The elements that constitute a complete pediatric ophthalmology examination parallel those that make up an adult examination but often require different techniques and devices. The following sections focus on these differences.
Visual Acuity Assessment
Visual acuity assessment requires different approaches depending on the age, developmental level, and cooperativeness of the child. In children, detection of amblyopia is of particular concern (see Chapter 6). Amblyopia is a developmental disorder of the central nervous system due to the abnormal processing of visual images, which leads to reduced visual acuity. Amblyopia is responsible for more cases of childhood-onset unilateral decreased vision than all other causes combined but is preventable or reversible with timely detection and intervention. Early detection of reduced vision from amblyopia is possible with the techniques described herein.
Ideally, accurate measurement of monocular distance visual acuity using a linear display of Sloan letters would be possible in all pediatric patients. Commonly, however, the child is preverbal, preliterate, or not fully cooperative. In these cases, clinical options include assessment of fixation behavior or testing with alternative eye charts designed for preliterate children.
In infants and toddlers, fixation behavior is observed to qualitatively assess visual acuity. Preferential looking and visual evoked potential testing may allow quantitative assessment of visual acuity in this young population (see the section “Alternative methods of visual acuity assessment in preverbal children”). Fixation and following (tracking) behavior is observed as the child’s attention is directed to the examiner’s face or to a small toy in the examiner’s hand. Fixation preference is determined by observing how the patient responds to having one eye covered compared with the other eye covered. Children typically resist occlusion of the eye with better vision. Determining whether each eye can maintain fixation through smooth pursuit or a blink provides additional information; strong fixation preference for one eye indicates decreased vision in the nonpreferred eye.
Fixation behavior may be characterized by the CSM (Central, Steady, and Maintained) method. Central refers to foveal fixation, tested monocularly. If the fixation target is viewed eccentrically, fixation is termed uncentral (UC). Steady refers to the absence of nystagmus and other motor disruptions of fixation (see Chapter 13). The S assessment is also performed monocularly. Maintained refers to fixation that is held after the opposite eye is uncovered. An eye that does not maintain fixation may be presumed to have lower visual acuity than the opposite eye. Maintained fixation is easier to identify in a patient with strabismus than in one without this defect. For children without strabismus or with a small angle of strabismic deviation, the induced tropia test may be useful (see Chapter 2 for strabismus terminology). First, the examiner directs the child’s attention to a target. Then a 10–20 prism diopter base-down prism is placed in front of 1 eye, and the eyes are observed. The prism is then placed in front of the opposite eye. If the eyes move up, the child is fixating with the eye under the prism. If the child alternates fixation during the test, a strong preference is not present. If the child consistently fixates with the same eye, the opposite eye likely has decreased vision. The visual acuity of an eye that has eccentric fixation and nystagmoid movements when attempting fixation would be designated uncentral, unsteady, and unmaintained (UC, US, UM).
Monocular recognition testing—which involves identifying letters, numbers, or symbols (all termed optotypes) with each eye separately—is the preferred method of assessing visual acuity. Optotypes may be presented on a wall chart, computer monitor, or handheld card. The acuity test should be calibrated for the test distance used. Because of the potential for variable or inaccurate viewing distances when near vision is tested, measurement at distance is preferred.
In eye charts used for testing preliterate children, the optotypes may be symbols or letters for matching. Table 1-1 lists the expected recognition visual acuity, as measured by an ophthalmologist, for children at different ages; these visual acuity levels may differ from those used in primary care vision screening criteria. Copies of appropriate optotypes may be given to the parent before the test for at-home rehearsal to improve testability, as well as the speed and reliability of responses.
Various optotypes are available for recognition visual acuity testing in preliterate children. LEA symbols (Fig 1-2) and the HOTV test (Fig 1-3) are reliably calibrated and have high testability rates for preschool-aged children. For a shy child, testability may be improved by having the child point to match optotypes on a chart with those on a handheld card rather than verbally identify them. Several symbol charts, such as Allen figures and the Lighthouse chart, are not recommended by the World Health Organization and the National Academy of Sciences because the optotypes are considered confusing, culturally biased, or nonstandardized. The Tumbling E chart is conceptually difficult for many preschool-aged children.
Table 1-1 Monocular HOTV Visual Acuity Test Results in Preschool Children
Figure 1-2 LogMAR (logarithm of the minimum angle of resolution) visual acuity chart with LEA symbols.
(Courtesy of the Good-Lite Company and Robert W. Hered, MD.)
Figure 1-3 Crowded HOTV optotypes.
(Courtesy of the Good-Lite Company and Robert W. Hered, MD.)
Because visual acuity may be overestimated when measured with isolated optotypes, particularly in amblyopia (see Chapter 6), a line of optotypes (linear acuity) or single optotypes surrounded by contour interaction bars (“crowding bars”; see Fig 1-3) should be used whenever possible. The optotypes should be spaced such that the distance between each optotype is no greater than the width of the optotypes on any given line. The design of the Bailey-Lovie and ETDRS (Early Treatment of Diabetic Retinopathy Study) charts incorporates appropriate linear-optotype spacing, a consistent number of optotypes on each line, and a logarithmic (logMAR) change in letter size from one line to the next (see Fig 1-2). See also BCSC Section 3, Clinical Optics, for further discussion of visual acuity charts.
By convention, visual acuity is determined first for the right eye and then for the left. A patch or other occluder is used in front of the left eye as the acuity of the right eye is checked and vice versa. An adhesive patch is the most reliable occluder because it reduces the possibility that the child will “peek” around the occluder. Computerized visual acuity test systems allow for randomization of optotype presentation, preventing patients from memorizing optotypes and thus increasing test accuracy. Patients with nystagmus may show better binocular than monocular visual acuity. To assess monocular distance visual acuity in this situation, the fellow eye, rather than being occluded, should be fogged by using a translucent occluder or a lens +5.00 diopters (D) greater than the refractive error in that eye. Patients with poor vision may need to move closer to the chart until they can see the 20/400 line. In such cases, visual acuity is recorded as the distance in feet (numerator) over the size of the letter (denominator); for example, if the patient is able to read 20/400 optotypes at 5 ft, the acuity is recorded as 5/400. See the visual acuity conversion chart on the inside front cover of this book, which provides conversions of visual acuity measurements for the various methods in use.
The line with the smallest optotypes in which most of the optotypes are identified by the patient is recorded; if the patient misses a few optotypes on a line, a notation is made. For adults and children, visual acuity test results may vary depending on the chart used. The clinician should document the type of test performed, specifying the optotype and whether crowding was used, to facilitate comparison of measurements obtained at different times.
Age-appropriate optotypes are also used to determine uncorrected and corrected near visual acuity. Measuring near visual acuity in children with reduced vision is helpful for determining how they may function at school.
Alternative methods of visual acuity assessment in preverbal children
Two major methods are used to quantitate visual acuity in preverbal infants and toddlers: preferential looking (PL) and visual evoked potential (VEP).
Preferential looking tests
In these tests, the child’s response to a visual stimulus is observed to assess visual acuity. Teller Acuity Cards II (Stereo Optical, Inc, Chicago, IL), the LEA Grating Acuity Test (Good-Lite Company, Elgin, IL), and Patti Stripes Square Wave Grating Paddles (Precision Vision, Woodstock, IL) measure grating acuity, a form of resolution acuity, demonstrated by the subject’s ability to detect patterns composed of uniformly spaced black and white stripes on a gray background. Grating acuity can be measured as early as infancy. Movement of the eyes toward the stripes indicates that the child can see them. Seeing narrower stripes denotes better vision (Fig 1-4, Video 1-1). The Cardiff Acuity Test, popular in Europe, uses vanishing optotypes for preferential looking.
Figure 1-4 Teller Acuity Cards can be used to measure visual acuity in a preverbal child. If the pattern is visible to the child, the eyes gaze toward the grating; otherwise, the stripes blend into the gray background.
(Left image courtesy of John W. Simon, MD; right image courtesy of Lee R. Hunter, MD.)
Teller Acuity Card.
Courtesy of Lee R. Hunter, MD.
Access all Section 6 videos at www.aao.org/bcscvideo_section06.
Visual evoked potential
Sweep visual evoked potential (VEP) can also be used to quantitatively assess visual acuity in preverbal patients. In this test, electrodes are placed over the occipital lobe to measure electrical signals produced in response to a visual stimulus. The child views a series of bar or grid patterns. If the stimulus is large enough for the child to discriminate, a visual impulse is recorded. Visual acuity is estimated based on the smallest stimulus width that produces a response.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.