In a Marcus Gunn pupil, the consensual pupillary light reflex is stronger than the direct pupillary light reflex.
Examination
A Marcus Gunn pupil can best be detected using the swinging flashlight test.
Interpretation
The presence of the consensual pupillary light reflex indicates integrity of the Oculomotor nerve[^1].
Causes of a Marcus Gunn pupil
A Marcus Gunn pupil is caused by an ipsilateral lesion of the retina or the Optic nerve anterior to the Optic chiasm. Possible causes include:
Retrobulbar neuritis or optic neuritis in the context of multiple sclerosis or viral infection.
Direct or indirect trauma to the optic nerve
Compression caused by a tumor anterior to the optic chiasm