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Pupils constrict with opioid exposure. They also tend to dilate when someone is in pain, and this autonomic phenomenon has been studied in adults as a way to measure pain. Researchers studied 30 children (age range, 9–17 years) undergoing pectus excavatum repair to determine whether pupillary response, measured with a noninvasive handheld pupillometer, was associated with self-reported postoperative pain.
For each 1-point increase on a 10-point pain visual analog scale, there was a statistically significant linear increase in maximum pupil constriction velocity (0.11 mm/second) and percent change in pupil size (0.4%) with a standard light stimulus (while controlling for opioid dose). Not surprisingly, there was a significant linear decrease …