Harvey's Garden

Pupillary light reflex

2 min read

  1. 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 retina → ipsilateral optic tract → ipsilateral pretectal nucleus in the midbrain
      • Fibers from the nasal half of the retina crossover → contralateral optic tract → contralateral pretectal nucleus
  2. Fibers from each pretectal nucleus innervate both the ipsilateral and contralateral Edinger-Westphal nuclei.
  3. efferent limb arises from Edinger-Westphal nucleus → oculomotor nerve (CN III) → ciliary ganglion. See Horner syndrome if ciliary ganglion is damaged.
  4. ciliary ganglion → short ciliary nerves → iris sphincter muscle → direct and consensual pupillary constriction

Mnemonic

2 priests educated 3 cili (silly) pupils:

  • Optic nerve (cranial nerve 2)
  • Pretectal nucleus
  • Edinger-Westphal nuclei
  • Oculomotor nerve (cranial nerve 3)
  • Ciliary sphincter → pupillary constriction

Pathology

  • Relative Afferent Pupillary Defect (RAPD) / Marcus Gunn Pupil
    • Lesion: CN II (Optic neuritis, Retinal detachment).
    • Finding: Paradoxical dilation with Swinging Flashlight Test in affected eye.
      • Step 1 (Light on Good Eye): Strong optic nerve signal Brain perceives bright light Strong constriction of both pupils.
      • Step 2 (Swing to Bad Eye): Damaged optic nerve sends a weaker signal.
      • The “Paradox”: The brain compares the new weak signal to the previous strong signal, perceiving it as a decrease in light intensity (like entering a dimmer room).
      • Result: The brain withdraws parasympathetic tone, causing both pupils to dilate, even though light is shining on the eye.
  • CN III Palsy (Efferent Defect)
    • Finding: “Down and Out”, Ptosis, Dilated Pupil.
    • Pupil-Involving: PComm Aneurysm (compression).
    • Pupil-Sparing: Diabetes (ischemia).
  • Argyll Robertson Pupil
    • Etiology: Tertiary Syphilis.
    • Finding: Accommodates but does not react to light.
  • Parinaud Syndrome
    • Etiology: Pinealoma (dorsal midbrain compression).
    • Finding: Vertical gaze palsy + Light-Near Dissociation.

Graph View

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