A novel high-frequency visual acuity chart

A high-frequency eye test chart using letters or figures of alternating black and white stripes (or dots) on a grey background was developed. Any cross-section of the letters has a Fourier transform with a zero frequency component equal to the luminance of the grey background. When the letters are out of focus, their image on the retina fades rapidly into the grey background, rendering them invisible rather than merely blurred as in a standard chart. The chart was calibrated by simulating refractive errors with defocusing lenses applied to a photographic camera and to subjects' eyes. No constant ratio was found between the size of the Snellen letters and the size of the high-frequency letters for equal visibility. The new chart requires letters for 20/200 acuity to be only 3.6 times larger than those for 20/20 vision. Results confirm the arbitrary nature of the Snellen fraction and warn about the accuracy of visual acuity determined by using charts of different letter types, calibrated by Snellen's system.     

Visual acuity with reversed-contrast charts: I. Theoretical and psychophysical investigations.

When the contrast polarity of a visual acuity chart is reversed by using bright letters on a dark background instead of dark on light, the shape and height of the resolution feature in the retinal image is identical but inverted compared with the normal situation. However, the contrast is different in the two cases because the background light level, which is a dividing factor when contrast is calculated, is much less when only the letters are bright. To the extent that contrast is a limiting factor in visual acuity, reversed-contrast acuity would be expected to be better, and proportionally more so in eyes where light scatter and aberrations widen and flatten the point-spread function. In a careful psychophysical study of Landolt C resolution, the minimum angle of resolution was found to be significantly smaller for white letters on a dark background than for the traditional dark on bright situation.     

Comparison of visual acuity levels in pediatric patients with amblyopia using Wright figures, Allen optotypes, and Snellen letters.

PURPOSE: To compare and correlate the clinical performance of Wright figures in visual acuity assessments of pediatric patients with amblyopia to those obtained through Allen cards and Snellen letters. SUBJECTS AND METHODS: Best-corrected visual acuity of 26 amblyopic children were measured with the Wright figures(c), Snellen letters, and isolated Allen optotypes, respectively. Amblyopia was defined as two lines of visual acuity difference or a visual acuity level of 20/30 or lower as determined by Snellen chart. The results were evaluated for statistical intergroup differences using the Wilks' Lambda multivariate analysis of variance and for correlation using the Pearson correlation coefficient test. RESULTS: The mean age of the subjects was 8.27 +/- 2.46 years (range: 5 to 15 years). The mean logMAR values for the Wright figures(c), Snellen letters, and Allen optotypes were 0.40 +/- 0.20, 0.47 +/- 0.23, and 0.29 +/- 0.28, respectively. When compared with Snellen letters, the Wright figures correlated to a higher degree ( r = 0.46, P < 0.001) than Allen optotypes ( r = 0.67, P < 0.001). With a visual acuity of 20/40 or worse on Snellen letter testing, the sensitivity of Wright figures(c) and Allen cards in diagnosing amblyopic eyes was 87.0 and 56.5%, respectively ( P = 0.016). CONCLUSIONS: Wright figures, designed primarily to evaluate the vision in the preliterate pediatric population, correlate more closely to Snellen letters and have a higher rate of correctly identifying amblyopia than isolated Allen optotypes in pediatric patients.     

Performance of video-displayed letter charts

A software package was developed for an Apple-II microcomputer to display Snellen letters as test charts for high speed measurement of visual acuity. With direct viewing of a 30 cm (12 inch) monitor, letter sizes can range from 6/100 to 6/6. This range can be extended by using a modified monitor to display reversed letters for indirect viewing, to increase the testing distance. To circumvent patient learning, random presentation order of different charts (recalled rapidly from diskette) can be used. The video charts were compared with a Bailey-Lovie (IogMAR) Snellen chart by duplicating the letter sizes and format. Acuities were on average one line less for the video chart compared to the wall chart.     

Modeling the effect of forward scatter and aberrations on visual acuity after endothelial keratoplasty

Purpose. To evaluate the effects on visual acuity of forward scatter and aberrations typical of those after Descemet stripping endothelial keratoplasty (DSEK). Methods. Twenty normal eyes of 20 subjects (ages 22-57 years) were examined with best spectacle correction. Under photopic conditions, high-contrast visual acuities (HCVAs) were measured by using ETDRS charts. Visual acuity was also measured by using aberrated charts that simulated the typical high-order aberrations at 12 months after DSEK. Forward scatter was induced by viewing the eye charts through a 1-mm-thick layer of scattering solution (Amco Clear, at a concentration of 4000 nephelometric turbidity units) and was measured with a straylight meter. Results. Forward scatter increased from 1.19 ± 0.11 log straylight parameter (log[s]; mean ± SD) without induced scatter to 1.57 ± 0.06 log(s) with induced scatter (P < 0.001). Induced scatter reduced HCVA on the nonaberrated chart by 2.7 Snellen letters, from 20/19 (Snellen equivalent) to 20/21 (P < 0.001) and by 2.1 letters on the aberrated chart, from 20/25 to 20/28 (P = 0.005). Addition of aberrations reduced HCVA by more than twice the number of Snellen letters than did induced scatter, by 6.4 letters with low scatter (P < 0.001), and by 5.8 letters with high scatter (P < 0.001). Conclusions. Under typical clinical testing conditions, increased forward scatter has minimal effect on visual acuity. High-order aberrations are a more likely cause of degraded visual acuity than is forward scatter in eyes with clear corneas after DSEK.     

Development of a reading speed test for potential-vision measurements.

PURPOSE. Previous studies suggest that optimal reading speed is unaffected by cataract, yet is significantly reduced in age-related macular degeneration (ARMD ). This raises the question of whether a reading speed test could be developed to assess potential vision after cataract surgery. METHODS. Nineteen subjects with cataract, 15 with ARMD, and 13 control subjects with normal, healthy eyes read Bailey-Lovie word charts aloud, and subsequently, critical print size and optimal reading speed were calculated. Measurements were also taken with the charts in reversed-contrast polarity and after pupillary dilation. RESULTS. Although the subjects with cataract had reduced word acuity and increased critical print size, optimal reading speed was similar to that of the control group at a mean of approximately 100 wpm. Optimal reading speed in the subjects with ARMD was substantially worse (mean of 39 wpm). Reversing the contrast polarity of the charts slightly increased the word acuity and optimal reading speed of the subjects with cataract. CONCLUSIONS. The results suggest that optimal reading speed would be useful as a potential-vision test. The proposed test would use text size of at least 1.32 degrees (1.2 log minimum angle of resolution [logMAR]), and pupil dilation would be unnecessary. A reading test with black letters on a white background would be adequate, because charts with reversed-contrast polarity made minimal difference in reading speed.     

Design of an Arabic near visual acuity chart

This paper describes the design of an Arabic test chart for measurement of visual acuity at near. The chart was designed employing specially selected Arabic letters and was based on the logMAR principle devised by Bailey and Lovie, Ten Arabic letters of nearly equal legibility values (0.92–1.05) (mean = 1.00), (SD = 0.05) were used in the design of the chart. Each row of the chart has 5 letters and row legibility values range from 4.82 to 5.03 with a mean of 4.92 (SD = 0.06). The logMAR method of visual acuity scaling was used, hence the sizes of letters in the rows progress in a uniform step of 0.1 log unit. The inter letter spacing is equal to the width of each letter in the row, while inter–row spacing is equal to the height of letter in the subjacent row. The height of letters ranged from 3.67 to 0.46mm corresponding to visual acuity of 2.4 M to 0.3 M which is equivalent to reduced Snellen 6'36 to 6:4.5 at 0.4 m. The chart is designed for use at 40cm with a recommended luminance level of 160cd/m².     

Comparison of Snellen and interferometer visual acuity in an aging noncataractous population

Using standard clinical procedures we have compared visual acuity (VA) estimates made with a hand-held white light interferometer to those obtained with a Snellen acuity chart. Fifty noncataractous patients with a mean age of 45 years (SD = 18) were tested. Snellen and interferometric acuity measures were obtained with and without refractive correction. On average, aided Snellen VA's were better (decimal acuity = 0.98) than the unaided interferometric VA's (decimal acuity = 0.67). Although we found a statistically significant p less than 0.01) correlation between unaided interferometric and aided Snellen VA's, the correlation was poor (r = 0.36). This poor correlation may account for the often observed failure to estimate postoperative aided Snellen VA with preoperative interferometric VA in cataract patients.     

A Calculator for LogMAR Visual Acuity Measurements

A sliding-scale calculator is presented that indicates the conventional (6m) Snellen acuity fraction, given any logMAR chart standard viewing distance and the size of the smallest letters on that chart read correctly by a patient.     

Computer-based test to measure optimal visual acuity in age-related macular degeneration

PURPOSE: The authors present a computer-based method for evaluating the visual acuity of patients with age-related macular degeneration (AMD). It incorporates four features known to improve visual acuity: high contrast, white optotypes on a black background to reduce intraocular scatter, proportional layout to reduce the effects of crowding, and multiple optotypes to minimize the effects of fixation instability and to maximize the likelihood of optotype detection. METHODS: Experiment 1 evaluated the best-eye acuity of 24 patients with AMD using the ETDRS chart and three versions of the Tumbling E acuity test: multiple black optotypes on a white background, single white optotype on a black background, and multiple white optotypes on a black background. Experiment 2 compared the two White E optotype tests with the ETDRS in patients with AMD, and Experiment 3 measured probability summation in persons with normal vision. RESULTS: Multiple white optotypes on a black background yielded the highest acuity estimates and the ETDRS the lowest. The Single E test yielded a lower estimate of acuity than the two Multiple E tests. The effect of polarity-white on black was better than black on white-was consistent with results found in persons with healthy retinas. For patients with AMD, acuity measured with the Multiple E test was independent of that measured with the ETDRS, but acuity measured with the Single E test decreased as acuity worsened. For the participants with normal vision, the differences between the Multiple and Single E tests were within the known limits of test-retest variability. CONCLUSIONS: The multiple-optotype, reversed-polarity test provides a measure of the optimal visual acuity of which a person is capable and, in this sense, may be a useful tool for assessing rehabilitation progress.     

Visual outcomes in children in Malawi following retinopathy of severe malaria

AIM: To investigate whether retinal changes in children with severe malaria affect visual acuity 1 month after systemic recovery. METHODS: All children with severe malaria admitted to a research ward in Malawi during one malaria season were examined by direct and indirect ophthalmoscopy. Visual acuity was tested in those attending follow up by Cardiff cards, Sheridan-Gardiner single letters, or Snellen chart. RESULTS: 96 (68%) children attended follow up, of whom 83 (86%) had visual acuity measured. Cardiff cards were used in 47 (57%) children, and Sheridan-Gardiner letters or Snellen chart in 29 (35%). There was no significant difference in the mean logMAR visual acuity between groups with or without macular whitening (0.14 versus 0.16, p = 0.55). There was no trend for worse visual acuity with increasing severity of macular whitening (p = 0.52) including patients in whom the fovea was involved (p = 0.32). Six (4.2%) children had cortical blindness after cerebral malaria, and all six had other neurological sequelae. Ophthalmoscopy during the acute illness revealed no abnormalities in four of these children. CONCLUSION: Retinal changes in severe malaria, in particular macular whitening, do not appear to affect visual acuity at 1 month. This supports the hypothesis that retinal whitening is due to reversible intracellular oedema in response to relative hypoxia, caused by sequestered erythrocytes infected by Plasmodium falciparum. Impaired visual functioning after cerebral malaria is not attributable to retinal changes and appears to be a cortical phenomenon.     

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 electro-physiological 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 arc 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.     

Effects of luminance and contrast on visual acuity, ages 16 to 90 years.

Visual acuity of persons aged 16 to 90 years was measured with Snellen letters of varied contrast at 10, 1, 0.1, and 0.01 fl chart luminances. Percentage losses of seeing with age were computed. At 10 fl luminance, perception of high- and medium-contrast letters has changed little at age 40, but about twice as much light is needed to see low-contrast letters as at age 20. By age 70, no 2-min-subtense (20/40) letters were seen at 0.01 fl luminance. The need of older people for increased lighting during indoor tasks and night driving is discussed.     

New visual acuity chart for patients with macular hole.

PURPOSE: To evaluate the usefulness of a new multiple-letter visual acuity chart (MLAC) for the measurement of visual acuity in patients with macular hole. METHODS: Visual acuity was measured using a standard visual acuity chart (Landolt rings, also referred to as C's) and with the MLAC in normal subjects and in patients with a cataract or a macular hole. The MLAC has 14 plates (45 x 45 cm), and on one plate, many Landolt C's were printed with the gaps pointing in the same direction and all of one size. The sizes of the letters and gaps were made to give equivalent visual acuities of 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 1.5, and 2.0. The spacing between the letters was 33.3% of the diameter of the C's. Each chart projected many C's onto the macular area (5 degrees x 5 degrees ), which permitted the measurement of visual acuity at an extrafoveal point without the patient having to search for the extrafoveal point with the best acuity. RESULTS: There was no difference in the acuity measurement determined with the standard chart and the MLAC in normal subjects and patients with cataracts. Twelve of 16 patients with open macular hole, however, demonstrated higher acuity measurement (more than two lines) on the MLAC than on the standard chart. The improvement of visual acuity measurement after successful macular hole surgery was significantly less with the MLAC than with the standard chart. CONCLUSIONS: Our results suggest that the standard acuity chart, when administered before surgery, underestimates the patient's potential visual acuity after surgery, whereas the MLAC provides a better estimate of the patient's postoperative acuity. The MLAC can be a useful tool for measuring visual acuity in patients with macular hole.     

Factors affecting long term results of successfully treated amblyopia: initial visual acuity and type of amblyopia.

AIMS--The study aimed to assess the effect of initial visual acuity and type of amblyopia on the long term results of successfully treated amblyopia. METHODS--The visual acuity of 94 patients, who had been successfully treated for unilateral amblyopia by occlusion of the good eye and followed up to the age of 9 years, was examined 6.4 years, on average, after cessation of treatment. Patients were divided into two groups according to the depth of amblyopia before occlusion therapy was started: those with visual acuity between 20/60 and 20/100 and those with visual acuity of 20/100 or worse. RESULTS--Deterioration of visual acuity was observed in 42% of patients in the first group and in 63% of patients in the second group. Their average deterioration, as measured by the Snellen chart, was 0.58 and 1.54 lines, respectively. The results were also assessed by the division of patients into three groups according to the type of amblyopia: strabismic, strabismic anisometropic, and anisometropic. Deterioration of visual acuity occurred in 46%, 79%, and 36% of patients in these three groups, with an average deterioration on the Snellen chart of 0.70, 2.04, and 0.64 lines, respectively. CONCLUSION--It is concluded that low initial visual acuity and strabismic anisometropic amblyopia are risk factors for deterioration of visual acuity in the long term, following the successful earlier treatment of eyes with amblyopia.     

The Eye Phone Study: reliability and accuracy of assessing Snellen visual acuity using smartphone technology

Purpose

Smartphone-based Snellen visual acuity charts has become popularized; however, their accuracy has not been established. This study aimed to evaluate the equivalence of a smartphone-based visual acuity chart with a standard 6-m Snellen visual acuity (6SVA) chart.

Methods

First, a review of available Snellen chart applications on iPhone was performed to determine the most accurate application based on optotype size. Subsequently, a prospective comparative study was performed by measuring conventional 6SVA and then iPhone visual acuity using the ‘Snellen'' application on an Apple iPhone 4.

Results

Eleven applications were identified, with accuracy of optotype size ranging from 4.4–39.9%. Eighty-eight patients from general medical and surgical wards in a tertiary hospital took part in the second part of the study. The mean difference in logMAR visual acuity between the two charts was 0.02 logMAR (95% limit of agreement −0.332, 0.372 logMAR). The largest mean difference in logMAR acuity was noted in the subgroup of patients with 6SVA worse than 6/18 (n=5), who had a mean difference of two Snellen visual acuity lines between the charts (0.276 logMAR).

Conclusion

We did not identify a Snellen visual acuity app at the time of study, which could predict a patients standard Snellen visual acuity within one line. There was considerable variability in the optotype accuracy of apps. Further validation is required for assessment of acuity in patients with severe vision impairment.     

The component of gaze selection/control in the development of visual acuity in children

Visual acuity was tested for 180 eyes of 90 children in four age groups using three types of test charts. Subjects read the same 10 high-contrast letters in Snellen (line) format, as isolated-letter flash cards, and as repeat-letter flash cards. Group mean line and group mean isolated-letter acuity showed similar progressive improvements with age. A subgroup of 24 of 50 eyes of 4- to 5-year-olds (15 of 25 subjects) and 3 of 50 eyes of 6- to 7-year-old (2 of 25 subjects) had low Snellen acuity. Of this low-acuity subgroup of 27 eyes, 10 scored above average for their age group on the repeat-letter chart. We concluded that abnormal lateral interactions were not the explanation for the immaturity of Snellen acuity in these 10 eyes. We suggest that an important factor in the low acuities of these 10 eyes is delayed development of the selection and/or control of gaze direction. Some eyes with excellent Snellen acuity showed high crowding. For example, there were five such eyes in the oldest group. We suggest that the excellent acuities of at least two of these eyes are limited by minor inaccuracies in gaze selection and/or control rather than by lateral interaction.     

Ocular toxicity caused by tamoxifen: case report

To report tamoxifen ocular toxicity. The best visual acuity was measured in both eyes with Snellen chart, slit-lamp examination of anterior segment, refraction, dilated fundus examination, fluorescein angiography and retinography in a 63-year-old patient, female, white, using tamoxifen 20 mg/day for 4 years, with 20/70 and 20/40 corrected visual acuity. The anterior segment examination showed corneal linear subepithelial opacity inferior to the visual axis and nuclear and posterior cortical cataract (1+/4) in both eyes. Fundus examination showed alteration of macular color in both eyes. Fluorescein angiography presented hyperfluorescence in the macular area at an early phase (window defect). Report of keratopathy and maculopathy caused by tamoxifen.     

First clinical experience with anecortave acetate (Retaane)

BACKGROUND: Anecortave acetate is an angiostatic cortisene which is injected as a posterior juxtascleral depot and has been shown to be effective in the treatment of exudative age-related macular degeneration (AMD). The compound is not yet approved in Switzerland but can be used as "compassionate use" in individual cases. PATIENTS AND METHODS: An uncontrolled case series with standardised documentation of ETDRS visual acuity, near acuity, need for magnification and fluorescein angiography was performed. RESULTS: 22 eyes of 19 patients (8 male, 11 female, average age 78.8 years) were treated with a posterior juxtascleral depot injection (PJD) of 15 mg anecortave acetate. The mean change in visual acuity after 3 months in eyes treated with anecortave acetate was -2.6 ETDRS letters corresponding to 0.52 Snellen lines. 3/20 eyes gained more than 1 line. 11/20 eyes showed stable visual acuity (+/- 1 Snellen line, +/- 5 ETDRS letters). 5/20 eyes developed moderate vision loss (one to two Snellen lines, 6-10 ETDRS letters). 1/20 lost 18 ETDRS letters (> 3 Snellen lines). There were no moderate or severe adverse events. CONCLUSIONS: A PJD of 15 mg anecortave acetate is safe and well tolerated. In eyes with occult CNV without recent progression or with residual neovascular activity after photodynamic therapy anecortave acetate may be an alternative therapeutic option before considering intravitreal anti-VEGF agents due to the much less invasive character and lower risk profile.     

Preschool visual acuity tests: A comparison between two prototype charts and two visual acuity charts in current use

The lack of standardised visual acuity charts with established norms for preschool children has resulted in a diverse variety of visual acuity tests in use for the paediatric patient. All available tests appear to have some limitations, including inadequate norms, long test times, poor reliability, and frequently use test parameters and stimuli which are different from those used with adults. This study compares results for four visual acuity test charts. A Bailey-Lovie Illiterate E chart and an abbreviated Sheridan-Gardiner test were compared with two prototype acuity charts; a modified Bailey-Lovie letter chart and an Arrow chart. A novel symbol such as an arrow constructed as a Snellen optotype incorporated into a chart on the Bailey-Lovie principle, was the test of choice. This test was interesting to preschool children and results showed significant correlation with those obtained using charts based on adult standards.