• Clinical science

Physiology and abnormalities of the pupil


The pupil is an opening in the center of the iris through which light enters the eye. Pupillary size can vary in response to light intensity and neurologic stimuli. Increasing brightness causes pupillary constriction (miosis) while increasing darkness causes pupillary dilation (mydriasis). Pupillary abnormalities can be caused by a variety of conditions.


Accommodation and convergence

Pupillary response

Pupillary control

Pupillary control is mediated by both parasympathetic and sympathetic innervation


Pupillary light reflex

Description: constriction of the pupils in response to light

Neurons Anatomical pathway of nerve fibers

Retinal ganglion cell neurons (first order)

  • Impulses from the retina travel in the afferent optic nerve (CN II) to the optic chiasm
  • At the optic chiasm
    • Fibers from the temporal half of the retinaipsilateral optic tractipsilateral pretectal nucleus in the midbrain
    • Fibers from the nasal half of the retina crossover → contralateral optic tractcontralateral pretectal nucleus
Internuncial neurons (second order)
  • Fibers from each pretectal nucleus innervate both the ipsilateral and contralateral Edinger-Westphal nuclei
Preganglionic parasympathetic motor neurons (third order)
Postganglionic parasympathetic motor neurons (fourth order)

Afferent pupillary defect


Anisocoria is the difference in size between the pupils without differential illumination of either eye.


Extraocular causes Pupillary constriction Pupillary dilation

Anisocoria is not associated with afferent pupillary defects!


  • Check the light reflex
    • If the light reflex is poor in one eye → Perform a slit lamp examination of the eye to rule out intraocular causes.
      1. If slit lamp examination is normal → administer low-dose pilocarpine (0.1%) or methacoline (2.5%) eye drops
    • If the light reflex is adequate in both eyes → Perform a slit lamp examination to look for a dilation lag.
      1. If no dilation lag is present → administer cocaine (2–10%) or apraclonidine (0.5–1%) eye drops
      2. If dilation lag is present → suspect Horner syndrome


Drugs affecting pupillary size

Receptor Drug class Agents
Dilation M3 receptor antagonism Anticholinergic


α1 receptor agonism α1 receptor agonist
Constriction M3 receptor agonism Parasympathomimetics
α1 receptor antagonism α-blocker
opioid receptor agonism Opioids

Other pupillary defects