Marcus Gunn Pupil

If you have optic nerve or retinal damage in one eye, your provider might use a flashlight to go back and forth between your eyes. Healthy pupils get smaller (constrict) in bright light. A Marcus Gunn pupil doesn’t get bigger and smaller in the right way.


What is Marcus Gunn pupil?

Marcus Gunn pupil means that your pupils don’t respond in the same way when your eye care provider shines a light on them. It’s the result of a problem with your retina or your optic nerve.

Usually, pupils exposed to light get smaller (constrict), while pupils faced with less light get bigger (dilate). When both eyes are healthy, they respond in the same way to light. So, if your provider shines a light into one eye, both pupils get smaller. If the light is taken away from one eye, both pupils get bigger. Both pupils should respond to light together.

If you have Marcus Gunn pupil, when light shines into your affected eye, both pupils won’t constrict as they should. By itself, this doesn’t present a health risk. However, another condition that may require treatment usually causes it.

The condition is named after Robert Marcus Gunn, an ophthalmologist in the late 1800s. It might also be called relative afferent pupillary defect (RAPD), Gunn’s syndrome or Marcus Gunn sign.

Types of Marcus Gunn pupil or RAPD

There are different ways to classify RAPD. Sometimes, professionals refer to a clinical grading system ranging from one to five based on how much your pupil contracts or dilates when exposed to light.

Providers may also classify Marcus Gunn pupil based on how much brighter the light has to be to make your affected pupil react the same as your normal pupil.


Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Possible Causes

What causes Marcus Gunn pupil?

Marcus Gunn pupil can be caused by problems with your retina or optic nerve. Conditions that may result in this type of RAPD include:

  • Glaucoma: A group of eye conditions that damage your optic nerve, primarily from a buildup of fluid in the front of your eye. Usually, glaucoma affects both eyes almost equally, so RAPD would be less common.
  • Optic neuritis: Inflammation of your optic nerve.
  • Ischemic central retinal vein occlusion: A blockage in the main retinal vein.
  • Retinal detachment: A very serious condition in which your retina pulls away from the tissues supporting it. The retinal detachment would have to be very severe to cause RAPD.
  • Retinitis pigmentosa (RP): A group of inherited eye conditions that affect the retina. Usually, RP affects both eyes almost equally, so RAPD would be less common.
  • Optic atrophy: Damage to the optic nerve that can’t be treated.

Other conditions that have been associated with Marcus Gunn pupil include:

  • Amblyopia: One eye is weaker than the other. It’s sometimes called lazy eye.
  • Cataracts: The lens of your eye becomes cloudy.
  • Eye tumors: These can be benign (not cancer) or malignant (cancerous).
  • Eye injuries: Your optic nerve and retina can be damaged by wounds, burns or infections.

Care and Treatment

What tests diagnose Marcus Gunn pupil?

Your healthcare provider will perform certain tests to find Marcus Gunn pupil, including the swinging light — or swinging flashlight — test. This is also known as the Marcus Gunn test. In this test, your provider shines a light into one eye and then the other, back and forth, to see how they respond to the light and then lack of light. It’s done in a semi-darkened room.

If one eye doesn’t respond correctly to the light, you may have two pupils of different sizes at some point during the test.

Your provider may order other tests depending on what they suspect is causing Marcus Gunn pupil.


How is Marcus Gunn pupil treated?

Treatment for Marcus Gunn pupil depends on what’s causing it. Treatments may include IV medications, eye drops, laser procedures or surgery.

For example, if you have optic neuritis, your provider may suggest IV steroids. If you have glaucoma, your provider may prescribe eye drops.

What can I expect if I have Marcus Gunn pupil?

If you have Marcus Gunn pupil, your outlook (prognosis) depends on what’s causing the RAPD. By itself, Marcus Gunn pupil doesn’t present a health risk. However, another condition usually causes it. Most diseases associated with Marcus Gunn pupil are treatable. Getting diagnosed early, when you first notice vision changes, is associated with an improved outlook.


When To Call the Doctor

When should I see my eye care provider?

Always see your healthcare provider if you have any change in your vision. This includes:

  • Pain.
  • Blurriness.
  • Problems seeing at night.

Additional Common Questions

What’s the difference between Marcus Gunn pupil and Argyll Robinson pupil?

Both Marcus Gunn pupil and Argyll Robinson pupil involve the response of pupils to light. But in Argyll Robinson pupil, both pupils remain small when either eye is exposed to light. In Marcus Gunn pupil, your pupils won’t be as small as they should be when the affected eye is exposed to a bright light.

A note from Cleveland Clinic

It’s not likely that you’ll notice if you have Marcus Gunn pupil. But you may notice changes in your vision or new problems with your vision. It’s important to get these checked out as soon as you notice them. With any of the problems related to Marcus Gunn pupil, it’s best to start treatment as soon as you can.

Medically Reviewed

Last reviewed on 02/25/2024.

Learn more about our editorial process.

Appointments 216.444.2020