Crowding and contrast in amblyopia

Crowding under scotopic and photopic vision in albino and normal-sighted participants

Crowding is a phenomenon in which the ability to recognize an object in a clutter deteriorates. It is, therefore, a fundamental aspect of object recognition and crucial in deciphering resolution. For visually impaired individuals, deficiency in crowding has a tremendous effect on vision and may reflect and predict the amount of deterioration in vision. It is well established that albinos suffer much more from crowding than normally sighted individuals under daylight luminance conditions. However, to our knowledge, this study is the first to investigate crowding in albino participants under low light conditions. In this study, we explored the crowding effect in a group of albino participants (n = 9) and a control group of normally sighted participants (n = 9). Crowding was conducted under daylight (photopic vision) and low light (scotopic vision). We measured the visual acuity threshold under crowding in three-letter spacing (0.5, 1, and 1.5) and compared it to a single target. Results indicate that albino participants experienced stronger crowding than the control under the photopic condition, while crowding under the scotopic condition was apparent in the albino but abolished for the control group. These findings highlight the importance of considering luminance when discussing the visually impaired population in general. In particular, it suggests that crowding in albinism is based on a peripheral-like mechanism and may indicate a cessation in visual development.

High- and Low-contrast Letter Acuity during Image Motion in Normal Observers and Observers with Infantile Nystagmus Syndrome

SIGNIFICANCE

High-contrast acuity in individuals with infantile nystagmus syndrome (INS) is poorer than expected from their ongoing retinal image motion, indicating a sensory loss. Conversely, acuity for larger low-contrast letters in these observers may be limited by image motion alone.

PURPOSE

The aim of this study was to assess visual acuity for letters of different contrast in normal observers and individuals with idiopathic INS under conditions of comparable retinal image motion.

METHODS

Visual acuity was measured using projected Landolt C charts in 3 normal observers and 11 observers with presumed idiopathic INS. Normal observers viewed each chart after reflection from a front-surface mirror that underwent continuous 4-Hz ramp motion with amplitudes ranging from 4 to 9.6° and simulated foveation durations of 20 to 80 milliseconds. Observers with INS viewed the charts directly. By reciprocally varying the luminance of the projected charts and a superimposed veiling source, Landolt C's were presented on a background luminance of 43 cd/m2 with Weber contrasts between -12 and -89%.

RESULTS

Whereas normal observers' high-contrast acuity during imposed image motion depends only on the duration of the simulated foveation periods, acuity for low-contrast optotypes also worsens systematically as motion intensity (frequency × amplitude) increases. For comparable parameters of retinal image motion, high-contrast acuity in all but one of the observers with INS was poorer than in normal observers. On the other hand, low-contrast acuity in the two groups of observers was similar when the retinal image motion was comparable.

CONCLUSIONS

Reduced high-contrast acuity in observers with INS appears to be attributable primarily to a sensory deficit. On the other hand, the reduction of low-contrast acuity in observers with INS may be accounted for on the basis of retinal image motion.

Reduced fixation stability induced by peripheral viewing does not contribute to crowding

Attending to peripheral visual targets while maintaining central fixation, a process that involves covert attention, reduces fixation stability. Here, we tested the hypothesis that changes in fixation stability induced by peripheral viewing contribute to crowding in peripheral vision by increasing positional uncertainty. We first assessed whether fixation was less stable during peripheral versus central (foveal) viewing for both crowded and uncrowded stimuli. We then tested whether fixation stability during peripheral viewing was associated with the extent of crowding. Fourteen participants performed a tumbling E orientation discrimination task at three different eccentricities (0°, 5°, 10°). The target was presented with or without flankers. Fixational eye movements were measured using an infrared video-based eyetracker. A central fixation cross was provided for the two peripheral viewing conditions, and optotype size was scaled for each eccentricity. Discrimination of appropriately scaled uncrowded stimuli was unaffected by eccentricity, whereas discrimination of crowded stimuli deteriorated dramatically with eccentricity, despite scaling. Both crowded and uncrowded peripheral stimuli were associated with reduced fixation stability, increased microsaccadic amplitude, and a greater proportion of horizontal microsaccades relative to centrally presented stimuli. However, these effects were not associated with the magnitude of crowding. This suggests that reduced fixation stability due to peripheral viewing does not contribute to crowding in peripheral vision.

Retinal Sensitivity and Fixation Analysis Using Microperimetry in Children With Anisometropic Amblyopia

PURPOSE

To determine retinal sensitivity and fixation in children with anisometropic amblyopia using macular analyzer integrity assessment (MAIA) microperimetry (CenterVue).

METHODS

Amblyopic and non-amblyopic eyes of 39 children with the diagnosis of anisometropic amblyopia were compared using MAIA microperimetry in terms of average threshold, fixation indexes (P1 and P2), and bivariate contour ellipse area (BCEA) for 95% and 63% of points. The correlations between visual acuity and microperimetry parameters were also analyzed.

RESULTS

The mean average threshold was 26.6 ± 5.9 dB for amblyopic eyes and 28.6 ± 1.9 dB for fellow eyes (P = .003). The mean P1 index was 80.2% ± 20.3% for amblyopic eyes and 87.6% ± 8.7% for fellow eyes (P = .032). The mean P2 index was 92.5% ± 9.6% and 94.3% ± 4.8% for amblyopic eyes and fellow eyes, respectively. The mean BCEA95 value was 12.9^°2 ± 17.2^°2 and 9.4^°2 ± 8.2^°2 and the mean BCEA63 value was 4.4^°2 ± 5.6^°2 and 3.1^°2 ± 2.7^°2 for amblyopic eyes and fellow eyes, respectively. No statistically significant difference was found for those parameters (P > .05).

CONCLUSIONS

Retinal sensitivity and P1 fixation index were significantly lower in amblyopic eyes compared to non-amblyopic eyes. Because measuring visual acuity does not represent all functions of the visual system, microperimetric evaluation may give extra information about total visual function of amblyopic eyes. [J Pediatr Ophthalmol Strabismus. 2020;57(4):246-250.].

Foveal Crowding Resolved

Crowding is the substantial interference of neighboring items on target identification. Crowding with letter stimuli has been studied primarily in the visual periphery, with conflicting results for foveal stimuli. While a cortical locus for peripheral crowding is well established (with a large spatial extent up to half of the target eccentricity), disentangling the contributing factors in the fovea is more challenging due to optical limitations. Here, we used adaptive optics (AO) to overcome ocular aberrations and employed high-resolution stimuli to precisely characterize foveal lateral interactions with high-contrast letters flanked by letters. Crowding was present, with a maximal edge-to-edge interference zone of 0.75-1.3 minutes at typical unflanked performance levels. In agreement with earlier foveal contour interaction studies, performance was non-monotonic, revealing a recovery effect with proximal flankers. Modeling revealed that the deleterious effects of flankers can be described by a single function across stimulus sizes when the degradation is expressed as a reduction in sensitivity (expressed in Z-score units). The recovery, however, did not follow this pattern, likely reflecting a separate mechanism. Additional analysis reconciles multiple results from the literature, including the observed scale invariance of center-to-center spacing, as well as the size independence of edge-to-edge spacing.

Factors Contributing to Discrepancy Between Visual Acuity Fractions Derived From a Snellen Chart and Letter Scores on the Early Treatment Diabetic Retinopathy Study Chart

PURPOSE

To report factors influencing the relationship between visual acuity (VA) fractions measured on Snellen chart and letter scores on the Early Treatment Diabetic Retinopathy Study (ETDRS) chart.

DESIGN

This was a retrospective review from a single ophthalmology outpatient clinic.

METHODS

All patients had routine consecutive VA testing in the right eye using a Snellen chart (1-6 m) and the ETDRS chart (4 m and/or 1 m), by the same optometrist, using a standardized testing protocol for each chart. Both acuity fractions and letter scores were converted to their equivalent logarithm of minimum angle of resolution (logMAR) for comparison. Multiple regression analysis was performed.

RESULTS

A total of 237 patients with a wide range of ocular disease and VAs were enrolled. Mean age was 63 years (range, 18-95 years). Recorded VA (logMAR) was better on Snellen chart by a mean (95% limits of agreement) of -0.07 (-0.33 to +0.18, P < 0.001). There was a trend for subjects with VA worse than 6/36 to read 3 to 6 letters more and those better than 6/36 to read 4 to 5 letters less on the ETDRS chart. Visual acuity level and amblyopia contributed to a small portion (17%-26%) of the total variance in the difference between logMAR equivalents derived from the 2 charts.

CONCLUSIONS

The discrepancy in VA derived from Snellen and ETDRS charts was nonuniform across VA range. This has implications on interpretation of published studies converting Snellen fractions to logMAR for analysis and reporting of VA outcomes.

Factors affecting crowded acuity: eccentricity and contrast

PURPOSE

Acuity measurement is a fundamental method to assess visual performance in the clinic. Little is known about how acuity measured in the presence of neighboring letters, as in the case of letter charts, changes with contrast and with nonfoveal viewing. This information is crucial for acuity measurement using low-contrast charts and when patients cannot use their fovea. In this study, we evaluated how optotype acuity, with and without flankers, is affected by contrast and eccentricity.

METHODS

Five young adults with normal vision identified the orientation of a Tumbling-E presented alone or in the presence of four flanking Tumbling-Es. Edge-to-edge letter spacing ranged from 1 to 20 bar widths. Stimuli were presented on a white background for 150 ms with Weber contrast ranging from -2.5% to -99%. Flankers had the same size and contrast as the target. Testings were performed at the fovea, 3°, 5°, and 10° in the inferior visual field.

RESULTS

When plotted as a function of letter spacing, acuity remains unaffected by the presence of flankers until the flankers are within the critical spacing, which averages an edge-to-edge spacing of 4.4 bar widths at the fovea and approximately 16 bar widths at all three eccentricities. Critical spacing decreases with a reduction in contrast. When plotted as a function of contrast, acuity only worsens when the contrast falls below approximately 24% at the fovea and 17% in the periphery, for flanked and unflanked conditions alike.

CONCLUSIONS

The letter spacing on conventional letter charts exceeds the critical spacing for acuity measurement at the fovea, at all contrast levels. Thus, these charts are appropriate for assessing foveal acuity. In the periphery, the critical spacing is larger than the letter spacing on conventional charts. Consequently, these charts may underestimate the acuity measured in the periphery because of the effects of crowding.

Adult discrimination performance for pediatric acuity test optotypes

PURPOSE

To compare adult discrimination performance on nine pediatric visual acuity tests to determine the consistency of optotype design.

METHODS

After their binocular acuity was measured with each test, eight adult observers (mean age, 27 years ± 6.3 SD; three emmetropes and five corrected myopes) were shown isolated single optotypes from the Allen figures, HOTV, Landolt C, Lea Numbers, Lea Symbols, Lighthouse, Patti Pics, Precision Vision numbers, and Tumbling E tests. A one-interval, two-alternative forced-choice protocol was used at a single distance, and each optotype was paired with all optotypes from the same chart. Confusion matrices were generated for each test and Luce's (1963) biased-choice model was fit to each matrix to derive measures of pairwise similarity between the optotypes.

RESULTS

The acuities from the Allen figures (P < 0.001) and HOTV (P = 0.029) were the only ones to differ significantly from the reference Landolt C. The choice-model analyses of the confusion matrices revealed that the Allen figures, HOTV, Lighthouse, Patti Pics, and Precision Vision numbers tests all had significant differences in discriminability of optotypes within the test.

CONCLUSIONS

Pediatric acuity test optotypes are not all equally discriminable to adult observers with normal vision and no ocular disorders. The current data suggest that care must be taken when presenting limited numbers of optotypes, as is done with young patients.

Amblyopia: a mini review of the literature

Amblyopia is a form of cerebral visual impairment in the absence of an organic cause. It is considered to derive from the degradation of the retinal image associated with abnormal visual experience during the developmental period of the visual system in infancy and early childhood. Amblyopia is a significant cause of unilateral visual deficit in childhood and is still considered as one of the most common causes of persistent unilateral visual impairment in adulthood. The following review aims at presenting the contemporary literature regarding the prevalence, the aetiology, the neural correlates, the period of critical development, the treatment, the prognosis and the disability associated with this visual deficit.

Fixation patterns evaluation by means of MP-1 microperimeter in microstrabismic children treated for unilateral amblyopia

PURPOSE

The aim of the study was to evaluate the fixation patterns of microstrabismic children previously treated for unilateral amblyopia.

METHODS

Thirty-three children (mean age 7.3+/-1.5 years) were included in the study. Visual acuity (VA) was measured using the Early Treatment of Diabetic Retinopathy Study charts. Fixationwas assessed by MP-1 microperimeter. Differences in position and stability of fixation between the fellow and the microstrabismic eyes were calculated by using the percentage of the preferred fixation points within central fixation and the percentage of the fixation points within target fixation, respectively. For statistical analysis Mann-Whitney test was used. To evaluate the influence of age and duration of anti-amblyopic treatment on microstrabismic eyes fixation, linear regression analysis was performed.

RESULTS

In the microstrabismic eyes VA was significantly reduced when compared to the fellow eyes (0.1236+/-0.0204 vs 0.0042+/-0.0032 logMAR; p<0.001). Position and stability of fixation were significantly better in the fellow eyes (93.21+/-0.65% vs 70.91+/-4.80%; p=0.002, and 89.88+/-0.94% vs 71.73+/-2.94%; p<0.001, respectively). A significant correlation was found between fixation stability and both the duration of anti-amblyopic treatment and pretreatment VA (p=0.024 and p=0.009, respectively) and between fixation centrality and pretreatment VA (p<0.001).

CONCLUSIONS

VA, centrality, and stability of fixation were significantly impaired in the microstrabismic eyes. Pretreatment VA was a risk factor for fixation impairment. The severity of fixation stability impairment was linked to the duration of anti-amblyopic treatment.

Monocular and binocular reading performance in children with microstrabismic amblyopia

AIM

To evaluate if functionally relevant deficits in reading performance exist in children with essential microstrabismic amblyopia by comparing the monocular and binocular reading performance with the reading performance of normal sighted children with full visual acuity in both eyes.

METHODS

The reading performance of 40 children (mean age 11.6 (SD 1.4) years) was evaluated monocularly and binocularly in randomised order, using standardised reading charts for the simultaneous determination of reading acuity and speed. 20 of the tested children were under treatment for unilateral microstrabismic amblyopia (visual acuity in the amblyopic eyes: logMAR 0.19 (0.15); fellow eyes -0.1 (0.07)); the others were normal sighted controls (visual acuity in the right eyes -0.04 (0.15); left eyes -0.08 (0.07)).

RESULTS

In respect of the binocular maximum reading speed (MRS), significant differences were found between the children with microstrabismic amblyopia and the normal controls (p = 0.03): whereas the controls achieved a binocular MRS of 200.4 (11) wpm (words per minute), the children with unilateral amblyopia achieved only a binocular MRS of 172.9 (43.9) wpm. No significant differences between the two groups were found in respect of the binocular logMAR visual acuity and reading acuity (p>0.05). For the monocular reading performance, significant impairment was found in the amblyopic eyes, whereas no significant differences were found between the sound fellow eyes of the amblyopic children and the control group.

CONCLUSION

In binocular MRS, significant differences could be found between children with microstrabismic amblyopia and normal controls. This result indicates the presence of a functionally relevant reading impairment, even though the binocular visual acuity and reading acuity were both comparable with the control group.

Amblyopia characterization, treatment, and prophylaxis

Amblyopia has a 1.6-3.6% prevalence, higher in the medically underserved. It is more complex than simply visual acuity loss and the better eye has sub-clinical deficits. Functional limitations appear more extensive and loss of vision in the better eye of amblyopes more prevalent than previously thought. Amblyopia screening and treatment are efficacious, but cost-effectiveness concerns remain. Refractive correction alone may successfully treat anisometropic amblyopia and it, minimal occlusion, and/or catecholamine treatment can provide initial vision improvement that may improve compliance with subsequent long-duration treatment. Atropine penalization appears as effective as occlusion for moderate amblyopia, with limited-day penalization as effective as full-time. Cytidin-5'-diphosphocholine may hold promise as a medical treatment. Interpretation of much of the amblyopia literature is made difficult by: inaccurate visual acuity measurement at initial visit, lack of adequate refractive correction prior to and during treatment, and lack of long-term follow-up results. Successful treatment can be achieved in at most 63-83% of patients. Treatment outcome is a function of initial visual acuity and type of amblyopia, and a reciprocal product of treatment efficacy, duration, and compliance. Age at treatment onset is not predictive of outcome in many studies but detection under versus over 2-3 years of age may be. Multiple screenings prior to that age, and prompt treatment, reduce prevalence. Would a single early cycloplegic photoscreening be as, or more, successful at detection or prediction than the multiple screenings, and more cost-effective? Penalization and occlusion have minimal incidence of reverse amblyopia and/or side-effects, no significant influence on emmetropization, and no consistent effect on sign or size of post-treatment changes in strabismic deviation. There may be a physiologic basis for better age-indifferent outcome than tapped by current treatment methodologies. Infant refractive correction substantially reduces accommodative esotropia and amblyopia incidence without interference with emmetropization. Compensatory prism, alone or post-operatively, and/or minus lens treatment, and/or wide-field fusional amplitude training, may reduce risk of early onset esotropia. Multivariate screening using continuous-scale measurements may be more effective than traditional single-test dichotomous pass/fail measures. Pigmentation may be one parameter because Caucasians are at higher risk for esotropia than non-whites.

Visual acuity assessment: a comparison of two tests for measuring children's vision

PURPOSE

To compare the Kay Picture Test and the logMAR Crowded Test.

METHODS

Monocular visual acuity measurement was attempted on 103 paediatric subjects (aged between 2.5 and 16 years) attending eye clinics, using each of the visual acuity tests.

RESULTS

The results obtained with the two tests were found to be comparable as confirmed with the Intra-class Correlation Coefficient, which revealed good agreement between the two tests. There was significant correlation between the visual acuity results measured and high conformity in the results from the weaker eye. There was a statistical difference in acuity scores between the tests; the Kay Picture Test gave a lower logMAR score with a mean difference of 0.08 logMAR. This 0.08 difference represents an actual difference of less than one line of acuity measure which can be considered to be clinically insignificant. There was a significant difference in the ability to perform each of the tests under binocular conditions, with more of the 50 children, aged 5 and under, able to perform the Kay Picture Test than the logMAR Crowded Test.

CONCLUSIONS

The results obtained with the two tests are comparable. Both tests can be considered to be appropriate for amblyopia detection and the use of picture-based logMAR tests should be considered for younger children.

Sensitivity and specificity of visual acuity screening for refractive errors in school children

PURPOSE

To examine the optimal cutoff point for the use of the visual acuity test to screen for refractive errors in schoolchildren.

METHODS

In a sample of schoolchildren between 7 and 9 years old, visual acuity testing was performed using modified ETDRS charts monocularly without optical aids by trained personnel. Cycloplegic autorefraction was performed in each eye. The screening efficacy of using various cutoff points for referring children for further optometric/ ophthalmic assessment was studied. Myopia was defined as a spherical equivalent of at least -0.5 D, hyperopia a spherical equivalent of at least +2.0 D, and astigmatism a cylinder of at least -1.0 D in at least one eye. The sensitivity, specificity, and predictive values were calculated using each patient as a case; a receiver operator curve was plotted.

RESULTS

A total of 1,028 children were tested. A satisfactory sensitivity/specificity profile was obtained using a referral criterion of visual acuity worse than or equal to 0.28 logarithm of the minimum angle of resolution in at least one eye. In this scenario, the sensitivity and specificity of this screening test were 72% (95% confidence interval [CI], 68 to 76) and 97% (95%CI, 95 to 98), respectively. The positive and negative predictive values were 96% (95%CI, 93 to 98) and 78% (95%CI, 75 to 82), respectively.

CONCLUSIONS

The modified ETDRS visual acuity chart can be used to predict refractive errors in schoolchildren in Singapore in a sensitive and specific manner using a referral criterion of worse than or equal to 0.28 logarithm of the minimum angle of resolution.

Visual function before and after the removal of bilateral congenital cataracts in adulthood

Subject Peter Doyle (PD) had congenital bilateral cataracts removed at the age of 43. Pre-operatively PD's visual acuity was 20/80, with a resolution limit around 15 cpd, and he experienced monocular diplopia with high contrast stimuli. Post-operatively PD's visual acuity improved to approximately 20/40, with a resolution limit around 25 cpd. Using a variety of pre- and post-operative tests we have documented a wide range of neural adaptations to his limited and distorted visual input, and have found a limited amount of post-operative adaptation to his newly improved visual input. These results show that the human visual system is capable of significant adaptation to the particular optical input that is experienced.

Visual acuity assessment in infants and young children

The accurate and reliable assessment of visual function in infants and young children is important for ensuring optimal management of those at risk of abnormal visual development. Visual acuity is the aspect of visual function most commonly assessed by optometrists and can be measured in infants and children using appropriate techniques. Acuity measurements obtained using different techniques may show considerable disagreement and may mislead the optometrist when monitoring acuity development. It is important for the practitioner to appreciate these differences so that reasonable comparisons may be made between acuity estimates made using different techniques. With this in mind, we discuss methodological differences between some of the techniques used in visual assessment of very young patients and the effects those differences may have on acuity estimates.

Efficacy of occlusion for strabismic amblyopia: can an optimal duration be identified?

BACKGROUND/AIMS

The study of occlusion efficacy in amblyopia has been hampered by the use of non-logMAR acuity tests and a failure to assess threshold acuity for both eyes. These issues are addressed in the current study which compares the effect of spectacles alone and spectacles in combination with occlusion, with the use of the logMAR crowded test.

METHODS

Changes in uniocular and interocular acuity differences were compared for two age matched groups of previously untreated children with strabismic amblyopia: one compliant with spectacles only (n = 17, mean 6.2 (SD 2.5) years) and the other with spectacles and occlusion (n = 69, mean 5.1 (1) years) over a 1 year period. Changes in logMAR acuity were also analysed for a larger occluded group (n = 119) in response to successive 200 hour blocks of occlusion up to > or =1000 hours, in an attempt to isolate an optimal occlusion regime.

RESULTS

Visual acuity improved for more of the amblyopic eyes of the occluded (74%) than the spectacles only group (59%), and only one child from the latter group deteriorated. Mean visual acuity improved for both eyes of both treatment groups, but the change was significantly larger for the strabismic eyes of the occluded group overall and within the first 6 month period (p <0. 05). Occlusion was only effective for the first 400 hours worn. Subsequent visual improvement was bilateral and symmetrical.

CONCLUSION

Occlusion is more effective in the treatment of strabismic amblyopia than spectacles alone, and the effect is optimal within the first 6 months of wear. In terms of occlusion duration, maximal improvement occurs in response to 400 hours of occlusion wear or less, and to full time occlusion. Visual maturation continues, but is retarded for amblyopic eyes.

Contrast letter thresholds in the non-affected eye of strabismic and unilateral eye enucleated subjects

To investigate the effects of visual disruption on contrast letter thresholds of the non-affected eye, subjects with one eye enucleated, strabismic subjects using the non-deviating eye and normal control subjects were asked to identify letters on eye charts and single letter cards which varied in contrast (between 4 and 96%) and size. At all contrast, contrast letter acuity of eye enucleated subjects was superior to both normal control subjects and strabismic subjects. Early onset strabismic subjects (onset < 24 months) showed inferior performance to normal control subjects at all contrasts of 25% and above. Late onset strabismic subjects showed normal performance at all contrasts, except for high contrast single letters, where performance was inferior to normal control subjects. Further, for all subjects groups, performance on letter charts was similar to performance on single letter cards. We conclude that disruption to the visual system caused by eye enucleation or strabismus is not equivalent. These differences may be due to intrinsic differences between the visual systems of eye enucleated subjects and strabismic subjects and/or to the profound differences in deprivation caused by the two conditions.

Preschool vision screening: rationale, methodology and outcome

Although population outcome studies support the utility of preschool screening for reducing the prevalence of amblyopia, fundamental questions remain about how best to do such screening. Infant photoscreening to detect refractive risk factors prior to onset of esotropia and amblyopia seems promising, but our current understanding of the natural history of these conditions is limited, thus limiting the prophylactic potential of early screening. Screening for strabismic, refractive and ocular disease conditions directly associated with amblyopia is more clearly proven, but the diversity of equipment, methods and subject populations studied make it difficult to draw precise summary conclusions at this point about the efficacy of photoscreening. Sensory-based testing of preschool-age children exhibits a similar combination of promise and limitations. The visual acuity tests most widely used for this purpose are prone to problems of testability and false negatives. Moreover, the utility of random-dot stereograms has been confused by misapplication, and new small-target binocularity tests, while attractive, are as yet inadequately field-proven. The evaluation standard for any screening modality is treatment outcome. However, variables in amblyopia classification and quantitative definition differences, timing of presentation, nonequivalent treatment comparisons, and compliance variability have been uncontrolled in virtually all extant studies of amblyopia treatment outcome, making it difficult or impossible to evaluate either the relative efficacy of different treatment regimens for amblyopia or the effects of age on treatment outcome within the preschool age range. The latter issue is a central one, since existence of such an age effect is the primary rationale for screening at younger rather than older preschool ages. The relatively low prevalence of amblyopia makes it difficult to achieve a high screening yield in terms of predictive value, but functionally increasing prevalence by selective screening of high risk populations causes further problems. Unless a "supertest" can be devised, with very high sensitivity and specificity, health policy decisions will be required to determine which of these two characteristics should be emphasized in screening programs. Performance of screening tests can be optimized, however, with adequate training, perhaps via instructional videotapes.

Glasgow Acuity Cards: a new test for the measurement of letter acuity in children

Problems with design and format of the traditional Snellen chart have led to the development of alternative charts for the measurement of visual acuity in adults. However, advances in chart design for the measurement of visual acuity in pre-school children have not paralleled those used in adult test charts. Visual acuity can be measured in infants and very young children using behavioural and electrophysiological techniques but clinical measurement of letter acuity tends to commence when the child is old enough to interact and co-operate with the examiner. Charts which are commercially available in the UK for measuring letter acuity in pre-school children (3-5 years old) either use single optotypes or are derivatives of the Snellen format. There is a need for a test of letter acuity for use with this young age group which is accurate, reliable and based upon sound and established design principles. Glasgow Acuity Cards have been designed to include features that should allow change in letter acuity to be detected in pre-school children, especially those undergoing vision therapy training. The test is performed at 3 m and incorporates several design features which have been used previously in adult charts but are new to childrens' test charts. These include: linear progression of letter sizes using a log scale; letters of approximately equal legibility; equal number of letters per line; control of contour interaction; screening cards to determine initial level of acuity. The test is quick and easy to perform and should provide a means of detecting change in letter acuity, with increased confidence.