Normally, light directed at 1 pupil causes equal constriction of both pupils (see Chapters 1 and 10). When light is shined into an eye with impaired conduction of the afferent pupillomotor signal along 1 optic nerve, pupillary constriction in both eyes is slower and smaller in amplitude compared with the response that occurs when light is shined into the eye with normal optic nerve conduction. In other words, both the direct and the consensual response are sluggish. This difference between the 2 eyes in pupillary response after light stimulation is known as a relative afferent pupillary defect (RAPD).
The most popular clinical method for detecting an RAPD is the swinging flashlight test (Video 3-1). This test, which compares the pupillary response in the 2 eyes when tested with the same light source, involves swinging a bright light (which is shined directly into the pupil along the visual axis) between the 2 eyes. If the afferent input is significantly asymmetric, the pupils redilate immediately when the light is shined into the affected eye (Fig 3-1).
Left relative afferent pupillary defect.
Courtesy of M. Tariq Bhatti, MD.
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Figure 3-1 Assessing for relative afferent pupillary defect (RAPD) in a patient with left traumatic optic neuropathy; the left pupil is pharmacologically dilated. A, The right pupil constricts in response to light directed at the right eye only. B, The right pupil dilates in response to light directed at the left eye only, indicating a left RAPD.
(Courtesy of Michael S. Lee, MD.)
An RAPD is an extremely reliable and sensitive indicator of optic nerve dysfunction. Its absence generally indicates either a lack of an optic neuropathy or of bilateral optic nerve involvement. Less commonly, an RAPD may also result from substantial retinal disease, such as a central retinal artery occlusion or retinal detachment. Chiasmal lesions may produce an RAPD secondary to asymmetric optic nerve involvement. In patients with a unilateral optic tract lesion, a mild RAPD may exist on the side of the temporal visual field defect. This has been explained by the difference in light sensitivity between the intact temporal and nasal hemifields. In addition, the proportion of fibers that cross (53%) is greater than the proportion of those that do not cross (47%). In rare cases, an RAPD may result from media opacities such as cataract or vitreous hemorrhage or from amblyopia.
Certain factors, such as dark irides or sluggish, dilated, or miotic pupils, can make it difficult to elicit an RAPD. In these circumstances, perceived color intensity and subjective sense of brightness can be used to identify and quantitatively grade an RAPD. Red perception testing requires a red target, such as a cap of a cycloplegic eyedrop bottle. Brightness sense testing requires a bright light source such as a muscle light or an indirect ophthalmoscope set at 6 volts (V). The clinician directs the patient to look at the target, which is held 30 cm from the eye and is exposed to each eye for the same length of time. The clinician then asks the patient whether the light is of equal brightness (or is equally red) in both eyes. If the patient indicates a difference between the eyes, the target is held in front of the “brighter” or “redder” eye first, before the target is switched to the other eye. The patient is asked, “If the light is 100% bright (the target is 100% red) in this eye, then what percentage is it in the other eye?”
Danesh-Meyer HV, Papchenko TL, Savino PJ, Gamble GD. Brightness sensitivity and color perception as predictors of relative afferent pupillary defect. Invest Ophthalmol Vis Sci. 2007;48(8):3616–3621.
Kawasaki A, Miller NR, Kardon R. Pupillographic investigation of the relative afferent pupillary defect associated with a midbrain lesion. Ophthalmology. 2010;117(1):175–179.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.