Portable infrared pupillometers provide an objective measure of pupil size and pupillary reflexes, which for most clinicians was previously only a visual impression. But despite the fact that pupillometry can uncover aspects of how the human pupil reacts to drugs and noxious stimulation, the use of pupillometry has not gained widespread use among anesthesiologists and critical care physicians. The present review is an introduction to the physiology of pupillary reflexes and the currently established clinical applications of infrared pupillometry, which will hopefully encourage physicians to use this diagnostic tool in their clinical practice. Portable infrared pupillometry was introduced in 1989. The technology involves flooding the eye with infrared light and then measuring the reflected image on an infrared sensor. Pupil size, along with variables of the pupillary light reflex and pupillary reflex dilation, is calculated by the instrument and displayed on a screen immediately after each time-stamped measurement. Use of these instruments has uncovered aspects of how the human pupil reacts to drugs and noxious stimulation. The primary clinical applications for portable pupillometry have been in the assessment of brainstem function. Portable pupillometry is useful in the management of pain because it allows for assessments of the effect of opioids and in the titration of combined regional-general anesthetics.

The observed pattern of thermoregulatory impairment is similar to that produced by most general anesthetics: a slight increase in the sweating threshold and a substantial, linear decrease in the vasoconstriction and shivering thresholds.

Dilation of the pupil in response to a noxious stimulus is a measure of opioid effect in isoflurane-anesthetized volunteers. In contrast, the pupillary light reflex is unaffected by alfentanil during isoflurane anesthesia. These data suggest that stimulus-induced pupillary dilation may be used to evaluate the analgesic component of a combined volatile and opioid anesthetic.

We studied the effects of noxious stimuli on arterial blood pressure, heart rate, pupil size, and the pupillary light reflex in 13 volunteers anesthetized with either isoflurane or propofol. Those given isoflurane (n = 8) were anesthetized twice, in a randomly selected order, once at an end-tidal concentration of 0.8% and once at 1.2%. An intense noxious stimulus was provided by electrical stimulation applied to skin of the abdominal wall (65-70 mA, 100 Hz). Hemodynamic values and pupillary responses were recorded immediately before stimulation and at 15-60-s intervals during 8 subsequent min. In the volunteers given isoflurane (both concentrations), stimulation significantly increased pupil size (265 +/- 44%) and the amplitude of the light reflex (233 +/- 23%). In contrast, mean heart rate and systolic blood pressure increased only 19 +/- 7% and 13 +/- 7% after stimulation. Five additional volunteers were anesthetized twice with propofol (approximately 3 micrograms/mL plasma concentration) and 60% nitrous oxide. The same electrical stimulus was applied, and hemodynamic and pupillary measurements were obtained. During one propofol anesthetic, an esmolol infusion (100 micrograms.kg-1 x min-1) was started 10 min before stimulation to determine whether this agent would blunt the pupillary response. The pupillary light reflex increased more than 200% during both propofol anesthetics with or without esmolol; once again, heart rate and blood pressure changed little. We conclude that with these experimental conditions, the pupil is a more sensitive measure of noxious stimulation than the commonly used variables of arterial blood pressure and heart rate.

Opioid administration with significant accompanying hypercarbia and hypoxia results in pupil diameters of 2 to 3 mm and a reduced but quantifiable pupillary light reflex. The authors conclude that the pupillary examination and evaluation of the light reflex remain useful for neurologic assessment during opioid toxicity.