Afferent pupillary defect (aka Marcus Gunn pupil)

• swinging flashlight test
• swing light back/forth from eye to eye. The lit eye will normally constrict.
• The normal pupillary light reflex pathway causes both pupils to constrict when light shines into one eye. The response in the stimulated eye is called the direct pupillary light reflex; the response in the opposite eye is called the consensual pupillary light reflex
• In this test, an abnormal eye will dilate under the flashlight. (Both eyes actually dilate). The afferent limb (outgoing nerve to muscles) of the pupillomotor response is defective, so the "light" signal doesn't get to the muscles of the abnormal eye, and the eye dilates. It is receiving no 'over-ride' order to constrict because the other, good eye is in darkness and dilated.
• So in normal environmental situations with both eyes receiving the same (light or dark) stimulus, both eyes tend to constrict and dilate at the same time.
• This test isolates each eye.
• Indicates disease affecting the optic nerve.