I have been instructed as an expert witness for hundreds of medico-legal and regulatory fitness to practise investigations. Repeatedly when the patient has a unilateral or asymmetrical eye disease, the clinical records indicate no relative afferent pupillary defect (RAPD). The swinging-flashlight test looks for differences between the visual afferent pathways of the two eyes.

My own experience of working with novice and experienced practitioners is that they conduct the test too quickly. A famous three-line adage used by ophthalmologists when describing the tempo for testing the RAPD is to:

• Slow down
• Take your time
• Don’t rush.

Here is the procedure I recommend to look for a relative afferent pupil defect. It is often referred to as the swinging flashlight test:

• This test should be performed in a room as dark as possible in order to start with large pupils.
• The patient fixates a distant target.
• A bright light, such as that from a pen torch is directed with an angle of about 45° to the optical axis from below to the upper peripheral retina.
• It is convenient to start with a distance that allows the pupil to constrict by one-third of its initial diameter. If the light is too strong, the pupil will constrict vigorously until it reaches the mechanical endpoint and a subtle defect will be missed.
• With the pen torch approximately 5 cm from the eye, move the light between the eyes rapidly, leaving it on each eye for 3 to 5 seconds. The rapid part is the moving of the light between the eyes. The light needs to remain on the eye for 3 to 5 seconds. This is the slow part of the procedure.
• Observe the response (dilation or constriction) and the size of each pupil at the moment when the light first arrives there and during the 3 to 5 second observation period. Be sure to shine an equal intensity of light into each eye.
• The swinging flashlight test should be repeated for 2 to 3 complete cycles

Throughout the test, judge the roundness of each pupil.

If the pupils are unequal in size, perform the dim-bright pupillary test.

If one or both pupils fail to respond directly or consensually, or if their responses are sluggish, test the accommodative responses of the pupil.

Let’s get better at this procedure.