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Chapter 029. Disorders of the Eye (Part 4)

Stereopsis Stereoacuity is determined by presenting targets with retinal disparity separately to each eye using polarized images. The most popular office tests measure a range of thresholds from 800–40 seconds of arc. Normal stereoacuity is 40 seconds of arc. If a patient achieves this level of stereoacuity, one is assured that the eyes are aligned orthotropically and that vision is intact in each eye. Random dot stereograms have no monocular depth cues and provide an excellent screening test for strabismus and amblyopia in children. Color Vision The retina contains three classes of cones, with visual pigments of differing peak spectral sensitivity:. | Chapter 029. Disorders of the Eye Part 4 Stereopsis Stereoacuity is determined by presenting targets with retinal disparity separately to each eye using polarized images. The most popular office tests measure a range of thresholds from 800-40 seconds of arc. Normal stereoacuity is 40 seconds of arc. If a patient achieves this level of stereoacuity one is assured that the eyes are aligned orthotropically and that vision is intact in each eye. Random dot stereograms have no monocular depth cues and provide an excellent screening test for strabismus and amblyopia in children. Color Vision The retina contains three classes of cones with visual pigments of differing peak spectral sensitivity red 560 nm green 530 nm and blue 430 nm . The red and green cone pigments are encoded on the X chromosome the blue cone pigment on chromosome 7. Mutations of the blue cone pigment are exceedingly rare. Mutations of the red and green pigments cause congenital X-linked color blindness in 8 of males. Affected individuals are not truly color blind rather they differ from normal subjects in how they perceive color and how they combine primary monochromatic lights to match a given color. Anomalous trichromats have three cone types but a mutation in one cone pigment usually red or green causes a shift in peak spectral sensitivity altering the proportion of primary colors required to achieve a color match. Dichromats have only two cone types and will therefore accept a color match based upon only two primary colors. Anomalous trichromats and dichromats have 6 6 20 20 visual acuity but their hue discrimination is impaired. Ishihara color plates can be used to detect red-green color blindness. The test plates contain a hidden number visible only to subjects with color confusion from red-green color blindness. Because color blindness is almost exclusively X-linked it is worth screening only male children. The Ishihara plates are often used to detect acquired defects in color vision although .