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Rachel Eshima McKay1, Merlín D Larson1

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Opioid administration significantly reduces pupillary unrest in ambient light (PUAL), serving as a specific indicator of central opioid effects. This finding is crucial for monitoring patients receiving opioids.

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Area of Science:

  • Neuroscience
  • Pharmacology
  • Ophthalmology

Background:

  • Opioids are known to cause pupillary constriction.
  • The effects of opioids on pupillary unrest and dynamic pupillary light reflex parameters remain uncharacterized.
  • Distinguishing opioid-induced pupillary changes from brain insults is vital, especially with portable pupillometry in critical care.

Purpose of the Study:

  • To investigate the impact of remifentanil infusion on pupillary unrest and dynamic pupillary light reflex parameters.
  • To determine which pupillary responses are most consistently affected by remifentanil.
  • To assess the utility of pupillary unrest in identifying central opioid effects.

Main Methods:

  • Eighteen volunteers were studied using two portable infrared pupillometers.
  • Pupillary unrest in ambient light (PUAL) was measured.
  • Neurological pupillary index (NPi), constriction velocity (CV), pupil diameter (PD), latency, and % reflex (% reflex) were assessed following light stimulation.
  • Remifentanil was administered progressively to achieve opioid effect concentrations associated with respiratory depression.

Main Results:

  • Remifentanil administration ablated pupillary unrest in ambient light (PUAL), decreasing it by 94% from baseline.
  • Other pupillary measurements (PD, CV, % reflex) decreased by 50-65%, with wide scattering at oxyhemoglobin desaturation.
  • Neurological pupillary index (NPi) remained unchanged.
  • PUAL consistently approached zero at maximum drug effect.

Conclusions:

  • Pupillary unrest in ambient light (PUAL) is a highly specific indicator of central opioid effects.
  • PUAL offers a non-invasive method for clinicians to monitor patients receiving opioids.
  • This finding aids in differentiating opioid-induced pupillary changes from neurological insults.