Corneal Light Reflex Tests
Corneal light reflex tests rely on the location of the first Purkinje image of a fixation light to assess the alignment of the eyes. The Hirschberg and Krimsky tests are the main tests of this type. Although they are not as accurate as cover tests, they are a useful alternative 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 22Δ/mm but can vary by as much as 20% 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: 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 7-4).
Figure 7-4 Hirschberg test. The extent to which the corneal light reflex is displaced from the center of the pupil provides an approximation of the angular size of the deviation (in this example, a left esotropia). (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.)
The Krimsky test uses prisms to quantify the decentration of the corneal reflections and is best suited for near fixation measurement. This is done by holding a prism over either eye. By adjusting
the prism power to center the corneal reflection symmetrically in each eye, it is possible to approximate the near deviation (Fig 7-5).
Figure 7-5 Krimsky test. The right exotropia is measured by the size of the prism required to center the 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.)
The angle kappa (Fig 7-6) can affect corneal light reflex measurements. Angle kappa is the angle between the visual axis and the anatomical pupillary axis of the eye. If the fovea is slightly 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. A large positive angle kappa 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 is termed negative angle kappa; it simulates esotropia. The angle kappa does not affect any of the cover tests.
Figure 7-6 Angle kappa. A positive angle (in which the corneal light reflex is medial to the vertical line) simulates exotropia, whereas a negative angle (in which the light reflex is lateral to the vertical line) simulates esotropia. (Reprinted with permission from
Parks MM. Ocular Motility and Strabismus. Hagerstown, MD: Harper & Row; 1975.)
The Brückner test uses the red reflex to test for bifoveal fixation. 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. The Brückner test is used mainly by primary care practitioners to screen for strabismus; observation of the red reflex in this setting can also identify opacities in the visual axis and refractive errors. See also Chapter 1.
Subjective Tests
The Maddox rod test uses a device consisting of a series of parallel cylinders that converts a point source of light into a line image. The optical properties of the cylinders cause the streak of light to be situated 90° to the orientation of the parallel cylinders. Because fusion is precluded by the Maddox rod, heterophorias and heterotropias cannot be differentiated. The Maddox rod can be used to test for horizontal and vertical deviations and, when used in conjunction with another Maddox rod, for cyclodeviations.
To test for horizontal deviations, the clinician places the Maddox rod in front of 1 eye (for this example, the right eye) with the cylinders positioned horizontally. The patient fixates on a point source of light; he or she sees a vertical line with the right eye and the point source of white light with the left eye. If the light superimposes on the line, orthophoria is present. If the light is to the left of the line, an esodeviation is present; if the light is to the right of the line, an exodeviation is present. To measure the amount of deviation, the examiner finds the prism that superimposes the point source on the line. The Maddox rod test cannot be used satisfactorily to quantitate horizontal deviations, however, because accommodative convergence cannot be controlled. A similar procedure, with the cylinders aligned vertically, is used to test for vertical deviations.
The double Maddox rod test (Fig 7-7) is used to measure cyclotropia. Maddox rods are placed in a trial frame or phoropter and positioned in front of each eye, with the rods aligned vertically so that the patient sees 2 horizontal lines. The patient or examiner rotates the axes of the rods until the lines are perceived to be parallel. To facilitate the patient’s recognition of the 2 lines, it is often helpful to dissociate the lines by placing a small prism base-up or base-down in front of 1 eye. The angle of rotation that causes the line images to appear parallel determines the magnitude and direction
(intorsion or extorsion) of cyclotropia. Traditionally, a red Maddox rod is placed before the right eye and a white Maddox rod before the left, but evidence suggests that the different colors can cause fixation artifacts; these artifacts do not occur if the same color is used bilaterally. In congenital conditions such as congenital superior oblique palsy, the patient often does not subjectively appreciate the torsion and will not indicate any torsion with the double Maddox rod test. In these cases, observation of the fundus with an indirect ophthalmoscope for torsion can aid diagnosis.
Figure 7-7 Double Maddox rod test. A, The cylinders are aligned vertically to produce 2 horizontal lines. B, Top: View of a patient with a small left hypertropia and no torsion. Bottom: View of a patient with a small left hypertropia and extorsion. (Part A
courtesy of Scott Olitsky, MD; part B courtesy of Steven Archer, MD.)
The Lancaster red-green test (and variations such as the Hess, Harms, and Lee screening tests) uses red-green goggles that can be reversed, a red-slit projector, a greenslit projector, and a screen ruled into squares. The patient’s head is held steady; by convention, the test is begun with the red filter in front of the right eye. The examiner projects a red slit onto the screen, and the patient is asked to place the green slit so that it appears to coincide with the red slit. The relative positions of the 2 streaks are then recorded. The test is repeated for the diagnostic positions of gaze, and the goggles are then reversed so that the deviation that occurs with the fellow eye fixating can be recorded. The Lancaster red-green test is used primarily for patients with complicated incomitant strabismus and requires that the patient have normal retinal correspondence (see Chapter 6).
The major amblyoscope (Fig 7-8A) is a versatile instrument that can be used to measure ocular alignment subjectively or with cover testing. For subjective measurements, dissimilar targets are presented to each eye, and the patient is asked to superimpose them. If the patient has normal retinal correspondence, the horizontal, vertical, and torsional deviations can be read directly from the calibrated scale of the amblyoscope (Fig 7-8B). Also see the section Amblyoscope Testing, later in this chapter.