Decreased initial pupil size and shortened constriction latency due to negative mood states and mental fatigue in clinical subacute pain models.

Previous clinical studies on pupillary light reflex parameters showed shortened constriction latency (LAT) and decreased initial pupil size (INIT) due to increased numeric rating scale (NRS) scores, reflecting subjective pain intensity, in subacute pain models. As noxious stimulation causes pupil dilation, decreased INIT (pupil constriction) was an unexpected finding. Pain-related negative moods and mental fatigue might have caused this finding. We aimed to investigate how psychological states affect LAT/INIT in two subacute pain models. For psychological assessment, we used the Profile of Mood States (POMS) and calculated the total mood disturbance (TMD) score using six mood categories. We collected data on NRS scores, POMS-related scores, LAT, and INIT through a longitudinal study. Using linear mixed-effects models, we evaluated the association of TMD scores with NRS scores, LAT, and INIT. Furthermore, we performed subanalyses targeting two mood categories that represent mental fatigue: Fatigue-Inertia (Fatigue) and Vigor-Activity (Vigor). We investigated whether obtained results can be identical in both models. In total, 141 patients were enrolled. Increased NRS scores were associated with increased TMD/Fatigue scores and decreased Vigor scores, being associated with shortened LAT/decreased INIT in both models. Score changes in NRS, TMD, Fatigue, and Vigor indicated that increased subjective pain intensity can be associated with negative mood states, especially mental fatigue. This study revealed that negative moods and mental fatigue induced by acute pain can be associated with shortened LAT/decreased INIT in both models, indicating clinical potential of LAT/INIT as objective indicators reflecting the psychological states for pain assessment.