What is a Marcus Gunn pupil?

Marcus Gunn pupil is also known as Marcus Gunn sign and relative afferent pupillary defect (RAPD). It’s when the eye’s pupil only slightly contracts or dilates when light shines over it, instead of shrinking immediately like it should. The condition was named after Robert Marcus Gunn, a Scottish ophthalmologist in the 1800s who introduced systematized teaching of eye disorders.

Marcus Gunn pupil indicates that there may be an underlying health problem.

What are the causes of a Marcus Gunn pupil?

There are a number of different causes of Marcus Gunn pupil. These include:

Cerebrovascular diseases are disorders that can cause temporary or permanent damage to the brain as a result of ischemia or bleeding. Different types of cerebrovascular disease include strokes, aneurysms, and vascular malformations.

Retinal diseases or damage can also cause Marcus Gunn pupil. These conditions include retinal detachment, retinal infections, and ischemic retinal disease.

Types of Marcus Gunn pupil

A Marcus Gunn pupil doesn’t typically have symptoms of its own. It’s often a symptom of other conditions. Other symptoms that may accompany Marcus Gunn pupil include blurry vision, vision loss, and headaches.

There are different severities of Marcus Gunn pupil. These include:

  • Mild RAPD: The affected pupil shows a weak initial contraction before dilating to a larger size.
  • Moderate RAPD: The affected pupil shows an unchanged contraction before dilating.
  • Severe RAPD: The affected pupil dilates to a larger size immediately.

How is the cause of a Marcus Gunn pupil diagnosed?

Doctors in a number of specialties test for RAPD during their routine examinations. This includes general doctors, ophthalmologists, optometrists, and neurologists. The “swinging light test” is relatively quick and not painful.

The doctor will perform the test in a semi-darkened room and will ask you to look at a particular object while they move a small beam of light from side to side. If your pupil dilates instead of contracting, the doctor will perform the test again to get a positive diagnosis.

You may not be aware that you have Marcus Gunn pupil until the test is performed.

Can a Marcus Gunn pupil cause any complications?

Some doctors perform the swinging light test when a person has been diagnosed with a condition that may cause RAPD. In other cases, RAPD can alert the doctor of a health complication that needs to be addressed.

Marcus Gunn pupil alone will not cause additional problems. However, its presence can demonstrate signs of an underlying issue that needs attention, such as an optic nerve infection or optic neuropathy.

How is a Marcus Gunn pupil treated?

There is no treatment for Marcus Gunn pupil itself, as it’s a symptom of a larger issue. However, a person can receive treatment or management of the underlying issue, which may relieve the RAPD.

For instance, if the presence of Marcus Gunn leads to a glaucoma diagnosis, your doctor may give you eye drops or prescribe laser surgery or microsurgery. If RAPD is caused by a tumor in the optic nerve, such as optic nerve glioma, your treatment plan may include radiation and surgery.

The cause of the Marcus Gunn pupil will determine when or if it can be resolved.

What is the outlook for a Marcus Gunn pupil?

Doctors began including ocular tests in their routine exams in the 19th and 20th centuries, due to the increased awareness of the eye’s relationship to the rest of the body. Not all doctors perform this test, but many do. This is especially true when a person has already been diagnosed with a condition that may have RAPD as an additional symptom.

Since Marcus Gunn can provide clinicians important information regarding systemic health, the swinging light test will likely continue to be an important part of examinations. Individual outlook depends on the underlying cause and can vary greatly.