IMAGING METHODS
Relative afferent pupillary defect (RAPD)
Created 01/05/2022, last revision 06/01/2023
Definition
Definition- a relative afferent pupillary defect (RAPD) or Marcus-Gunn pupil are the terms referring to a specific aberrant response of the pupils to light stimuli
- RAPD is characterized by bilateral dilatation of pupils instead of constriction during swinging flashlight test
- RAPD is the hallmark of a unilateral or bilateral asymmetric afferent sensory lesion
- many factors may influence the identification and appropriate quantification of RAPD; subtle abnormalities are difficult to detect
Examination
- initially, inspect pupils’ size, equality, and regularity during a standard eye exam
- test reaction to standard light stimulus
- under normal circumstances, both pupils constrict equally and briskly
- light stimulus of the Marcus-Gunn pupil (with aberrant afferent part of the visual pathway) shows a weak bilateral pupil constriction
- alternating light stimulus to both eyes (“swinging flashlight test”)
- shine a penlight toward one eye, then swing to the other, alternate quickly, and observe the patient’s pupils’ response
- both pupils should constrict quickly and equally during intermittent exposure to direct light
- after swinging of the light stimulus from the healthy eye to the Marcus-Gunn eye (swinging test), bilateral pupil dilation appears
- swinging flashlight test showing RAPD is most valuable in the detection of optic nerve lesions because retinal lesions usually have pathological fundoscopic findings
- the only difficulty is with the bilateral symmetric afferent optic pathway defect anterior to the chiasm, which does not lead to the detection of a relative afferent defect
- RAPD may, however, be present in bilateral asymmetric prechiasmatic, chiasmatic and optic tract lesions (contralateral to the lesion because crossing fibers outnumber those non-crossing fibers)
- lesions of the last neuron of the optic pathway (tractus geniculocalcarinus) do not lead to pupillary reflex abnormalities
Conditions Leading to a RAPD
Optic nerve disorders
- optic neuritis (multiple sclerosis, neuromyelitis optica spectrum disorder, anti-MOG, etc.)
- even mild optic neuritis with a minimal loss of vision or normal vision can lead to RAPD
- ischemic optic neuropathies (arteritic and non-arteritic causes)
- glaucoma (if one optic nerve is particularly severely damaged, an RAPD can be seen)
- other optic nerve inflammations (sarcoidosis, systemic lupus erythematosus, Sjögren’s syndrome)
- other optic nerve infections (cat scratch disease, syphilis, Lyme disease, toxoplasmosis, cytomegalovirus, cryptococcus and TB, etc.)
- hereditary optic neuropathies, such as Leber’s optic neuropathy (usually eventually bilateral) and other inheritable 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 optic neuropathy
- optic atrophy (e.g., post papilledema)
- post-surgical damage to the optic nerve
- idiopathic optic neuropathy
Retinal Causes of RAPD
- the symmetrical bilateral retinal disease will not show RAPD
- ischemic retinal disease (ischemic central retinal vein occlusion, central retinal artery occlusion, severe ischemic branch retinal or arterial occlusions, 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)
- an intraocular tumor (retinal and choroidal tumors including melanoma, retinoblastoma, and metastatic lesion)
- retinal infection (cytomegalovirus, herpes simplex, and other causes of retinitis)
Other Causes of RAPD
- amblyopia (RAPD can be present in severe amblyopia; usually, the visual acuity would be worse than 20/400
- cerebral vascular disease
