eye under binocular conditions is shown. Examples of the types of strabismus in which these responses are commonly found are given.
4Δ Base-Out Prism Test
The 4Δ base-out prism test is a diagnostic maneuver performed primarily to document the presence of a small facultative scotoma in a patient with monofixation syndrome and no manifest deviation (see Chapter 6). In this test, a 4Δ base-out prism is quickly placed before 1 eye and then the other during binocular viewing, and motor responses are observed (Fig 7-13). Patients with bifixation usually show a version (bilateral) movement away from the eye covered by the prism followed by a unilateral fusional convergence movement of the eye not behind the prism. A similar response occurs regardless of which eye the prism is placed over. Often in monofixation syndrome, no movement is seen when the prism is placed before the nonfixating eye. A refixation version movement is seen when the prism is placed before the fixating eye, but the expected subsequent fusional convergence does not occur.
Figure 7-13 The 4Δ base-out prism test. A, When a prism is placed over the left eye, dextroversion occurs during refixation of that eye, indicating absence of foveal suppression in the left eye. If a suppression scotoma is present in the left eye, neither eye will move when the prism is placed before the left eye. B, Slow fusional adduction movement of the right eye is then observed, indicating absence of foveal suppression in the right eye. C, In a second patient, the right eye stays abducted, and the absence of adduction movement, as shown in part B, indicates foveal suppression in the right eye. D, Weak fusion is another cause of absence of adduction movement; such patients experience diplopia until refusion occurs spontaneously.
(Modified with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:220.)
The 4Δ base-out prism test is the least reliable method of documenting the presence of a macular scotoma. Occasionally, a patient with bifixation recognizes diplopia when the prism is placed before
an eye but makes no convergence movement to correct for it. Patients with monofixation syndrome may switch fixation each time the prism is placed and show no movement, regardless of which eye is tested.
Afterimage Test
This test involves the stimulation, or labeling, of the macula of each eye with a different linear afterimage, 1 horizontal and 1 vertical. Because suppression scotomata extend along the horizontal retinal meridian and may obscure most of a horizontal afterimage, the vertical afterimage is placed on the macula of the deviating eye and the horizontal afterimage on the macula of the fixating eye by having each eye fixate on a linear light filament separately. The test can also be performed by covering a camera flash with black paper and exposing only a narrow slit; the center of the slit is covered with black tape to serve as a fixation point, as well as to protect the fovea from exposure. In the presence of eccentric fixation (see Chapter 4), there is no assurance that the afterimage will be aligned with the fovea; thus, the test cannot be interpreted. The patient is then asked to draw the relative positions of the perceived afterimages. Possible perceptions are shown in Figure 7-14.
Figure 7-14 Afterimage test. A, Normal retinal correspondence. B, Crossed ARC in a case of esotropia. C, Uncrossed ARC
in a case of exotropia. (Modified with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:227.)
Amblyoscope Testing
Although its use has declined, the major amblyoscope (discussed earlier; see Fig 7-8A) was a mainstay in the assessment and treatment of strabismus for decades. The amblyoscope can measure horizontal, vertical, and torsional deviations. Because it can neutralize torsion, it is still useful for distinguishing between central fusion disruption (see Chapter 6) and inability to fuse because of a large cyclodeviation. It can also assess suppression, retinal correspondence, fusional amplitudes, and stereopsis and may be used for exercises designed to overcome suppression and increase fusional amplitudes.
Worth 4-Dot Test
The Worth 4-dot test (Fig 7-15) is often considered a test of sensory fusion; however, it does not test sensory fusion directly as there is no fusible feature in the test. Its best use is to test for a suppression scotoma. The test uses red-green glasses—traditionally, the red lens is placed in front of the right eye and the green lens in front of the left—and a target consisting of 4 dots: 1 red, 2 green, and 1 white. The red lens blocks the green light, and the green lens blocks the red light, so the red and green dots are each seen by only 1 eye. The white dot is the only feature seen by both eyes, but it is seen in color rivalry in a patient with fusion ability. The polarized Worth 4-dot test uses polarized glasses rather than red and green ones. The stimulus dots can be presented in a wall-mounted display or by a handheld flashlight. The test should be administered in good ambient light so that peripheral features in the room can stimulate motor fusion. The patient then reports the number of dots seen:
Seeing 2 dots indicates a suppression scotoma in the left eye. Seeing 3 dots indicates a suppression scotoma in the right eye.
Seeing 4 dots indicates that there is some degree of sensory fusion and that the patient has NRC (if there is no manifest strabismus) or harmonious ARC (if there is manifest strabismus). If there is a scotoma, the perception of 4 dots indicates that the scotoma must be smaller than the test target.
Seeing 5 dots is a diplopia response. The patient has manifest strabismus without suppression or ARC.
