Corneal Light Reflex and Red Reflex Tests
Corneal light reflex tests assess eye alignment using the location of the first Purkinje image, the image formed from reflection of a fixation light by the anterior corneal surface, which acts as a curved mirror. The Hirschberg and Krimsky tests are the main tests of this type. Though not as accurate as cover tests, they are useful for uncooperative patients and those with poor or eccentric fixation, in whom cover testing is not possible.
The Hirschberg test is based on the correlation between the decentration of the corneal light reflection and the ocular deviation. The ratio is about 22 prism diopters (Δ) per millimeter of decentration but can vary between 12Δ and 27Δ from one individual to the next. With an uncooperative child, it is not always possible to accurately measure the light reflex displacement, so gross estimates of the deviation are often used (although these are highly dependent on pupil size): 30Δ if the reflex is at the pupil margin, 60Δ if the reflex is in the middle of the iris, and 90Δ if the reflex is at the limbus (Fig 7-3).
The Krimsky test uses prisms to quantify the decentration of the corneal reflections from a handheld torch. This is done by holding a prism over either eye and adjusting the prism power until the corneal reflection is positioned symmetrically in each eye to approximate the near deviation (Fig 7-4; Video 7-5).
The Krimsky test.
Animation developed by Steven M. Archer, MD, and Kristina Tarczy-Hornoch, MD, DPhil.
The angle kappa, the angle between the visual axis and the anatomical pupillary axis of the eye (Fig 7-5), can affect corneal light reflex measurements. The fovea is usually slightly temporal to the pupillary axis, making the corneal light reflection slightly nasal to the center of the cornea. This is termed positive angle kappa. A large positive angle kappa (eg, from temporal dragging of the macula in cicatricial retinopathy of prematurity) can simulate exotropia. If the position of the fovea is nasal to the pupillary axis, the corneal light reflection will be temporal to the center of the cornea. This negative angle kappa simulates esotropia. The angle kappa does not affect any of the cover tests.
In the Brückner test, the direct ophthalmoscope is used to obtain a red reflex simultaneously in both eyes. Foveation of the ophthalmoscope filament dims the red reflex. If strabismus is present, the deviated eye will have a lighter and brighter reflex than the fixating eye. Media opacities and refractive errors can also cause unequal red reflexes. Simultaneously, the positions of the corneal light reflexes can be assessed. This test is used mainly by primary care practitioners to screen for vision disorders.
Figure 7-3 Hirschberg test, left eye. The extent to which the corneal light reflex is displaced from the center of an average-sized pupil provides an approximation of the angular size of the deviation (here, a left esotropia). Δ = prism diopter.
(Modified with permission from Simon JW, Calhoun JH. A Child’s Eyes: A Guide to Pediatric Primary Care. Gainesville, FL: Triad Publishing Company; 1997:72.)
Figure 7-4 Krimsky test. The magnitude of the right exotropia is estimated by the power of the prism required to produce symmetric pupillary reflexes, as shown at bottom.
(Reprinted with permission from Simon JW, Calhoun JH. A Child’s Eyes: A Guide to Pediatric Primary Care. Gainesville, FL: Triad Publishing Company; 1997:72.)
Figure 7-5 Angle kappa. A positive angle kappa (in which the corneal light reflex is nasally displaced; shown in cross-section for the right eye), if large enough, simulates exotropia, whereas a negative angle (in which the light reflex is temporally displaced) simulates esotropia.
(Modified with permission from Parks MM. Ocular Motility and Strabismus. Hagerstown, MD: Harper & Row; 1975.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.