mydriasis

physiology

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

July 22, 2026

Quick Summary

Mydriasis refers to the excessive or prolonged dilation of the pupil of the eye. The pupil is the opening in the iris through which light enters the eye to reach the lens and retina.12

The size of the pupil is controlled by the muscles of the iris. In bright light, the pupil constricts, and in dim light, it dilates. This light reflex is bilateral, meaning both pupils react even if only one eye is exposed to light. Pupillary constriction also occurs when focusing on near objects, which helps reduce eye aberrations and increase depth of focus.23

Dilation can occur due to strong emotional stimuli, pain, or stimulation of sensory nerves. Certain drugs, like atropine, can cause pupillary dilation by paralyzing the nerve supply. Evaluating pupil size is an important part of medical examinations, as variations can indicate age, disease, trauma, or other neurological issues.32

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

3 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

epinephrine
Physiological actions
… in the eye, resulting in mydriasis (dilation of the pupil) and improved visual acuity. The physiological actions of epinephrine are terminated by metabolic breakdown with catechol-O-methyltransferase (COMT) or monoamine oxidase (MAO), by reuptake into nerve endings, and by diffusion from active sites.
Read More
… in the eye, resulting in mydriasis (dilation of the pupil) and improved visual acuity. The physiological actions of epinephrine are terminated by metabolic breakdown with catechol-O-methyltransferase (COMT) or monoamine oxidase (MAO), by reuptake into nerve endings, and by diffusion from active sites.
Read More