The acuity card procedure [13, 14] was developed to shorten the time needed to obtain an estimate of acuity for an individual infant and thus enable preferential looking procedures to be incorporated into clinical settings. It consists of a series of gray cards (Fig. 10.2) 25.5 × 51.0 cm, with a black and white grating (measuring 12.5 × 12.5 cm) located on one side of a central peephole.

The observer or tester, who is masked to the location of the grating (striped pattern), presents the card to the infant enough times to determine with confidence whether the child can distinguish stripes on one card before presenting the card with the next narrower pattern on it. Increasingly, narrow gratings are presented until the tester determines that the child can no longer resolve the gratings. The infant’s threshold acuity is determined by the smallest stripes (highest spatial frequency) that it can resolve at the testing distance. Efficiency of the acuity card procedure is enhanced by the subjective judgments that the tester makes concerning the infant’s looking behavior, thus shortening the time necessary to reach threshold acuity. As shown in Fig. 10.3, the acuity card procedure maximizes the child’s interest in the task by allowing the tester to interact with the infant between presentations. In addition, the threshold acuity obtained using the acuity card procedure is in good agreement with those obtained using traditional methods of preferential looking procedures. However, there are also limitations of the acuity card procedure, including potential tester bias and artifacts in the cards themselves [13].

Early clinical studies of preferential looking techniques showed high success rates for monocular testing, usually better than 90% in infants older than 6 months [15].

Incorporation of the acuity card procedure into clinical practice requires knowledge of the normal range and the development of grating acuity in infants and young children (Fig. 10.4). Mayer et al. [16] provided normative values for monocular testing using rigorous standardized techniques of subjects from 1 month to 4 years. The acuity card procedure has been used in various clinical conditions, in addition to ROP, such as cataracts and aphakia, Leber’s congenital amaurosis, macular colobomas, nystagmus, and various central nervous system abnormalities, such as periventricular leukomalacia and delayed visual maturation. In strabismic amblyopia , it appears that amblyopic eyes show greater acuity deficits when tested using recognition acuity tests compared to acuity results of grating acuity tests. In addition, a fixation preference for one eye may not be reflected by decreased grating acuity in the fellow amblyopic eye [16].

Interpretation of acuity values obtained in infants and young children using resolution acuity methods must be carefully reported and interpreted. A key consideration is that resolution acuity values are not necessarily equivalent to recognition acuity [17] and there are, as yet, no methods for assessing recognition acuity in infants and very young children. However, several studies in older children and adults suggest that measures of recognition acuity and measures of resolution acuity give similar values. The largest to date is from the CRYO-ROP study [18] that compares results of letter acuity testing, using a crowded HOTV chart and results of grating acuity testing, using the acuity card procedure. Visual acuity scores were obtained from 1694 eyes at age 3½ years to 2101 eyes at 4½ years. There was a significant correlation between letter acuity and grating acuity in young children with birth weights of less than 1251 g, whether or not the eyes had ROP residua. Eyes that had acuity below 20/150 generally showed better grating acuity than letter acuity, and it was possible to provide the 90% confidence intervals for letter acuity scores for specific grating acuity results (see Table 10.2).

The clinician must remember that resolution acuity results will often underestimate the degree of visual loss present in a child that may be documented when a more standard recognition acuity measure can be obtained. As Kushner [17] suggests, care must be taken in reporting grating acuity values in terms of Snellen acuity values, especially to agencies making determination of need for visual services. It is also important to remember that an acuity in the normal range at an early age does not necessarily predict normal acuity at a later stage later.

Incorporation of the acuity card procedure into a busy clinical practice requires adaptations and individuals responsible for testing must be trained in the technique. A well-lit quiet space should be provided for testing, and one should also be wary of bias, since in a clinical setting the same tester may test a child several times and remember which is the “bad” eye and which is the “good” one [19]. The tester must also be familiar with nonstandard techniques needed to test the child with low vision, nystagmus, and visual field abnormalities [20].

The Visual Evoked Potential (VEP) (recorded at the occipital region of the scalp) can be elicited with varying stimuli and has a correlation with visual acuity. Checkerboards and gratings of varying spatial frequency are the most common stimuli used in recording pattern visual evoked potentials (PVEP) . In normal children, an estimate of visual acuity may be based on the amplitude of the PVEP although it is not realistic to expect a direct correlation [21, 22]. A recordable PVEP to a 50′ or smaller check size indicates good vision, while a flash VEP response with no recordable PVEP suggests rudimentary vision. PVEPs may be useful for serial monitoring and interocular comparisons in infancy [23]. Sweep VEP techniques allow rapid acquisition of responses to a stimulus that varies in spatial frequency and may provide a more easily acquired estimation of acuity [24, 25].

In addition to estimating acuity, serial VEP monitoring may be helpful in detecting early optic nerve dysfunction in children with craniofacial anomalies at risk of raised intracranial pressure [23, 26].

The technique for measuring visual evoked potential requires intense interaction between the tester and the subject with the tester ensuring that the child is attending the test patterns. This interaction between subject and tester is especially important since the computer equipment, electrodes placed on the head, and eye patches can be very intimidating for the young patient.

Comparisons of the results of VEP estimated acuity to measured acuity with letter recognition tests show the same underestimation of an acuity deficit using VEP techniques. This is similar to results comparing recognition acuity tests to grating acuity tests and is likely due to the simpler task of resolving grating patterns compared to the higher order form discrimination task of letter recognition.

ISCEV (International Society for Clinical Electrophysiology of Vision ) sets the technical standards for visual electrophysiological testing. Each laboratory however must develop its own normative database and maintain high-quality control over data collection and interpretation. As this technology becomes more user-friendly and widely available, it is reasonable to expect increasing widespread clinical use of these techniques [28].

Recognition acuity tests rely on the form recognition of symbols, pictures, or letters of decreasing spatial frequency to determine the threshold acuity. Many optotypes in various arrangements have been proposed and several are in widespread use.

The Landolt C has been designated as the standard optotype for recognition acuity testing by the World Health Organization and the National Research Council of the National Academy of Science. However, no test has been designated standard for use in verbal, preschool children. The Allen and Snellen picture charts, which have simple recognizable pictures, tumbling E, crowded HOTV, and Snellen letters, have all been used for very young children, along with numerous variations of numbers of symbols per line and spatial relation of the symbols [29]. Lea symbols were introduced in 1980 and provide similar results to the Landolt C acuity testing and are now in widespread use [30]. Kay pictures provide comparable acuity assessment to logMAR acuity testing but may underestimate amblyopia in children with lower acuity [31].