What Is a Marcus Gunn Pupil?

Also known as relative afferent pupillary defect (RAPD)

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Marcus Gunn pupil—also known as "relative afferent pupillary defect," or RAPD—refers to when a person's pupil only gets slightly smaller in response to light, instead of dilating as expected. It can be caused by a number of different eye conditions, including retinal detachment, optic nerve damage, and very severe macular degeneration.

Man having an eye exam at ophthalmologist's office

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Marcus Gunn pupil is the name of an eye defect impacting a person's pupil. While a normal pupil will constrict when exposed to bright light, one with Marcus Gunn pupil has a much weaker response. Because of this, when light is moved quickly between the normal eye and the one with Marcus Gunn pupil, the one with the defect dilates (or gets bigger) instead of constricting.

The abnormal response to light in the eye with Marcus Gunn pupil is a result of reduced stimulation of the visual pathway. Because the affected eye isn't able to accurately read the intensity of the light, it causes its visual pathway to incorrectly respond to the decrease in stimulation—as if the light itself were not as bright as it actually is. The healthy eye, meanwhile, is still able to respond normally to light.

Who Was Marcus Gunn?

Robert Marcus Gunn (1850-1909) was a Scottish ophthalmologist who first described this type of abnormal pupillary response in 1902 after observing it in a patient with unilateral retrobulbar optic neuritis.

Along with this reaction, Gunn is also known for a rare condition characterized by when a person's upper eyelid moves rapidly when they open or close their jaw. This is called "Marcus Gunn syndrome" or simply, "jaw-winking."


Marcus Gunn pupil can be caused by an array of different disorders and diseases, frequently including those that involve damage to the optic nerve or retina. It is not more prominent in one sex or age group over others. The eye conditions associated with Marcus Gunn pupil can include:

  • Lesions of the optic nerve
  • Optic neuritis
  • Direct optic nerve damage (trauma, radiation, tumor)
  • Orbital cellulitis
  • Lesions of the optic chiasm
  • Lesions of the optic tract
  • Lesions of the pretectum
  • Ischemic optic disease or retinal disease
  • Glaucoma
  • Visual field defect
  • Retinal detachment
  • Retinal infection (CMV, herpes)
  • Central retinal vein occlusion (CRVO)
  • Central serous chorioretinopathy(CSCR)
  • Macular degeneration
  • Retinitis pigmentosa (RP)
  • Endophthalmitis
  • Dense cataract
  • Eye patching of one eye
  • Dark adaptation of one eye
  • Amblyopia
  • Anisocoria

Prevalence of Marcus Gunn Pupil in Eye Conditions

Marcus Gunn pupil is more closely associated with some of these conditions than others.

For example, it occurs in more than 90% of cases of acute unilateral cases of optic neuritis, 91% cases of ischemic central retinal vein occlusion (CRVO), more than 50% cases of retinal detachment involving the macula, and 23% cases of primary open-angle glaucoma (POAG).


The primary way of diagnosing Marcus Gunn pupil is through what's referred to as the "swinging light test" or the "swinging flashlight test"—and it's exactly what it sounds like.

To perform the test, an eye doctor will swing a light back and forth in front of both of a person's pupils and compare the reaction to stimulation in both eyes.

Typically, our pupils have two ways of responding to light:

  • Direct response: This is when the pupil gets smaller when light is shone on it directly.
  • Consensual response: This is when a pupil constricts reflexively when a light is shone directly into the other eye, demonstrating that the eyes are linked.

During a normal swinging light test, the pupils of both eyes constrict equally, regardless of which eye is receiving direct light. But during a swinging light test involving someone with Marcus Gunn pupil, the person's affected pupil will only get slightly smaller, rather than being proportional to the opposite pupil.

Given how prevalent Marcus Gunn pupil is with certain eye conditions—particularly those that involve the retina or optic nerve (but only optic nerve disease that occurs in front of the optic chiasm)—the swinging light test is a useful tool in detecting these diseases.

And while it may be tempting to conduct your own test to find your pupillary response, this is something that is best left to professionals in the context of a comprehensive eye exam—rather than a flashlight and your bathroom mirror.


The treatment for Marcus Gunn pupil is directed towards the underlying cause of the eye defect, given that it is a symptom of other conditions. For example if Marcus Gunn pupil is a result of:

  • Orbital cellulitis, the treatment would involve intravenous antibiotics orsurgical procedures, including those to drain the sinuses or an abscess of orbital fat.
  • Glaucoma, treatment may include medication, laser surgery, or other types of non-laser surgical procedures, like a trabeculectomy, cataract surgery, or the implantation of a drainage device.
  • Retinal detachment, treatment might include a freeze treatment, laser surgery, or a more involved surgery required to move the retina back in place.
  • Optic nerve lesion or optic neuritis, treatment options could involve a short course of steroids, which are typically injected into your veins.

Talk to Your Healthcare Provider Before Starting Any New Regimen

As is the case with basically every condition involving our body or brain, people with Marcus Gunn pupil should consult their healthcare provider before beginning any type of new health or fitness regimen. This includes activities we typically think of as being gentle—like yoga.

For example, if you are experiencing a flare-up of a condition that causes Marcus Gunn pupil, you probably want to hold off on taking up yoga until it has cleared up. But again, before adding anything to your health or wellness routine, talk to your healthcare provider first.

A Word From Verywell

If it feels as though something is wrong with your eye—or any other part of your body for that matter—it's always a good idea to discuss it with your healthcare provider. Listen to your body; what might seem like no big deal could turn out to be the sign of something more serious, and—in many situations—the earlier a problem is caught, the more can be done to help.

Of course, that doesn't mean we should panic and assume the worst anytime we experience something slightly different relating to our body. It's more about being aware and noticing when something feels off.

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Pearce J. The Marcus Gunn pupilJournal of Neurology, Neurosurgery & Psychiatry. 1996;61(5):520. doi:10.11362Fjnnp.61.5.520

  2. Stanford Medicine. Pupillary responses.

  3. Pearce, J., 1996. The Marcus Gunn pupil. Journal of neurology, neurosurgery, and psychiatry, 1996; 61(5): 520.

  4. Broadway DC. How to test for a relative afferent pupillary defect (RAPD). Community Eye Health. 2012;25(79-80):58-59.

  5. Merrow AC, Linscott LL, O’Hara SM, et al., eds. Orbital cellulitis. In: Diagnostic Imaging: Pediatrics (Third Edition). Diagnostic Imaging. Elsevier; 2017:1160-1163. doi:10.1016/B978-0-323-44306-7.50426-6

  6. American Academy of Ophthalmology. Glaucoma treatment.

  7. National Eye Institute. Retinal detachment.

  8. Johns Hopkins Medicine. Optic neuritis.

By Elizabeth Yuko, PhD
Elizabeth Yuko, PhD, is a bioethicist and journalist, as well as an adjunct professor of ethics at Dublin City University. She has written for publications including The New York Times, The Washington Post, The Atlantic, Rolling Stone, and more.