Corneal light reflex tests are useful in assessing ocular alignment in patients who cannot cooperate sufficiently to allow cover testing or who have poor fixation. The main tests of this type are the Hirschberg, modified Krimsky, Brückner, and major amblyoscope methods.
The Hirschberg method is based on the premise that 1 mm of decentration of the corneal light reflection corresponds to approximately 7°, or 15Δ, of ocular deviation of the visual axis. Therefore, a light reflex at the pupillary margin is about 2 mm from the pupillary center (with a 4-mm pupil), which corresponds to 15°, or approximately 30Δ, of deviation. A reflex in the mid-iris region is about 4 mm from the pupillary center, which is roughly 30°, or 60Δ, of deviation; similarly, a reflex at the limbus is about 45°, or 90Δ, of deviation (Fig 6-3).
The Krimsky method uses reflections produced on both corneas by a penlight and is ideally used at near fixation. The original method involved placing prisms in front of the deviating eye. More common modifications today involve holding prisms before the fixating eye or split between the 2 eyes. By adjusting the prisms to center the corneal reflection in the deviated eye, it is possible to approximate and quantitate the near deviation (Fig 6-4). The Hirschberg and Krimsky methods can be inaccurate even when used by experienced strabismologists. Therefore, their use is often limited to patients who are uncooperative or have vision that is too poor to allow for a measurement with other techniques.
The angle kappa can affect light reflex measurements. Angle kappa is the angle between the visual axis and the anatomical pupillary axis of the eye. If the fovea is temporal to the pupillary axis (as is usually the case), the corneal light reflection will be slightly nasal to the center of the cornea. This is termed positive angle kappa and simulates exodeviation. If the position of the fovea is nasal to the pupillary axis, the corneal light reflection will be slightly temporal to the center of the cornea. This is termed negative angle kappa and simulates esodeviation (Fig 6-5). An angle kappa will 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. If strabismus is present, the deviated eye will have a lighter and brighter reflex than the fixating eye. Note that this test detects, but does not measure, the deviation. This test also identifies opacities in the visual axis and moderate to severe anisometropia. The Brückner test is primarily used by primary care practitioners to screen for strabismus and anisometropia.
The major amblyoscope method uses separate target illumination, which can be moved to center the corneal light reflection. The amount of deviation is then read directly from the scale of the amblyoscope (see Chapter 4).
, KushnerBJ. The accuracy of experienced strabismologists using the Hirschberg and Krimsky tests.1998;105(7):1301–1306.