Swinging flashlight test simulator

The swinging flashlight test is a pupil check that moves a bright light back and forth between the eyes to find a relative afferent pupillary defect (RAPD), a weaker light signal from one eye.

Words to know

• Pupil: the black opening in the colored iris.
• Direct response: the lit pupil gets smaller.
• Consensual response: the other, unlit pupil gets smaller too, at the same time.
• Afferent: the incoming signal from the eye to the brain (through the optic nerve).
• RAPD (Marcus Gunn pupil): one eye's incoming signal is weaker than the other's.

Steps

1. Dim the room. Turn the room lights down so the pupils are larger and easier to watch.
2. Give a distant target. Ask the patient to look at a far target. Looking up close makes the pupils shrink on their own and spoils the test.
3. Check size and shape. Look at both pupils in the dim light: are they round and equal in size?
4. Light the first eye. Shine a bright penlight into one eye from slightly below for about 3 seconds. Both pupils should get smaller.
5. Swing quickly. Move the light straight across to the other eye and hold about 3 seconds. Keep the same distance, angle and time on each eye.
6. Watch what the lit pupil does. Normal: the pupil stays small or gets a little smaller. RAPD: both pupils get bigger (dilate) when the light lands on the weak eye.
7. Swing several times and record. Swing back and forth a few times to confirm. Record normal pupils, or RAPD with the eye, for example RAPD OS.

Normal vs RAPD

• Normal: both pupils constrict equally whichever eye is lit, and stay small through the swing.
• RAPD: light in the good eye makes both pupils small. Swing to the affected eye and both pupils dilate. The eye being lit when they dilate is the eye with the RAPD.

If one pupil cannot react (for example it is fixed or hard to see), you can still test: watch the pupil that does react as you swing the light (reverse RAPD check).

In this simulator the pupil sizes are simplified to show the pattern. Real RAPDs range from subtle (a weak first squeeze, then widening) to obvious. Any pupil finding goes to the doctor.

FAQ

What is an RAPD?

A relative afferent pupillary defect (RAPD) means one eye sends a weaker light signal to the brain than the other, usually from optic nerve or severe retina disease. It is also called a Marcus Gunn pupil.

What is a normal swinging flashlight test?

Both pupils get smaller equally whichever eye is lit, and they stay small as the light swings from eye to eye.

What does a positive swinging flashlight test look like?

When the light swings from the good eye to the affected eye, both pupils dilate instead of staying small. The eye being lit at that moment is the eye with the RAPD.

Does an RAPD make the pupils different sizes?

No. Each pupil responds to light in either eye, so the two pupils stay about the same size. An RAPD shows up as a change during the swing, not as unequal pupils.

Why should the patient look at a distant target?

Looking at something near makes the pupils constrict on their own (the near response). That hides the light reaction you are trying to compare.

How long should the light stay on each eye?

About 3 seconds on each eye, the same time, distance and angle for both. Leaving it longer on one eye can create a false difference.

Do cataracts cause an RAPD?

An RAPD points the doctor toward the optic nerve or retina rather than a cataract. Whether a very dense cataract can cause a small effect is debated, so the doctor sorts that out.

Can you test for an RAPD if only one pupil can move?

Yes. Watch the pupil that can react while you swing the light. If it dilates when the light moves to the other eye, that other eye has the RAPD. This is sometimes called checking by reverse.

Studying for the COA exam? See the COA certification guide. More clinic simulators: applanation tonometer, visual acuity test, lensometer and keratometer.