Afferent Pupillary Defect Marcus Gunn Pupil

A specific sign for optic nerve dysfunction is the presence of an afferent pupillary defect (Marcus Gunn pupil). In testing for an afferent pupillary defect, room illumination should be decreased, the patient should be fixating at a distant target, and a bright light source should be used. The test compares the pupillary constriction, and hence optic nerve function, of the two eyes. The eye with the optic nerve dysfunction perceives the light source as being less bright and the pupils constrict less as compared to when the same light is shown in the "normal" eye. Therefore, the direct response to light (light shown into the involved eye) is less than the consensual response (light shown into the uninvolved eye). Clinically, the test is performed by swinging a flashlight between the two eyes, resting approximately 2 s at each eye.12 During the swinging flashlight test it appears to the observer as if the involved eye is exhibiting pupillary dilation to the light source (see Fig. 2-3). This seemingly "paradoxical dilatation to light" is the hallmark of an afferent pupillary defect, and the eye with the pupil that "dilates" to light has either an optic nerve or retinal abnormality.

Performing the swinging flashlight test on young children is often troublesome because distance fixation cannot be

Marcus Gunn Pupil

FIGURE 2-3. Testing for an afferent pupillary defect. The "swinging flashlight test" is performed by shining a light in the right eye [top right) and noting the pupil size of both eyes. The light is then "swung" to the left eye and both pupils observed [top left). The poorer response when shining the light in the right eye indicates a right afferent pupillary defect. The test can also be performed using room illumination by alternatively covering each eye [middle left and right). The larger pupil on the right eye indicates a right afferent defect. Bottom: testing for an afferent defect with a fixed right pupil. Compare the size of the left pupil when light is shone in the right eye [bottom left) as compared to the left eye [bottom right). The better response from the left eye indicates a right afferent defect.

FIGURE 2-3. Testing for an afferent pupillary defect. The "swinging flashlight test" is performed by shining a light in the right eye [top right) and noting the pupil size of both eyes. The light is then "swung" to the left eye and both pupils observed [top left). The poorer response when shining the light in the right eye indicates a right afferent pupillary defect. The test can also be performed using room illumination by alternatively covering each eye [middle left and right). The larger pupil on the right eye indicates a right afferent defect. Bottom: testing for an afferent defect with a fixed right pupil. Compare the size of the left pupil when light is shone in the right eye [bottom left) as compared to the left eye [bottom right). The better response from the left eye indicates a right afferent defect.

TABLE 2-2. Pupil Evaluation

in Low-Vision Infant.

Location of disorder

Anterior pathway

Cortical

Pupil reactivity to light

Sluggish

Brisk

Afferent defect

Possible

Never

Paradoxical pupil

Possible

Never

Iris defects

Possible

Never

maintained and, with near fixation, accommodative miosis makes interpretation difficult. In such circumstances, normal room illumination can be used to assess optic nerve function10 (Fig. 2-3 demonstrates this technique). This approach is especially helpful in the infant or toddler who invariably fixates on the light source, making traditional testing impossible. An afferent pupillary defect can even be detected if one of the pupils is non-reactive secondary to trauma, pharmacological dilation, or ocular inflammation (Fig. 2-3). In such cases, the direct and consensual responses of the single reactive pupil must be compared. If the consensual response is less than the direct response there is an abnormality in that eye. The afferent pupillary defect is a sensitive indicator of optic nerve dysfunction and, more specifically, visual field loss.17 Retinal disease or amblyopia can also cause an afferent pupillary defect, but the disease process is usually severe and visual function is quite poor. Because the afferent pupillary defect is a relative measure of optic nerve function between eyes, bilateral symmetrical optic nerve involvement will result in a negative test.

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Responses

  • connie
    How to perform the marcus gunn flashlight test?
    6 years ago

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