Later it became clear that not only pupillary escape but also the initial constriction needed to be considered ( Cox, 1986). When the swinging-flashlight test was introduced, pupillary escape was mentioned as the typical sign of a RAPD ( Levatin, 1959), although some patients with neuroretinal disease were noted to lack it. Pupillary escape is a very clear and specific sign of RAPD. (Note that the term “pupillary escape” is occasionally also used in pupillography to mean the opposite of pupillary capture, i.e., the absence of capture is named “escape” this has nothing to do with the swinging-flashlight test.) This behavior is called pupillary escape ( Levatin, 1959 Cox, 1992). If a marked RAPD is present, for example in a case of optic neuritis, the pupil of the involved eye will dilate when the light returns, and the initial constriction has been lost. This initial constriction is a very sensitive sign for subtle interocular differences in the pupillary light reaction ( Cox, 1986). When one eye is illuminated, the fellow eye begins to dark-adapt, its light sensitivity increases, and the pupils will constrict when the light comes back, even if the changeover is done very quickly. The light should be shined for 2–4 seconds at each eye and then rapidly moved to the other eye. In (A) direct and consensual reactions are the same, whereas in (B) there is a difference between direct and consensual reactions.
The reaction of the illuminated eye is called direct pupillary light reaction, while the accompanying reaction of the fellow eye is called consensual pupillary light reaction. Again, there is a relative afferent pupillary defect on the left side. The right pupil now reacts better when the right eye is illuminated than when the left eye is illuminated. (B) The left iris is damaged, and the pupils are unequal. If the left eye is illuminated, both pupils constrict less: there is a relative afferent pupillary defect on the left side. (A) If the right eye is illuminated, both pupils constrict normally. Therefore, it is advisable not to constrict the pupil below this range when performing the swinging-flashlight test.įig. For pupils with diameter of less than 3–4 mm, pupillary light responses are no longer linearly correlated with the light flux. Best results will be obtained if the illumination remains in a range that allows the pupil to respond linearly. If the light is too strong, the pupil will constrict vigorously until it reaches the mechanical endpoint and a subtle defect will be missed. It is convenient to start with a distance that allows the pupil to constrict by one-third of its initial diameter. By varying the distance from the eyes, the examiner can adjust the cornal illumination. A bright light, such as an indirect ophthalmoscope or a flashlight, is directed with an angle of about 45° to the optical axis from below to the upper peripheral retina. This test should be performed in a room as dark as possible in order to start with large pupils ( Wilhelm, 1998). The swinging-flashlight test looks for differences between the visual afferent pathways of the two eyes. If this is the case, a different approach is necessary. Preconditions are two pupils with the ability to react equally, i.e., no fixed or poorly reacting pupil, no marked anisocoria ( Fig. The swinging-flashlight test compares the direct light reactions of both eyes ( Levatin et al., 1973).