Marcus Gunn pupil
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| Marcus Gunn pupil Classification and external resources |
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| DiseasesDB | 29599 |
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Marcus Gunn pupil is a medical sign observed during the swinging-flashlight test whereupon the patient's pupils constrict less (therefore appearing to dilate) when the light swings from the unaffected "good" eye to the affected "bad" eye. There is no anisocoria, and the "bad eye" still senses light and produces pupillary constriction to some degree, albeit reduced.
The commonest cause of Marcus Gunn pupil is a lesion of the optic nerve (before the optic chiasm) or severe retinal disease. It is named after British ophthalmologist Robert Marcus Gunn.[1]
[edit] Mechanism
The Marcus Gunn phenomenon is a relative afferent pupillary defect. That is to say, the "bad eye" can still perceive light and respond to it, but not as much as the "good eye"; the bad eye is relatively less responsive than the good eye, but both eyes are still responsive to light. If you shine the light in the bad eye, both pupils will constrict (due to the still-intact consensual light response). However, if you shine the light in the "good eye", the pupils will constrict even more. It is as if you are shining a light of lesser intensity at the bad eye.
In context of the swinging flashlight test, you first shine the light in the good eye, causing full pupillary contraction in both eyes. Then you move the light to the bad eye. The bad eye perceives this same light as if were not as bright, and thus causes the pupils to constrict less. This gives the illusion that both pupils are now dilating as a response to the light. They are actually still constricting in response to the light, but constricting less than when the light was shining at the good eye, because the bad eye perceives a dimmer light. But relative to the previous maximal constriction from shining the light at the good eye, the pupils now dilate. Had you started with the light shining on the bad eye first, you'd see both eyes constrict slightly. This distinguishes the Marcus Gunn Pupil from a total CN II lesion, in which the bad eye perceives no light. In that case, shining the light at the bad eye produces no effect. In any case the patient themselves should report that they are totally blind in the unreactive eye
[edit] References
- Bickley L. Bates' Guide to Physical Examination, 9ed. Lippincott Williams & Wilkins
- Kanski J. Clinical Ophthalmology: A Systematic Approach, 5ed. Butterword
[edit] External links
- 1611005962 at GPnotebook
- EYE38 at FPnotebook
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