What is an Afferent Pupillary Defect (APD)?
An afferent pupillary defect (APD) is an abnormal pupil response caused by reduced afferent (sensory) input from the retina and optic nerve to the brain’s pupillary light reflex pathway. In day-to-day eyecare, the finding most commonly discussed is a relative afferent pupillary defect (RAPD), also called a Marcus Gunn pupil, which means one eye is providing a weaker light signal than the other eye.
In a normal pupil light reflex, shining light into one eye causes both pupils to constrict (a direct response in the illuminated eye and a consensual response in the other eye). With a RAPD, when the light is moved from the healthier eye to the affected eye, both pupils constrict less and may appear to dilate (“paradoxical dilation”) because the brain is receiving less afferent input from that eye than it received a moment earlier.
A RAPD is a clinical sign, not a diagnosis. It most often indicates unilateral or asymmetric disease affecting the optic nerve or retina, and it is detected during a routine pupil assessment using the swinging flashlight (swinging light) test.
What is the Difference Between an APD and a RAPD?
An afferent pupillary defect (APD) is a general term for a problem in the afferent (sensory) input to the pupillary light reflex. A relative afferent pupillary defect (RAPD) (also written rAPD) is the clinical finding that occurs when afferent input is unequal between the two eyes. RAPD is detected by comparing responses during the swinging flashlight test: when the light is moved from the less affected eye to the more affected eye, both pupils constrict less and may appear to dilate because the afferent signal is weaker from that eye.
In published writing, APD is sometimes used as shorthand for this relative finding, but RAPD is the more precise term for the typical bedside sign because it is defined by an inter‑eye difference.
How is an Afferent Pupillary Defect Detected?
An afferent pupillary defect is detected using the swinging flashlight test, a quick, non-invasive assessment performed as part of a routine pupil examination.
The test is carried out by first dimming the room and asking the patient to fixate on a distant target. A bright light is then directed into one eye for approximately three seconds. In a healthy visual system, both pupils constrict equally because the afferent signal from the illuminated eye triggers a bilateral pupillary response.
The eyecare professional (ECP) then swings the light to the fellow eye. If both eyes are functioning normally, the pupils remain constricted because the level of perceived light entering the visual system remains unchanged.
A positive RAPD is present when the pupils dilate rather than remain constricted as the light is moved to one eye, despite that eye being directly illuminated. This apparent paradox occurs because the affected eye is sending a weaker signal to the brain than the fellow eye. When the light is transferred to the affected eye, the brain interprets the change as a reduction in overall light intensity and responds by dilating both pupils.
The magnitude of the RAPD can also be graded using neutral density filters, allowing clinicians to monitor disease severity and progression over time.
What Causes an Afferent Pupillary Defect?
A positive RAPD indicates unilateral or asymmetric disease affecting the retina or optic nerve and most often reflects pathology in the prechiasmal afferent visual pathway (retina/optic nerve). Less commonly, asymmetric lesions involving the optic chiasm, optic tract, or midbrain pretectal region can also produce a RAPD. The defect reflects reduced afferent sensory input from one eye compared with the other.
Common causes include:
- Optic neuritis, an inflammatory condition of the optic nerve that is frequently associated with multiple sclerosis.
- Ischaemic optic neuropathy, which occurs when blood flow to the optic nerve is reduced.
- Glaucoma when optic nerve damage is significantly more advanced in one eye than the other.
- Severe retinal disease, including retinal detachment, central retinal artery occlusion, and central retinal vein occlusion.
- Optic nerve compression caused by conditions such as orbital tumours or thyroid eye disease.
Importantly, media opacities such as cataracts and corneal scarring do not cause a RAPD, provided the retina and optic nerve remain healthy. However, dense cataract has been reported to produce an apparent or contralateral RAPD through optical effects (for example, light scatter and altered retinal illumination), so a RAPD should not automatically be attributed to media opacity without considering retinal or optic nerve disease. This distinction is clinically useful because it helps differentiate reduced vision caused by refractive or media-related problems from vision loss resulting from retinal or neurological disease.
A positive RAPD is one of the most important objective signs of unilateral or asymmetric optic nerve and retinal disease. Because the test evaluates the integrity of the afferent visual pathway, it can reveal pathology even when visual acuity, colour vision, or fundus findings are only mildly affected.
The significance of a RAPD depends on the underlying cause. Conditions such as optic neuritis, ischaemic optic neuropathy, retinal artery occlusion, and compressive optic neuropathy can require urgent investigation, imaging, or referral to an ophthalmologist or neuro-ophthalmologist.
When a RAPD is detected, it is recommended to perform additional assessments, which may include visual acuity testing, colour vision testing, visual field assessment, retinal examination, optical coherence tomography (OCT), and neuroimaging where indicated. Identifying the cause promptly is important because some conditions can threaten vision permanently if left untreated.