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A Marcus Gunn pupil is a condition where the affected pupil constricts less when light is shined on it compared to the healthy pupil. It is not an issue with the pupil itself but with the nerve that sends light signals from the eye to the brain. It signals a problem in the visual pathway.
read more about Marcus Gunn pupil ...This condition develops when damage occurs to the optic nerve or the retina. The eye cannot properly sense light, meaning the brain does not receive the signal to constrict the pupil strongly.
Eye doctors diagnose Marcus Gunn pupil using the swinging flashlight test. The doctor shines a light into the unaffected eye, causing both pupils to constrict. When the light is quickly swung to the affected eye, both pupils appear to dilate instead of constrict because the damaged nerve cannot sense the light as brightly.
The condition signals reduced communication between the eye and the brain. It is an objective sign of a problem in the afferent (sensory) pathway of the vision system. Doctors use this test to confirm damage to the optic nerve.
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No, it requires a specific light test by a doctor to detect the subtle reaction difference.
No, the pupil response itself is not painful, but the cause (optic neuritis) may be.
No, but it indicates reduced function or vision loss in that eye.
A Marcus Gunn pupil (Afferent Pupillary Defect) is a serious finding. If a doctor notes this during an exam, it often means the optic nerve is damaged. You may need an MRI or a referral to a neurologist to check for conditions like Multiple Sclerosis or glaucoma.
StatPearls. Marcus Gunn Pupil (ncbi.nlm.nih.gov). 2024.
AAO. What is an APD? (aao.org). 2024.
Cleveland Clinic. Optic Nerve Disorders (clevelandclinic.org). 2023.
Mayo Clinic. Optic Neuritis Symptoms and Diagnosis (mayoclinic.org). 2024.