What is RAPD (Relative Afferent Pupillary Defect)
RAPD is a relative afferent pupillary defect—an asymmetric pupillary response to light indicating unilateral or asymmetric optic nerve dysfunction, retinal disease, or other afferent visual pathway pathology. 1
Clinical Definition and Mechanism
RAPD represents impaired afferent input from one eye compared to the fellow eye, detected through the swinging flashlight test where light directed at the affected eye produces less pupillary constriction than when directed at the normal eye 1
The pupils themselves are structurally normal; the defect lies in the afferent visual pathway (retina, optic nerve, or optic tract) rather than the efferent pupillomotor pathway 1
Clinical Significance and Measurement
An RAPD of 0.3 or more log units (easily visible clinically) is NOT typically seen in amblyopia alone, making it a critical distinguishing feature 1
A subtle RAPD may occur with dense amblyopia, but the presence of a large RAPD should prompt immediate investigation for compressive optic neuropathy or significant retinal pathology 1
Diagnostic Approach in Pediatric Patients
Pupillary examination should assess size, symmetry, shape, direct and consensual light responses, and presence of relative afferent defect 1
Testing in infants and children may be challenging due to hippus (physiologic pupillary oscillation), poorly maintained fixation, and rapid accommodative changes 1
Anisocoria greater than 1 millimeter warrants evaluation for pathological processes including Horner syndrome, Adie tonic pupil, or third-cranial-nerve palsy 1
Critical Clinical Pitfall
The presence of a large RAPD mandates a thorough search for compressive optic neuropathy or other etiologies of visual impairment such as significant retinal abnormalities, optic neuritis, or ischemic optic neuropathy 1
Irregular pupils may indicate traumatic sphincter damage, iritis, or congenital abnormalities like coloboma, which should be distinguished from RAPD 1