Evaluating Vision in Preverbal and Preliterate Infants and Children

Introduction

Vision screening should begin in infancy and progress to optotype (figures or letters) acuity testing by age 4 years. The American Academy of Pediatrics, the American Academy of Ophthalmology, and the American Association for Pediatric Ophthalmology and Strabismus have put forth a consensus statement regarding the evaluation of vision in children. These groups recommend attempting optotype acuity testing in cooperative 3-year-olds and then repeating yearly. Specialized techniques for visual acuity quantification are necessary when evaluating children younger than 3–4 years of age. These techniques include observation, fixation targets, optokinetic nystagmus, visual evoked potentials, forced-choice preferential looking, specially constructed graded optotypes, and digital photoscreeners.

Historical and Observational Techniques

Much can be learned from the description of a child’s visual behavior from family members. Parents or caregivers should be asked routinely whether the child responds to visual stimuli such as a silent smile or silent toys and follows objects around the environment. Pertinent observations include strabismus, nystagmus, persistent staring, and inattention to objects. A younger sibling’s visual behavior may be compared with that of an older child.

The pupillary light response, although not equivalent to visual ability, indicates intact afferent visual neurological pathways to the level of the brachium of the superior colliculus and efferent pathways to the iris sphincter. This reflex is present in premature babies with a gestational age of between 30 and 32 weeks. Visualization in very young children sometimes requires magnification because their pupils are smaller than those of older children (decreased sympathetic tone), and the light responses are of small amplitude. Dilatation to direct illumination (paradoxic pupillary response) has been described in Leber’s congenital amaurosis, optic nerve hypoplasia, congenital achromatopsia, and congenital stationary night blindness, along with other cone dysfunctions. Nystagmus, which, in children, can be thought of as a sign of binocular visual impairment, is typically absent in cortical blindness, and its presence should guide the observer to an evaluation for a bilateral visual deficit. Despite this, some children with nystagmus may still have quite good visual function.

The blink to a bright light is a response noted by 30 weeks of gestational age and occasionally is present in decorticate infants. Children typically develop a blink reflex to threatening gestures by 5 months of age; however, care must be taken not to push air against the child’s corneas and elicit a blink resulting from corneal sensation.

In addition to pupillary reflexes, performance of the Brückner (or red reflex) test can serve as a screening and diagnostic test. Performed by looking at both eyes simultaneously through a direct ophthalmoscope, this test can provide diverse information about a child, including eye alignment, refractive error, anisocoria, and media opacities. A normal result will show equal red reflexes in both eyes, which can provide reassurance to the examiner.

Fixation Targets

Visual fixational abilities may be demonstrated in full-term newborns by using a high-contrast target. A flashlight is a poor target because it has no defined edge; stripes, dots, or checkerboards are preferred. Full-term infants younger than 3 months of age “follow” a small target by using hypometric saccades ; these infants may generate smooth pursuit movements to a large target, such as an optokinetic drum. Horizontal saccades are first developed, and vertical saccades become identifiable by about 2 months of age. Because saccadic palsies are common in young children who have central nervous system damage, spinning an upright child demonstrates the presence of saccades as the rapid recovery phase of the spin-induced nystagmus. If no rapid phase can be stimulated, the child’s vision cannot be evaluated by his or her ability to “follow” a small target because neither a saccadic nor a smooth pursuit system is yet available. In addition, a child who has normal fixational behavior should dampen spin-induced nystagmus in 3–5 seconds; a blind or poorly sighted child cannot use fixational dampening and beats for 15–30 seconds until mechanical dampening occurs.

In slightly older children, small, colorful, familiar toys generate the best, albeit often momentary, interest. Two methods of evaluating visual acuity using toys are “Fix & Follow” (F&F) and “Central, Steady, & Maintained” (CSM) testing. For F&F testing, a small toy or fixation target is moved slowly and silently in front of the child to observe the child’s fixation and motility. Asymmetry in monocular testing can give an indication of amblyopia. CSM notation provides information regarding potential eccentric fixation (central), ocular motility or nystagmus (steady), and the ability to keep fixation (maintained).