Opioids Cause Miosis (Pupillary Constriction), Not Dilated Fixed Pupils
Opioids characteristically produce miosis (pupillary constriction), not pupillary dilation—this is one of the most reliable and sensitive clinical signs of opioid exposure. 1
The Classic Opioid Pupillary Response
Opioids cause "pinpoint pupils" (miosis) through activation of μ- and δ-opioid receptors, which stimulate the pupillary sphincter muscle and produce characteristic pupillary constriction 1, 2, 3
Miosis is strongly correlated with opioid exposure and response to naloxone administration, making it a key diagnostic feature in suspected opioid overdose 1
The miotic effect persists even during severe opioid-induced respiratory depression with accompanying hypercarbia and hypoxia—studies demonstrate that pupils remain constricted (2-3 mm diameter) with a quantifiable light reflex even when oxygen saturation drops to ≤85% 4
Clinical Characteristics of Opioid-Induced Miosis
Peak miosis is best detected under moderately dim interior lighting (4-16 foot-lamberts) approximately 90 minutes after opioid administration 2
The pupillary light reflex remains present and quantifiable during opioid toxicity, though the amplitude is reduced and linearly related to the smaller pupil diameter 4
Pupil diameter decreases approximately 1.0 mm with each log unit increase in lighting intensity during opioid exposure 2
What Actually Causes Dilated Fixed Pupils
If you encounter dilated fixed pupils, consider anticholinergic toxicity, not opioid exposure. 5
Anticholinergic medications (scopolamine, atropine, cyclopentolate, tropicamide) cause pupillary dilation through blockade of parasympathetic innervation to the iris sphincter muscle 5
Phenothiazines (promethazine, prochlorperazine, chlorpromazine) possess anticholinergic effects that can cause pupillary dilation 5
Anticholinergic agents can precipitate acute angle-closure glaucoma in susceptible individuals with narrow iridocorneal angles 5
Critical Clinical Pitfall
Do not confuse opioid toxicity with anticholinergic toxicity—these syndromes produce opposite pupillary findings and require different management approaches. 1, 5 Opioid overdose presents with the classic triad of CNS depression, respiratory depression, and miosis, while anticholinergic toxicity presents with agitation, hyperthermia, dry mucous membranes, and mydriasis (dilated pupils). 1, 5