Relative afferent pupillary defect (RAPD)

David Goldemund M.D.
Updated on 20/01/2024, published on 01/05/2022


  • 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
Relative afferent pupillary defect (RAPD) - swinging flashlight test

Conditions associated with RAPD

  • 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
  • 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

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Relative afferent pupillary defect (RAPD)