Reliability of a modified logMAR distant visual acuity chart for routine clinical use.

BACKGROUND/AIM: Acuity charts that follow the principle of logarithmic size progression (logMAR charts) are considered to be the gold standard for the assessment of distant vision. But it is not well accepted for routine eye examinations due to increased testing time and the complexity of scoring. This study was designed to check whether a modified logMAR chart with three optotypes would provide a reliable acuity assessment compared to standard logMAR charts for routine eye examination.
MATERIALS AND METHODS: Two versions of modified and standard logMAR charts were designed, constructed, and used to assess the visual acuity of 50 individuals drawn from a typical out-patient population. Timed test-retest variability and limit of agreement (95% confidence limit of agreement) of the modified chart are compared to the standard logMAR chart using Bland-Altman method. A comparison of the testing time was carried out using paired t-test.
RESULTS: The test-retest variability of the charts was comparable, with 95% confidence limit of the mean difference being ±0.08 for standard logMAR and ±0.10 for modified logMAR. Both the versions of modified logMAR charts produced the results which agreed well with those of the standard logMAR charts. The mean testing time required to complete the acuity measurements with the modified chart was significantly lesser compared to the standard chart (P < 0.001).
CONCLUSION: The outcomes of the current study demonstrates that the modified logMAR chart with three optotypes offers a comparable result to the standard logMAR charts for assessing distant visual acuity in routine clinical examination set up with a much lesser testing time.

Introduction

Visual acuity (VA) measurement is the most commonly adapted method for the assessment of visual function in clinical as well as research settings.[1] It is an integral part of routine eye examinations as it contributes a lot to the diagnosis and/or prognosis of eye disorders as well as to assess the responsiveness of the management. In the year 1862, Snellen developed the first letter acuity chart to assess visual function which worked on the principle of measuring the finest spatial details that the visual system can discriminate. The reliability of the Snellen chart later came under criticism due to (1) disparity in the number of letters in each line causing a varying level of difficulty for the subjects; (2) huge variation in the contour interaction throughout the chart due to non-uniformity in spacing between the letters and lines, (3) varying size progression of optotypes between the acuity levels, (4) lack of standardized scoring system, (5) poor repeatability and reliability, etc.[23] Despite all these limitations, Snellen's chart is still the commonly preferred option for VA assessment in eye examinations due to the ease of its use. The development of logarithmic progression charts in the 1960s have negated the limitations pertaining to design and precision, but failed to acquire the preference of clinicians mainly due to unfamiliar scoring system and extended time taken to complete the test.[4] Therefore, in this study, we aimed to compare the performance of a modified logMAR chart (with three optotypes) against the standard ETDRS chart (considered as the gold standard) in a routine eye examination set-up by assessing the limit of agreement, test-retest variability, and the time taken to complete the test.

Materials and Methods

50 subjects were recruited from the outpatient department of OEU Institute of Ophthalmology, Kasturba Hospital, Manipal, between January 2011 to March 2011. The group included patients with cataract, pseudophakia, glaucoma, myopia, and emmetropia. Two each sets of modified logMAR (m-LMR1 and m-LMR2) and standard logMAR (s-LMR1 and s-LMR2) charts were designed and all the acuity assessments were carried out on the right eye (RE) of the subjects. The study was approved by the institutional review board and all the participants signed the informed consent before taking part in the study.

Chart design

The acuity charts were designed, constructed, and printed on white panels based on Bailey Lovie's principle to test the VA at 4 m.[5] The only difference between the chart designs were the number of optotypes per line. Standard logMAR (s-LMR) had five optotypes and modified logMAR (m-LMR) had three. The combination of Sloan letters used to construct both versions of modified and standard charts was different. All the four charts had 14 acuity levels with 0.1 log units of size progression ranging from +1.0 to −3 logMAR. Figure 1a (standard logMAR), 1b (modified logMAR), and Table 1 summarizes the design principle of both the charts.

Figure 1

(a) Standard logMAR distant visual acuity chart. (b) Modified logMAR distant visual acuity chart

Table 1

Design features of two logMAR charts

VA testing procedure

VA assessment was carried out monocularly (RE) with subject's habitual refractive correction (if any). Each of them was tested using all the four charts after giving initial instructions on testing procedure and ensured that sufficient break was given between the tests. Charts were presented in a random order in order to avoid the effect of learning and getting accustomed to the test charts. The whole procedure was conducted under consistent indoor environments with artificial lighting.

Total time taken to complete the acuity measurements was documented using a stop watch. The time was measured from the start of first optotype until the subject erroneously read all the five (or three) letters in a line. Monitoring of the time and documentation of acuity scores were done by two individual examiners. Subjects were prompted to read the letters one-by-one from the left upper corner and the end point was defined as the subject misread a line completely.

Scoring

An interpolated logMAR acuity scoring (“single-letter” scoring) method[6] was employed for documenting the VA using the formula:

+1.10 − Tc × Lv logMAR,

where Tc is the total number of optotypes correctly identified in the chart and Lv is the logMAR value of each letter on the chart.

As the scale increment in a standard logMAR chart is 0.1 log units and each line contains five optotypes, the logMAR value for a single letter (Lv) would be 0.02 (0.1/5). But in modified logMAR, it was 0.033 (0.1/3). The VA of a patient who had read all the optotypes correctly upto 0.8 line and three letters on 0.7 line on a standard ETDRS chart, his/her VA was documented as 0.74 (+1.10 − 18 × 0.02) logMAR.

Statistical analysis

The agreement between two sets of charts and test-retest variability was performed using the Bland and Altman analysis (95% range, ±1.96 SD).[7] A paired t-test was used to compare the testing duration between the two chart designs.

Results

The mean age of the subjects recruited for the study was 45.52 ± 2.08 years (range between 18 and 75 years). Among the subjects chosen, 30 subjects had cataract, 13 were pseudophakic, 5 were emmetropic, 11 were myopic, and 4 had glaucoma. VA of the participants, as measured with the standard logMAR (s-LMR1 and s-LMR2), ranged between +0.50 and − 0.17 (mean: 0.09 ± 0.19, median: 0.00). Correspondingly, the acuity as measured with the modified logMAR charts ranged between +0.50 and − 0.30 (mean: 0.07 ± 0.20, median: 0.00)

The mean difference in logMAR between the two sets of charts was s-LMR: +0.003 ± 0.04; m-LMR: +0.004 ± 0.05. The 95% confidence limit for test-retest variability for the standard logMAR was ±0.08 and that for the modified log MAR was ± 0.10, indicating slightly higher variability with the modified logMAR [Table 2]. This demonstrates that 95% of repeated acuity measurements with the modified log MAR chart can produce a result within ± 0.10 logMAR of one another.

Table 2

Test-retest variability of the two chart designs

To further illustrate the reliability of the modified logMAR chart, the Bland–Altman analysis was applied. The difference between the paired data taken from the standard and modified logMAR charts (s-LMR1 versus m-LMR1 and s-LMR2 versus m-LMR2) was plotted against their mean [Figures 2 and 3] to assess the extent of agreement in their performance. From the data [Table 3], it can be observed that the mean difference between the two designs is very less and the confidence limits constructed around the mean difference are very narrow. Hence, the results illustrate that the modified logMAR charts produce a valid assessment of VA which is comparable with the standard one.

Figure 2

Differences between logMAR visual acuity scores for individual subjects obtained with the standard logMAR1 and modified logMAR1

Figure 3

Differences between logMAR visual acuity scores for individual subjects obtained with the standard logMAR2 and modified logMAR2

Table 3

Limit of agreement between the standard and modified logMAR chart designs

The mean and range of testing time to complete the acuity measurements were noted as 51.05;(29-91) seconds with standard logMAR chart and as 33.91;(18-62) seconds with modified logMAR chart. Time needed to complete the acuity measurements with the modified logMAR chart was significantly lesser (P < 0.001, paired t-test) compared to the standard logMAR chart.

Discussion

The clinicians all over the globe are in consensus about the type of VA chart to be used for research purpose.[8] Disappointingly, for routine clinical use, the advantages of logarithmic progression acuity charts are conveniently eluded by blaming the time required to administer the procedure.[9] The importance of having the same standard of acuity measurement in research as well as in clinical situation is highly significant when we apply the outcomes from clinical trials (which typically assess the VA with logarithmic charts) to predict the outcomes in clinical practice (which prefers to use Snellen's chart).[10]

This study demonstrated that the modified logMAR chart can produce acuity data that agree very well to the data produced by the gold standard logMAR chart [Table 3]. A mean difference of VA measurements between the two designs which is closer to zero and the narrowness of the confidence limits constructed surrounding the mean justifies our view [Figures 2 and 3]. This observation is consistent with the previous studies.[41112]

The test-retest variability achieved with the modified logMAR chart (95% CL of ± 0.10) is in agreement with reliability scores published in the literature, which varies between ± 0.07 to ± 0.20.[1] However, the test-retest variability in this study was close to the lower range. This could probably be due to the fact that all the acuity measurements were done with best-corrected visual acuity (BCVA) and by the same examiner. Moreover, all the four charts were presented to the subjects in a single visit with specific interval between the tests. Subsequent to the points mentioned above, the data were recorded using an interpolated single-letter acuity score method compared to the line assignment method favored in routine clinical practice. The line assignment method is known to demonstrate more test-retest variability compared to the single-letter scoring technique.[13] But a better clinical situation could have been replicated had the uncorrected as well as habitual visual acuities were also included and analyzed, which may be considered in future studies.

In this study, a reduction of 30% in testing time was observed to complete the modified logMAR chart compared to the standard logMAR. Results of this study are supported by the literature too.[41112] Moreover, the widely accepted line assignment method in clinical practice can even bring down the testing time.

Although this study did not involve a comparison of logMAR designs with the Snellen type, previous articles have already documented its poor test-retest variability especially for the line assignment method.[1112] However, each study population may differ in the way they respond to acuity charts and lack of published data on this aspect in Indian population brings up an opportunity for future work. As per the literature, the testing time observed to complete the Snellen chart was only marginally better than the time required for reduced logMAR charts.[412] Apart from that, the assessment with the Snellen chart cannot be relied in low-vision patients due to its higher variability in determining poor VA.[10] Hence, the modified logMAR design discussed in the present study would be a reliable alternative to standard logMAR and most importantly can replace the Snellen chart from our routine clinical settings.