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Relative Afferent Pupillary Defect (Marcus Gunn Pupil)
Ambient Light
Bright
- In bright ambient light, both pupils fully constrict (normal response).
- In dim ambient light, both pupils fully dilate (normal response).
Swinging Flashlight Test
Normal Pupil
- When a flashlight shines on the normal pupil, both pupils fully constrict. The direct and consensual reflexes are intact.
- When a flashlight shines on the abnormal pupil, both pupils dilate to some degree (mid-dilate). In essence, less afferent signal reaches the midbrain, so the pupils don’t fully constrict, making them seem to “dilate” in the abnormal eye. But really, they simply constrict less because of the weak afferent input.
- The swinging flashlight test elicits the defect, which only becomes apparent when comparing responses between the two eyes.