Comparison of Snellen and Early Treatment Diabetic Retinopathy Study charts using a computer simulation

AIM: To compare accuracy, reproducibility and test duration for the Snellen and the Early Treatment Diabetic Retinopathy Study (ETDRS) charts, two main tools used to measure visual acuity (VA). METHODS: A computer simulation was programmed to run multiple virtual patients, each with a unique set of assigned parameters, including VA, false-positive and false-negative error values. For each virtual patient, assigned VA was randomly chosen along a continuous scale spanning the range between 1.0 to 0.0 logMAR units (equivalent to 20/200 to 20/20). Each of 30 000 virtual patients were run ten times on each of the two VA charts. RESULTS: Average test duration (expressed as the total number of characters presented during the test ±SD) was 12.6±11.1 and 31.2±14.7 characters, for the Snellen and ETDRS, respectively. Accuracy, defined as the absolute difference (± SD) between the assigned VA and the measured VA, expressed in logMAR units, was superior in the ETDRS charts: 0.12±0.14 and 0.08±0.08, for the Snellen and ETDRS charts, respectively. Reproducibility, expressed as test-retest variability, was superior in the ETDRS charts: 0.23±0.17 and 0.11±0.09 logMAR units, for the Snellen and ETDRS charts, respectively. CONCLUSION: A comparison of true (assigned) VA to measured VA, demonstrated, on average, better accuracy and reproducibility of the ETDRS chart, but at the penalty of significantly longer test duration. These differences were most pronounced in the low VA range. The reproducibility using a simulation approach is in line with reproducibility values found in several clinical studies.     

Contrast sensitivity for letter and grating targets under various stimulus conditions

Measurement of visual acuity for letters of different contrasts has been suggested as a clinical way to evaluate contrast sensitivity in patients with vision abnormalities. If variable-contrast letter acuity provides information similar to the contrast sensitivity function (CSF), then comparable effects should be seen in stimulus manipulations which simulate decreased vision. Using both our own and published data, we compared the effects of diffusive blur, dioptric blur, and eccentric viewing on contrast sensitivity for letter and grating targets. A diffuser placed close to the eye reduces contrast sensitivity fairly evenly across all spatial frequencies, with similar results for letters and gratings. However, dioptric blur reduces sensitivity substantially more to letters than to comparably fine gratings. Eccentric viewing also produces a larger sensitivity loss for letters than for gratings. Because some stimulus manipulations produce dissimilar changes in contrast sensitivity for letters and gratings, it is questionable whether the results of one measure can be used to draw inferences about the other. It is proposed that local or relative phase discrimination has an important role in explaining the different responses to letter and grating targets.     

Low-contrast letter charts as a test of visual function

Visual pathway disorders can cause visual loss that is not detected by the Snellen test: visual sensitivity to coarse detail may be depressed, even when visual sensitivity to fine detail is unaffected. Sinewave grating test targets can detect such hidden visual loss. However, electronic apparatus for generating sinewave gratings is expensive, while the inexpensive Arden plates provide no check on the patient's accuracy. We have tested 10 patients and 10 control subjects with a set of five letter charts (including the standard Snellen chart). These letter charts were of different contrasts, namely 10%, 22%, 31%, 64% and 93%, but otherwise were substantially alike. Subjects were also tested with sinewave gratings. We found good agreement between sinewave grating and letter chart findings. In particular, the charts picked up visual loss that was not detected by the standard Snellen chart: they detected visual pathway dysfunction in all seven patients whose sinewave data were abnormal. Our findings suggest that even one low-contrast letter chart could provide a valuable supplement to the standard Snellen chart. Compared with other available devices, these charts have the advantages of cheapness, simplicity and of providing the ophthalmologist with an immediate check on patients' accuracy.     

Effects of dioptric blur on Snellen and grating acuity

We compared the effects of dioptric blur on Snellen acuity and grating acuity. Dioptric blur had a strong negative effect on Snellen acuity, consistent with previous studies, but had little effect on grating acuity. Between 1 and 12 D both types of acuity were reduced as a linear function of blur. However, 12 D of blur reduced grating acuity to only 6/24 (20/80), whereas letter acuity was worse than 6/300 (20/1000). We suggest that these differences are due to the presence of "spurious resolution" in which phase-reversed gratings are readily detectable. But the phase reversals so distort the relative positions of linear segments within the letters that the letters become unrecognizable. These results indicate that Snellen letters are more sensitive than gratings to a patient's refractive errors, emphasize the differences between Snellen and grating acuity, and indicate that the minimum angle of resolution (MAR) concept is not applicable to letters.     

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.     

Reliability of high- and low-contrast letter charts

The aim of this study was to measure the reliability, in test score units, of several clinical tests which use high- and low-contrast letters, and to provide an estimate of what constitutes a significant change in performance over time. Patients with normal vision and with early or subtle eye disease were recruited so that the results would be representative of the population likely to present for primary vision screening. Patients were tested on the Bailey-Lovie logMAR chart, the Regan low-contrast letter charts and the Pelli Robson low-contrast letter chart on two occasions: the two test sessions were separated by at least four weeks to give an estimate of reliability appropriate for the conditions under which the tests are likely to be used. A 'significant change', i.e. one which would be observed in only about 5% of patients with stable visual performance, was about ±2 'steps' of the measurement scale, i.e. ±2 lines for the Bailey-1.m it-and Regan charts and ±2 letter groups for the Pelli-Robson chart.     

The usefulness of Vistech and FACT contrast sensitivity charts for cataract and refractive surgery outcomes research

AIM: To investigate the repeatability and sensitivity of two commonly used sine wave patch charts for contrast sensitivity (CS) measurement in cataract and refractive surgery outcomes. METHODS: The Vistech CS chart and its descendant, the Functional Acuity Contrast Test (FACT), were administered in three experiments: (1) Post-LASIK and age matched normal subjects; (2) Preoperative cataract surgery and age matched normal subjects; (3) Test-retest repeatability data in normal subjects. RESULTS: Contrast sensitivity was similar between post-LASIK and control groups and between the Vistech and FACT charts. The percentage of subjects one month post-LASIK achieving the maximum score across spatial frequencies (1.5, 3, 6, 12, 18 cycles per degree) were (50, 33, 13, 13, 0 respectively) for FACT, but only (0, 0, 13, 4, 0 respectively) for Vistech. A small number of cataract patients also registered the maximum score on the FACT, but up to 60% did not achieve the minimum score. Test-retest intraclass correlation coefficients varied from 0.28 to 0.64 for Vistech and 0.18 to 0.45 for FACT. Bland-Altman limits of agreement across spatial frequencies were between +/-0.30 and +/-0.85 logCS for Vistech, and +/-0.30 to +/-0.75 logCS for FACT. DISCUSSION: The Vistech was confirmed as providing poorly repeatable data. The FACT chart, likely because of a smaller step size, showed slightly better retest agreement. However, the reduced range of scores on the chart due to the smaller step size led to ceiling (post-LASIK) and floor (cataract) effects. These problems could mask subtle differences between groups of patients with near normal visual function as found post-refractive or cataract surgery. The Vistech and FACT CS charts are ill suited for refractive or cataract surgery outcomes research.     

Comparison of Snellen acuity, VER acuity, and Arden grating scores in macular and optic nerve diseases.

Patients with various macular and optic nerve abnormalities underwent Snellen acuity, transient VER acuity, and Arden grating testing. Snellen acuity was the coarsest of the 3 evaluations, generally falling after Arden scores and VER acuity had already undergone significant degradation. The Arden gratings appeared to be the most sensitive of the 3 tests, equalling VER performance in optic nerve diseases and surpassing it in macular diseases. Variations in results between the different tests are generally understandable if one considers the functions tested by each and the anatomical derangements caused by the diseases in question. The Arden grating test appears to be an excellent and sensitive screening test for central visual disturbances.     

The role of contrast sensitivity charts and contrast letter charts in clinical practice

Practitioners frequently make clinical decisions based on changes in visual acuity. However, acuity is recognised to be only one aspect of visual performance. In recent years, a number of clinical chart-based tests which measure different aspects of vision based on contrast sensitivity have become available. These tests allow a more subtle investigation of visual problems. In this review of currently available contrast sensitivity charts and contrast letter charts, we examine their clinical application and some of the problems which may be encountered in the use of these tests in practice. This should permit a better understanding of some patients' visual problems, better clinical decision making and a better understanding of studies which report visual performance measurements. Information on currently available equipment and a list of suppliers in Australia is included. (Clin Exp Optom 1995; 78: 2: 43–57)     

Comparison of visual acuity estimates using three different letter charts under two ambient room illuminations

Background:

Visual acuity is an essential estimate to assess ability of the visual system and is used as an indicator of ocular health status.

Aim:

The aim of this study is to investigate the consistency of acuity estimates from three different clinical visual acuity charts under two levels of ambient room illumination.

Materials and Methods:

This study involved thirty Malay university students aged between 19 and 23 years old (7 males, 23 females), with their spherical refractive error ranging between plano and –7.75D, astigmatism ranging from plano to –1.75D, anisometropia less than 1.00D and with no history of ocular injury or pathology. Right eye visual acuity (recorded in logMAR unit) was measured with Snellen letter chart (Snellen), wall mounted letter chart (WM) and projected letter chart (PC) under two ambient room illuminations, room light on and room light off.

Results:

Visual acuity estimates showed no statistically significant difference when measured with the room light on and with the room light off (F_1,372 = 0.26, P = 0.61). Post-hoc analysis with Tukey showed that visual acuity estimates were significantly different between the Snellen and PC (P = 0.009) and between Snellen and WM (P = 0.002).

Conclusions:

Different levels of ambient room illumination had no significant effect on visual acuity estimates. However, the discrepancies in estimates of visual acuity noted in this study were purely due to the type of letter chart used.     

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.     

Measurement of glare sensitivity in cataract patients using low-contrast letter charts*

The Committee on Ophthalmic Procedures Assessment of the American Academy of Ophthalmology suggested using low-contrast visual acuity measured before and after adding a glare source as a test for assessing overall visual disability from immature cataracts. We have developed a test that follows the Committee's three principles of design, and we report that the effect of glare on visual acuity is considerably greater for recognizing low-contrast letters than for recognizing high-contrast letters. The effect of glare on visual acuity increases progressively as letter contrast is reduced in the stages 96%, 50%, 25%, 11% and 4%, The 25% chart (and possibly the 11% chart) gives the most suitable sensitivity for eyes with immature cataracts. Age-related brunescence and aging itself do not necessarily produce high sensitivity to glare. Sensitivity to glare was markedly different in eyes with different kinds of cataract.     

Comparison of Snellen acuity and objective assessment using the spatial frequency sweep PVER

Background: We compared the vision objectively assessed by spatial frequency sweep pattern-reversal visual-evoked response (SPVER) with the Snellen acuity in patients. Methods: SPVER acuity and Snellen acuity were measured in 100 patients with various ocular pathologies, including macular diseases, diffuse retinal degeneration, optic nerve diseases, glaucoma, and high myopia. For SPVER, 10 sinusoidally modulated vertical gratings were presented as stimuli. The responses were averaged and displayed through the discrete Fourier transform on the monitor display. The PVER acuity was determined by extrapolating the SPVER amplitude-spatial frequency function to baseline. Results: Vision ranged from 20/15 to 20/400 with Snellen acuity, and from 20/25 to 20/190 with SPVER. The overall correlation between the two acuities wasr=0.666. The correlation varied fromr=0.895 in eyes with glaucoma tor=0.436 in eyes with optic nerve disease. Seventy-seven eyes (77%) had a visual acuity agreement of within 1.0 octave between the two measurements. Conclusion: The SPVER acuity and the Snellen acuity correlated to a certain degree. Discrepancies were found in certain diseases, with the highest disparity in patients with optic nerve disease. We conclude that the SPVER is effective in estimating vision objectively, particularly in patients in whom the standard Snellen test is impossible to perform or yields unreliable results.This study was presented in part at the American Academy of Ophthalmology Annual Meeting, San Francisco, November 1994     

LASIK前后角膜地形图与散瞳验光测量结果的比较分析

目的 观察激光原位角膜磨镶术(laser in situ keratomileusis,LASIK)前后角膜地形图仪与散瞳验光测量结果的差异.方法 以我院行LASIK的患者103例(200眼)为研究对象,进行回顾性分析,测量LASIK术前、术后6个月的散光状态,以角膜地形图仪记录角膜地形图形态、角膜屈光度、角膜散光度;散瞳后应用全自动电脑验光仪结合带状光检影及试镜,确定球镜度数、柱镜度数和轴位.所有数据用SAS 6 12分析软件进行统计学分析.结果 术前散瞳验光测得散光度为(-0.71±0.42)D,角膜地形图测得散光度为(-0 86±0.54)D,2种方法测得散光度有显著统计学差异(t=-5 10,P<0 01);散瞳验光测得散光轴位为(95.09±64.83)°,角膜地形图测得散光轴位为(93.72±71.92)°,2种方法测得散光轴位比较差异无统计学意义(t=-0.72,P>0.05).术后3个月、6个月散瞳验光测得散光度为平均(-0.54±0.38)D、(-0.43±0.33)D,角膜地形图测得散光度分别为平均(-0 51±1.94)D、(-0.42±1.92)D,2种方法测得散光度比较差异无统计学意义(r=-0.11、-0.07,P>0.05),且术后3个月与6个月比较差异无统计学意义(t=0.15,P>0 05);散瞳验光测得散光轴位分别为(75.11±51.17)°、(76.25±51 22)°,角膜地形图测得散光轴位为(76.08±54 96)°、(76 39±55.58)°,2种方法测得散光轴位差异有统计学意义(r=0.78、0.70,P<0.01),且术后3个月与6个月比较差异无统计学意义(t=0.43,P>0.05).结论 LASIK术前角膜地形图仪和散瞳验光记录的散光度有差异性,轴位无显著性差异;LASIK术后角膜形态改变,眼部参数之间的关系也发生改变,角膜地形图仪和散瞳验光记录的散光度无统计学意义,轴位有统计学意义.术后3个月与6个月测量结果证明,LASIK在矫正散光上有较高的精确性,在设计屈光性手术时有指导意义.     

成人近视眼低阶像差与主客观验光结果的比较

目的 :比较成人近视患者的低阶像差测量结果与用主客观不同验光方法所获得的球镜屈光度、柱镜屈光度和散光轴向之间的差异 ,并对其相关因素进行分析。方法 :首先对成人近视患者 96例 (192只眼 )进行散瞳电脑验光、视网膜检影、波前像差检查及瞳孔恢复后的主观屈光检查。然后分别以屈光度、年龄及性别为区分条件分组 ,比较用不同方法及在不同条件下测得的球镜 (DS)、柱镜 (DC)屈光度和散光轴位 (AA)的差别并进行相关因素分析。结果 :四种方法所获得的球镜、柱镜屈光度和散光轴位均值分别为 :散瞳电脑验光是 (- 5 .11± 2 .16 )D、(- 0 .6 1± 0 .16 )D和 86 .6 0± 73.85 ;散瞳视网膜检影是 (- 5 .13± 2 .6 7)D、(- 0 .6 2± 0 .70 )D和 79.77± 79.38;主观屈光检查是 (- 5 .30± 2 .15 )D、(- 0 .6 4± 0 .73)D和 81.6 7± 75 .2 2 ;波前像差检查的低阶像差值为收稿日期 :2 0 0 4-0 8-15 ;修回日期 :2 0 0 4-11-0 2作者简介 :王晶 (1970 -) ,女 ,江苏人 ,主治医师 ,讲师 ,研究方向 :屈光性角膜手术。E -mail:sifanjean @yahoo.com .cn(- 4 .91± 2 .2 0 )D、(- 0 .6 9± 0 .5 3)D和 89.88± 6 8.95。在散瞳条件下 ,低阶像差测得的球镜值明显低于用视网膜检影及电脑验光所测得的球镜屈光度 (P <0 .0 5 )