{{short description|When one eye's exposure to light creates a muted pupil response in both eyes}} {{for|the jaw-winking syndrome|Marcus Gunn phenomenon}} {{Infobox medical condition (new) | name = Relative afferent pupillary defect | image = Gray773.png | caption = The left [[optic nerve]] and the [[optic tract]]s. A Marcus Gunn pupil indicates an ''afferent'' defect, usually at the level of the [[retina]] or [[optic nerve]]. Moving a bright light from the unaffected eye to the affected eye would cause ''both'' eyes to dilate, because the ability to perceive the bright light is diminished. | pronounce = | field = [[Ophthalmology]], [[Optometry]] | synonyms = Marcus Gunn pupil | symptoms = | complications = | onset = | duration = | types = | causes = | risks = | diagnosis = | differential = | prevention = | treatment = | medication = | prognosis = | frequency = | deaths = }}

A '''relative afferent pupillary defect''' ('''RAPD'''), also known as a '''Marcus Gunn pupil''' (after [[Robert Marcus Gunn]]), is a [[medical sign]] observed during the [[swinging-flashlight test]]<ref>{{cite web |url=http://stanfordmedicine25.stanford.edu/the25/pupillary.html |title=Pupillary Responses |website=Stanford University School of Medicine |access-date=2015-11-04}}</ref> whereupon the patient's [[pupils]] excessively [[mydriasis|dilate]] when a bright light is swung from the unaffected eye to the affected eye. The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.

Depending on severity, different symptoms may appear during the swinging flash light test:{{citation needed|date=October 2025}}

Mild RAPD initially presents as a weak pupil constriction, after which dilation occurs.{{citation needed|date=October 2025}}

When RAPD is moderate, pupil size initially remains same, after which it dilates.{{citation needed|date=October 2025}}

When RAPD is severe, the pupil dilates quickly.{{citation needed|date=October 2025}}

==Presentation== {{Empty section|date=October 2025}} ==Cause== Normally, the optic nerve is responsible for sensing light while the oculomotor nerve is responsible for contracting the pupil in response to this light.

The most common cause of Marcus Gunn pupil is a [[lesion]] of the [[optic nerve]] (between the retina and the [[optic chiasm]]) due to [[glaucoma]], a severe [[retina]]l disease, or due to [[multiple sclerosis]]. It is named after [[Scottish people|Scottish]] [[ophthalmologist]] [[Robert Marcus Gunn]].<ref>{{WhoNamedIt|doctor|2687}}</ref> A second common cause of Marcus Gunn pupil is a contralateral [[optic tract]] lesion, due to the different contributions of the intact nasal and temporal hemifields.<ref name="pmid8937350">{{cite journal |author=Pearce J |title=The Marcus Gunn pupil |journal=J. Neurol. Neurosurg. Psychiatry |volume=61 |issue=5 |pages=520 |date=November 1996 |pmid=8937350 |pmc=1074053 |doi= 10.1136/jnnp.61.5.520|url=}}</ref>

==Diagnosis== The Marcus Gunn pupil is a relative afferent pupillary defect indicating a decreased pupillary response to light in the affected eye.<ref name="pmid8937350"/>

In the [[swinging flashlight test]], a light is alternately shone into the left and right eyes. A normal response would be equal constriction of both pupils, regardless of which eye the light is directed at. This indicates an intact direct and consensual [[pupillary light reflex]]. When the test is performed in an eye with an afferent pupillary defect, light directed in the affected eye will cause only mild constriction of both pupils (due to decreased response to light from the afferent defect), while light in the unaffected eye will cause a normal constriction of both pupils (due to an intact efferent path, and an intact consensual pupillary reflex). Thus, light shone in the affected eye will produce less pupillary constriction than light shone in the unaffected eye.{{cn|date=June 2022}}

[[Anisocoria]] is absent. A Marcus Gunn pupil is seen, among other conditions, in unilateral [[optic neuritis]].<ref name=":0">{{Cite journal |last1=Petzold |first1=Axel |last2=Fraser |first2=Clare L |last3=Abegg |first3=Mathias |last4=Alroughani |first4=Raed |last5=Alshowaeir |first5=Daniah |last6=Alvarenga |first6=Regina |last7=Andris |first7=Cécile |last8=Asgari |first8=Nasrin |last9=Barnett |first9=Yael |last10=Battistella |first10=Roberto |last11=Behbehani |first11=Raed |last12=Berger |first12=Thomas |last13=Bikbov |first13=Mukharram M |last14=Biotti |first14=Damien |last15=Biousse |first15=Valerie |date=2022 |title=Diagnosis and classification of optic neuritis |url=https://discovery.ucl.ac.uk/10156457/3/Petzold_on-v7-R3c.pdf |journal=The Lancet Neurology |volume=21 |issue=12 |pages=1120–1134 |doi=10.1016/s1474-4422(22)00200-9 |pmid=36179757 |hdl=2078.1/276419 |s2cid=252564095 |issn=1474-4422 }}{{Dead link|date=September 2025 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> It is also common in retrobulbar optic neuritis due to [[multiple sclerosis]] but unreliable in bilateral optic neuritis.<ref name=":0" /><ref>Mumenthaler Neurology 4ed, Thieme 2004, page 486 Demyelinating diseases</ref>

A total [[CN II|optic nerve (CN II)]] lesion, in which the affected eye perceives ''no'' light, is very similar to a Marcus Gunn pupil; to distinguish them, in a complete optic nerve lesion shining the light in the affected eye produces zero dilation nor constriction.{{citation needed|date=October 2025}}

==See also== {{col div|colwidth=30em}} * [[Argyll Robertson pupil]] * [[Adie syndrome]] * [[Cycloplegia]] * [[Miosis]] * [[Parinaud's syndrome]] * [[Syphilis]] {{colend}}

==References== {{reflist}}

== External links == {{Medical resources | DiseasesDB = 29599 | ICD10 = | ICD9 = | ICDO = | OMIM = | MedlinePlus = | eMedicineSubj = | eMedicineTopic = | MeshID = }}

{{Eye pathology}}

[[Category:Eye diseases]] [[Category:Human pupil]]