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PupillaEye Exam Lab

RAPD and the swinging flashlight test

Updated

Quick answer

A relative afferent pupillary defect (RAPD), or Marcus Gunn pupil, means one eye sends a weaker light signal to the brainstem than the other. It is found with the swinging flashlight test: when the light swings from the healthy eye to the affected eye, both pupils dilate instead of staying small. An RAPD points to asymmetric optic nerve disease or extensive retinal damage, and on its own it does not make the pupils unequal in size.

Open the simulator

How to perform the swinging flashlight test

  1. Dim the room and ask the patient to look at a distant target, so the near response does not constrict the pupils.
  2. Use a bright, even light (a good penlight or an indirect ophthalmoscope) held slightly below the line of sight.
  3. Shine the light into the right eye for about 2–3 seconds and watch the pupil constrict.
  4. Swing the light quickly across the bridge of the nose to the left eye, hold it for the same 2–3 seconds, and watch the first movement of that pupil.
  5. Swing back and forth several times, spending exactly the same time on each eye, and compare the response each time the light arrives.

How to interpret the result

  • Normal: each pupil constricts, or stays constricted, when the light arrives. A small, equal redilation on both sides (pupillary escape) is normal.
  • RAPD: when the light arrives on the affected eye, its pupil dilates — "paradoxical" dilation — because the brainstem suddenly receives less light signal and both pupils redilate together.
  • Both pupils always move together. The light reflex is consensual, so an afferent defect never causes anisocoria. If the pupils are unequal, look for a second, efferent problem.
  • One pupil fixed or pharmacologically dilated? Watch only the pupil that works while you swing the light (the reverse swinging flashlight test). If it dilates when the light moves to the other eye, that other eye has the RAPD.

How to grade an RAPD

Bedside grading describes what the affected pupil does when the light arrives. Grading scales differ slightly between sources; the plus scale below is the most widely used.

GradeWhat you see when the light reaches the affected eye
1+Weak initial constriction, then greater redilation
2+Initial stall (no visible constriction), then dilation
3+Immediate dilation
4+Amaurotic pupil: no reaction to direct light at all

To quantify an RAPD, place neutral density filters of increasing density (0.3 log unit steps: 0.3, 0.6, 0.9, 1.2…) over the healthy eye until the swinging response looks balanced. The filter that balances it is the size of the RAPD in log units. Small asymmetries of up to about 0.3 log units occur in healthy people.

What causes an RAPD?

  • Optic nerve disease (the usual cause): optic neuritis, ischemic optic neuropathy (arteritic or non-arteritic), compressive or infiltrative optic neuropathy, traumatic optic neuropathy, asymmetric advanced glaucoma and optic atrophy.
  • Extensive retinal disease: central retinal artery occlusion, ischemic central retinal vein occlusion or a large retinal detachment. Macular disease alone usually gives only a small RAPD.
  • Optic tract lesion: an RAPD in the eye opposite the lesion (the eye that loses its temporal field), together with a contralateral homonymous hemianopia.
  • Dense amblyopia can cause a mild RAPD.
  • Not causes: cataract, corneal scars, refractive error, and efferent problems such as a third nerve palsy or a pharmacologically dilated pupil.

Common mistakes

  • Letting the patient look at the light, which triggers the near response.
  • Staying longer on one eye than the other, so the two retinas are unequally light-adapted.
  • Swinging too slowly, so each pupil has time to escape and the difference is lost.
  • Calling hippus (small rhythmic pupil oscillations) or symmetric escape an RAPD.
  • Giving up when one pupil does not react, instead of using the reverse test.

Practice it on the virtual patient

  1. Open the exam simulator with an RAPD in the right eye.
  2. Select the penlight and swing it between the eyes, 2–3 seconds on each.
  3. Watch both pupils dilate together when the light reaches the right eye, then compare with the left-eye version and with complete optic atrophy (CN II).
  4. Open the clinical explanation under the eyes to check your interpretation.
Open the simulator

Frequently asked questions

Does an RAPD cause unequal pupils?

No. An RAPD is an afferent (input) defect, and because each pupil receives input from both eyes, the pupils stay equal in size. Anisocoria together with an RAPD means there is a second, efferent problem such as a third nerve palsy, Horner syndrome or a pharmacologic pupil.

Can a cataract cause an RAPD?

No. Even a dense cataract lets enough light reach the retina for a normal pupil response, so an RAPD in an eye with cataract points to optic nerve or retinal disease and must be explained. A dense vitreous hemorrhage can occasionally produce a small RAPD.

Is a Marcus Gunn pupil the same as Marcus Gunn jaw-winking?

No, they only share a name. A Marcus Gunn pupil is an RAPD. Marcus Gunn jaw-winking is a congenital ptosis in which the drooping eyelid lifts when the jaw moves, caused by abnormal connections between the trigeminal and oculomotor nerves.

How do you record an RAPD?

Record the eye and the size, for example "RAPD OS 2+" or "RAPD OS 0.6 log units", together with whether the pupils are equal and react to light and near. Write "no RAPD" when the swinging flashlight test is normal.

Is a new RAPD an emergency?

An RAPD is a sign, not a diagnosis, but a new RAPD with vision loss needs same-day assessment to exclude causes such as giant cell arteritis, compressive optic neuropathy or retinal artery occlusion.