Question: What is the swinging light test?

Answer: The swinging-light-test is used to check for relative afferent pupillary defect (RAPD). If one of the eyes isn’t detecting light well, then the pupils will not contrict as well when light is shined in that eye. Often, however this pupil defect is subtle. The swinging light test enables you to pick this up by comparing the pupil response between the eyes. Here’s how you do it!

Swing the light back-and-forth between the eyes. Normally, the pupil will contrict with one eye, dilate a little as the light passes over the nose, and contrict again with the other eye. Thus, you get constrict - constrict - constrict - contrict.

However, when one eye doesn’t see as well, the pupils don’t constrict quite as well. In fact, the pupils appear to dilate a little when you hop over to the bad eye with that flashlight. Thus, you get constrict - dilate - constrict - dilate.