Neurological exams never fail to amaze me with how simple they are to perform given how much information they provide to localize a lesion – take the swinging-flashlight exam for example.
What happens when you stare into a bright light? Well your parasympathetic nervous system (PSNS) constricts both your pupils. A relatively simple neural circuit is behind pupillary constriction when faced with a stimulus (like bright lights), so what if something goes wrong? What if, for example, there’s damage to the retina or optic nerve itself? First, one should realize that if you covered one eye and shined a bright light in the other, both pupils will constrict. This is due to the bilateral nature of the aforementioned circuit.
The swinging-flashlight exam employs this fact. In short, you briefly shine light in one pupil causing both pupils to constrict (if healthy). Then you quickly switch the light to the other pupil (which should still be constricted). If there’s sensory (afferent) damage in that eye, you’ll observe a phenomenon dubbed the “afferent pupillary defect” (AFD).
Our main objective is observing the illuminated pupil’s size. When we move the light from one eye to the other… back and forth… a pupillary circuit with damage to the afferent component will actually dilate in the presence of light. How can that be?
Simple. You’ve knocked out the sensory component. 🙂 The pupil can’t “sense” the light, and therefore has no reason to constrict (or remain constricted) in the presence of the stimulus. It reestablishes its normal size after having been constricted by shining light in the other eye. This net realignment towards equilibrium creates a visible dilation in the affected pupil. Another way to think about it is that a pupil with a damaged sensory circuit can constrict only by exploiting the opposite eye’s sensory circuitry. Remember, if one pupil constricts, both constrict.
It’s hard to put into words, so here’s a helpful video describing this clinical finding (the relevant information begins around 45 seconds into the clip).