Abstract
Purpose:
The Pelli-Robson and LEA contrast sensitivity charts are commonly used in clinical settings to measure contrast sensitivity. Although the Pelli-Robson chart is considered the gold standard, it is limited by its bulky size. The LEA chart, on the contrary, offers a more practical and portable option that is still reliable. This has led to questions about whether we can predict Pelli-Robson scores based on LEA scores. This study developed a conversion method to help transition from the LEA chart to the Pelli-Robson chart and validate the conversion score.
Methods:
In this retrospective study, we analyzed the relationship between LEA and the Pelli-Robson contrast sensitivity test. Our study examined a total of 120 eyes. We developed a conversion table through the equipercentile equating method. Subsequently, we assessed the reliability and accuracy of this algorithm for converting LEA results into Pelli-Robson contrast sensitivity scores.
Results:
The study used a conversion table to convert LEA scores to Pelli-Robson scores. The conversion table achieved a reliability of 0.91 based on intraclass correlation, and the algorithm had an accuracy of 81.6% within a 1-point difference from the raw score.
Conclusions:
This study reported a reliable and comparable conversion algorithm for transforming LEA scores into converted estimated Pelli-Robson scores, thereby improving the usefulness of existing data in both clinical and research contexts.
Keywords: Contrast sensitivity test, conversion, LEA number chart, Pelli-Robson chart
Contrast sensitivity is an important and routine examination in neuro-ophthalmology clinical practice. It can detect and quantify vision loss even when visual acuity is normal.[1,2] Loss of contrast sensitivity can lead to visual difficulties and may predict performance impairment compared to standard acuity measurements.[2,3,4,5,6]
Contrast sensitivity assessment is different from visual acuity testing in that it uses targets with varying contrast levels instead of targets that vary in size but have a single high level of contrast.[1,2,5] The subject must distinguish between visible and invisible increments of luminance from the background.[5] Contrast sensitivity assessment serves as a valuable tool for early detection of issues related to the optic nerve, irregularities in the eye’s media, and macular lesions.[7,8,9,10,11,12] However, interpreting the data from contrast sensitivity tests is more intricate compared to visual acuity data, especially when it comes to distinguishing subtle abnormalities from what is considered a normal presentation.[1,5]
There are two primary types of contrast sensitivity tests: grating tests and letter tests.[1,2,5,6] In clinical settings, the Pelli-Robson chart is a widely used tool for letter testing. It consists of eight rows of letters, each row containing two sets of three letters, with contrast levels varying among these groups of letters.[1,2,3,13] Each letter on the Pelli-Robson chart subtends a visual angle of 2.86 degree when viewed from a distance of 1 meter. The Pelli-Robson score quantifies a subject’s contrast sensitivity on a logarithmic scale, ranging from 0 to 2.25. The original scoring system grants 0.15 credit if at least two out of three letters in a set are correctly identified, while a modified scoring system grants 0.05 credit per correctly identified letter.[1]
The Pelli-Robson contrast sensitivity test is considered a reliable test with good test-retest repeatability. This chart can maintain its reliability across various factors such as background luminance within the photopic range, testing distances spanning from 0.25 to 4 meters, and even when there is a 2-D defocus due to the large letters on the chart. However, it does come with some logistical limitations due to its wall-mounted design, bulky size, and the chart’s tendency to fade over time.[1,3]
The LEA contrast sensitivity chart is another letter test used in clinical practice in recent years. It is available in two formats: the LEA symbol chart and the LEA number chart. The chart is available in the form of flip cards, but the letter subtends a different angle for different distances. The LEA chart has five pages with five numbers or five symbols on each page, with contrast ranging from 25% to 1.2%.[14]
While the LEA contrast sensitivity chart offers a reliable test in a way more practical and portable chart,[15] another study reported that the LEA chart is less reliable and has relatively less test-retest repeatability compared to Pelli-Robson.[16] This condition led to a question of whether we can predict the Pelli-Robson contrast sensitivity score based on the LEA contrast sensitivity score. This study aimed to develop a conversion method to facilitate the transition from the LEA to the Pelli-Robson and validate the conversion score.
Methods
A retrospective observational study of 120 eyes (69 patients) in a referral hospital was held. Inclusion criteria were all patients registered as outpatients in the neuro-ophthalmology clinic from December 2021 to May 2022 who underwent contrast sensitivity test using Pelli-Robson and LEA contrast sensitivity test at one time of visitation. Exclusion criteria were patients who were examined with only one of those contrast sensitivity tests.
Medical records were reviewed to obtain the demographic characteristics of patients, including sex, age, and contrast sensitivity score. Contrast sensitivity was examined by a neuro-ophthalmologist consultant using the Pelli-Robson and LEA contrast sensitivity test. The study was approved by the research ethics committee of the hospital.
Data were analyzed using the statistical software program SPSS V23.0 and Microsoft Excel V16.70. First, the LEA score was converted into logarithmic, according to the manufacturer’s stated contrast levels. The relationship between LEA and the Pelli-Robson contrast sensitivity test was evaluated using Spearman correlation. The equipercentile equating method was used to convert LEA to the Pelli-Robson score. The reliability and repeatability of this algorithm were evaluated using Bland-Altman and intraclass correlation coefficient (ICC).
Results
There were 120 eyes of 69 patients examined using the Pelli-Robson and LEA contrast sensitivity test from December 2021 to May 2022. Demography characteristics of the patients and characteristics of LEA and Pelli-Robson contrast sensitivity scores are shown in Table 1.
Table 1.
Demography characteristics and contrast sensitivity score
| Age |
Sex (n) |
LEA Score | Pelli-Robson Score | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Range | (n) | Male | Female | |||||||
| 6–19 | 13 | 7 | 6 | 1.66±0.33 | 1.29±0.33 | |||||
| 20–39 | 24 | 12 | 12 | 1.57±0.46 | 1.21±0.42 | |||||
| 40–59 | 28 | 7 | 21 | 1.66±0.32 | 1.31±0.34 | |||||
| ≥60 | 4 | 2 | 2 | 1.58±0.53 | 1.27±0.35 | |||||
| Average | 36.2±14.85 | 1.63±0.38 | 1.26±0.37 | |||||||
The relationship between LEA and the Pelli-Robson chart was evaluated using the Spearman correlation coefficient. They indicated a strong correlation (r = 0.817; P < 0.01). Based on their strong correlation, this study constructed the LEA to Pelli-Robson conversion score by using the equipercentile equating method, wherein the LEA and Pelli-Robson scores were considered equivalent if their corresponding percentile ranks were equal.
The plot of equipercentile equivalents of LEA and Pelli-Robson contrast sensitivity scores is presented in Fig. 1. For example, an LEA score of log 1.6 is equivalent to a Pelli-Robson score of log 1.2, with both of these scores falling at approximately the same percentile rank. Table 2 shows LEA scores and their respective equivalents on the Pelli-Robson score.
Figure 1.
Equipercentile equating of LEA and Pelli-Robson contrast sensitivity scores. Red line: Pelli-Robson contrast sensitivity scores. Blue line: LEA contrast sensitivity scores
Table 2.
Equipercentile equating table for potential conversion of LEA to Pelli-Robson scores
| LEA score |
Equivalent Pelli-Robson Score |
|||
|---|---|---|---|---|
| % | Log | |||
| 25 | 0.6 | 0.3 | ||
| 10 | 1 | 0.6 | ||
| 5 | 1.3 | 1.05 | ||
| 2.5 | 1.6 | 1.2 | ||
| 1.25 | 1.9 | 1.5 | ||
In the development of this conversion algorithm, reliability and agreement were important factors taken into account. The research demonstrated a strong ICC of 0.911 (P < 0.01) between the raw and converted Pelli-Robson scores. The Bland-Altman plot indicated a mean difference of 0.005 between the raw and converted Pelli-Robson scores, with upper and lower limits of agreement (LOA) of 0.414 and − 0.404, respectively [Fig. 2]. The accuracy of the Pelli-Robson conversion score compared to the raw Pelli-Robson score was 35.8% showing no difference (perfectly matched score), 81.6% (±log 0.15, with 1 line of difference), and 95% (±log 0.3, with 2 lines of difference) [Table 3].
Figure 2.
A Bland-Altman Plot of the difference in the raw and converted Pelli-Robson scores. LOA: Limit of Agreement
Table 3.
Analysis of accuracy and reliability between the converted and raw Pelli-Robson scores
| Method | Difference between the raw score and the converted Pelli-Robson score (Accuracy) |
*Intraclass Correlation Coefficient | ||||||
|---|---|---|---|---|---|---|---|---|
| 0 | ±log 0.15 | ±log 0.30 | ||||||
| Current Study | 35.8% | 81.6% | 95% | 0.911 (0.872–0.938) | ||||
| Leat et al.[14] | 18.3% | 63.3% | 95% | 0.906 (0.865–0.934) | ||||
Notes: These differences in accuracies and intraclass correlation coefficients were evaluated using this study’s participants. *Values are presented as intraclass correlation coefficients (95% confidence intervals)
Discussion
The relationship between LEA and Pelli-Robson was evaluated to construct a reliable algorithm to convert scores between these two scales. Spearman correlation coefficient for the LEA and Pelli-Robson contrast sensitivity chart indicated a strong correlation in this study. This might be because both LEA and Pelli-Robson charts are standardized measurement tools for contrast sensitivity testing. A previous study also reported that LEA symbols and Pelli-Robson have a strong correlation.[14]
By employing a conversion table, it becomes possible to represent LEA scores in Pelli-Robson terms. This conversion algorithm facilitates the comparison of LEA and Pelli-Robson results in clinical environments and promotes the compatibility of data across longitudinal studies. This method has been successfully applied to standardize various tests, including the conversion of scores from assessments such as the Mini-Mental State Examination, Montreal Cognitive Assessment, and Dementia Rating Scale-2. Notably, this method ensures that the equated scores consistently remain within the range of achievable scores.[17,18]
The LEA scores were required to be transformed into logarithmic scores not only because the Pelli-Robson score is stated in logarithmic terms for equal unit denomination but also to smooth the raw scores and create a normal distribution without irregularities that are attributable to sampling. Log-linear transformation enhances the accuracy of the equating method.[17,18]
The LEA chart is suggested as reliable. Although it is less reliable and has less test-retest repeatability compared to the Pelli-Robson chart,[16] it offers a more portable way to measure contrast sensitivity in settings where Pelli-Robson could not be a choice of measurement tool. The conversion of LEA to the Pelli-Robson score enables giving an estimation of the Pelli-Robson score in children or adults who could not perform the Pelli-Robson contrast sensitivity test.[14]
A previous study reported a conversion from the LEA symbol chart to the Pelli-Robson chart by using a different method conversion formula in the normal population. The formula stated in the previous study was PR = (0.775 × LEA) −0.075, where PR is the Pelli-Robson log contrast sensitivity score and LEA is the contrast sensitivity score calculated from the manufacturer’s stated contrast levels. The study did not evaluate the correlation between the raw and measured conversion scores.[14] We tried to convert our LEA score from this study participant into a Pelli-Robson score by using this formula and evaluated its accuracy. The differences between these two conversion methods were evaluated [Table 3].
Based on our study, we made a conversion score and evaluated both the reliability and agreement of the conversion algorithm. Our study showed an excellent (ICC = 0.911; P < 0.01) between the raw and converted Pelli-Robson scores. The Bland-Altman plot showed that the mean difference between the raw and converted Pelli-Robson scores was 0.005 with an upper and a lower LOA of 0.414 and − 0.404, respectively [Fig. 2]. It showed no proportional bias and a comparable conversion score. The accuracy of the Pelli-Robson conversion score compared to the raw Pelli-Robson score was 35.8% showing no difference (perfectly matched score), 81.6% (±log 0.15, with 1 line of difference), and 95% (±log 0.3, with 2 lines of difference). The accuracy of our conversion score was higher compared to the previous formula [Table 3]. It can be concluded that this conversion algorithm offers a reliable, repeatable, and high-accuracy algorithm among neuro-ophthalmology patients.
There were several limitations associated with this study. First, it was susceptible to the inherent constraints associated with a retrospective study design. A prospective study is, therefore, warranted to validate the results. Second, the data in this study were collected from neuro-ophthalmology patients only with various age groups, sex, BCVA, and diagnosis.
The examination needs to be done with more stringent inclusion criteria and/or in normal populations. The number of samples needs to be expanded, specifically in the over 60-year-old age group. The equipercentile equating method itself could lead to an irregular distribution of scores.
Conclusion
This study reported a reliable and comparable conversion algorithm for transforming LEA scores into converted estimated Pelli-Robson scores for neuro-ophthalmology patients. thereby improving the usefulness of existing data in both clinical and research contexts. Limitations of this study included that it was a retrospective study in nature. Further research can be carried out in a prospective way with more stringent inclusion criteria and/or in normal populations.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
Acknowledgement
The authors would like to extend their gratitude and acknowledgments to all study participants and study team members for their time and energy spent on this project.
References
- 1.Owsley C. Contrast sensitivity. Ophthalmol Clin North Am. 2003;16:171–7. doi: 10.1016/s0896-1549(03)00003-8. [DOI] [PubMed] [Google Scholar]
- 2.Rubin GS. Visual acuity and contrast sensitivity. In: Ryan SJ, Schachat AP, Wilkinson CP, Hinton DR, Sadda SR, Wiedemann P, editors. Retina. Philadelphia: Elsevier/Saunders; 2013. pp. 300–6. [Google Scholar]
- 3.Thurman SM, Davey PG, McCray KL, Paronian V, Seitz AR. Predicting individual contrast sensitivity functions from acuity and letter contrast sensitivity measurements. J Vis. 2016;16:15. doi: 10.1167/16.15.15. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Wei H, Sawchyn AK, Myers JS, Katz LJ, Moster MR, Wizov SS, et al. A clinical method to assess the effect of visual loss on the ability to perform activities of daily living. Br J Ophthalmol. 2012;96:735–41. doi: 10.1136/bjophthalmol-2011-300093. [DOI] [PubMed] [Google Scholar]
- 5.Pelli DG, Bex P. Measuring contrast sensitivity. Vision Res. 2013;90:10–4. doi: 10.1016/j.visres.2013.04.015. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Ginsburg AP. Contrast sensitivity and functional vision. Int Ophthalmol Clin. 2003;43:5–15. doi: 10.1097/00004397-200343020-00004. [DOI] [PubMed] [Google Scholar]
- 7.Ulrich K, Palmowski-Wolfe A. Comparing three different contrast sensitivity tests in adults and in children with and without amblyopia. Klin Monbl Augenheilkd. 2019;236:434–7. doi: 10.1055/a-0853-1683. [DOI] [PubMed] [Google Scholar]
- 8.Williamson TH, Strong NP, Sparrow J, Aggarwal RK, Harrad R. Contrast sensitivity and glare in cataract using the Pelli-Robson chart. Br J Ophthalmol. 1992;76:719–22. doi: 10.1136/bjo.76.12.719. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Thakur S, Ichhpujani P, Kumar S, Kaur R, Sood S. Assessment of contrast sensitivity by Spaeth Richman contrast sensitivity test and Pelli Robson chart test in patients with varying severity of glaucoma. Eye (Lond) 2018;32:1392–400. doi: 10.1038/s41433-018-0099-y. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Hilmi MR, Khairidzan MK, Azemin ZC, Azami MH, Ariffin AE. Measurement of contrast sensitivity using the M and S smart system II compared with the standard Pelli–Robson chart in patients with primary pterygium. Makara J Health Res. 2018;22:167–71. [Google Scholar]
- 11.Gupta L, Cvintal V, Delvadia R, Sun Y, Erdem E, Zangalli C, et al. SPARCS and Pelli-Robson contrast sensitivity testing in normal controls and patients with cataract. Eye (Lond) 2017;31:753–61. doi: 10.1038/eye.2016.319. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Vingopoulos F, Wai KM, Katz R, Vavvas DG, Kim LA, Miller JB. Measuring the contrast sensitivity function in non-neovascular and neovascular age-related macular degeneration: The quantitative contrast sensitivity function test. J Clin Med. 2021;10:2768. doi: 10.3390/jcm10132768. doi: 10.3390/jcm10132768. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Mäntyjärvi M, Laitinen T. Normal values for the Pelli-Robson contrast sensitivity test. J Cataract Refract Surg. 2001;27:261–6. doi: 10.1016/s0886-3350(00)00562-9. [DOI] [PubMed] [Google Scholar]
- 14.Leat SJ, Wegmann D. Clinical testing of contrast sensitivity in children: Age-related norms and validity. Optom Vis Sci. 2004;81:245–54. doi: 10.1097/00006324-200404000-00010. [DOI] [PubMed] [Google Scholar]
- 15.Moganeswari D, Thomas J, Srinivasan K, Jacob GP. Test re-test reliability and validity of different visual acuity and stereoacuity charts used in preschool children. J Clin Diagn Res. 2015;9:NC01–5. doi: 10.7860/JCDR/2015/14407.6747. doi: 10.7860/JCDR/2015/14407.6747. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Firdous A, Ayesha S. Comparative reliability of Pelli-Robson and lea number chart for contrast sensitivity measurement. Ophthalmol Pakistan. 2016;6:29–32. [Google Scholar]
- 17.van Steenoven I, Aarsland D, Hurtig H, Chen-Plotkin A, Duda JE, Rick J, et al. Conversion between mini-mental state examination, montreal cognitive assessment, and dementia rating scale-2 scores in Parkinson’s disease. Mov Disord. 2014;29:1809–15. doi: 10.1002/mds.26062. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 18.Yang H, Yim D, Park MH. Converting from the montreal cognitive assessment to the mini-mental state examination-2. PLoS One. 2021;16:e0254055. doi: 10.1371/journal.pone.0254055. doi: 10.1371/journal.pone. 0254055. [DOI] [PMC free article] [PubMed] [Google Scholar]