Normal Light Reflex and Relative Afferent Pupillary Defect (RAPD)

A Relative Afferent Pupillary Defect is an examination finding in patients who have an asymmetric pupillary reaction to light when it is shined back and forth between the two eyes. It is most commonly a sign of asymmetric optic nerve disease or damage but can also present in widespread asymmetric retinal disease. The disease or condition causing the RAPD has to be asymmetric because if both eyes are equally affected than the pupillary reaction is symmetric. This exam finding is best assessed with the swinging light test:; 1. In a dim room, have the patient fixate on a distant point; 2. Shine a light in one eye and allow pupil diameter to stabilize, shining the light directly into their eye for about 3 seconds; a. Both pupils should constrict equally; 3.Quickly swing the light to the other eye and observe pupil diameter; a. If that eye is normal, both pupils will constrict slightly; b. If RAPD is present in that eye, both pupils will dilate; 4. Quickly swing the light back to the first eye; a.; If that eye is normal, both pupils will constrict slightly; b.; If RAPD is present in that eye, both pupils will dilate; 5.; Repeat these steps several times to confirm your findings - be careful not to overexpose one eye and induce an RAPD. Spend about 3 seconds on each eye and then switch to the other eye so that the light exposure remains equal between the two eyes. Working up a positive RAPD: Although an RAPD is almost always a result of an asymmetrical defect of the optic nerve or retina, any pathology that decreases the amount of light sensed by one eye relative to the other will result in this finding. Because of its potential urgency, it is important to determine the etiology of the RAPD. The first step is to take a careful history that focuses on the timing of visual changes, prior eye diseases, eye pain and any recent trauma. If visual loss is present (which is almost always the case with an RAPD), the speed of onset is an important clue. A hyperacute onset of seconds to minutes points to a traumatic or vascular cause such as a fracture compressing the optic nerve, a large retinal detachment or ischemic optic neuropathy from something like a central retinal artery occlusion. In these cases, an urgent ophthalmic consultation is warranted because immediate intervention may preserve vision. An acute onset of visual loss of hours to days points to inflammation or infection of the optic nerve, such as optic neuritis. A subacute onset of days to months has a larger differential such as asymmetric glaucoma, severe macular degeneration or a mass in the orbit compressing on the optic nerve. An RAPD is almost always correlated with some visual loss, however some patients with optic neuritis may retain 20/20 vision. This typically presents acutely with pain with eye movements and in a young patient, often female.; In summary, patients presenting with eye pain or unilateral vision loss need a careful pupillary exam, looking for an RAPD. If present, this typically localizes to the optic nerve, though widespread retinal diseases are also a possibility. Accompanied pain suggests an optic neuritis while painless vision loss with an RAPD are concerning for ischemic optic neuropathies. In a young patient without any recent trauma, a mass or tumor compressing on the optic nerve is amongst the most concerning causes of painless vision loss with an RAPD. In the elderly, this same presentation could be Giant Cell Arteritis, which if not caught and treated carries a high risk of having a stroke or causing vision loss in the other eye by the same mechanism.