An rAPD points to disease of the optic nerve or the retina. The retina is the light-sensing layer at the back of the eye. The optic nerve carries its signal to the brain. Together they form the afferent pathway, the "in" road for light. An rAPD is also called a Marcus Gunn pupil. When that in road is damaged on one side, the brain receives a weaker light signal from that eye.
The test compares the two eyes. Light shone in one eye makes that pupil constrict, the direct reaction. The other pupil constricts at the same moment, the consensual reaction. The consensual reaction happens even if the second eye is blind. Here the muscle works normally, yet the pupil fails to constrict to direct light. The problem is the signal going in, not the muscle. That points to optic nerve or significant retinal damage. On the swing, the weaker eye's pupil appears to widen because it had been getting a stronger signal from its healthy partner.
Timing matters in clinic. Do pupil testing before any dilating drops, because drops make the reading useless. Report an abnormal pupil, such as one that widens to direct light, to the ophthalmologist right away. Screening for an rAPD and reporting it is a core part of the COA pupil assessment content area.
Incorrect Answers
- B. Strabismus is a problem of eye alignment. It is found with cover testing, not with a penlight swung between the pupils.
- C. A weak consensual response in the non-stimulated eye is what suggests an iris sphincter or nerve pathway problem. An rAPD is about light getting in, and the sphincter is working.
- D. A dense cataract clouds the lens and blurs the picture. The textbook ties an afferent defect to optic nerve or retinal damage, not to a cloudy lens.
Testing Pearls
- rAPD and Marcus Gunn pupil mean the same thing.
- Direct reaction is the lit eye; consensual reaction is the fellow eye.
- Record resting pupil size in millimeters for each eye first.
- Up to 1 mm of difference between pupils can be normal physiologic anisocoria.
- Test pupils before dilating drops and report any abnormal result to the doctor.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 8, Comprehensive Medical Eye Examination.
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 2, Anatomy and Physiology of the Eye.
