When determining the angular size of a strabismus, any participation of binocular fusion must be blocked. This allows measurement of the strabismus without interference from fusional vergence movements.
The examination is done with the alternating cover test, while the patient fixes attention on an object at least 3 m away. The standard gaze positions are achieved by turning the head in various directions. (For example, a head turn to the right requires the patient to fix on a position in the left field of gaze). By noting the saccadic movements with each alternation of the cover, the examiner can detect even small disparities in ocular alignment, measuring the size, as well as the type of the incomitance. Following such observations the angle of deviation can be quantified for a given gaze direction by placing corrective prisms in front of the paretic eye (the alternating prism cover test).
Subjectively, the angle of deviation can usually be determined more quickly and reliably by having the patient report the locations and separations between the doubled images. Prerequisites for this method are normal retinal correspondence, adequate visual acuity, and patient cooperation. For patients with spontaneous diplopia the motil-ity deficit can be estimated without additional equipment during fixation on an object that has sufficient contrast with its background. For example, consider uncrossed di-plopia at the primary position (the image from the right eye is seen to the right, while the image from the left eye is seen to the left) that increases in right gaze and decreases in left gaze. This means that gaze to the right produces an increasing esodeviation. To judge cyclodeviations the examiner must provide a straight line (a yardstick, for example) that the patient can use to describe the angle of tilt. The perceived image rotation is the opposite of the eye's rotation. (This is the same reversal of image direction found in horizontal and vertical strabismus). Thus, an excyclotropic right eye will see the image rotated counterclockwise, or the image seen by the left eye will appear to be rotated clockwise. Usually, the deviating eye sees the image as tilted, unless it happens to be strongly dominant.
If the doubled images are not sufficiently clear, red/ green goggles can be used with a point source of light as a fixation target. The relative location and orientation of the red and green images can usually be described by patients with average cognitive skills. The contrasting colors of the two images inhibit fusion, allowing full expression of the faulty ocular alignment. This will be true for even small deviations, which would otherwise be quickly compensated by fusional vergence movements, allowing the deviation to remain latent, rather than manifest. Additionally, the double-Maddox rod test can be used to quantify rotational disparities. Within a trial frame, a red Maddox rod is placed on the right side and a green one on the left. The orientation of the Maddox rods is set vertical, to produce horizontal lines. Parallel lines indicate no rotational deviation, while tilted lines that cross one another mark the presence of a cyclodeviation. Rotation of one or both Maddox rods to bring the lines into parallel alignment allows quantification of the cyclodeviation.
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