Exact measurement of the angle of deviation is crucial to prescribing the proper prism correction to compensate for the angle of deviation and to the corrective surgery that usually follows. A measurement error may lead to undercorrection or overcorrection of the angle of deviation during the operation. Example: Esotropia of + 15 degrees is corrected by shifting the medial rectus 4.0 mm posteriorly and shortening the lateral rectus 5.0 mm.
The angle of deviation is measured with a cover test in combination with the use of prism lens of various refractive powers. The patient fixates on a certain point with the leading eye at a distance of 5 m or 30 cm, depending on which angle of deviation is to be measured. The examiner place prism lenses of different refractive power before the patient's deviant eye until the eye no longer makes any adjustment. This is the case when the angle of deviation corresponds to the strength of the respective prism and is fully compensated for by that prism. The tip of the prism must always point in the direction of deviation during the examination.
Prism bars simplify the examination. These bars contain a series of prisms of progressively increasing strength arranged one above the other.
Maddox's cross (Fig. 17.9) is a device often used to measure the angle of deviation. A light source mounted in the center of the cross serves as a fixation point. The patient fixates the light source with his or her leading eye. The objective angle of deviation is measured with prisms as described above. In children, often only the objective angle of deviation is measured as this measurement does not require any action on the part of the patient except for fixating a certain point, in this case the light source at center of the cross. In adults, the examiner can ask the patient to describe the location of the area of double vision (double vision may be a sequela of paralytic strabismus, which is the most common form encountered in adults). This uses the graduations on the Maddox's cross. The cross has two scales, a large numbered scale for testing at five meters and a fine scale for testing at one meter (see Fig. 17.9). The patient describes the location of the area of double vision according to a certain number on this scale. The examiner selects the appropriate prism correction according to the patient's description to correct the angle of deviation of the paralyzed eye. This superimposes the images seen by the deviating eye and the nondeviating eye to eliminate the double vision.
Fig. 17.9 A Maddox cross is frequently used only as a fixation object when examining children. The patient fixates on the light source in the center. The two scales (a large numbered scale for testing at five meters and a fine scale for testing at one meter) are only relevant for verbal patients asked to describe the location of the area of double vision, for example in paralytic strabismus. (See text for examination procedure.)
The angle of deviation can be measured in prism diopters or degrees. One prism diopter refracts light rays approximately half a degree so that two prism diopters correspond to one degree.
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