What is Miosis?
Miosis is pupillary constriction (a smaller pupil diameter), caused by activation of the parasympathetic nervous system in response to light and near visual stimuli, or by certain drugs and pathologic conditions 1.
Physiologic Mechanism
The parasympathetic nervous system mediates miosis through supranuclear pathways that project to the Edinger-Westphal nuclei, causing contraction of the pupillary sphincter muscle 1. This occurs normally in response to:
- Light exposure - The pupillary light reflex involves rods, cones, and intrinsically photosensitive retinal ganglion cells using melanopsin 1
- Near visual stimuli - Accommodation reflex
- Decreased arousal states - Lower sympathetic outflow as sedation sets in 2
Clinical Contexts Where Miosis Occurs
Drug-Induced Miosis
Certain medications and substances cause marked pupillary constriction:
- Opioids - One of the most sensitive objective indices of opioid effects 3. Peak methadone miosis is best detected under moderately dim lighting (4-16 foot-lamberts) approximately 90 minutes after administration 3
- Parasympathomimetics - Used therapeutically in acute angle-closure crisis to open the angle by making the iris less bowed 4
- Narcotics - 88% frequency of miosis in comatose children 5
- Phenothiazines - 72% frequency in pediatric overdose 5
Important Clinical Caveat
Pupil diameter is systematically affected by lighting conditions - pupils decrease 1.0 mm with each log unit increase in lighting intensity, and are 0.35 mm larger with monocular versus binocular viewing 3. This must be considered when assessing miosis clinically.
Pathologic Miosis
In acute angle-closure crisis, parasympathomimetic treatment may be ineffective when IOP is markedly elevated because pressure-induced ischemia of the pupillary sphincter makes the pupil unresponsive to miotic therapy 4. This represents a critical pitfall in emergency management.
Congenital Miosis
Rare inherited condition with pupils 0.5-2.5 mm diameter that react normally to light and accommodation but dilate poorly with mydriatics, often associated with enlarged corneas and other ocular abnormalities 6.