How to test for a relative afferent pupillary defect (RAPD)

Light source The ‘swinging light test’ is used to detect a relative afferent pupil defect (RAPD): a means of detecting differences between the two eyes in how they respond to a light shone in one eye at a time. The test can be very useful for detecting unilateral or asymmetrical disease of the retina or optic nerve (but only optic nerve disease that occurs in front of the optic chiasm). The physiological basis of the RAPD test is that, in healthy eyes, the reaction of the pupils in the right and left eyes are linked. In other words, a bright light shone into one eye leads to an equal constriction of both pupils. When the light source is taken away, the pupils of both eyes enlarge equally. This is called the consensual light reflex. To understand how the pupils react to light, it is important to understand the light reflex pathway (Figure 1). This pathway has two parts.