Afferent Pupillary Defect (Marcus Gunn Pupil): Definition and Clinical Significance
An afferent pupillary defect (APD), also known as a Marcus Gunn pupil, is an important clinical sign that indicates damage to the optic nerve or severe retinal disease. It is most commonly identified during the swinging flashlight test and reflects an abnormal afferent (sensory) visual pathway.
Recognizing an afferent pupillary defect is critical in trauma evaluation, neurologic assessment, and head and neck examinations, as it often signals underlying optic nerve injury.
What Is an Afferent Pupillary Defect?
In a normal pupillary light reflex, shining a light into one eye causes both pupils to constrict:
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Direct response in the stimulated eye
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Consensual response in the opposite eye
In an afferent pupillary defect, the affected eye does respond to direct light, but the response is weaker than expected. When the contralateral (normal) eye is stimulated, both pupils constrict more briskly, including the pupil on the affected side.
This asymmetry indicates reduced sensory input from the affected optic nerve.
Key Clinical Finding
An afferent pupillary defect is characterized by:
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A pupil that reacts to direct stimulation
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A stronger consensual response when the opposite eye is illuminated
This paradoxical finding occurs because the damaged optic nerve transmits less visual signal to the brain, resulting in diminished activation of the pupillary reflex pathway.
What Does an Afferent Pupillary Defect Indicate?
The presence of an afferent pupillary defect is highly suggestive of:
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Optic nerve injury
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Optic neuritis
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Compressive optic neuropathy
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Severe retinal pathology
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Traumatic optic neuropathy
It does not result from oculomotor nerve dysfunction, iris pathology, or pharmacologic dilation, making it a valuable localization sign.
Clinical Relevance in Trauma and Surgery
In facial trauma, orbital injuries, and head and neck surgery, identifying an afferent pupillary defect may be the first indication of optic nerve compromise. Early recognition is essential, as timely intervention can influence visual outcomes in certain conditions.
Because the pupil still reacts, an APD can be overlooked unless specifically tested for, emphasizing the importance of performing a proper swinging flashlight exam.
Alternate Name: Marcus Gunn Pupil
The afferent pupillary defect is also known as the Marcus Gunn pupil, named after the physician who first described the phenomenon. Both terms are used interchangeably in clinical practice and medical education.
Conclusion
An afferent pupillary defect is a subtle but powerful diagnostic sign indicating optic nerve dysfunction. Although the pupil reacts to direct light, a stronger consensual response during contralateral stimulation reveals impaired afferent signaling. Proper identification of this finding is essential in neurologic, ophthalmologic, and trauma evaluations.
