Marcus Gunn Pupil

What Is It, Causes, Treatment, and More

Author: Lily Guo

Editors: Alyssa Haag, Ian Mannarino, MD, MBA

Illustrator: Jillian Dunbar

Copyeditor: David Walker

What is Marcus Gunn pupil?

Marcus Gunn pupil, also referred to as a relative afferent pupil defect (RAPD), is a rare condition in which an individual’s response to light is different than expected in only one eye. The presence of a Marcus Gunn pupil is indicative of a unilateral dysfunction in the optic nerve, which transmits information from the retina to the brain, or dysfunction of the retina, which is the light sensitive area at the back of the eye. 

In typical circumstances and in the absence of disease, a pupil should constrict, or become smaller, in the presence of increased light. The other eye simultaneously constricts due to a consensual light reflex. When the light source is removed, both eyes should dilate, or enlarge, equally. This is due to the pathway that the light takes when it reaches the eyes. The afferent pathway sends messages from the pupil to the brain along the optic nerve to the optic tracts, and the efferent pathway sends the message back from the brain to the pupil via nerves, resulting in pupil constriction and dilation. When the pathway is disrupted in one of the eyes, the Marcus Gunn pupil is observed. A Marcus Gunn pupil will remain dilated despite exposure to bright light, and the individual’s two pupils may become different sizes.

What causes Marcus Gunn pupil?

Marcus Gunn pupil can be caused by diseases of the retina, including retinal detachment (i.e. where the retina at the back of the eye pulls away from its normal position) and retinal ischemia (i.e. a conduction characterized by chronic reduced blood flow to the retina). It may also be due to optic nerve disease, specifically occuring before the optic chiasm (ie. the point where the two optic nerves cross over each other). Examples of optic nerve diseases include optic neuritis, or inflammation of the optic nerve commonly due to multiple sclerosis, which is an autoimmune disease that leads to attack of the coating around the optic nerve. 

Another underlying condition that results in Marcus Gunn pupil is severe glaucoma, a condition where fluid build-up in the eye leads to increased pressures in the eye and resultant damage to the optic nerve. Trauma to the eye or head or tumors compressing the optic nerve can likewise result in a non consensual pupillary reflex.

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What are the signs and symptoms of Marcus Gunn pupil?

When a light is shone into an eye with a Marcus Gunn pupil, it will not constrict as much as an unaffected eye, signaling that there is an underlying problem. Due to retinal detachment, other symptoms may include the appearance of floaters in one’s eyes, sudden flashes of light, or a shadow in the vision field. Optic neuritis can present with pain with movement of the eyes or temporary loss of vision in one eye. Lastly, glaucoma can present with headaches, nausea, vomiting, eye redness, and blurred vision.

How is Marcus Gunn pupil detected?

The Marcus Gunn pupil can be detected by the swinging light test, a diagnostic test in which a flashlight is moved quickly between both eyes and the reaction of both pupils to the light is observed. This is typically done by a neurologist or an optometrist in a semi-darkened room using a bright and narrow beam of light. The specialist holds the light in front of one eye for around three seconds before moving it to the other eye. They then observe the pupil’s reaction to light.

How is Marcus Gunn pupil treated?

Treatment of Marcus Gunn pupils is dependent on the underlying cause. If it is due to optic neuritis, it may improve without any treatment; however, oral steroids can be prescribed if symptoms do not resolve on their own. Pain may be managed with over-the-counter pain medication, such as acetaminophen and ibuprofen, if necessary. To treat underlying glaucoma, eye drops, laser treatment, or surgery may be indicated. In the case of retinal detachment, surgical repair is effective 80-90 percent of the time; however, it is not always a viable option if there is too much scar tissue present. If the retina cannot be reattached, eventual blindness may result. Lastly, in the case of an optic nerve tumor, treatment depends on whether the tumor is benign (ie. noncancerous) or malignant (ie. cancerous). If it is benign and stable, or non-growing, imaging can be used periodically to track its growth, and the tumor may be removed surgically if optic nerve compression persists. In the case of malignant tumors, surgery, radiation, and chemotherapy may be indicated.

What are the most important facts to know about Marcus Gunn pupil?

Marcus Gunn pupil refers to the unequal pupillary response to light due to damage or disease in the retina or optic nerve. Examples include retinal detachment, retinal ischemia, optic neuritis, severe glaucoma, trauma, and tumor of the optic nerve, among other causes. If an individual has a Marcus Gunn pupil, the affected pupil will continue to dilate after a light is shone into either and can be detected by an eye specialist swinging a flashlight in front of the individual's eyes. Treatment involves treating the underlying conditions that cause it and can vary from taking steroid medications to undergoing surgery to correct the defect in the eye.

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Related links

Optic pathways and visual fields
Anatomy of the eye
Eye conditions: Retinal disorders: Pathology review
Eye conditions: Refractive errors, lens disorders and glaucoma: Pathology review
Anatomy of the olfactory (CN I) and optic (CN II) nerves

Resources for research and reference

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

Mayo Foundation for Medical Education and Research. (2020, August 28). Retinal detachment. Mayo Clinic. Retrieved from

MediLexicon International. (n.d.). Marcus Gunn pupil: Causes, diagnosis, and treatment. Medical News Today.

Stanford Medicine 25. (n.d.). Do you know Marcus Gunn? Stanford Medicine 25.