Everything you need to know about the relative afferent pupillary defect.

The relative afferent pupillary defect (rAPD) is an important finding on the neurological exam, and is essentially characterized by alternating pupillary dilation and constriction as you move a bright light between the patient’s eyes.

Let’s turn to the rock band the Eagles to help demonstrate – specifically, the statue commemorating the song “Take It Easy” in the “Standin’ on the Corner Park” on route 66 in Winslow, Arizona.

Standing on a corner, in Winslow Arizona…

Now, lets imagine shining a light into the eyes of the man standing on the corner. We see bilateral pupil constriction when we shine our light into his right eye, and bilateral pupil dilation when we shine our light into the left eye, as demonstrated below. The man on the corner has a relative afferent pupillary defect.

First – which eye do you think is the normal eye, the man on the corner’s right eye or his left eye? (scroll down for the answer).

The answer is that the right eye is the normal eye, and the left eye is the one with the afferent defect. Remember that afferent means “conducting towards something.” The eye and optic nerve “conduct” the light signal to the brain, and the “defect” lies somewhere along that path.

So, in a rAPD, there is a lesion that blocks or reduces the transmission of visual information from being “conducted” through one of the eyes and/or optic nerves to the brain.

The midbrain – specifically, the Edinger-Westphal nucleus evaluates the total “brightness” of light received from both eyes, and send a corresponding signal back to the pupils to constrict or dilate in response. Think of the Edinger-Westphal nucleus as the brain’s light meter.

The Edinger-Wesphal “light meter” perceives less light, or an overall darkening in the environment, when light is shined into an eye with a dysfunctional afferent pathway compared to when the flashlight was shone in the good eye, in which MORE of the light signal made it to the brain. The response to the brain’s perceived “darkening” of the room is pupillary dilation.

What makes the afferent defect relative? The pupils respond differently because the amount of light reaching the brainstem is different when the light is moved from eye to e ye. If both afferent pathways are dysfunctional, such as in bilateral optic neuritis, then the signal reaching the brainstem would be equal, hence, no rAPD!

Anything that blocks transmission of the afferent light signal in one eye can cause an rAPD, including a retinal tear or hemorrhage, but as neurologist’s we’re more often concerned with dysfunction of the optic nerve, such as optic neuritis.

Optic neuritis – inflammation of the optic nerve – is a common symptom in multiple sclerosis. In fact, one in four MS patient’s has optic neuritis as the presenting symptom.

Remember to test for a rAPD by checking the pupillary response in each eye individually, with the swinging flashlight test.


A rAPD is also known as a Marcus Gunn pupil.

Robert Marcus Gunn (1850 – 1909), Scottish ophthalmologist. Photo not taken while standing on a corner in Winslow, Arizona.

Find more tips on the neurological exam in the medical student and advanced practice provider books.