A relative afferent pupillary defect (rAPD) is an eye exam finding whose presence (or absence) is found during the pupillary exam. A rAPD is also known as a Marcus Gunn pupil and typically indicates the presence of either optic nerve or retinal dysfunction.
The presence of a rAPD is found with the "swinging flashlight test." A good demonstration of the swinging flashlight test can be found here. An rAPD can only be found by this test since it is comparing the difference in pupillary reactions between the two eyes.
The basic mechanics/rationale of the swinging flashlight test are:
- Shine the light in the right eye. If there is no optic nerve or retinal pathology, both pupils will constrict equally. If one of the eyes has optic nerve or retinal pathology, then its direct pupillary response will be weaker than its consensual pupillary response.
- As you shine the light in the right eye, quickly "swing" the light over to the left eye. If both eyes have equal optic nerve and retinal function, then you should see no significant change in the size of the left pupil. This occurs since the direct and consensual pupillary responses are equal.
- Let's say the left eye has optic nerve or retinal dysfunction. Then when you swing the light from the right to the left eye, the left pupil should dilate.
- Let's say the right eye has optic nerve or retinal dysfunction. Then when you swing the light from the right to the left eye, the left pupil should constrict.
- Now quickly shine the light back to the right eye (from the left eye). There are 3 possible responses:
- If there is no relative damage between the two eyes, then there will be no change in pupil size as you swing the light back to the right eye.
- If the right eye has optic nerve or retinal damage, then the right pupil will dilate as you swing the light back to the right eye.
- If the left eye has optic nerve damage, then the right pupil will constrict as you swing the light back to the right eye.
Yes, we know that this is information overload! The basic gist is that if you see significant changes in pupil size as you swing the light back-and-forth between the eyes, then you should alert your eye care providerso (s)he can double check the pupils prior to any dilation.
