Marcus Gunn pupil is a rare condition that affects the eyes, causing each to respond differently when a light is shone into them.

This response indicates that there is dysfunction or disease in the optic nerve or retina. It is a type of relative afferent pupil defect (RAPD).

After being exposed to a bright light, a normal pupil will constrict or become smaller. A Marcus Gunn pupil, on the other hand, continues to dilate, and a person's two pupils become different sizes.

A person with Marcus Gunn pupil on its own is not at any risk, but the condition can signal other eye problems that may need to be treated.

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With Marcus Gunn pupil, the pupils may not react as expected to light or in the same way as one another.

Marcus Gunn pupil is named after Robert Marcus Gunn, a Scottish ophthalmologist from the 1800s.

Someone with Marcus Gunn pupil may complain of visual impairment in one eye. A doctor will need to determine whether the discrepancy is caused by Marcus Gunn pupil or by a cataract or other non-RAPD related condition.

There are different types of RAPD, as well as non-RAPD conditions that can cause symptoms of Marcus Gunn pupil:

  • No RAPD: There are some other conditions, such as hippus (fluctuation in pupil size) and non-reactive pupils, that can make spotting RAPD difficult.
  • Mild RAPD: The affected pupil shows a weak initial constriction, before dilating to a larger size.
  • Moderate RAPD: The affected pupil shows a stable or unchanged level of constriction, before dilating to a larger size.
  • Severe RAPD: The affected pupil dilates to a larger size immediately.

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An affected pupil that dilates immediately when exposed to bright light may indicate severe RAPD.

There are many causes of Marcus Gunn pupil, including:

  • unilateral optic neuropathy, usually causing loss of vision
  • optic neuritis
  • severe glaucoma, which can damage the optic nerve
  • traumatic optic neuropathy, including direct ocular trauma, orbital trauma, and head injuries that damage the optic nerve
  • an optic nerve tumor, which is rare
  • orbital diseases, such as compressive damage to the optic nerve from thyroid-related orbitopathy, orbital tumors, or vascular malformations
  • optic atrophy, or degeneration of the optic nerve
  • optic nerve infections or inflammation
  • severe ischemic retinal disease, such as ischemic central retinal vein occlusion, central retinal artery occlusion, or sickle-cell retinopathy
  • retinal detachment
  • severe unilateral macular degeneration
  • retinal infection
  • severe lazy eye, otherwise called amblyopia

When a light is shone into a normal functioning eye, both the left and right eyes should constrict equally, as they are linked. This is called consensual light reflex, and the test is commonly known as the Marcus Gunn test, or the "swinging flashlight test."

Pupils react to light being shone into them in this way because of the light reflex pathway, which is split into two parts:

  • The afferent pathway: The pupil sends messages to the brain along the optic nerve.
  • The efferent pathway: The message is sent back from the brain to the pupils via nerves, which causes the pupils to constrict.

The light test involves an eye doctor shining a bright and narrow beam of light into the eyes in a semi-darkened room. The doctor will hold the light in front of one eye for about 3 seconds and then move it to the other eye.

When a light is shone into an eye that has Marcus Gunn pupil, it will not constrict as much as an unaffected eye, signaling that there is a problem.

A person is often unaware they have Marcus Gunn pupil until a doctor performs the swinging flashlight test. The discovery can alert the doctor to an underlying condition.

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Eye drops are a common treatment option for Marcus Gunn pupil.

Marcus Gunn pupil is not treated directly, as it is a symptom of another issue. Therefore, treatment will depend on the cause of Marcus Gunn pupil.

Some of the conditions that can cause Marcus Gunn pupil may be treated in the following ways:

  • Optic neuritis: Treated with steroids but will often improve without any treatment. Pain can be managed with over-the-counter pain medication, if necessary.
  • Severe glaucoma: Treated with eye drops, laser treatment, or surgery, depending on the underlying cause.
  • An optic nerve tumor: Treated according to whether the tumor is benign or malignant (cancerous).
  • Optic atrophy: Is irreversible, so treatment focuses on stopping its progression.
  • Retinal detachment: Can only be repaired with surgery, which is not possible in every case due to remaining scar tissue. The surgical procedure has an 80-90 percent success rate, although a second operation is sometimes needed. Surgery does not always improve vision, and if the retina cannot be reattached, a person will eventually go blind in that eye.
  • Severe unilateral macular degeneration: Currently, there is no cure, but vision aids and treatment can help slow down progression.
  • Retinal infection: Treated with an injection in the eye, and may require oral or intravenous drugs. Sometimes, doctors may recommend laser treatment and surgery.
  • Lazy eye: Treated with specific glasses, eye drops, vision therapy, and patches.