Subjective Tests of Ocular Alignment
The Maddox rod test assesses ocular alignment using the patient’s perception of the relative position of the images seen by each eye. For the eye viewing through the Maddox rod, a series of parallel cylinders convert a point source of light into a line image perpendicular to the cylinders. Like alternate cover testing, the test is dissociating and precludes fusion; thus, heterophorias and heterotropias cannot be differentiated. The test cannot assess alignment in patients with anomalous retinal correspondence (ARC) or suppression.
To test for horizontal deviations, the Maddox rod is held in front of 1 eye (eg, right eye) so that the cylinders are horizontal. The patient, fixating on a point source of light, sees a vertical line with the right eye and the point source of light with the left. Assuming normal retinal correspondence (NRC), in orthophoria, the point superimposes on the line; in esodeviations, the light is seen to the left of the line; and in exodeviations, the light is to the right of the line (see the section The Red-Glass (Diplopia) Test). The deviation is measured by finding the prism power that superimposes the point source on the line. Note, however, that unlike in cover testing with an accommodative target, accommodative convergence is not controlled by this technique. Vertical deviations can be assessed by orienting the cylinders vertically.
The double Maddox rod test (Fig 7-6) is used to measure cyclodeviations. Two Maddox rods are placed in a trial frame or phoropter and aligned vertically so that the patient sees 2 horizontal lines. A small vertical prism may be introduced to help separate the lines. The rod axes are rotated until the patient sees parallel lines. The angle of rotation indicates the magnitude and direction (intorsion or extorsion) of cyclodeviation. Traditionally, a red Maddox rod and a clear one were paired, but this was thought to bias the patient’s localization of the cyclodeviation toward the eye with the red rod. Using the same color bilaterally avoids this bias. In congenital conditions such as congenital superior oblique palsy, the patient may not subjectively appreciate torsion or indicate any torsion with the double Maddox rod test. In these cases, seeing fundus torsion on indirect ophthalmoscopy can aid diagnosis.
The Lancaster red-green test (and variations such as the Hess, Harms, and Lees screen tests) is useful for assessing ocular alignment in complicated incomitant strabismus in cooperative patients with NRC and no suppression (see Chapter 5), such as adults with acquired strabismus. Reversible red-green goggles, red-slit and green-slit projectors, and a grid projected or marked on a screen or wall are used in the test. With the red filter in front of the patient’s right eye, the examiner projects a red slit onto the grid; the patient places the green slit so that it appears superimposed on the red slit. The relative positions of the streaks are recorded. The test is repeated for the 9 diagnostic positions of gaze, and the goggles are reversed to record deviations with the fellow eye fixating.
Figure 7-6 Double Maddox rod test. A, The cylinders are aligned vertically, such that a patient with normal binocular vision sees 2 superimposed horizontal lines. B,Top: View seen by a patient with a small left hypertropia and no torsion. Bottom: View seen by a patient with a small left hypertropia and extorsion.
(Part A courtesy of Scott Olitsky, MD; part B courtesy of Steven M. Archer, MD.)
The major amblyoscope (Fig 7-7A) can be used to measure ocular alignment both objectively and subjectively. It may be particularly useful in adult strabismus, as it allows neutralization of torsional diplopia to assess fusional responses and can help characterize ARC. Separate, dissimilar targets are presented to each eye simultaneously, and the amblyoscope is adjusted until the patient sees the images superimposed. If the patient has NRC, the horizontal, vertical, and torsional deviations can be read directly from the calibrated scale of the amblyoscope (Fig 7-7B). See also the section Amblyoscope Testing, later in this chapter.
A, The major amblyoscope. Targets can be placed in each arm of the device to be presented separately to each eye. The arms can then be moved to compensate for ocular misalignment. B, Testing with a major amblyoscope for retinal correspondence in a patient with 20Δ of esotropia. NRC = normal retinal correspondence, with a fused percept when the angle of strabismus is fully compensated, presenting targets to the fovea of each eye; ARC = anomalous retinal correspondence (harmonious), with a fused percept in the absence of any compensation for the angle of strabismus, with one eye viewing the target foveally and the other extrafoveally; UHARC = unharmonious anomalous retinal correspondence, with a fused percept when the angle of strabismus is partially compensated.
(Part A courtesy of Steven M. Archer, MD; part B modified with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:229.)
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.