What is a Relative Afferent Pupillary Defect (RAPD)?
A relative afferent pupillary defect (RAPD) is an abnormal pupillary response where the affected eye paradoxically dilates instead of constricting when light is directly shined on it, indicating unilateral dysfunction of the optic nerve or anterior visual pathway. 1, 2
Clinical Definition and Pathophysiology
An RAPD represents asymmetric afferent input to the midbrain pretectal nuclei, resulting from unilateral or asymmetric damage to the retina, optic nerve, or anterior visual pathway structures. 1, 3
The defect occurs because the affected eye transmits less light signal to the brain compared to the normal eye, even though the efferent pupillary motor pathways remain intact bilaterally. 2, 3
RAPD is usually accompanied by other signs of visual dysfunction including decreased visual acuity, visual field defects, and abnormal color vision, though rare cases of isolated RAPD with normal vision have been reported with lesions affecting the contralateral brachium of the superior colliculus or pretectal nucleus. 4, 5
Detection Method: The Swinging-Light Test
The swinging-light test is the standard clinical method for detecting RAPD and should be performed in a darkened room with the patient fixating on a distant target. 1, 3
Step-by-Step Technique:
Shine a bright penlight directly into the right eye for less than 5 seconds while observing the pupillary response—the pupil should constrict normally. 1, 3
Quickly swing the light across the bridge of the nose to the left eye and observe the pupillary response as the light reaches it. 1, 3
Repeat this alternating pattern several times, swinging back and forth between eyes. 1, 3
Interpretation:
Normal response: Pupillary constriction or no change in pupil size when light is directed at either eye. 1, 2
Abnormal response (RAPD present): The affected eye's pupil dilates when light is shined directly on it, rather than constricting. 1, 2, 3
Clinical Significance and Threshold Values
An RAPD of 0.3 log units or greater is clinically significant and not typical of amblyopia alone—this magnitude should prompt investigation for optic nerve pathology, retinal disease, or compressive optic neuropathy. 2, 3
Studies of normal subjects show that small RAPDs (<0.3 log units) can be present in a minority of healthy individuals (approximately 15% by observation, up to 48% by sensitive pupillography), but RAPDs exceeding 0.39 log units are pathologic. 6
The presence of any detectable RAPD should alert clinicians to possible optic nerve or extensive retinal pathology requiring further investigation. 2, 3
Common Clinical Pitfalls to Avoid
Do not confuse anisocoria (difference in pupil size) with RAPD (difference in pupillary response to light)—these represent entirely different pathophysiologic processes. 2, 3
Anisocoria refers to unequal pupil sizes and relates to efferent pathway dysfunction or structural abnormalities. 2
RAPD specifically indicates afferent pathway dysfunction with preserved efferent responses. 2, 3
Pupillary evaluation in infants and children is particularly challenging due to frequent shifts in fixation and focusing, as well as physiologic pupillary oscillations (hipus). 1, 2, 3
Objective Quantification Methods
Infrared video pupillometry (IVP) is the gold standard for objective RAPD quantification, allowing precise measurement in log units. 7, 8
Ultrasound pupillometry has shown similar performance characteristics to IVP and may detect subclinical optic nerve damage in patients with multiple sclerosis who show no RAPD on clinical swinging-light testing. 7
The infrared pupillary asymmetry (IPA) test using standard OCT devices provides rapid (<30 seconds) quantitative RAPD assessment and correlates with structural retinal nerve fiber layer damage. 8