Pupillometry to guide postoperative analgesia.

P OSTOPERATIVE pain, by its very nature, is unpleasant for patients and can prolong their recovery. Although it provokes a variety of autonomic responses that are likely to be harmful, pain by definition is subjective. In conscious subjects, pain is thus best evaluated simply by asking. And of course this is the routine clinical approach in which visual analog or verbal response scores are used to guide therapy. However, Aissou et al. make the valid point that many patients in the immediate postoperative period have difficulty evaluating and/or communicating pain intensity. Infants, the demented, nonverbal patients, and those with delirium cannot accurately convey pain intensity. Patients who are obtunded from residual effects of anesthetics may also be unable to distinguish pain from other sensations, or to express the amount of pain they experience. In addition, some patients will relate pain scores that are inconsistent with their behavior, as mentioned by Aissou et al. Adverse effects of untreated acute postoperative pain include limited mobility, impaired ventilation, and increased stress hormones. Untreated perioperative pain may lead to a greater risk for chronic postsurgical pain. Conversely, overtreatment of drug-seeking or especially expressive patients promotes respiratory toxicity and aggravates opioidinduced side effects such as nausea and vomiting, ileus, sedation, and hyperalgesia. An objective measure of perioperative pain that is independent of patient consciousness and communication would thus help guide postoperative opioid administration. Aissou et al. evaluated a simple and well-known bedside test: pupillary dilation in response to a standardized noxious stimulus (PDR). Although their proposed use is to guide opioid administration in patients who cannot accurately convey their need, these investigators compared verbal response pain scores with the PDR in conscious and communicative patients. They found a direct relationship between the magnitude of the PDR brought about by a controlled amount of pressure on the surgical wound and the patient’s requirements for morphine in the postanesthesia care unit. Magnitude of the PDR was also directly related to the patient’s own verbal assessment of their pain. These are important findings, and readers who are interested in this technique might find reviews of the subject useful. The PDR has been studied extensively since it was described more than 300 yr ago by Philippe de La Hire. The imminent 19 century physiologist Moritz Schiff (1823–1896) thought the PDR was an accurate measure of pain and promoted the reflex as an “anesthesiometer.” His work on the pupil and nociception are a logical starting point for those interested in this fascinating subject. There is copious literature on the PDR, and several contemporary investigators have studied the PDR as a measure of nociception and analgesia. There are several overriding themes. First, the PDR is not specific for pain. Rather it is an alerting response that can be elicited by any stimulus that is strong enough to increase the level of arousal. The PDR can thus be used as a measure of pain, but only in controlled situations when confounding factors are well controlled. Only under these circumstances is the PDR magnitude closely related to noxious stimulus intensity and reliably demonstrates dose-dependent depression by opioids and nitrous oxide. Second, it would be a mistake to conclude that “pain dilates the pupil and opioids ablate pain, and thus decrease pupillary dilation.” Anyone who makes acute pain rounds will observe patients with constricted pupils consequent to opioid administration who nonetheless have severe pain. And as Aissou et al. have shown, patients with severe pain in the postanesthesia care unit had constricted pupils just like other patients. It was thus not their pupil size that differed; instead, it was their pupillary response to an evoked stimulus,