ADD-ONS / GENERAL NEUROLOGY

Relative afferent pupillary defect (RAPD)

Created 01/05/2022, last revision 05/11/2023

Definition

relative afferent pupillary defect (RAPD), also known as Marcus-Gunn pupil, is the term referring to a specific abnormal response of the pupils to light stimuli
RAPD is characterized by bilateral dilation of the pupils, instead of constriction, during the swinging flashlight test
RAPD serves as the hallmark of a unilateral or bilateral asymmetric afferent sensory lesion
many factors may influence the identification and accurate quantification of RAPD; subtle abnormalities are difficult to detect

initially, assess the size, equality, and regularity of the pupils during a standard eye examination
test response to a standard light stimulus
- under normal conditions, both pupils constrict equally and briskly
- a single light stimulus of the Marcus-Gunn pupil (with an aberrant afferent part of the visual pathway) results in weak bilateral pupil constriction
alternating light stimulus to both eyes (“swinging flashlight test“)
- shine a penlight into one eye, then swing it to the other
- alternate quickly and observe the response of the patient’s pupils
- both pupils should constrict quickly and equally during intermittent exposure to direct light
- after swinging the light stimulus from the healthy eye to the Marcus Gunn eye (swinging test), bilateral pupil dilation occurs

the swinging flashlight test, showing RAPD, is most valuable in the detection of optic nerve lesions, as retinal lesions usually present with pathologic fundoscopic findings
the only challenge arises with the bilateral symmetric afferent optic pathway defects anterior to the chiasm, which do not lead to the detection of a relative afferent defect
however, RAPD may be present in bilateral asymmetric prechiasmatic, chiasmatic, and optic tract lesions (contralateral to the lesion, as crossing fibers outnumber noncrossing fibers)
lesions affecting the final neuron of the optic pathway (tractus geniculocalcarinus) do not lead to abnormalities in the pupillary reflex

optic neuritis (multiple sclerosis, neuromyelitis optica spectrum disorder, anti-MOG, etc.)
- even mild optic neuritis with minimal vision loss or normal vision can lead to RAPD
ischemic optic neuropathies (arteritic and nonarteritic causes)
glaucoma (if one optic nerve is particularly severely damaged, RAPD may be observed)
other optic nerve autoimmune disorders (sarcoidosis, systemic lupus erythematosus, Sjögren’s syndrome)
optic nerve infections (cat scratch disease, syphilis, Lyme disease, toxoplasmosis, cytomegalovirus, cryptococcus, and tuberculosis, etc.)
hereditary optic neuropathies, such as Leber’s optic neuropathy (usually bilateral) and other inherited optic neuropathies
optic nerve tumor (glioma, meningioma of the optic nerve, or tumors compressing the optic nerve)
compressive optic neuropathy, with or without the orbital disease (thyroid-related orbitopathy, orbital tumors, or vascular malformations)
traumatic optic neuropathy
radiation-induced optic neuropathy
optic atrophy (e.g., due to papilledema)
postsurgical optic nerve damage
idiopathic optic neuropathy

the symmetrical bilateral retinal disease does not show RAPD
ischemic retinal disease
- central retinal vein occlusion (CRVO)
- central retinal artery occlusion (CRAO)
- severe ischemic branch retinal artery occlusion (BRAO)
- severe ischemic diabetic or sickle-cell retinopathy)
Ischemic ocular disease (Ocular ischemic syndrome due to ophthalmic or carotid artery stenosis/occlusion)
retinal detachment
severe macular degeneration (if unilateral and severe)
intraocular tumor (retinal and choroidal tumors including melanoma, retinoblastoma, and metastatic lesions)
retinal infection (cytomegalovirus, herpes simplex, and other causes of retinitis)

amblyopia (RAPD can be present in severe amblyopia; the visual acuity would usually be worse than 20/400
cerebral vascular disease