The Light Reflex
The pupillary light reflex is an automatic constriction of the pupils in response to light. It protects the retina from excessive light and optimizes image quality.
Pathway
- Afferent limb (sensory): Retinal photoreceptors detect light and send signals via the optic nerve (CN II) to the pretectal nucleus in the midbrain
- Interneurons: The pretectal nucleus sends signals to both Edinger-Westphal nuclei (right and left)
- Efferent limb (motor): Parasympathetic fibers from the Edinger-Westphal nuclei travel via CN III to the sphincter pupillae muscles of both eyes
Because the pretectal nucleus connects to both Edinger-Westphal nuclei, shining light into one eye constricts both pupils:
- Direct response: Pupil of the illuminated eye constricts
- Consensual response: Pupil of the opposite eye also constricts
The Near Reflex
The near reflex (accommodation reflex) involves three simultaneous responses when shifting focus from distance to near:
- Accommodation: Crystalline lens power increases
- Convergence: Both eyes turn inward
- Miosis: Pupils constrict
This triad improves near image quality by increasing depth of focus (miosis), maintaining binocular fixation (convergence), and focusing the image (accommodation).
Relative Afferent Pupillary Defect (RAPD)
The RAPD (also called Marcus Gunn pupil) is detected using the swinging flashlight test. When light is alternated between the two eyes:
- Both pupils constrict when the normal eye is illuminated
- Both pupils dilate (or constrict less) when the affected eye is illuminated
An RAPD indicates damage to the afferent pathway (optic nerve) on the affected side. The retina or optic nerve is not transmitting as strong a signal, so the brain perceives less light and reduces the constriction response.
Common causes of RAPD:
- Optic neuritis (often from multiple sclerosis)
- Optic nerve compression (tumor)
- Severe retinal disease (unilateral)
- Optic nerve trauma
Abnormal Pupil Responses
| Condition | Light Response | Near Response | Cause |
|---|---|---|---|
| RAPD (Marcus Gunn) | Reduced (afferent defect) | Normal | Optic nerve damage |
| Argyll Robertson | Absent | Present ("light-near dissociation") | Neurosyphilis (historical) |
| CN III palsy | Fixed, dilated pupil | Absent | Aneurysm, diabetes, trauma |
| Horner's syndrome | Miotic (small) pupil, responds to light | Normal | Sympathetic pathway disruption |
| Adie's tonic pupil | Sluggish or absent | Slow, tonic response | Ciliary ganglion damage |
Pharmacological Effects on Pupils
- Mydriatics (sympathomimetics like phenylephrine): Stimulate the dilator muscle, causing dilation
- Cycloplegics (parasympatholytics like tropicamide, cyclopentolate, atropine): Block the sphincter, causing dilation AND paralysis of accommodation
- Miotics (parasympathomimetics like pilocarpine): Stimulate the sphincter, causing constriction
Key Takeaways
- Light reflex: CN II afferent, CN III efferent; produces direct and consensual responses
- Near reflex triad: accommodation, convergence, miosis
- RAPD (Marcus Gunn pupil) indicates optic nerve damage; detected by swinging flashlight test
- Mydriatics dilate only; cycloplegics dilate AND paralyze accommodation
- New anisocoria warrants investigation for neurological causes