Relative afferent pupillary defect (RAPD)

Created 01/05/2022, last revision 06/01/2023


  • 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


  • 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
Relative afferent pupilary defect (RAPD)

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
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Relative afferent pupillary defect (RAPD)