What is a relative afferent pupillary defect (RAPD)?

An RAPD (Marcus Gunn pupil) is detected with the swinging flashlight test. When light is alternated between eyes, both pupils dilate when the affected eye is illuminated (instead of constricting). It indicates damage to the optic nerve or retina on the affected side, reducing the strength of the afferent signal.

What is the difference between the direct and consensual pupil response?

The direct response is constriction of the illuminated eye's pupil. The consensual response is simultaneous constriction of the opposite eye's pupil. Both occur because the pretectal nucleus in the midbrain sends signals to both Edinger-Westphal nuclei, which innervate both eyes' sphincter muscles.

What is light-near dissociation?

Light-near dissociation occurs when the pupil does not respond to light but does constrict during the near reflex (accommodation/convergence). The classic example is the Argyll Robertson pupil, historically associated with neurosyphilis. The light reflex pathway and near reflex pathway have different neural routes, so damage can affect one without the other.

Pupillary Reflexes for ABO

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 afferent (sensory) pathway uses CN II (optic nerve). The efferent (motor) pathway uses CN III (oculomotor nerve). Damage to CN II on one side eliminates both direct and consensual responses when that eye is illuminated, but the consensual response works when the OTHER eye is illuminated.

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

If you notice a significant difference in pupil size (anisocoria) during dispensing, note it in your records and mention it to the patient's eye care provider if it appears new. Anisocoria of 1 mm or less is physiological (present in ~20% of people), but larger or new-onset anisocoria can indicate neurological problems.

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

Confusing mydriatics and cycloplegics. Mydriatics ONLY dilate the pupil. Cycloplegics dilate the pupil AND paralyze accommodation. A patient dilated with cyclopentolate cannot focus for near work until the drug wears off (several hours). Pure mydriatics (phenylephrine) do not affect accommodation.

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