Marcus Gunn pupil, also known as afferent pupillary defect, is a non-specific finding that indicates partial optic nerve dysfunction. It is mainly due to unilateral optic neuropathy, or rarely optic chiasm or optic tract lesions.
In response to light input to the affected eye, both eyes do not constrict to a normal extent but the affected pupil shows less constriction than the unaffected pupil. But if light is shone into the normal eye both eyes constrict normally and equally. It is 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 fibres to the affected eye are delivered via cranial nerve III.
History and etymology
Robert Marcus Gunn (1850-1909), was a Scottish ophthalmologist 2.