paired t-test and analysis of variance (ANOVA) were used to compare the data, and Pearson's correlation coefficient was calculated to assess the bivariate correlation between VAS score and pupillary diameter. Results: Mean +/- standard deviation (SD) of VAS scores at baseline, 6 h, 12 h, 18 h and 24 h postoperatively were 0, 4.88 +/- 0.88, 3.39 +/- 0.86, 2.19 +/- 0.66 and 1.40 +/- 0.49, respectively. Mean change was significant at all time intervals (P < 0.001). The mean +/- SD change in average pupillary diameter was 0.59 +/- 0.64, 0.44 +/- 0.54, 0.22 +/- 0.34 and 0.07 +/- 0.22 mm, respectively, showing a significant change from baseline (P < 0.05). On overall evaluation, mean pupillary diameter showed a significant incremental trend with an increasing VAS score (P < 0.001). Conclusion: Change in pupillary diameter correlated well with the pain scores (VAS) and thus pupillary diameter can be chosen as an objective measurement of postoperative pain severity.