Also known as a “Marcus-Gunn Pupil,” the Afferent Pupillary Defect is due to dysfunction of Cranial Nerve 2 (Optic Nerve). CN-2 governs sight, and is thus stimulated by light.
Find it by a “Swinging Flashlight Test” (a medical term; not my own coinage):
- In a very dark room, shine a penlight in the Left eye
- The pupil constricts.
- Shine it over to the Right, pupil dilates!!!
- Swing back to the Left, pupil constricts.
- Back to theRight, pupil dilates!!!!!
- Etc… Patient has a Right APD.
Pathology: CN-2 carries the impulse to the brain (occipital lobe). But en route, it synapses to the Edinger-Westphal nucleus in mid-brain. There, CN-3 (Oculomotor Nerve) travels back to constrict the pupil (direct response), and also to the contralateral pupil (consensual response).
Let’s say the Right CN-2 is weak, maybe due to Multiple Sclerosis, or any lesion. Both CN-3’s are normal.
When you shine a penlight in the normal Left Eye, the Pupil constricts briskly. The Right Pupil also constricts briskly, but you don’t see it, because the room is very dark.
Quickly swing the penlight to the Right Eye. The diseased CN-2 senses less light, so the Right Pupil dilates. Left Pupil also dilates, but you don’t see it [dark room].
Swing back to the Left Pupil. Now both pupils constrict briskly, but you only see the Left.
Back to the Right, both dilate (less CN-2 stimulus), but you only see the Right.
Etc. It’s a “Right APD”.
The Swinging Flashlight Test should be performed every time you check pupils. A fascinating case in the New England Journal of Medicine discussed the importance of the swinging flashlight test, & also the pinhole exam. An obscure ultimate diagnosis was initially missed, the main point being how clinicians omitted these two salient tests. At, of all places, the E.D. of Massachusetts Eye and Ear Infirmary. See N Engl J Med 2008;359:2825-33.