[Emergency] ATLAS OF AFFERENT PUPILLARY DEFECT (MARCUS GUNN PUPIL)

ATLAS OF AFFERENT PUPILLARY DEFECT, AFFERENT PUPILLARY DEFECT, MARCUS GUNN PUPIL, The Atlas of Emergency Medicine, Fourth Edition, 2016, OPHTHALMIC CONDITIONS

AFFERENT PUPILLARY DEFECT (MARCUS GUNN PUPIL) 


Clinical Summary
Pupil size is controlled in the midbrain, from which efferent nerves travel to both pupils resulting in symmetric pupils, even with a unilateral light stimulus. The perception of the light stimulus, however, may be decreased by disease within anterior visual structures such as the retina, optic nerve, chiasm,
optic tract, and midbrain pathways. With diminished afferent stimulation, less light is “perceived,” and pupil contraction diminishes (ie, dilates) as a result. In this situation, the affected side is said to demonstrate an afferent pupillary defect (APD), and the pupil is called a Marcus Gunn pupil.

A Marcus Gunn pupil is best appreciated by the swinging flashlight test, which discloses differences in afferent stimuli between the two eyes. A flashlight is directed onto one pupil and then the other. The normal response is a prompt constriction. (Generally, this is followed by a slight “release” dilation.) During the brief interval required to move the light from one to the other eye, both pupils begin to dilate. Thus, under normal circumstances, when the light reaches the second eye, reflex constriction again occurs. With a diseased eye or pathway, the diseased side perceives less light and the midbrain therefore sets the pupil size to be larger, and as a result the pupils dilate (or continue to dilate following the “release” dilation).

Although sensitive, the swinging flashlight test is not specific, since the pathology may be anywhere in the visual pathway from the retina to the midbrain. The test cannot distinguish between unilateral afferent disease and asymmetric bilateral disease. Typically, a positive finding indicates optic nerve disease such as ischemic optic neuropathy, optic neuritis (including that seen with multiple sclerosis), retrobulbar optic neuritis, and glaucoma.
Afferent Pupillary Defect
FIGURE 2.56 ■ Afferent Pupillary Defect. Schematic representation of an 
afferent pupillary defect (APD) due to neurologic lesion in
the anterior visual pathway.  


Marcus Gunn Pupil.
Marcus Gunn Pupil.
FIGURE 2.57 ■ Marcus Gunn Pupil. These photographs demonstrate 
an afferent pupillary defect of the left pupil. When the light shines
in the affected left eye, the pupils are less constricted than when the 
light shines in the right eye. Thus, when the light is swung from the right
pupil to the left, the pupils appear to dilate. The anisocoria here is subtle.  


Management and Disposition
The finding of a Marcus Gunn pupil is nonspecific. A fundoscopic exam may show occlusion of the central retinal vein or central retinal artery, or a dense hemorrhage in the vitreous.
Otherwise, a neurologic evaluation (history, physical examination, and CT scan) is important to assess for treatable conditions. In the absence of gross ocular or neurologic disease, a clinically stable patient may be discharged from the ED with ophthalmology follow-up 

Pearls
1. An APD does not cause anisocoria because any change in light input results in a bilaterally symmetric output from the midbrain.
2. In the swinging flashlight test, both pupils will dilate when the flashlight moves from the normal pupil to the Marcus Gunn pupil, since pupil size is centrally mediated in the midbrain.
3. Bilaterally symmetric disease produces no APD, since the basis for the swinging flashlight test is asymmetry of disease.
4. A dense vitreous hemorrhage is capable of producing a mild defect. Lesser ocular pathology such as branch retinal vein or artery occlusion or a cataract is unlikely to produce a clinically detectable APD.
5. Dim room illumination may be helpful when performing the swinging flashlight test. The patient should focus on an object 15 feet away to avoid the pupil constriction normally seen with accommodation. 

REFERENCES
The Atlas of Emergency Medicine, Fourth Edition, 2016. 

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CLINICAL ATLAS,118,DERMATOLOGY ATLAS,10,EMERGENCY ATLAS,44,HAEMATOLOGY ATLAS,23,HUMAN ANATOMY,1,MICROBIOLOGY ATLAS,66,PARASITOLOGY ATLAS,4,PATHOLOGY ATLAS,22,PEDIATRIC ATLAS,42,STDs,19,SUBCLINICAL ATLAS,116,
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Free Medical Atlas: [Emergency] ATLAS OF AFFERENT PUPILLARY DEFECT (MARCUS GUNN PUPIL)
[Emergency] ATLAS OF AFFERENT PUPILLARY DEFECT (MARCUS GUNN PUPIL)
ATLAS OF AFFERENT PUPILLARY DEFECT, AFFERENT PUPILLARY DEFECT, MARCUS GUNN PUPIL, The Atlas of Emergency Medicine, Fourth Edition, 2016, OPHTHALMIC CONDITIONS
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