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Normal Visual Development
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The visual system is immature at birth. Achievement of normal vision is dependent on key anatomic changes during development that involve both the retina and the visual pathways. At the level of the retina, connections between the photoreceptors and the inner retinal cells are not fully formed until a few months after birth.1 At the level of the brain, myelination of the optic radiations takes place in the first year of life. Normal visual development also depends on well-focused input from the anterior segment of the eye. Hubel and Wiesel found that, if this normal input is not present, then visual deprivation leads to maldevelopment in the cerebral cortex.2
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Vision Testing in Children
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Estimates of visual acuity at birth range from 20/2000 to 20/400. A newborn starts fixating and regarding the mother's face by 2 weeks of age. Between 8 and 10 weeks of age, a normal infant can generally fix and follow a large object over an arc of 180°. A visual problem exists if an infant cannot fix and follow a lighted toy by 3 months of age.
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Normal ranges of visual acuity in children vary based on age. Methods to measure visual acuity are also age-dependent. In general, recognition acuity measures are favored over other methods (see "Recognition acuity tests: Allen pictures, HOTV and Snellen letters," p. 380). To assess visual acuity in a preverbal child, motor behavioral responses can be used, such as optokinetic nystagmus or preferential looking tests. When needed, a visual evoked potential test can quantify the visual sensory response. Table 29-1 lists normal ranges of visual acuity by age using the preferential looking test or visual evoked potentials. When children are old enough to verbalize their responses, visual acuity is done by recognition acuity testing. If a child is too shy to talk, he or she can be asked to match pictures or letters with a hand-held card.
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Examination Procedures and Guidelines for Referral
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Optokinetic Nystagmus
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This test consists of a cylindrical drum with alternating black and white rectangular stripes. The drum is rotated in the infant's visual field while the examiner observes for smooth pursuit eye movements in the direction of the rotating drum followed by saccadic eye movements in the opposite direction. Horizontal saccades can be elicited at birth in a full-term child, but vertical saccades do not develop until 4 to 6 weeks of age.
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Visual Evoked Potentials
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