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What is a Marcus Gunn Pupil?

The "swinging flashlight test" whereby a light is shone alternately back and forth between both eyes, can be used to elicit this finding. A normal response results in constriction of both pupils, which indicates intact direct and consensual pupillary light reflex. An abnormal response due to RAPD, however, results in minimal constriction of both pupils when the light in shone in the affected eye, causing one to perceive the presence of pupillary dilation.

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 Back to the Basics, Part 5: My Patient has an RAPD! Now What?

A relative afferent pupillary defect can be an ominous exam finding. These cases illustrate the proper steps to take in managing your patients.

 What is the relative afferent pupillary defect(RAFD)?

The RAPD is also called the Marcus-Gunn pupil. It indicates optic nerve or retinal disease. It also requires that the disease be only in one eye ie., bilateral disease may not produce a defect. It should be performed in a dimly(not dark) lit room, with a strong light source. Swing the light from eye to eye. Shining the light on either pupil should make both pupils constrict. This is the direct and consensual response. A positive RAPD is diagnosed when the pupil that has the light directed at it, does not constrict, but dilates.

Articles of Interest

How to Perform the Swinging Flashlight Test

Prerequisites and procedure for performing the swinging flashlight test to detect an RAPD (Relative afferent pupillary defect).

Here's the Easiest Way to Test for an Afferent Pupillary Defect

Relative afferent pupillary defect testing focuses on the afferent light pathway and can play an integral part of clinical testing. Here are two effective testing techniques.

How to test for a relative afferent pupillary defect (RAPD

In a normal swinging light test (i.e. there is no RAPD) the pupils of both eyes constrict equally regardless of which eye is stimulated by the light. In an abnormal swinging-light test (i.e. there is a RAPD) there is less pupil constriction in the eye with the retinal or optic nerve disease.

Marcus Gunn Pupil

Marcus Gunn Pupil: (AKA relative afferent pupillary defect or RAPD) A pupil that responds by constricting more to an indirect than to a direct light, seen with unilateral optic nerve or retinal disease,

Relative Afferent Pupillary Defect

Any disorders that affect the optic nerves regardless of the its pathology can lead to a RAPD.

Relative Afferent Pupillary Defect (RAPD)

A Relative Afferent Pupillary Defect is an examination finding in patients who have an asymmetric pupillary reaction to light when it is shined back and forth between the two eyes. It is most commonly a sign of asymmetric optic nerve disease or damage but can also present in widespread asymmetric retinal disease. The disease or condition causing the RAPD has to be asymmetric because if both eyes are equally affected than the pupillary reaction is symmetric.

Swinging Flashlight Test

The Swinging Flashlight Test aims to reveal any asymmetry of afferent input in the pupillary light reflex, i e, a relative afferent pupil defect (RAPD). The display aims to illustrate the test procedure and the outcomes for different degrees of damage. The model also shows physiological unrest (hippus). The abnormal side is to the right on the screen. Click on the display to start swinging, click again to stop.

What is the Marcus Gunn Pupil?

Robert Marcus Gunn was a Scottish physician from the 1800’s. He studied under other well known physicians such at Douglas Argyll Robertson. Dr. Gunn is known for the Marcus Gunn pupil (AKA relative afferent pupillary defect or RAPD).

“What is Marcus Gunn Pupil and What Does it Indicate?”

Treatment is not directed at the pupil anomaly itself but rather at the underlying pathology, which could range from demyelinating diseases like multiple sclerosis to direct optic nerve injury, ischemic optic neuropathy, or severe retinal degeneration.

Resources

Radiopaedia

This sign is classically elicited on the swinging light test (or Marcus Gunn test). If light is shone into the normal eye both eyes constrict normally and equally. However, in response to light input to the affected eye, both pupils do not constrict to a normal extent, and may be seen to paradoxically dilate as the light moves quickly over it. This occurs because of a decrease in afferent input reaching the pretectal pathway responsible for the pupillary light response in the midbrain via a damaged optic nerve, but efferent fibers to the affected eye are delivered via cranial nerve III.

StatPearls

Marcus Gunn pupil is indicative of a defect in the afferent pathway of the light reflex. To avoid the high morbidity associated with this condition, it must be promptly diagnosed and the cause should be treated.