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Development of a clinically feasible logMAR alternative to the Snellen chart: performance of the "compact reduced logMAR" visual acuity chart in amblyopic children

¹ Vitreo-Retinal Unit, St Thomas’s Hospital, Lambeth Palace Road London SE1 7EH, and Kent County Ophthalmic and Aural Hospital, Church Street, Maidstone, Kent, UK
² Kent County Ophthalmic and Aural Hospital, Church Street, Maidstone, Kent, UK
³ Department of Epidemiology and International Eye Health, Institute of Ophthalmology, University College London, Bath Street, London EC1V 9EL, UK

Correspondence to:
Correspondence to:
Mr D A H Laidlaw, Vitreo-Retinal Unit, St Thomas’s Hospital, Lambeth Palace Road London, SE1 7EH, UK;
allaidlaw{at}btinternet.com

ABSTRACT
Background/aim: The "compact reduced logMAR" (cRLM) chart is being developed as a logMAR alternative to the Snellen chart. It is closer spaced and has fewer letters per line than conventional logMAR charts. Information regarding the performance of such a chart in amblyopes and children is therefore required. This study aimed to investigate the performance of the cRLM chart in amblyopic children.

Methods: Timed test and retest measurements using two versions of each chart design were obtained on the amblyopic eye of 43 children. Using the methods of Bland and Altman the agreement, test-retest variability (95% confidence limits for agreement, TRV) and test time of the cRLM and the current clinical standard Snellen chart were compared to the gold standard ETDRS logMAR chart.

Results: No systematic bias between chart designs was found. For line assignment scoring the respective TRVs were 0.20 logMAR, 0.20 logMAR, and 0.30 logMAR. Single letter scoring TRVs were cRLM (95% CL 0.17) logMAR, ETDRS (95% CL 0.14) logMAR, and Snellen (95% CL 0.29) logMAR. Median testing times were ETDRS 60 seconds, cRLM 40 seconds, Snellen 30 seconds.

Conclusion: The sensitivity to change of the cRLM equalled or approached that of the gold standard ETDRS and was at least 50% better than that of Snellen. This enhanced sensitivity to change was at the cost of only a 10 second time penalty compared to Snellen. The cRLM chart was approximately half the width of the ETDRS chart. The cRLM chart may represent a clinically acceptable compromise between the desire to obtain logMAR acuities of reasonable and known sensitivity to change, chart size, and testing time.

Keywords: visual acuity; vision test; amblyopia

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