Evaluation of Letters versus Numbers (ELEVEN Study) ETDRS visual acuity chart in English alphabet illiterate population

Abstract

Objective: Current standard for visual acuity measurement is the use of ETDRS alphabet chart in most if not all International clinical trials. We aimed to evaluate the repeatability and agreement of the Best Corrected Visual Acuity (BCVA) using the ETDRS number chart compared with ETDRS alphabet charts in healthy eyes of patients who do not recognise the English Alphabet. Methods: A prospective, randomised controlled trial. Participants were tested for literacy by survey and recognition of tested alphabets. They were then randomised into 2 groups, where one group’s BCVA was measured with the ETDRS PV Numbers charts first, followed by the ETDRS Revised Sloan Letters charts while the second group would be reversed. Participants were asked to read as many optotypes as possible according to the ICO Visual Acuity Measurement Standard (1984). The 4 chart measurements per participant were performed and recorded by a single optometrist in the same testing room with similar lighting conditions. Additionally, the test duration for each chart was also recorded. Results: 120 eyes in 60 patients were included, 30 patients in the illiterate group and 30 patients in the literate group. In the illiterate group, the BCVA mean score was significantly higher for reading numbers, 52.35, while the BCVA score for letters was 43.75 (p=0.0001). The time taken to perform the BCVA was also significantly faster in the numeric group, 85.71 seconds per eye, versus 301.42 seconds per eye in the letters group (p<0.0001). In the literate group, the BCVA mean score was not significantly higher for reading numbers, 56.00 versus reading letters 51.88 (p=0.383). However, the time taken to perform the BCVA was significantly faster in the numeric group, 28.38 seconds, versus 43.88 seconds per eye in the letters group (p=0.004) There were no significant differences noted between the two eyes of the same patient for test duration and score. There were also no significant differences between test duration and score whether the letters or numbers were tested first. Conclusion: In conclusion, the ETDRS best corrected visual acuity score may not be suitable for patients who are unable to read the English alphabet due to prolonged test duration and significantly lower scoring.