Detecting an Afferent Pupillary Defect (APD), also called a “Marcus-Gunn pupil,” is the best way of all to diagnose a problem with the optic nerve, which goes from the eye to the brain & lets us see (see Diagram – Anatomy of the Eye). Most clinicians don’t know about it. We test for it by the “swinging flashlight test,” shining a light in one eye, then immediately to the other, then back to the first, then other, etc., watching what happens to the pupil.
Normally, when there’s bright light, the pupil constricts (gets smaller), while in the dark it dilates (gets bigger). That’s how the body controls the amount of light entering. The optic nerve (2nd Cranial Nerve) senses light going into the eye, the 3rd Cranial Nerve controls the pupil. There’s a normal reflex for the 3rd nerve that acts on both eyes together, so shine a light in one eye, & both pupils constrict.
Let’s say something’s wrong with the optic nerve of the Right eye (Multiple Sclerosis is a common cause, but any destructive eye disease can do it). In a dark room, shine a light in the Left eye, both pupils constrict. Swing it to the ill Right eye, it dilates because it senses less light (actually both pupils dilate, but since the room is dark, we only see the Right). Swing it back to the Left, the pupil constricts (both do, but we only see the Left). Back to the Right, dilates. Left, constricts. Right, dilates. Weird, but diagnostic.
If your vision goes bad in just one eye, especially if there’s eye pain but even if not, ask your medical provider if they did a swinging flashlight test for an APD, to diagnose Optic Neuritis. If they don’t know what it is, explain it to them (or show them this).