Assessing Pain Using the Variation Coefficient of Pupillary Diameter.

Pupillary diameter (PD) varies under the influence of the sympathetic as well as parasympathetic systems, increasing proportionally with pain intensity. Such variations however, should not be confused with pupillary fluctuations, which refer to the fast and permanent PD fluctuations induced by the ongoing interplay between the sympathetic and parasympathetic systems, which we propose to measure using the variation coefficient of PD (VCPD). This study aimed first at correlating PD, PD increase during a contraction, and VCPD, with pain rated using a numeric rating scale (NRS) during obstetrical labor, and then at comparing such correlations with each other. Forty patients were included in the study, and 160 simultaneous ratings (NRS, PD, and VCPD) were taken: 40 in the presence of uterine contractions and 40 in the absence of such contractions, before and 20 minutes after epidural analgesia. VCPD correlates more strongly (r = .77) than PD increase (r = .42) with pain rated using a NRS. The ability of VCPD to predict the occurrence of NRS scores ≥4 during obstetrical labor is .97 (confidence interval, .93-1.0). When measured over 10 seconds during contraction, VCPD correlates more strongly than PD increase with pain rated using the NRS. Such stronger correlation allows for an easy assessment of antinociception-nociception balance. PERSPECTIVE: The VCPD allows for an objective assessment of pain in laboring women. It could allow for an easy assessment of pain in noncommunicating patients: newborns or very old patients, patients with serious psychological conditions, assessment during the immediate postoperative period, or in intensive care units.