Pupillary constriction and convergence of the eye accompany accomodation to near vision. The Marcus-Gunn pupil is also known as the pupillary escape reflex, and is a relative afferent pupillary defect. The mechanism by which this phenomenon occurs is thought to be by a reduction in the number of fibers subserving the light reflex on the affected side. The Marcus-Gunn pupil is caused by optic nerve or chiasm damage as is seen with multiple sclerosis and lesions compressing the optic nerve. Defined as convergence, accomodation, and supranuclear gaze palsy.
Upward gaze is impaired, buth there is a normal response to vertical doll's eyes.
There is also lid retraction. May also have fixed pupils, dissociated light-near response, convergence spasms, and nystagmus retractorius. The superior collicular bodies are usually infiltrated or compressed by mass lesions or by enlarged ventricles. The differential diagnosis includes the following. Mass lesion compressing quadrigeminal plate.