The sympathetic and parasympathetic systems lead to permanent pupillary fluctuations. These fluctuations are different from pupillary diameter variations (PD) that increases proportionally with pain intensity. The pupillary fluctuations are measured using the variation coefficient of PD (VCPD). David J.Charier et al. and his colleagues conducted a prospective study based on finding the correlation between PD, the increase of PD  during a contraction, and VCPD with pain and then undertook a comparative analysis among such associations. The pain was rated using a numeric rating scale (NRS) during obstetrical labor. 

The study included 40 patients and 160 simultaneous ratings (NRS, PD, and VCPD). Among these 160 concurrent scores, 40 were in the presence of uterine contractions and 40 were in the absence of such contractions, before and 20 minutes after epidural analgesia. The results indicated that there occurs a strong correlation between  VCPD (r = .77) and pain rated using an NRS as compared to PD increase (r = .42). The potential of VCPD to predict the occurrence of NRS scores ≥4 during obstetrical labor is .97 (confidence interval, .93–1.0).  VCPD correlates more strongly than PD, and it increases with pain rated using the NRS when measured over 10 seconds during contraction. This stronger correlation enables to assess antinociception-nociception balance easily.

It is concluded that VCPD allows for an objective assessment of pain in laboring women. It is helpful in evaluating pain in noncommunicating patients: newborns or very old patients, patients with acute psychological conditions, assessment during the immediate postoperative period, or in intensive care units.