Swinging Flashlight Test

The Swinging Flashlight Test aims to reveal any asymmetry of afferent input in the pupillary light reflex, i e, a relative afferent pupil defect (RAPD). The display aims to illustrate the test procedure and the outcomes for different degrees of damage. The model also shows physiological unrest (hippus). The abnormal side is to the right on the screen. Click on the display to start swinging, click again to stop.

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Swinging Flashlight Demo     © L. Frisén 2007


The objectives of the Swinging Flashlight Test are to arrange for symmetrical stimulation in terms of geometry and duration and to provide a recovery phase of constant duration (when the stimulus is located in-between the eyes). Personally, I prefer using a hand ophthalmoscope rather than a flashlight. Using a dim room light and a low-power setting, I look through the scope, using my right, dominant eye. The pupil under stimulation will then be seen at high contrast, retro-illuminated by the red fundus glow. The patient is asked to look at a distant target, in line with my right ear. The light beam is then made to slowly swing back and forth between the patient's pupils, as many times as needed to ascertain or negate any differences in the rates of pupil contraction and relaxation between the two eyes.

In case of uncertainty of evaluation, it is useful to ask the patient to estimate the light's apparent brightness in each eye. One way to phrase the question is, "If the light is worth 100 cents in this eye, what is the worth in the other eye?" Quite subtle relative afferent defects apparently may cause quite pronounced differences in apparent brightness between the eyes.

The magnitude of a RAPD can be estimated quantitatively by attenuating the light reaching the better eye, e g, by holding a neutral density (ND) filter in front of that eye. Various filter densities are tried to find that ND value that makes the two pupils react symmetrically.

RAPDs are most commonly due to unilateral or asymmetric optic neuropathies and chiasmal disorders. Retrochiasmal causes are very much rarer and usually confined to unilateral lesions of the optic tract and/or the lateral geniculate body. Cataracts and retinopathies rarely cause a RAPD.

The presence of any efferent pupil defect, e g, pupillotonia, precludes a meaningful application of the Swinging Flashlight Test. Asking the patient for his or her subjective comparison of apparent brightness between the eyes is still possible, of course.

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